Hadronic Production of Charm Particles

FORTSCHRITTE DER PHYSIK VOLUME 39 1991 NUMBER 5 -6

~~ ~ ~ ~~~ ~

Fortschr. Phys. 39 (1991) 5-6, 347-391

Hadronic Production of Charm Particles

HANNELIES NOWAK

Institut fur Hochenergiephysik Berlin-Zeuthen

Abstract

Charm hadron production is reviewed from the view-poin of an experimentalist. Results are presented on total cross-section measurements for charm-anticharm pairs, on its dependence on the atomic num,ber as well as results on xF and p I * behaviour of charm particles produced in hadronic interactions. Correlation studies and charm hadron spectroscopy are also included in this review.

1. Introduction

Since the discovery of hidden charm in 1974 and open charm two years later a great effort has been made to understand the production of heavy flavours. Experimental difficulties as well as theoretical complications in studying charm hadro-production characterise the physical situation.

Experimentally, the immense background in hadronic production of charm particles (one charm event in about los inelastic interactions) as well as the short life times of these particles require experiments with appropriate reduction rates, i.e. fast intelligent triggers, and high statistics. Excellent particle identification is also needed because of the large variety of branching modes each with a ratio of a few percent.

Theoretically, charm hadro-production is a good testing ground for perturbative QCD, though perturbative theories are better suited for heavy quarks with higher masses like beauty or top. But nevertheless charm production has already led to next- to-leading order calculations and thus to a deeper understanding of the production properties of charm particles.

Additionally, to look for new phenomena a t the future accelerators like UNK in Ser- pukhov or the new storage rings planned for the end of this century like the Large Hadron Collider ar CERN and the SSC in the USA need a deep understanding of the backgroundprocesses. The hadro-production of charm will be one of the most important background sources in searching for new phenomena.

The aim of this article is to review our present knowledge on charm hadro-production from a more experimental view-point. Oneimportant motivationis to work out the open questions to be solved in the near future.

The data from two experiments, NA 27 a t CERN and E 743 a t FERMILAB, 60th working with thesmallLexan Bubble Chamber (LEBC) as vertex detector are often used as reference data because of their small systematic errors, low background, high reso lution and high efficiency of the accompaniedspectrometer. But it has to be mentioned

1 Fortschr. Phya: 39 (1991) 5-6

348 H. NOWAK, Hadronic Production of Charm Particles

already here that the collected statistics limits the physical possibilities in studying very rare phenomena.

In chapter 2 the present knowledge of charm particle spectroscopy is summarized with emphasis on charm baryons. In chapter 3 charm hadro-production is described. The dependence of the charm cross-section on the atomic number A is studied in chapter 4. In chapter 5 the inclusive spectra of charm particles in the variables Feynman x (xF) and transverse momentum (p12) are investigated. Chapter 6 is devoted to correlations between charm particles produced in the same interactions, and to the multiple production of charm pairs. The results of this review are summarized in chapter 7.

2. Charm Spectroscopy

2.1. Spectroscopy of Charm Mesons

Rapid progress has been made during the last years in charm particle spectroscopy, especially for charm mesons. In the framework of the standard quark parton model the three lowest lying pseudoscalar states are formed by a c-quark in combination with a light anti-quark. In the quark constituent model the Do(cu), D+(cd), andD,+(cs) [l] corre- spond to the lE0 states. Do and D+ form an isospin doublet, D,+ is an isospin singlet. Do [2] and D+ [3] mesons were first reported in 1976 from e+e- reactions. The D,+ has a more complicated history (see e.g. [4]) but the first real result was published by the CLEO collaboration [5 ] in 1983. To each ground state meson corresponds a vector state

Table 1

Main characteristics of charm mesons

Meson J p Quark Content Mass in MeV Lifetime x 10-13 s

DO 0- CQ 1864.5 f 0.6 4.26+:::: D+ 0- cd 1869.3 f 0.6 10.46+,0:$:

-

Ds+ 0- ci 1969.3 f 0.7 4.393f58 D*O 1- cu 2007.1 f 1.4 - D*+ 1- ca 2010.1 & 0.6 - D,*+ 1- CB 2112.7 f 2.3 - D**O 1+, 2+ cii 2422 4 -

These states denoted as D* are also well established by now [6-81. The first candidate for an orbitally excited state (D**O) has been observed by the ARGUS collaboration [9] already in 1986. This state is wellestablished today. In the photoemulsion experiment E 564 [lo] an indication was found for a radially excited D8*+ meson a t 2790 GeV decaying into D*OK+. However because of poor statistics this state needs confirmation. Table 1 summarizes the meson data like masses [ll] and lifetimes [12], quark content as well as spin and parity.

Fig. 1 contains all recently available measurements of the D,+ meson mass. There are 13 measurements from 12 different experiments showing the progress made during the last years. It is remarkable that the measurements with the smallest errors were obtained in the photon and hadron experiments E 691 [13], NA 11/32 [14], and NA 32/2 [5 ] .

Fortschr. Phys. 39 (1991) 5-6 349

Ds MESON MASS MEASUREMENTS

CLEO

TASSO

ARGUS1

ARGUS2

NA11132

NA3212

T PC

HRS

MARK 3

E 591

E LOO

WA 58

E 531

u- 1920 19LO 1960 1980 2000 M(McWc~)

Fig. 1. Results of different D,+ mass measurements together with the mean value of N ( D f ) = (1969.3 f 0.7) MeV

2.2. Spectroscopy of Charm Baryons

Introducing thefourth quark “charm” leads to an extension of the SU3 multiplets to those of SU,. The lowest lying charm baryon SU, multiplets are presented in fig. 2. The ordinary baryons are arranged in the ground plane (c = 0). In the (c = 1) - plane of the J p = 1/2+ multiplet the &+(cud), the S,+(esu) and S.,O(md) can be found. The IRc(css) belongs to the J P = 3/2+ multiplet. The experimental situation in charm baryon spectroscopy is more confusing than for charm mesons. Except for Acf only a few and sometimes even contradictory measurements exist. The first measurement of the Ac+ mass was reported in 1975 by CAZZOLI and co-workers from Brookhaven [16]. Fig. 3

Fig. 2. Schematical of SU,-representation of J p = 1/2+ and J = 312+ baryons

1*

350 H. NOWAR, Hadronic Production of Charm Particles

CAZZ

KNAPP

BALT

CNOPS

GlBO

MARK2

ALLA

CALK

K I T 1

RUSS

BOSE

KIT 2

81s- 2

E531

WAS8

NA 27

CLEO

NA32

ARGUS

E G91

R 608

E 400

& BARYON MASS MEASUREMENTS

-1

-4 -4

I - I I I 1 I I

2240 2260 22W 2300 2320 M,MeV,&

Fig. 3. Results of different&+ mass measurements together with the mean value of M&+) = (2286.3 & 0.5) MeV

summarizes all available A,+ mass measurements together with the mean value cal- culatedto be (2286.3 f 0.5) MeV. This mean value is clearly dominated by the recent measurements of NA 32 1151, E 691 [17] and ARGUS [MI.

Data on strange charm baryons are also very rare. CLEO has recently found a Sco state decaying into E-x+ [24]. (32 f 8) events a cluster around a mass value of (2471 f 3) MeV. In fig. 4a is shown the invariant mass distribution of E-x+ combinations with the decay angle 0 of the x+ in the E-x+ center-of-mass frame in the range -0.8 < cos 0 < 1.0. The data are fitted with a Gaussian having a fixed width of 26 MeV above a background approximated by a polynom. Using a further cut in the zp variable ( z p z = pZ/(E&,, - Mk,)) a much clearer peak is found. Fitting this distribution leads to a mass value of (2472 f 3) MeV coming from 16 events over a background of 2.1 events. The systematic error is determined to be +4 MeV.

The E: is a (csd) quark state. Therearetwo possible states one being flavour symmetric under s and d exchange having a mass of 2604 MeV [25], the other one being antisymmetric with a mass of 2505 MeV. The authors conclude that the Z,O found by the CLEO collaboration is close to the antisymmetric state because of its mass value.

A reanalysis of the data was published in the second article of ref. [24]. After cali- brating the mass scale using the decays of D+ -+ K-x+x+, DO -+ K-x+ and SO(1530) -+ E-x+ the E,O mass value has changed slightly to (2472 -f 3 -f 4) MeV. In the same article the CLEO collaboration has presented a Ec+ signal in the Z-x+x+ decay mode. A fit to this effective mass spectrum using a Gaussian (FWHM = 22 MeV) over a poly- nomial background results in a signal of (23.0 f 6.3) events with a central mass value of (2467 f 3) MeV. The systematic error turned out to be also A 4 MeV. The total syste-

Fortschr. Phys. 39 (1991) 5-6 351

matic uncertainty in measuring themass difference between these two states was deter- mained to be f 1 MeV. The measured value of the isospin splitting of (-5 & 4 & 1) MeV is in good agreement with theoretical predictions. The Ec+ was seen up to now in four experiments [15, 24, 27, 281 in four different decay modes.

600.

500.

v! 4 0 0 . r

16

% = 12 0 - !n

> w

( C I

225 2A5 2.65

Mass ( ~ - l T * l l G e V / c z l

Fig. 4. a) CLEO results on 3 , O decaying into Z-x+ with 0.8 < cos O(x+) < 1.0. The curve is the result of a fit leading to N(Zco) = (2472 f 3) MeV b) same as a) but with the additional cut xp > 0 c) A%- mass distribution from the same experiment showing a clear Z- signal

Three experiments have reported on the production of the charm baryons C,++ and C,O decaying strongly into h c + x f [19,20,21]. Earlier measurements based on very limited statistics [16,22] gave already an indication for the existence of these states. On the other hand neither NA 27 [23] nor ACCMOR [ 151 have seen a significant production of C, states. The spin parity assignment for the ZC baryon can be J p = 1/2+ as well as J P = 3/2+ (see fig. 2). The present statistics does not allow a final conclusion on spin and parity.

ARGUS has measured the mass difference between the Cc++ and the Cco caused by isospin splitting. The value of (1.2 0.7 f 0.3) MeV is compatible with zero within two standard deviations. CLEO [26] has also measured this mass difference and has obtained a result fully agreeing with the ARGUS measurement.

