A S S O C I A T E D P R O D U C T I O N OF S - M E S O N S A N D S T R A N G E P A R T I C L E S A N D D O U B L E C-MESON P R O D U C T I O N

BIS-2 Collaboration

A. N. ALEEV, V. A. AREFIEV, V. P. BALANDIN, V. K. BERDYSHEV, V. K. BIRULEV, A. S. CHVYROV, I. I. EVSIKOV, T. S. GRIGALASHVILI,

B. N. GUS'KOV, I. M. IVANCHENKO, M. N. KAPISHIN, N. N. KARPENKO, D. A. KIRILOV, I. G. KOSAREV, N. A. KUZ'MIN, M. F. LIKHACHEV,

A. L. LYUBIMOV, A. N. MAKSIMOV, P. V. MOISENZ, A. N. MoRozov, V. V. PAL'CHIK, A. V. POSE, T. B. PROGULOVA, A. PROKES, V. V. RYBAKOV,

L. A. SLEPETS, A. I. ZINCHENKO

Joint Institute of Nuclear Research, 141 980 Dubna, Russia

K. HILLER, J. KLABUHN, I-I. NOWAK, S. NOWAK, I-I. E. RYSECK

DESY, Berlin-Zeuthen, Germany

A. F. KAMBURYAN, A. A. LOKTIONOV, Yu. K. POTREBENIKOV

Institute of High Energy Physics, Acad. Sci. Kazakh., Alma-Ata, Kazakhstan

A. S. BELOUSOV, E. G. DEVITSiN, A. M. FOMENKO, V. A. KOZLOV, E. I. MALINOVSKY, S. YU. POTASHEV, S. V. RUSAKOV, P. A. SMIRNOV,

YU. V. SOLOVIEV, L. H. SHTARKOV, YA. A. VAZDYK, M. V. ZAVERTYAEV

Lebedev Physical Institute, Acad. Sci. Russia, Moscow, Russia

E. A. CHUDAKOV

Institute of Nuclear Physics, Moscow State University, Moscow, Russia

V. D. CHOLAKOV

Hilendarski University of Plovdlv, Plovdiv, Bulgaria

J. HLADKY, M. NOV2{K, M. SMIZANSKA, M. VECKO

Institute of Physics, Czechosl. Acad. Sci., Na Slovance 2, 180 40 Praha 8, Czechoslovakia

V. J. ZAYACHKY

Higher Chemical-Technological Institute, Sofia, Bulgaria

V. R. KRASTEV

Central Laboratory of Automation and Scientific Instrumentation, Bulg. Acad. Sci., Sofia, Bulgaria

Czechoslovak Journal of Physics, Vol. 42 (1992), No. 2 159

A. N. Aleev et al.

D: T. BURILKOV, P. K. MARKOV, G. G. SULTANOV, P. T. TODOROV, R. K. TRAYANOV

Institute of Nuclear Research and Nuclear Energetics, Bulg. Acad. Sci.,

Sofia, Bulgaria

L. N. ABESALASHVILI, N. S. AMAGLOBELI, M. S. CHARGEISHVILI, N. 0 . KADAGIDZE, V. D. KEKELIDZE, R. A. KVATADZE, G. A. KVIRIKASHVILI,

N. L. LOMIDZE, G. V' MELITAURI, G . I . NIKOBADZE, T. G. PITSKHELAURI, a . G. SHANIDZE, G. T. TATISHVILI

Institute of High Energy Physics, Tbilisi State University, Tbilisi, Georgia

Received 12 April 1991

Associated production of C-mesons and strange particles by neutrons at energies of 30-70 GeV has been analysed. It has been found that at least 67 % of inclusive C-meson production in the kinematic region XF > 0.1 and PT < 1 GeV/c proceeds via OZI-allowed processes with extra strange particles. Double C-meson production has also been observed and its cross section in the full kinematic region is estimated to be 15 =[= 9 #b.

In two previous papers we have investigated the properties of C-mesons produced by neutrons on hydrogen and nuclear targets [1]. Using the same data sample, we are now looking for strange particles produced in association with the C-meson production.