The E 691 collaboration has found in their data (yN interactions) also a clear I;? signal a t the same mass value as it was seen by ARGUS. The I;,++ signal is very poor.

The E 400 measurements for the Cco - Ac+ mass difference are in disagreement with the results of the other experiments. The E 400 value of (178.2 f 2.0) MeV leads to an isospin splitting of (- 10.8 f 2.9) MeV which is non-zero and negative within two standard deviations.

Several models try to explain the mass differences [29] taking into account in different ways the constituent quark masses, colour hyperfine interactions and QCD Coulomb quark force. The predictions vary from +6.5 to -18 MeV. So the only conclusion is again to wait for more accurate data.

Table 2 summarizes the charm baryon parameters including for the Cc states the mass differences to the At+ ground state as measured by the ARGUS experiment [19].


Table 2 Main characteristics of charm baryons

~~

Baryon J p Quark Mass Lifetime A M content [MeV1 x 10-13 8 [MeV1

A,+ 1/2+ cud 3286.3 f 0.5 1.52::; - zc+ 1/2+ MU 2460 f 19 4.32;:; - Zi 0 1/2+ cad 2472 f 3 - -

3/2+ e56 2740& 20 -6.0 - Qc

168.2 f 0.5 c,o cdd 2451.0 f 1.2 - 167.0 & 0.5 g;+i cuu 2452.2 f 1.7 -

3. Total Charm Cross Section in Hadronic Interactions

Charm production in e+e- reactions is caused by the electro-weak interaction, whereas in hadronic reactions the strong interaction is at work for heavy quark production. Therefore, it can be used toprobe the today’s theoryof the strong interaction, the quan- tum chromodynamics (QCD).

The existence of good experimental results on charm and beauty production has sti- mulated the development of theoretical descriptions of these processes. Planned experiments on future hadron-hadron colliders include the search for the sixth quark, the top. To separate this process from the background of charm and beauty production a realistic picture of the production of those lighter heavy quarks is needed.

In the first section of this chapter a short description of the basic principles of perturbative QCD is given from the view-point of an experimentalist. Then a comparison of total cross-section measurements with existing model calculations will be made. Further on, the experimental results on charm pair production cross-sections are presented (mainly from NA 27) and compared with other experimental results as well as with theoretical predictions. In this chapter we restrict ourselves to charm hadroproduction on Hydro- gen to exclude the A-dependence of the process.

3.1. Charm Quark Production in QCD

The hadroproduction of a heavy quark Q can be described as an interaction of two incoming strongly interacting particles. The produced quarks are coloured objects frag- menting into heavy particles (mesons or baryons). Assuming an inclusive production of a heavy quark Q with an energy E and a momentum p according to

HA(PI) + H B ( P 1 ) &(P) f (3.1)

leads to the standard perturbative QCD ansatz [30] :

The densityfunctions F i A , B of the light partons are evaluated a t a scale p. 6 is the short distance parton-parton cross-section including no more mass singularities calculated by standard perturbation QCD as perturbation series in a,(p2). The scale ,u is chosen to be of the order of the mass m of the produced heavy quark. The scalep includes some uncer-

Fortschr. Phys. 39 (1991) 5-6 353

t a in t i s which will manifest themselves in an uncertainty of the cross-section. Formula (3.2) is valid on the quark level and presumes a factorisation of the fragmentation process. Although there is no proof of this assumption the scheme works very well at SPS and Tevatron energies. Equation (3.2) includes no flavour excitation since the sum runs only over light partons. Interactions with spectator partons are suppressed by powers of the heavy quark mass and are therefore alao excluded.

Theoretical calculations for the charm pair cross-section are presently available in the second and third order of perturbative QCD. The subprocesses contributing to the inclusive cross-section are summarized in table 3.

Table 3 Parton subprocesses contributing to the inclusive cross-section

Subprocess Order of as

Starting with the lowest order processes glum and quark fusion are used to calculate the cross-section of charm pair production. The invariant matrix elemenis for these processes are given following the notation of ref. [30] :

(3.4)

The matrix elements are summarized and averaged over initial and final state properties like colour and spin. They depend on the SU(N) colour group ( V = N 2 - 1, N = 3 ) . The variables ti and e, combinations of the momenta of the partons and the mass m of the heavy quark, are defined as follows:

The coupling constant is denoted by g.

as Using these matrix elements the invariant parton-parton cross-section can be written

with s as the total parton centre-of-mass energy squared. In ref. [31] this formula was applied to fixed target energies. Fig. 5 taken from ref. [31]

includes the NA 27 and E 743 cross-section values (their determination will be described later). They indicate that these lowest order calculations are not sufficient to describe

354 H. Now=, Hadronic Production of Charm Particles

- n 1 0 ~ a

u -

1

1 0 0 - 1 1 1 1 1 1 I I 1 1 I , I 1 I - -

- - H N 1 P N ------__

- mc=12GeV -

-...--- -

____------ _____------ * ____-----

:; /--- ____----

- mc=1.8 GeV - __-__----- ___---

,.*- ' 1 * f I ' I I I " ' I , "

the experimental data. But also theoretical arguments exist suggesting that higher order corrections mostly due to the fragmentation of gluons could be large. The main process is

It is of the order 61,3 and can numerically be as important as the 0(ua2) cross-section. The reason for this effect is the large gluon production cross-section. Experimentally was observed that this process is especially important if the heavy quark is produced inside a high energy jet [32].

A full calculation of the inclusive cross-section for heavy quark production in the order o~~~ is described in [33]. The total cross-section for the production of a heavy quark pair (following the notation of ref. [30] again) is then

(3.8) dS) = ,L' J ds, dszaij(s1, x ~ , S , m2, p2) F i A ( Z i , P ) PiB(sz, PI - ij

It depends on S, the centre-of-mass energy squared of the incoming hadrons A and B. The total short distance cross-section 6 can be written as

The f i i are dimensionless and have the following perturbative expansion

(3.9)

(3.10)

The energy dependence of the cross section is given by the e parameter.

Fortschr. Phys. 39 (1991) 5-6 355

The running coupling constant u8 is determined by the renormalisation group. Ex- plicit expressionsand detailed analyses can be found in [34]. Fig. 6 presents the next-to- leading order calculation for the total cross-section [35] together with the data points from NA 27 and E 743. It is clearly seen that the large increase of the cross-section pro- vided by the O ( U , ~ ) contribution leads now to a reasonable description of the cross-section behaviour in the SPS and Tevatron region. In the ISR region the cross-section is still to small in comparison to the data. BERGER [35] has given a value in the order of

t /------

- ", 10 - b t / / ,' pp-CFX

Q'= c m t

DO1 - ---MRSl

200 coo 600 800 P l o b [GeVl

Fig. 6. Beam momentum dependence of u(&) in next-to-leading order calculation from ref. [35]. The data points are again from NA 27 and E 743

55- 100 pb for the total charm cross-section at fi = 63 GeV. The measured A,+B cross- section even from R 608 [36] exceeds this value at least by a factor of two. Another open point is the threshold behaviour of this cross-section. The BIS-2 collaboration has recently published [37] u - B values for a limited xF and p , region in the order of 1 pb indicating a total value of a t least 10 pb near threshold. This result can not be explained by presently available models.

3.2.

In this section the determination of the total charm cross-section in different experiments a t different energies will be discussed. The methods used for the cross-section calculations will be described in some detail together with the experimental set-ups. The experiments are ordered according to the increasing beam energy.

Experimental Determination of the Total Charm Cross-Section


3.2.1. The Experiment BIS-2

The results of a charm search on Carbon done with the BIS-2 spectrometer were already published in 1984 [38,39]. Recently the collaboration has presented their first result on neutron production of Ac+ baryons off Hydrogen [37]. The mean neutron energy was about 58 GeV. 11 - lo6 neutron-proton interactions were recorded. A schematical view of the BIS-2 spectrometer as used for this part of the experiment is shown in fig. 7. The target of 2.1 g/cm2 liquid Hydrogen has a cylindrical form with a diameter of 6 cm. It

1'

1 2 3 m

Fig. 7. Layout of the BIS-2 spectrometer as used for the runs with the Hydrogen target

was surrounded by a recoil proton detector (SOM). 14 two-coordinate proportional chambers were used for the coordinate measurements. The analysing magnet M1 was changing the transverse momentum of charged tracks by -0.625 GeV/c. Two multichannel threshold cerenkov counters (C1 and C2) were used to identify charged particles. A Lead glass counter (C3 and C4) was responsible for the identification of photons. Two scintillator hodoscopes were involved together with some MWPC's to trigger for four or more charged particles in the whole spectrometer.

To search for charm particles in the decay channels

A,+ -+ Aox+x+x- (a)

and Ac+ + Eopx+x- (b)

(3.11)

only events containing a K,O --f xi-x- or Ro -+ px- and three or more charged particles were used. About 3 . lo4 candidates for the d.:cay mJde (a) and 6 - lo4 candidates for the decay mode (b) were found. Only events were used with an invariant (x+x.- ) or (px-) mass within an interval of f 10 or &8 MeV of the KO or Ao mass, respectively. To exclude faked KO or Ao from primary tracks or other interactions further cuts were introduced. The vertex of the Vo had to be located at least five centimeters behind the end of the target and the minimum distance between theVO and the tracks from the primary vertex had to be smaller than 1 cm. After those cuts 4853 events containing a K,O plus three or

Fortschr. Phys. 89 (1991) 5-6 357

more tracks and 3 118 events with Ao plus three or more tracks were used. To exclude events from the wall of the Ironvessel 0.5 - lo6 triggers with an empty target were taken and analysed in the same way. After subtracting this background and with the help of particle identification 758 K,Opx+x- combinations and 645 AOx+x+x- combinations were found. As can be seen from fig. 8 some enhancement in the A,+ mass region is present in both distributions. A fit to the combined spectrum gave (35 10) A,+'s a t a mass value of (2.268 f 0.005) GeV. For both decay modes the invariant mass spectra of the charge conjugated mass combination does not show any peak in the Ac+ mass region.

'I 1

2000 2100 2 2 0 0 2300 2400

M I A" n%%- I G eV I c z I

30

t I 2 0 0 N

... z

10

I I 2000 2100 2200 2 3 0 0 2400

M I K z p lr*lr-) I G e V / c 2 )

Fig. 8. a) Effective (hox+x+x-)

and b) (Ezpx+n-) mass distributions from the BIS-2 Hydrogen data

The apertureof the BIS-2 spectrometer allows only to register A,+ decays in the kinematical region of zF > 0.5 and p , < 1 GeV. A detailedMonte Carlo study was performed to determine the efficiency of the spectrometer including reconstruction efficiency, efficiency of each detector, trigger conditions, multiple scattering and corrections for unseen Ao and KO decays.

The following values for IS - B were obtained

IS(ZF > 0.5) * B(A,+ -+ E0px+x-) = (1.0 f 0.3 f 0.2) pb

U(ZF > 0.5) * B(A,+ -+ A"x+x+x-) = (0.28 f. 0.15 -+ 0.04) pb. (3.12)

The first error is the statistical one. The second value represents the systematic error including the uncertainties coming from the neutron flux and from the determination of the exact number of wall interactions.


To calculate the total charm cross-section from these numbers needs some further assumptions concerning branching ratios and behaviour of the cross-section for X, < 0.5. Using a branching ratio of 1.4% for the decay mode At+ -+ AOx+x+x- (renormalised mean value of ARGUS and CLEO measurements [40]) leads to a value of

(3.13)

Following the arguments of [40] all the Ac+ branching ratios could be too small a t least by a factor of two using the normalisation to the Mark I1 (pK-x+) branching ratio. ARGUS and NA 27 results indicate also that there is still a big uncertainty in the data.

To extrapolate to the full xF region is difficult near threshold. Measurements on Carbon done with the BIS-2spectrometer [37,38] show that charm production a t these energies is dominated by diffractive dissociation. Under these assumptions the cross-section np -+ cB + X can be estimated to be between

(3.14)

~ ( x F > 0.5) = 20pb.

10 pb < o < 50 pb.

3.2.2. The Experiment NA 2'7

The experiment NA 27 [41-441 was designed to study the main features of charm production and decay. The combination of the high resolution bubble chamber LEBC with the European Hybrid Spectrometer (EHS) allows an excellent separation of production and decay vertices as well as a very good momentum determination of the charged particles and also a xo reconstruction over a wide kinematical region. The set-up of EHS as used in the experiment NA 27 is shown in fig. 9.

THE EUROPEAN HYBRID -SPECTROMETER

LEBC PIC SAD lSlS IGD INC FC TRO FGD FNC

U1 U 3 M1 W2 D1 D 2 D 3 M2 D4 D5 06

I I 1

0 10 2 0 30 40 rn

Fig. 9. Layout of the European Hybrid Spectrometer as used in the NA 27 experiment

The incoming proton beam had a momentum of (400.0 f 1.2) GeV/c. To reach a good resolution in the small LEXANBu'jble Chamber (LEBC) used as target and vertex detector a small bubblediameter and a large bubble density are needed. Table 4 summarizes the main properties of this chamber.

The chamber was placed in front of the first magnet (Ml). This allows an easier scan of the pictures because straight tracks were showing better the charm signature. The chamber was filled with Hydrogen to dlow the study of charm production on protons. The background on Hydrogen due to secondary interactions is negligible. Because of the proton beam a rejection of all interactions with an odd number of tracks reduces the background to a very small fraction.

Fortschr. Phys. 89 (1991) 5-6 359

Table 4 Main properties of LEBC

Temperature Expansion frequency Camera frequency Bubble density Bubble diameter Bubblesltrack (5 cm) Volume

29.0 K 30 Hz 15 H z 80 bubbles/cm 17 pm 400 120 - 50 25 mms

The measurement of particle momenta was performed by multiwire proportional and drift chambers including the big ionisation sampling drift chamber ISIS. Two magnets changed the fictive transverse momenta of charged tracks by 480 and 450 MeVIc, respectively. Charge particle identification was done over the whole enery range. In the first lever arm the cerenkov detector SAD and the ionisation sampling detector ISIS were used for the identification of charged particles. Neutral pions and photons were detected by the intermediate gamma detector IGD, a Lead glas calorimeter. Neutral hadrons were measured by the intermediate calorimeter INC. Fast particles entering the second lever arm of the EHS were identified by the forward cerenkov counter FC, the transition radiation detector TRD, the forward electro-magnetic calorimeter FGD and the forward hadronic calorimeter FNC. During the exposures in 198311984 2.3 Mill. pictures were taken. In 23laboratories films were scanned for secondary decays using the same criteria.

A secondary decay was defined to have at least one track with an impact parameter greater than 50 pm. The impact parameter y (schematically shown in fig. 10) is defined as

y = f j . y . c . t . - PL P

(3.15)

where pL is the transverse momentum and p the momentum of the decay product, and t the proper lifetime of the decaying particle. The impact parameter y is of the order of c . t due to the proportionality to p L / p - lly,!? and therefore independent of the momentum.

To avoid most of the strange particles already a t scanning stage events were measured only if the transverse decay length LT (see fig. lob) of the corresponding charm candidate was less than 2 mm.

After first measurements most of the strange particles among the two-body decays were removed by fitting the appropriate channels. The remaining candidates must have a charm topology (four or six charged decay tracks in a neutral decay or three or five

Fig. 10. Schematical repiesentation of a) the impact parameter y and b) the transverse decay 1engt.h L,


charged decay tracks in a charged decay) or at least one decay track with a transverse momentum of p, > 250 MeV with respect to the line of flight of the parent particle. All those events were measured a t the HPD measurement device.

The centre of each bubble was measured with a precision of 5 pm resulting in rms re- siduals of 1.8 pm for 25 master points used to reconstruct the track. The impact parameter precision was 2.5 pm at the main vertex.

The NA 27 experiment offers high quality data. The systematic error turns out to be negligible because of: - LEBC was filled with Hydrogen, therefore the background from secondary inter-

actions for the whole charm sample was (0.7 events. This limit is rather conser- vative since an invisible proton range up to 300 pm (> 10 bubble sizes) was required.

- The absence of a special charm trigger removed any trigger bias in data taking. - The high resolution optical system led to a high detection efficiency of (95 5)y0 for

more than two-prong decays and (90 f 5)% for the two-prong decays. - The high precision measurement device HPD ensured that tracks and vertices were

correctly related. Taking into account the HPD impact parameter resolution of 2.5 pm a background due to track misassignment of about 0.5% was deduced.

- Further cuts applied to the data removed almost all strange particle background. From detailed Monte Carlo calculations a background of less than 1 event for the whole sample at scanning stage was predicted.

The high particle identification level of the EHS was extremely important for the definition of the Act sample. Here the presence of a uniquely identified proton among the decay products having the same charge as the charm baryon was required. Further- more only unique 3 C fits, i.e. without ambiguity to D+ or D8+, were accepted. Analogous criteria were used to select unique D,+ mesons.

557 charm decays were found in the NA 27 pp experiment. For the determination of the cross-section only decays with a clear topology, definite decay length, and minimum and maximum impact parameter values as listed in table 5 were used. Furthermore for one-prong decays the decay has to be inside the spectrometer acceptance and for more- prong decays a t maximum one track was allowed to be outside of the spectrometer ac- ceptsnce (lei < 150mrad, (ill < 150mrad). The chosen y,,,interval guaranteesthat more than 90% of theD,+ mesons and more than 98% of the A,+ decays were excluded. Thus the remaining 217 decays were taken as D mesons. The corrections applied for unseen decays, remaining unclear topologies and possible D,+ and A,+ decays in this sample were

Table 5 Cut parameters and number of decays in the different decay topologies as used in the NA 27 analysis

Number of L/mm Ymin/pm Y m a x I ~ No. of decays charged decay tracks min. max. min. max.

1 2 90 100 - 1500 39 3 1 - 20 100 2 000 74 5 1 - 20 100 2 000 6

2 2 30 20 60 500 67 4 1 - 20 60 1500 30 6 1 - 20 60 1 500 1

Fortschr. Phys. 39 (1991) 5-6 361

1 I I 1 I I I 400 GeV pp A>- pK-n+

%

-..- NOFIT 1C-FIT 2C- FIT

- 3C-FIT -

- ----

-

-1.0 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6

xA c

400GeV pp A:- pK-n+no

V O 1 I I I I I I I

..-

NOFIT OC- FIT ---- 1C-FIT

-. .- . . . . . . .

_ - ZC-FIT 3C- FIT . .

... . . . . ..

0.8 1.0

b) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1.0

X A,

Fig. 11. Acceptance of the EHS for different A,* decay modes a) A,+ + ~ K - x + and b) A,+ + pK-x+xo

rather small, in average 2.5 per event. This has to be compared with correction factors between lo4 and 105 for counter experiments. As can be seen from fig. 11 for A,+ decays the spectrometer acceptance for charm particle decays was nearly 100% for XF > 0. D,+ and D mesons have shown the same behaviour. This means that no production model dependent corrections were necessary.

The following inclusive cross-sections were obtained :

a(D+/D-) = (11.9 f 1.2 f 0.8) pb

a(DO/DJ) = (18.3 f 1.9 f 1.6) pb

o(D/D) = (30.2 f 2.2 f 2.4) pb.

(3.16)


Using now the experimental fact that most of the charm decays were found in pairs a cross-section for DD pair production of

a(Db) = (14.6 & 2.0) pb (3.17)

The clearly identified A,+’s were produced also in pairs leading to a Ac*D cross-section

a(A,+D/K,D) < 6.1 pb a t 90% c.1. (3.18)

was obtained.

of

Analogously[an upper limit for the D,+D cross-section can be given as

a(Ds+D) < 1 pb a t 90% c.1. (3.19)

The charm pair production cross-section at fi= 27.4 GeV is in the region between 13 and 24 pb. This value is presented in figs. 5 and 6.

3.2.3. The Experiment E 743

The experiment E 743 [45] was performed 1985 at the FNAL Tevatron accelerator using the same small Lexan bubble chamber (LEBC) as NA 27 and an existing spectrometer, the Fermilab MPS, in a proton beam of 800 GeV/c. The LEBC chamber already described in section 3.2.2. served again both as liquid Hydrogen target and as high resolution vertex detector. Downstream of LEBC two proportional wire chambers like in NA 27 gene- rated an interaction trigger when more than two particles were registered in coincidence with the presence of an interacting beam particle. The overall trigger efficiency for events with more or equal four charged tracks was (88 f 2)%. For events containing charm this efficiency became > 99%. Atotal of 1.18 million triggers yielded 0.5 million proton- proton interactions in Hydrogen. The remaining triggers corresponded mainly to wall interactions. About 60% of the data were fully analysed.

The scanning and measuring procedure was analogous to the one in the NA 27 experiment. The measurements were performed a t CERN on ERASME machines and a t Mons on an Adam & Eva machine. The two-track resolution was 20 pm and a decay track could be clearly recognist:d if its impact parameter was greater than 10 pm.

Fig. 12 shows tht: s:t-up of this experiment. The MPS detector was used for momentum determination and particle identification. The MPS particle tracking system included wire chambers upstream and downstream of a magnet which imparted a 700 MeV/e transverse momentum kick to charged particles. The momentum error of a reconstructed track was determined to be A p / p = (0.3 + O.Olp)% with p in GeV leading to a momentum error of 1.3% for a 100 GeV track. Particle identificatioii was performed by two

4 E743 FNAI. MPS

10 15 2 0 25 30

z LMcter 1

Fig. 12. Layout of the Fermilab Multiparticle Spect.rometer as used in the E 743 experiment

Fortschr. Phys. 39 (1991) 5-6 363

cerenkov counters and a transition radiation detector covering the momentum region between 6 and 8@@GeV. Using the same method for cross-section determination as in the NA 27 experiment the following cross-sections were obtained

c(D+/D-) = (26 & 4) pb

a(DO/D) = (22:;) pb (3.20)

and - a(D/D) = (482i0) pb.

The quoted errors are statistical ones only. The estimated systematic uncertainty, mainly due to the branching ratio error, is 25%.

Taking into account the small part of Ac+ and D,+ produced in the experiment a charm pair cross-section between 27 and 57 pb at fi = 38.8 GeV was obtained.

3.2.4. ISR Experiments

In this section we compare two ISR experiments performed with different set-up's but nearly at the same energy. The first one (R 415) [46] was one of the first charm search experiments having successfully investigated A,+, D+, and DO. The data were published already in 198111982 and had a big influence on the development of theoretical descriptions. The second one (R 608) [36] has published its data in 1988 after a careful reanalysis using already the knowledge collected over the last six years.

The limited kinematical region and the high correction factor of 105 depending on acceptance and production mechanism are typical ingredients of both experiments.

3.2.4.1. The R 415 Experiment

The experiment R 415 [46 J was performed a t the CERN Intersection Storage Rings (ISR) using the Split Field Magnet (SFM) spectrometer. The main aim of the experiment was a study of charm production in proton-proton interactions at the highest available c.m.- energy fi = 62 GeV. For the experiment a trigger was designed making use of the relatively high semielectronic branching ratio ( M 10%) of charm particles.

TOF /

0 1 2 3 4 5 m

Fig. 13. Layout of the Split-Field-Magnet Spectrometer a8 used in the R 415 experiment

2 Fortschr. Phys. 39 (1991) 5-13


In fig. 13 is shown the set-up of the SFM spectrometer. The main features of the apparatus were - a powerful electron detector at 90" containing electromagnetic shower detectors,

brenkov threshold counters and some MWPC's with analog read-out to measure dE/dx

- a time-of-flight (TOF) system for particle identification up to 1.5 GeV (separation of x/K/p) and 2.0 GeV (proton separation from the rest)

- the usual SE?M MWPC system for the measurement of the momenta of charged particles.

Triggering for a single electron produced under 90" about 6 104 events were obtained The background from corresponding to a total integrated luminosity of 4.4 - loa6

60 70

N 0 "0 60

% 0 4 0 0 z

- - >

L 50

40

30 5 .- d 0 E

- Ln \

c 0 0

0 ._ c

._ c

n n

V 6 20

10

0.625 1.025 1.425 1825 2225 0.2 1 1.8 2.6 3.4 4.2

m ( K - r r * ) (GeV/c21 m (K-TOFPfB+) (GeVlc')

70

a~ 60 L

.... >

0 50 . 40

2 30 0 .-

C .;s 20 0

10

1.8 2.2 2.6 3.0 3.L 1.8 22 2.6 3.0 3.4

rn (K-pa') GeV / c 2 1

Fig. 14. Results of the R 415 experiment a) invariant (K-x+) mass showing a clear DO signal b) invariant (K-sc*x+) mass showing a D+ signal c) invariant (K-px+)-rnass showing a, clear A,+ signal d) same like c) but with an e+ trigger. No Ae+ is seen in the distribution

Fortschr. Phys. 39 (1991) 5-6 365

charged hadrons was less than 2%, that from neutral hadrons less than 50%. To identify also the charge of the electron only those particles with relative momentum error of less than 15% were retained. In the experiment the A,+ baryon and the Do and D+ mesons were obtained as peaks in the effective mass distributions of their decay particles (see fig. 14). No vertex detection was foreseen, so an increase of the signal to background ratios was reached by cutting in kinematical variables like the longitudinal momentum xL = 2 p L / f i and the rapidity y = 0.5 In [ ( E + pL)/(E - pL)]. The background was determined by plotting the same mass distribution in combination with an electron of the wrong charge. No peaks were found in these mass distributions. Fig. 14c presents the (pK-x+) invariant mass spectrum for events with an associated e- under the conditions IxL(p)] > 0.3 and K- and X+ identified. The peak in the A,+ mass region is a four standard deviation effect. For comparison Fig. 14d shows the same mass distribution accompagnied by a wrong charge electron. No peak in the Ac+ region can be seen.

The D+ was studied in its K-x+x+ decay mode. Again a peak of (39 f 11) events is visible (fig. 14a) if an associatedelectron of the right charge w@s present. Particle identification via TOF for K- and nf was again required. The Do was studied in the decay mode DO --f K-x+ (fig. l4b) with an associated decay of an anti-charm state decaying into e-K+ + anything. Again particle identification was required for all charged particles.

To calculate the total cross-section for the A,+, D+ and Do signals the following quan- tities must be known - for the observed particle

. the branching ratio of the observed decay mode . . the decay dynamics

. . . the production distributions (pl , xL)

- for the associated anti-charm particle . its mass

. . its inclusive semielectronic branching ratio . . . the decay dynamics

. . . . the production distributions (pl , xL) - correlations between the two particles.

Experiments measuring charm particle production in a limited phase space region have to correct their data by Monte Carlo calculations using a given model dependent xL and p , dependence as input. The obtained correction factor is often in the order of lo3 to 105. Three models concerning the xL behaviour were used for total charm pair cross- section calculation :

I. E(da/dzL) - (1 - \ x L ~ ) ~

11. dtrfdy = const. (3.21)

111. du/dxL = const.

Model I simulates a more central production of charm particles. The constant behaviour of the differential cross-section in dependence'on the rapidity y (model 11) was tried in this experiment but i t is not more used now. The model 111 assumes diffractive production of charm particles.

The p , dependence was always described as follows

dUldP, - exP ( - 2 P J . (3.22)

2*


No correlations between particle and antiparticle were assumed. The charm particle decay was treated according to phase space.

The obtained numbers vary over more than one order of magnitude in dependence of the production characteristics assumed. Using the knowledge of today central D production can be favoured. For the Ac+ we prefer to use the measured xL dependence.

The numbers given in Table 6 are recalculated according to the today’s branching ratios .-

A,+A,+, D,+D,+ and D,+D production were not taken into account. The first two processes are at these energies probably negligible but the third one could have a cross-section in the same order as that for D+D production (at least 1/10 of the total cross-section of the charm pairs). Taking into account a 50% error as it was assumed by the authors the CE cross-section is somewhere between 660 and 2000 pb. This value is much higher than that calculated by BERGER [34] including the next-to-leading order QCD calculations.

Table 6 Charm pair cross-section from R 415

CE xL for c xL for F

3.2.4.2. The Experiment R 608

In the experiment R 608 a t the CERN ISR a forward multiparticle spectrometer [36] was used at the highest available energy of fi = 63 GeV. The apparatus had an aperture of f150 mrad around the outgoing beam direction. It contained two threshold cerenkov counters used for particle identification and a mini-drift chamber system for tracking. The schematic view of the set-up is shown in fig. 15.

The reaction studied was

pp -+ Ac+ + X -+ (AOn+x+n-) + X. (3.23)

Wire C h a m b e r s B.

t-------r l

0 2 4 6 a m

Fig. 15. Layout of the R 608 Spectrometer

Fortschr. Phys. 39 (1991) 5-6 367

In the kinematical region of 0.5 < xF < 0.9 and p , < 1.1 GeV a signal of (621 f 103) events was found a t amass of (2305 3 f 6) MeV. Fig. 16 shows the acceptance corrected differential cross-section times branching ratio, B - do/dxF integrated over all p, . The acceptance correction was almost model independent (isotropic Ae+ decay was assumed). A fit of B . dx/dxF = (1 - x ~ ) ~ [black points in fig. 161 yields n = 2.1 f 0.3. The integrated cross-section for xF > 0.5 is

B * u = (2.84 f 0.50 f. 0.72) pb. (3.24)

In order to estimate the inclusive cross-section the branching ratio of the decay mode Ac+ --f AQx+x+x- must beknown. The authors have used the value of (2.8 f 0.7 f 1.1)0/, [47] as it was calculated by the CLEO collaboration. The results are shown as black squares in fig. 16. But using the branchingratios of 1.4%, as it was determined as an average of the renormalized CLEO and ARGUS measurements 1401, leads to a cross-section twice as high. The inclusive Ac+ cross-section in the given XF and p , interval is between 100 and 200 pb. No charm pair cross-section can be calculated from the R 608 data.

I 10

( 1 - x I F

Fig. 16. a&), a(&+) and B - a(&+) a8 a function of (1 - zp) as measured from the R 608 experiment

4. A-Dependence of otot

In this chapter all experimental results concerning cross-section dependence on atomic number, A, are reviewed and compared with theoretical descriptions. In the framework of perturbative QCD the cross-section of Drell-Yang processes depends on the number of quarks. For heavy quark production on nuclear targets the impuls approximation yields a linear A dependence of the cross-section. But due to the nuclear environment processes like Fermi motion, coherent scattering, rescattering and final-state interaction reduce the transparency of the nucleus leading to a “shadowing” and an A-dependence of the cross-section proportional to A213.

On the other hand it is well known from the production of ordinary hadrons that the A dependence of the cross-section depends on the variable xF (Feynman x) [48], that means on the production mechanism.

The first results on the A-dependence of charm particles were reported already in 1984 by the BIS-2 collaboration [49] and the E 613 experiment [50].


4.1. Direct Measurements of the A-Dependence


The A-dependence of the charm quark production was measured in a beam-dump experiment a t Fermilab using a 400 GeV/c proton beam directed onto thick targets of Tungsten, Copper, and Beryllium [51]. The hadronic cascade is absorbed inside the target, muons were either absorbed or deflected in magnetied Iron. The remaining neutrinos were detected 56 m downstream in a Lead-liquid scintillator calorimeter with a sub- sequent muon detector. Fig. 17 shows the set-up of the experiment.

I R O N VERTICAL P l T C H

CALORIMETER ToROIDS

MAGYET ,--I1 INCIDENT I P R O ~ O N TARGET: SBd' @EAM 3 h @ e , C U , o r W

DRIFT CHAMBERS

1 183ft.; 55,8m

Fig. 17. Layout of the E 613 spectrometer

To separate the prompt neutrinos produced in semileptonic decays of charm particles from the non-prompt background, the variation of the observed rate with target density was studied. The prompt rates were determined by a bin-by-bin extrapoIation of the corrected rates in the full and partial density targets. For each target was recorded the number of charged current events from vy and gy interaction together with the number of muonless (Op) events. To get from the Op event rate the prompt (v, + ge) rate all known background sources were subtracted. Table 7 summarizes the data used for the A-dependence determination.

The prompt (ve + ~j,) rates for each target were roughly the same. This means that the charm cross-section, if parametrised as A" must have an oc value similar to that of the inelastic proton cross-section on nuclei [52]. A fit of the data with a, = a, . A"' leads to a' = 0.03 0.05 with x 2 = 0.5 for one degree of freedom. Combining this with the A dependence of the inelastic proton cross-section on nuclei a = a, - A0.'2 yields

a = 0.75 f 0.05. (4.1)

Table 7 E 613 data as used for a determination

Target Protons Op events Op/L016 p (v, + Tie) prompt rate

Be 7.7. 10'6 958 155.3 & 4.8 41.9 f 5.4 c 11 2.1 * 1016 193 103.9 f 7.6 50.2 f 7.8 W 13.7 * 10" 1026 89 3 2.9 46.2 f 3.0

Fortschr. Phys. 39 (1991) 5-6 369

A linear A-dependence can be excluded by the data because of the bad x2 probability of 24.6 for two degrees of freedom. No variation of a with the neutrino energy can be seen within the limits of statistics. But for the highest energies (Ev > 120 GeV) a somewhat smaller OL value (0.58 0.12) was obtained.

4.1.2. The BIS-2 Experiment

In a special run the BIS-2 collaboration has studied the A-dependence of Ac+ production [53] using Carbon, Aluminium andCopper targets of 3.4 - All3 g/cm2 thickness. The spectrometer set-up and the selection criteria were described already in section 3.2.1. The Acf was detected by signals in the effective (KB0px+~-) and (Aox+x+x-) mass distribution. The kinematical region of A,+ detection was 0.5 < xF < 1 and p , < 1.0 GeV/c.

Table 8 Number of A,+ decays for different targets

~~~~~ ~~ ~

Decay mode Target

C Al c u

K,Opx+r 16.0 f 5.3 18.5 f 5.7 17.0 f 5.5 AOX+X-X- 8.0 f 3.3 7.5 & 3.2 7.0 f 2.8

SUM 24 f 6 26 & 7 24 f 6

To avoid any time dependent effect the targets were changed every 40-50 thousann interactions. Altogether 12. lo6 interactions were registered containing 3.9. 106, 4.2 - 106, and 3.8. lo6 interactions on C, Al, and Cu, respectively. In table 8 are summarized the obtained numbers of &+'s for the different decay modes and the different targets.

According to the measured neutron flux the following correction factors have to be taken into account :

C/Al/Cu = l/l.04/0.89. (4.2) The a value can be calculated from the following relation

Here are 6, = 1.04 and 6, = 0.88 the flux correction factors of A1 and Cu, respectively. The N i are the numbers of A<+ and the Ai the atomic numbers of the i-th target. This procedure results in

&(Ac+) = 0.73 -J= 0.20. (4.4)

4.1.3. The Experiment WA 78

The W A 78 collaboration has studied the production of charm particles on three different target materials (Aluminium, Iron and Uranium) using an incident 320 GeV X- beam [54] a t the CERN SPS.

Fig. 18 shows the set-up used. The essential parts of the spectrometer were a muon spectrometer and .a target calorimeter. The muon spectrometer consists of a bipolar


superconducting magnet and a system of drift and multiproportional wire chambers. The target calorimeter has two parts: an upstream expandable one and a downstream fixed one. The dense absorbers in the fixed part were made of Iron. The expandable part was constructed in a way that allows an easy replacement of absorber plates as well as changes in the target density. Threedifferent densities were used for each target. The vertex position was determined from the starting point of the first shower. Only events with a vertex point inside the target were used for the analysis.

Beam

"U uu I . . . , . . .

m 1 1 10 9 8 7 6 5 lo 3 2 1 0 1 2 3 4 5 6 7 m

Fig. 18. Layout of the CERN R-spectrometer as used for the experiment WA 78

The trigger required at least one muon in the spectrometer. To reduce background muons (from beam or halo) a cut in the muon momentum and in the calorimeter energy was made. Only muons with a momentum between 20 and 100GeV were accepted. Non- prompt muons from K or x decay were eleminated using different target densities and extrapolating afterwards to zero absorption length. Prompt muons from pfp- pair production with one undetected muon were also rejected with the help of Monte Carlo calculations.

Table 9 summarizes the event numbers for pf production from charm decays on different targets.

Table 9 Event numbers for pf production from WA 78

Target Interactions p+ rates p- rates

per lo6 pions

A1 31.106 55.0 f 14.0 93.0 -j= 16.0 Fe 33.106 73.0 f 9.0 104.0 f 11.0 U 70- lo6 68.0 f 5.0 115.0 f 6.0

Fitting these numbers with a power law in A leads to

"(pi) = 0.76 & 0.08 O L ( ~ - ) = 0.82 & 0.06.

(4-5)

Within the statistical significance no variation of OL with respect to xF was observed.

Fortschr. Phys. 39 (1991) 5-6 37 1

m

I


The E 400 collaboration has measured the A-dependence of Ecf production using a neutron beam of 640 GeV mean energy [28]. The spectrometer was located in the broad- band O'neutron beam of the FNAL in Batavia. Fig. 19 shows the layout of this apparatus. The experiment was run with three different targets (thin wafers of Tungsten, Silicon and Beryllium in a common vacuum chamber) separated along the beam by 2.5 cm. Long- lived charm decays located in between the different targets were detected by a high resolution vertex MWPC system consisting of nine wire,planes with 250 pm pitch in

. T1

-50 - CO

-100 -

TOP VIEW

O!

u CONCRETE Fe

1 1 1 1 1 1 , , 1 1 l , , I , l , , I I

0 500 1000 1500 2000 crn

Fig. 19. Layout of the E 400 spectrometer

three views. Good ability for high multiplicity events, i.e. reconstruction of events with up to 20 tracks, and good particleidentification by threecerenkov counters with 34 cells each were the advantages of this apparatus. The calorimeters were used for trigger purposes and neutron flux monitoring.

45 - lo6 events have entered the analysis. In this sample the following numbers of Ec+'s were found: (55.7 f 15.1) decaying into A°K-x+x+ and (46.7 f 15.7) decaying into Z°K-x+x-. To determine the functional dependence of the 9,+ cross-section on atornic number (and on xF andp,) a special minimum proper decay distance of 0.006 cm was required. Because of a steep acceptance drop for xF < 0.1 a further cut, XF > 0.15, was introduced reducing the number of EC's to (21 f 1) on W, (49 f 13) on Si and (34 f. 11) on Be. After correcting for the relative number of nuclei in the three target materials from the expression

(4.6) a(nAi 3 ..", + X)/U(7&Aj -+ EC + X ) = (A&$)"

01 = 0.90 f 0.13

a value of

(4.7)

was obtained for 0.15 < xF < 0.6 being consistent with a linear A dependence.

372 H. NOWAP, Hadronic Production of Charm Particles

0.4

4.2. Comparison of Experimental Results

Two of the four experiments described in section 4.1 have measured the A-dependence for charm baryon production, the other two for charm meson production. Within the statistical significance of the experimental data no difference can be found in the A- dependence of charm meson and charm baryon production. Therefore, as resonable approximation the weighted mean value from all measurements can be taken:

(a} = (0.79 f- 0.03)s (4.8)

This value favours anIA2I3 dependence. Fig. 20a shows the results of ,the four experiments together with the calculated weighted average.

-

d

Fig. 20. a) Results of the determination of the A-dependence of the charm cross- section from different experiments together with the mean value of a = (0.79 f 0.03). b) Same like a) but showing a as a function of the mean ZF of the corresponding

t I I I 1 I experiment 02 0.4 0.6 0.8 1.0 < x ~ >

Already from the production of ordinary hadrons it is known that the value of a depends strongly on Feynman xF. To study this effect in charm particle production the measured a values from the four experiments described above are shown in fig. 20b in dependence on the mean xF of the experiment. Even within the large errors the same tendency as for light flavour production can be seen. CHARM varies from 1 a t xF = 0 to about 213 at XF = 1.

For comparison the A-dependence of the cross-section determined for different light flavour particles[48] is shown in fig. 21. Here the parameter a is a much stronger function of xF with a value decreasing from 0.75 a t xF = 0 to 0.45 a t xF = 1. The p , dependence of a was demonstrated already in 1975 by CRONIN et al.

Fortschr. Phys. 39 (1991) 5-6

d.

as

373

or' 100 G e V u p 1 0 0 G e V 0 n 400 G e v - oh* 300 G e v

300 G e v - K O 300 Gev 0s' 400 Gev

24 Gev O K - 24 GeV v K ' 24 GeV V K - 24 GeV - p ZLGeV = P 2 4 G e V

0.6 0'7 t I

0 0.2 0.4 0.6 0.5

X F

Fig. 21. Dependence of the a coefficient from xF for different ordinary hadrons and different beam energies

The total inclusive cross-section can be described by the empirical formula

u(A) = KO * ~ ( p ) * A" (4.9)

where a(A) and u(p) are the nuclear and proton cross-sections, respectively. The normalisation parameter KO lies for light flavour data approximately in the range 1.5-2.0. It was introduced to avoid the experimental fact that the variation of the production cross- section with atomic number according to a simple power law is valid for heavy nuclei but seems to be problematic if Hydrogen data are included.

Another possibility to obtain the xF dependence of a C H A R M and a limitation for the KO factor was developed in ref. 1561. A direct comparison of NA 11 data 1571 (n- Be inter-

Comparison o f N A 11 and N A 2 7 In-1 xF- Distribution

Fig. 22. a) Comparison of xF distributions for D mesons from NA 11 (x-Be) and NA 27 (x-p) experiments. b) Dependence of a from XF normalised to a = 1.0 a t XF - 0


a5

actions a t 200 GeV) and NA27 data [58] (x-p data at 360 GeV) was done. The xF distributions of produced D mesons are presented in fig. 22a. Under the assumption that the production characteristics do not vary rapidly with the energy a xF-dependence of a can be deduced. Because of only two target materials a and KO cannot be determined inde- pendently. KO was estimated by fixing a = 1.0 at xF = 0. In fig. 22b the result is shown with KO = 1.5. It is important to notice that if KO = 1.0, OL would be greater than one in the central region. Finally, the authors mentioned that an A-dependence of this type tends to enhance D production in the central region when using nuclear targets. This could explain the absence of a leading D effect in the x- Be data [57] and the 7 ~ - Si data [59].

The problem of a and KO determination can be solved only by a high statistics experiment with different targets including Hydrogen. The experiment E 769 running a t Fermi- lab [60] since 1989 is the first one which can simultaneously measure a and KO.

DO - -

4.3. Theoretical Ideas to Explain the A-Dependence

The multiple scattering model was used successfully to describe high energy hadron-nucleus collisions. In this model the interaction with the nucleus can be taken as the sum of interactions with different nucleons. To calculate detailed values like inclusive spectra or the A-dependence it is necessary to choose a concrete formulation of the production model. In ref. [61] the author has used mainly the quark-gluon strong model [62, 631 to calculate the spectra of pions, kaons, nucleons, and ha hyperons produced in proton- nucleus and pion-nucleus interaction. The A-dependence of charm and light flavour particles is calculatedanalogously. Using the model of ref. [63] for the production of charm hadrons a is calculated as a function of xF for pA interaction a t 50 GeV. Fig. 23a presents the dependence of a on xF for DO mesons and &+ baryons in the A range of 12 and 64. For instance at xF M 0.7 OL is 0.5-0.6 which is thelowerlimit of the BIS-2 measurement. Comparing fig. 23a with fig. 20b indicates that the values theoretically calculated seem to be to low. Another surprising prediction is that the Do production a t small xF should have a smaller OL value than that of &+. The presently available data do not show any difference in the behaviour of charm mesons and charm baryons.

The A-dependence of the Ac+ and DO multiplicities (here defined as /? = uc,,,M/$'~l.)

Fig. 23. Predictions of the a and @ behaviour as a function of xF for DO end A0-F from the model used in ref. [61]

Fortschr. Phys. 39 (1991) 5-6 375

is presented in fig. 23b. The quark-gluon string model predicts an A-dependence very close to that of the ordinary hadrons.

Another attempt to describe the BIS-2 data and to extrapolate to SPS energies was made inref. [64]. This model works satisfactory in the fragmentation region and can describe theBIS-2 and the E 613 data quite well. The a values calculated in ref. [64] vary slowly with xF being 0.82 a t xF = 0.1 and 0.78 a t xF = 1. This leads again to a disagreement with the data and is comparable only with the lower limit of the E 400 data. Differences in the behaviour of Acf baryons and D mesons are negligible in this model.

In any model of hadron-nucleus interactions the A-dependence of charm particles (as well as of ordinary hadrons) is determined by the specific production mechanism which the model is based on. The A-dependence of charm particles indicates that more than one production mechanism is valid since the a parameter is neither 213 nor one. Each model taking only one production mechanism into account (central production or fragmentation) must fail. Of course, to work out more precise models more precise data are needed also. The data of E 769 [60] from Fermilab available 1991 will certainly help to solve this problem.

5.

In this section the inclusive distributions (xF andp,) of charm mesons and baryons will be compared. The existence of a leading component in the data will be discussed in some detail.

To measure the xF and pL behaviour of charm production, good particle identification and resonable acceptance over a wide xF and p, region are needed. Most of the spectro- meters presently used perform reasonable particle identification. Some of them are limited in acceptance. Amethod how toobtain a clean sample of D mesons will be described in some detail for the proton data of NA 27 [44]. Results of other experiments are summarized and comparad to work out the main features of inclusive charm production.

Inclusive Spectra of Charm Particles

5.1. NA 27 Results on Inciwive D-Meson Production

In the proton part of the NA 27 experiment a sample of 119 fully reconstructed D decays in the region xF > 0 was found [44]. It consists of 24 D+, 27 D-, 29 Do, 22 p, 16 D o / p ambiguous and 1 D* ambiguous. The events were corrected for bubble chamber visibility and spectrometer acceptance. The mean decay weight was 2.7. Figs. 24a and 24b present the measured do/dxF and du/dp,2 spectra for all 119 decays. The dash-dotted line represents the fit to the data according to

d2u

dxF d P ~ 2 - (1 - z F ) ~ * exp (-bpL2)

leading to n = (4.9 & 0.5) and b = (0.99 & 0.09) (GeV/c)-2. There was no need toin- voke a two-component fit to raproduce the XF dependence as it was the case for the x- data of the same experiment [58]. The overallxFspectrum of the D mesons is rather central. The ratio

indicates that only a very small fraction of the D mesons was produced in the fragmentation region.


n 1 - U

X D . 0 73

0 .2 .4 .6 .8 1.

r4-l a

.u '0

5!

100 '1 i

X F Fig. 24 a Fig. 24 b Fig. 24. a) zF and b) pL behaviour of the D cross-section as measured in the NA 27 experiment. The dashed curve is a gluon fusion calculation with &fragmentation. The curve da.shed by points is the results of a fit according to

The black curve is the result of a Lund model calculation [73]

The dashed curves infigs. 24a and 24b were obtained using a gluon fusion model, the black ones are the result of Lund Monte Carlo calculations [59].

Fit results according toformula (5.1) for the different types of D mesons uniquely identified are preawted in fig. 25 together with the experimental data. The Do and D- distributions agree well with each other and with the overall distribution. The D+ distribution is significantly harder, the no one softer. A possible explanation is a leading diquark production and will be discussed later in some detail.

Table 10 Fits to du/dxF - (1 f XF)" * exp ( -bpL2) and to (l/E) d ~ / d z p - (1 - ZF)'" * exp (--bpL2) for XF > 0

Particle No. of decays n

all D 119 4.9 5 1.5 D+ 24 3.1 f 0.8 D- 27 5.4 f 1.2 DO 29 5.5 f 1.2

Dambig 17 3.9 & 1.1 B 22 8.1 f 1.9

P+ + DO1 53 4.2 5 0.8 [D- + Go] 49 6.6 f 1.1

?n

3.2 & 0.6 1.8 f 0.7 3.5 &- 0.9 3.8 0.9 6.2 f 1.4 2.9 & 0.9 2.7 f 0.6 4.6 5.0.8

b

0.99 f 0.10 1.32 f 0.27 1.04 0.20 0.82 5 0.14 0.62 f 0.14 0.93 0.30 1.04 f 0.14 0.84 f 0.12

Fortschr. Phys. 39 (1991) 5-6 377

Feynrnon x

Fig. 25. XF and pL2 distributions from NA 27 for different types of D mesons. The curves are always the result of a fit

0 1 2 3 4 5

p: [GeVlc l ’

The exponents obtained from the xF distributions (both in non-invariant and invariant form) as well as the slopes of the corresponding p,2 distributions are listed in table 10. The analysis of the Do/B* distributions was complicated because of the existence of 16 ambiguous Do/@ decays with a relatively hard xF distribution. Splitting these decays equally between Do and Do brings the Do result closer to D+, and the Do closer to D-. The obtained exponents are then 5.1 and 6.5, respectively.

5.2. Summary of Experimental Results on Inclusive Charm Production

In an analogous way ot,her experiments a t different energies with different beam particles and target materials have determined their coefficients n and b . In table 11 are summarized all these results. A careful inspec-ticm of all these data reveals the following trends :

The p , behaviour of charm hadro-production does not depend on energy, type of beam particle or target material. The mean value of b is between 1 and 2 GeV-2. Within the experimental accuracy all data agree well with each other. The xF behaviour of charm hadro-production depends on energy, detected particle and target material. With increasing energy an increasing value of n for the same kind of charm particle and target ma.teria1 can be found. Heavier targets tend to weaken this effect. A leading particle effect can be observed for all reaction types. Heavier targets seem also weaken up this effect. Non-leading charm production is always central. These effects indicate the existence of more than one production mechanism at the quark level.


Table 11 Fit parameter from different experiments obtained from

d2a N (1 - /xr.i)". exp (-bp,*)

dxF d P ~ 2

Experiment Beam Target Detected n b Energy particle ( GeV-2) (GeV)

BIS-2 ~381 [391

NA 32 [591 NA 16 [681

[441 NA 27

E 400 C281 E 743 [451 R 608

NA 32 ~361

[591

NA32/2

NA 11 1671

~ 9 1

NA 32 1591

Na 3212 [67)

n 58

P 200

P 360

P 400

n 600

P 800

P 63 * 63 K- 200

K- 230

200 Tr-

x- 200

x- 230

C

Si

P

P

W, Be, Si

P

P

Si

cu

Be

Si

c u

A,+ Do D-

all D

alte D

all D

D+/Do D-/DO

Ec+

all D

A,+

all D

D leading D non-lead. D8* D, D8+ all D

all D

all D

D leading D non-lead.

all D D leading D non-lead.

XF > 0

XF > 0.2

A,+

all D, all D D leading D non-lead.

1.5 f 0.5 1.1 & 0.6 0.8 f 0.6

5 . 5 3

1.8 f 0.8

4.9 f 0.5 6.6 f 1.1 4.2 f 0.8

4.7 & 2.3

8.6 * 2.0

2.1 3 0.3

4.7 -& 0.9

4.6';:: 4.7:;:; 1.1:::;

4.93; 0.72:;

3.8'::;

2.9 f 0.6

1.2 f 0.5

1.1:;:;

2.1:;:: 3.33:

1.5'::;

2.5:;:;

3.0 f 0.4

3.7 & 0.7

3.6 f 0.3 4.4 f 0.3

3.9 * 0.2

1.2:::; 1.8:::;

1.42;::

0.99 f 0.11 0.84 & 0.12 1.04 f 0.14

0.97 f 0.21

1.1 f 0.3

3.4 & 0.4

2.710.7 -0.5

3.43:: 2.6+0,:: 0.6 f 0.2 0.5 f 0.2 1.0:;::

1.0:;::

1 .oT;:; 1.1 +;:;

1.22:;;;; 0.91:;:;;

1.27:::;

1.0 f 0.2

1.06:;:f

0.84+0,:0,;

0.84:;::; 0.82 f 0.02 0.74 f 0.04 0.94 f 0.05

Fortschr. Phys. 39 (1991) 5-6 379

Table 11 (Continued)

Experiment Beam Target Detected n b Energy particle ( GeV-2) (GeV)

E 595 7r- Fe D-/D 1.6 f 0.6 1701 278 E+/B 2.1 5 0.5

NA 16 x- P all D 2.8 f 0.8 1.1 f 0.3 [681 360 D leading 1. f 1.

NA 27 x- P all D 3.6 -j= 0.6 l.l8+:::: ~581 360

z, > 0.2

D leading 1.Sz::f D non-lead. 7.9:;::

To explain the relatively hard xF spectrum for D+ mesons found in NA 27 pp data i t must be assumed that protons are composed of a quark and a di-quark [65]. This was already suggested by other data [66]. If the di-quark exists as a single entity (3 of colour) it will carry on average a larger fraction of the proton momentum. After production of a cz quark pair by some hard process like gluon fusion, charm particles can be built up by the combination of thea with one valence quark and the c with the di-quark. An (uu) diquark can form an (uu) c combination leading to a pD+ final state (fig. 26a). The (ud) c combination will produce either leading &+'s (fig. 26b) or nD+ and pD" final states (fig. 26 c). Low mass combinations are prefered.These processes could cause charm mesons to be more leading than the anti-charm ones. This effect is reflected in the xF distribution of the D+. The hard D+ distribution is accompagnied by large ( ~ ~ 2 ) . Only a certain fraction of all D+ should be produced by this effect. A two-component fit t o the data is not justified with the statistics available. But already a contribution of 25% of leading D+ (n = 1) would be enough to produce the observed effect.

This effect would be in the same order of magnitude like the leadingparticle effect ob-

Fig. 26. Possible graphs to explain the obtained leading diquarkleffect in the NA 27 (PP) data

3 Fortschr. Phys. 39 (1991) 5-8


I T-BEAM

LEADING D

bl

6 8 EA M

20 NONLEADING 0

0. 0.2 0.4 06 08 1. 0. 02 04 06 08 1.

XF XF

Fig. 27. zF spectra, for D mesons from NA 32 pion data a) leading D mesons b) non-leading D mesons

served in the NA 27 n-p data. Another example for a leading particle effect in n- nucleus interactions isdemonstratedin fig. 27. The data are from the NA 32 experiment.

In ref. [31] were calculated the xF and p L 2 spectra for gluon fusion and quark anni- hilation using the pion structure function of OWENS [71] and the nucleon structure function of DUKE and OWENS "721. In figs. 28 and 29 the results of QCD calculations are compared with the NA 27 (pp) and E 743 data. As can be seen from the figures the xF and pL2 behaviour is described quite well, especially the increase of central production with increasing energy. However, the data include D meson production only but exceed already the theoretical predictions. This leads again (see also sect. 3.2) to the conclusion that next-to-leading order processes play an important role already a t SPS and Teva- tron energies.

6. Correlation Properties

In hadronic reactions charm particles are created always in pairs. In real experiments however, due to acceptancelosses and limited vertex resolution also "single" charm production is observed. In theNA 27 experiment about 314 of all observed charm containing events have two or more decays which could be a charm decay. Careful background esti- mates [43] can rule out strange particles or interactions faking a charm decay. The study of correlations between the two produced charm particles is of great interest for the understanding of the production mechanism.

The first study of DD correlations was reported in 1985 [74] based on the x-p data of the NA 27 experiment. A similar study using the proton data was published in ref. [U].

The events having a pair of charm candidates were ordered in two groups:

(i) both decays have a clear topological signature for charm (ii) both decays are fully reconstructed.

Fortschr. Phys. 39 (1991) 5-6 381

1000 JS= 27.4 GeV 7

100

- v) c L

," 10 a I

Y x '0 . u 1 '0

.1

.Ol

1000 I ' ' ' I ' ' I ' I ' I I I I I ' ' ' ' A= 38.8 GeV

- v) c 0 L

n a - Y

X 'D . t) U

'I 1

3 XF

Fig. 28. a) ZF and b) p , spectra from leading order QCD calculations [31] in comparison with NA 27 pp data.

3+


100

10

,001

.DO01

VI c O

1

L

n - 4

bj 0 10 20 30 40 50

p; (GeV* 1

Fig. 29. a) XF and b) p , spectra from leading order QCD calculations 1311 in coniparison with E 743 data

Fortschr. Phys. 39 (1991) 5-6 383

6.1. Correlations in CDl

To study angular correlations in the plane transverse to the beam the sample (i) cau be used, being statistically more significant than sample (ii), since no complete reconstruction of the decay is required. The angle @l to be examined is the angle between the two lines of flight of the D mesons in the plane mentioned above. For sniaI1 transverse decay lengths LL, the angle can be distorted due to the errors of the vertex position [42]. A correction procedure using Monte Carlo simulations was introduced. Charm candidates with an ambiguous decay topology and only one clearly identified charged decay track (X1 topology) were rejected from the sample because of the big uncertainty in the decay vertex position. A total of 53 pairs from the x- data and 107 pairs from the proton data was selected. In the pion data an enhancement of events around 120" was found. However, the statistical significance of this enhancement was less than two standard deviations. In the proton data as shown in fig. 30 no enhancement around 120" was seen.

Fig. 30. distribution from NA 27 pp data. The solid histogram is corrected according to uncertainties in the determination of the vertex posit.ions

0 30 60 90 120 150 180

QT (degrees)

The influence of the Monte Carlo corrections is demonstrated by showing in fig. 30 both the raw data (dashed histogram) and the corrected one (solid histogram). Most of the corrections appear a t small @L values. The distribution shows an increase towards 180" as it was expected from leading order processes like gluon fusion or quark annihila- tion if the partons have very small transverse momenta. The mean values of for the x- and proton data are included in table 12.

To study the influence of fragmentation schemes different fragmentation functions are used to describe the data. The variable z is defined as z = E,/EQ. Here are EH the energy of the produced hadron after fragmentation and EQ the energy of the heavy quark Q , i.e. of the produced charm quark. The following three fragmentation schemes were

384 H. NOWAR, Hadronic Production of Charm Particles

TabIe 12 Mean values of M(DD), zF, pL2, A Y, and @jI from NA 27 in compa.rison with data from NA 32 and WA 75

NA 27 Z-P 4.50 f 0.16 0.25 f 0.07 1.65 f 0.40 0.80 f 0.14 115 f 8 PP 4.65 f 0.13 0.18 f 0.03 1.50 f 0.30 1.02 f 0.12 105 f 5 WA 75 x-N 1 1 5 3 12 NA 32 x-cu 4.32 0.48 1.45 f 0.41

considered : (i) D(z) = 6(1 - z ) &fragmentation. (ii)

(iii) As can be seen from fig. 30 the data are well described by both the Peterson fragmentation and the string fragmentation as used in the Lund Monte Carlo.

A @I distribution was also published by the WA 75 collaboration [76] and is shown in fig. 31. 102 charm pairs were obtained from x- interactions with emulsion nuclei a t 350 GeV. Because this spectrometer did not detect neutral secondaries, not all background topologies could be rejected completely. &+ and D,+ decays were also not considered. However, the authors mentioned that for events with identified muons both kinds of background sources were small.

D(z) = const. {z[l - (l /z) - &/(1 - +)]}-I E can be interpreted as mq2/mQ2 and is of the order of 0.2 for D mesons. String fragmentation as used in the Lund Monte Carlo.

Peterson-fragmentation 1751

0- 0'

0' p 180'

d o '

Fig. 31. QL distributions from WA 75 for all DB pairs and different subsamples

Fortschr. Phys. 39 (1991) 5-6 385

Beside the 102 charm pairs (one of the decays contains a muon) 84 single muonic charm decays were found. The events were classified according to the charge of the muon and the decay topology as D+ and DO/DO. No enhancement around 120" in the GI distribution was found. The mean value (see also table 12) of this distribution agrees well with the NA 27 x- result. Furthermore, the WA 75 collaboration presents distributions for the different types of D meson pairs, but the low statistics do not allow any conclusion about differences in these distributions.

The curves in fig. 31 were computed according to a fusion model [77] with PETERSON [75] fragmentation (curveA) and &fragmentation (curve B), and to a cluster model [78] with a Boltzmann type fragmentation function (curve C).

6.2.

To search for correlations in the charm pair production the invariant mass M , xF, p12, and the rapidity gap AY of charm pairs with fully reconstructed decays were studied. In different experiments different cuts are applied to the data depending on the acceptance of the used spectrometer.

The NA 27 results both from the x- and proton data require charm pairs to have xF > -0.1 for each charm particle. This yields 12 events in the pion data and 17 events

Correlations in Fully Reconstructed Decays

4 . 5 . 6 .

I

2o t

p: (001 I G e V / c 1 2 AY (DD? Fig.32

Vig. 32. a) Effective mass, b) ZF, c) pL2, and d) AY for charm pairs with z p > 0 from NA 27 pp data. The meaning of the different superimposed curves is described in the t,ext

386 H. NOWAX, Hadronic Production of Charm Particles

in the proton data. A,+B and/or D,+D pairs were always excluded. The events were weighted by

WT = WTsl. WT,, . WT,,b - min(WTvl, WTV2).

Here WTsl and WTs,are the spectrometer weights of the corresponding decay correcting for acceptance losses. WTvl and WTv2 are visibility weights correcting for detection losses a t scanning stage. Theminimum valueof both is the visibility weight of the charm pair. Wamb7 the ambiguity weight is the inverse number of accepted fit hypotheses. The sum of the weights for the 17 charm pairs in the proton part of NA 27 is 45.9. Fig. 32 presents for the protondata of NA 27 the weighted distributions of the effective mass M , xF7 pL2, and the rapidity gap A Y of the pairs with zF(DD) > 0. The DD pairs are domi- nantly produced a t low xF values with a fairly small rapidity gap indicating the influence of short range correlations. It is interesting to remark that the obtained distributions in x-p interactions areverysimilar to those of the pp data. This can be concluded also from the mean values given in the table 12.

The NA 32 data used for the comparison in table 12 are still preliminary and not acceptance corrected. This does not influence significantly the mass and pL2 values. The agreement of the data is remarkable.

To compare the data with QCD predictions and to investigate the influence of the fragmentation scheme three different curves were superimposed to the histograms of fig. 32.

The calculations base on leading order QCD including an intrinsic transverse momentum k , of the partons [42]. The usual parametrisation

with (kL2) = 0.64 GeV2 was used. This leads to a more realistic description of the data weakening the back-to-back behaviour of the DD pair in the c.-m. system and making the p , values of the charm particles greater than zero.

The used fragmentation function introduced in section 6.1 can describe the data only very qualitatively. Even taking into account the very limited statistics all fragmentation schemes fail to describe the p12 distribution. The &fragmentation function prefers too low M(Dfi) and AY values. A resonable qualitative description can be reached only using the Lund Monte Carlo.

6.3. Multiple Production of Charm Pairs

The W A 75 experiment a t CERN has reported an evidence for the simultaneous production of four charm particles [79]. The experiment was designed to observe the decay of beauty particles and to study their properties. It was found one pair of B- and zo which was the first direct observation of beauty mesons in hadronic reactions [80]. As a by-product of this search two primary interactions each containing four charm decays were registrated.

A total of 80 litres of emulsion in form of stacks was exposed to a 350 GeV x- beam. The emulsion was placed in front of a vertex detector followed by an Iron/Tungsten dump and a magnetic spectrometer for the detection of muons not coming from the primary vertex 1.5. lo6 interactions were triggered out of 3 . 108 interactions in total requiring a t least one charged particle in the spectrometer behind the beam dump

Fortschr. Phys. 39 (1991) 5-6 387

together with a beam particle and three minimum ionising particles just behind the emulsion stacks.

After off-line selection and reconstruction of the muon tracks less than lo4 events remain for the physical analysis. Out of them 5000 interactions have been located in the emulsion. 92% from these 5000 were rejected because of the muon coming from the primary vertex or from a x or K decays.

About one half of the events with muons not originating from the primary vertex showed one or more charm decay topologies. Always one decay has to be compatible with a semi-muonic decay mode. Twelve events have been found with more than two decay vertices. Two cases have four independent secondary vertices each of which can be interpreted as a charm decay. Table 13 summarizes the results of the kinematical analysis of these two events.

Table 13 Summary of kinematical analyses of multiple charm pair events from WA 75

Event Decay Decay P PL Lifetime Com- Nr. vertex length GeV/c GeV/c 10-13s ment

-

mm

1 1W) 0.180 51':: 1.3 0.2 0.2 2074) 1.30 7.7 f 0.4 0.6 10.4+00:: 3W4) 2.70 23:: 1.2 7.43:; 4 V ) 3.97 I50f 90 4.0 f.6'::& cc-

2 W 4 ) 0.361 6.9 f 0.2 0.7 3.3 * 0.1 2(V2) 2.02 25 & 9 1.1 5.33:; e+ 3 W ) 4.22 5 7 % 12 1.7 4.6';:; v- 4(C1) 6.22 120:;; 2.6 3.2';;:

In WA 75 the measured rate of double to single charm pair production was about 10-2. Ana simple mechanism like two successive processes in the intranuclear cascade or for- mation of multiple pairs of heavy flavours in diffractive central clusters according to double Pomeron exchange [Sl] give rise to a ratio of a t most

Even lower rates are predicted [82] by gluon Compton scattering producing a Higgs boson and a c quark. The Higgs boson will then decay into a pair of charm particles.

A more phenomenological way is to compare the double charm pair production with the production of hidden charm. The NA 3 collaboration has reported [83]

uyy/uy 3.10-4

for x- interactions a t 280 GeV. Using the value of

uDE/uy N 2.5 * 102

estimated from NA 3 and NA 16 measurements [84] a value for the ratio of DD'DD to DD production

uDEDE/'yP ('DE/uy)2

can be estimated. It is a t least not in contradiction to the measured one of the WA 75 experiment. This very interesting experimental result needs aIso confirmation from a high statistics experiment.


7. Summary

The main results on charm hadro-production described in this review can be summarized as follows : - Rapid progress has been made during the last years in charm particle spectroscopy,

especially for charm mesons. The mass value of the D,+ meson is now determined as good as for the D+ and Do and has the value (1969.3 f 0.7) MeV.

- Six charm baryons Ac+, Zc+, Sco, Zc++, ZcO, and Qc are observed. Except for the Ac+ and the Zc+ all other mass values need conformation.

- Theoretical calculations for charm pair cross-sections are presently available in the second and third order of perturbation QCD.

- The second order calculations are not sufficient to describe the experimental data a t any energy.

- Third order calculations lead to a resonable description of the energy dependence of the cross-section in the SPS and Tevatron region. For higher energies the predicted cross-section is still to small.

- The following cross-sections were experimentally determined :

at f i w 10 Gew

a t I;= 21.6 GeV

10 pb < (T < 50 pb

13 pb < (T < 24 pb

a t I/;= 38.6 GeV

a t i s= 62 GsV

21 pb < (T < 27 pb

660 pb < u < 2 mb.

- Four experiments (BIS-2, WA 78, E 400, E 613) have measured the A-dependence of inclusive charm production. Within the statistical significance of the data no difference can be found in the A-dependence of charm meson and charm baryon production. The mean value of & (&) = 0.79 f 0.03) favours an A-dependence of A213.

- Studying the xF-dependence of this or parameter the same tendency as for ordinary hadron production was found. or varies from 1 a t xF = 0 to about 2/3 at xF near 1.

- Present theoretical models are not able to describe this A-dependence satisfactoryly. - A comparison of the xF andp,2 behaviour of charm particles as obtained in different

experiments leads to the following conclusions :

- The p , behaviour of charm hadro-production does not depend on energy, type of beam particle or target material. The mean value of b is between 1 and 2 GeV2. Within the experimental accuracy all data agree well with each other.

- The xF behaviour of charm hadro-production depends on energy, detected particle and target material.

- With increasing energy an increasing value of n for the same kind of charm particle and target material can be found. Heavier targets tend to weaken this effect.

- A leading particle effect can be observed for all reaction types. Heavier targets seem also to weaken this effect up.

- Non-leading charm production is always central.

- These effects indicate the existence of more than one production mechanism a t the quark level.

- Correlations in the variable between the produced charm and anti-charm particle can be described with leading order QCD calculations including an intrinsic transverse momentum k, of the partons. The influence of the different fragmentation schemes in negligible.

Fortschr. Phys. 39 (1991) 5-6 389

- To describe the charm pair correlation in the variables M , xF, pL2, and AY for charm pairs leading order QCD calculations together with different fragmentation schemes were used. The two fragmentation functions tried cannot describe the data satisfactoryly. Even taking into account the very limited statistics all fragmentation schemes fail to describe the pL2 distribution. The &fragmentation function prefers too low M(DD) and A Y values. A resonable qualitative description can be reached only using the Lund Monte Carlo.

- Experimental difficulties as well as theoretical complications in studying charm hadro-production have left over still a lot of open questions. Of course, to work out more precise models more precise data are needed also. The data of E 769 [60] from Fermilab available 1991 will certainly help to solve these problems.

- To look for new phenomena at the future accelerators like UNK in Serpukhov or the new storage rings planned for the end of this century like the Large Hadron Collider at CERN and the SSC in the USA needs a deep understanding of the background processes. The hadro-production of charm will be one of the most important background source in searching for new phenomena.

Acknowledgements

I want to thank all my collegues from the BIS-2, NA 27 and E 743 collaborations for the friendly, long-time collaboration. The stimulating discussions with my collegues from the IfH Zeuthen Profs. U. Kundt and S. Nowak, Drs. H. Biittcher and W.-D. Nowak are gratefully acknowledged. Special thanks for technical assistance go to Miss. Chr. Engelhardt, E. Kohs, S. Mietling and Mr. W. Bernard.

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Hadronic Production of Charm Particles