From the analysis of the associated strange particle production we expect a better understanding of the underlying C-production dynamics. Figure 1 shows possible quark line diagrams for the C-production. Of central importance for such processes is the Okubo-Zweig-Iizuka (OZI)-rule [2] which allows the production and decay to proceed through connected quark diagrams and forbids the annihilation of a quark-antiquark pair into a meson. According to this rule fig. la represents a fusion process of strange sea quarks into a C-meson. In this process, the C-meson should be accompanied by additional strange particles. Single C-meson production without additional strange particles can be described by an OZI-forbiddeu quark fusion process (fig. lb) or by a gluon fusion process (fig. lc).

The relative contribution of these different production mechanisms to the inclu- sive C-production rate depends on the quark content states of the C-meson. In the SU(3) broken quark model with an ideal mixing of octet and singlet states the C- -meson is a pure s~ state. However, experimentally a deviation from the ideal mix- ing was observed which can be explained by an admixture of nonstrange q~-t-pairs. This admixture allows single C-meson production while a pure s~-state should be always accompanied by additional strange particles according to the OZI-rule.

Early experiments have obtained contradictory results concerning the validity of the OZI-rule in hadronic C-meson production. While enhanced probability of the C-meson production with an extra K+K - pair has been observed in exclusive chan- nels [3], such an effect was not seen in inclusive reactions [4]. A later experiment

160 Czech. J. Phys. 42 (1992)

O-mesons , s t range part ic les , and Cq~-meson produc t ion . . .

with larger statistics has, however~ observed a clear excess of strange particle pro- duction in inclusive reactions with C-mesons [5]. A lower limit of 40 % was given for the contribution of the OZI-allowed processes to the inclusive C-meson production.

o}

. q ~ s . m q

b)

Fig. 1. Parton diagrams for C-meson production: a) OZI-allowed strange sea quark fusion, b) OZI-inhibited light quark fusion, c) gluon fusion.

As the C-meson is the first member of a series of hidden flavour vector mesons the study of its production characteristics might be quite useful for the understanding of the production mechanisms of the heavier vector mesons J/@ and Y. Taking into account their smaller cross sections and the complicated topologies of associated charm/bottom particle decays the observation of such processes is an extremely difficult experimental task.

According to the OZI-rule the double C-meson production without additional strange particles should be strongly suppressed for nonstrange beams. However, a surprisingly high production rate was found in exclusive reactions [6]. The partial wave or amplitude analyses of these data indicate candidates for a broad resonance state. However, the clarification of their physical nature is still open. A possible interpretation of these measurements could be the production of intermediate multi- gluon states with subsequent decays into r162

The present experiment was pe .rformed with the BIS-2 spectrometer at the Ser- pukhov accelerator. The averagemomentum of the incoming neutrons was about 40 GeV/c with a tail up to 70 GeV/c. The experiment was sensitive in the range of Feynman XF > 0.1 and the transverse momentum PT <~ 1 GeV/c. Details of the spectrometer and the trigger conditions are described elsewhere [7].

Analysing about 2.1 x 107 events from hydrogen and nuclear targets we ob- served about 7 000 C-mesons in the decay mode r --* K+K-[1]. For the selection of charged kaons the information from two multi-cell Cherenkov counters has been

Czech. J. Phys. 42 (lgg2) 1 6 1

A. N. Aleev et al.

used. Most of the charged particles have momenta lower than the kaon threshold of both counters. In such cases kaons cannot be separated from protons. Therefore in the following analysis the uniquely identified K-candidates as well as K / p ambi- guities were taken as K-mesons. Uniquely identified pions and r / K / p ambiguities were excluded.

3 . 1 0

I f i --]

2. F i I i

1.75 /

~ 1 . 5

1.25

LD z l .

z ~ 0.75

0 t,.9

0 .5

0 . 2 5

O. 11 i �9 1 ,02 1.04 1.~36 1.0~

E F F E C T I V E M A S S K ' K " , G e V / c 2

Fig. 2. The invariant mass spectrum M(K+K - ) of the hydrogen sample. The b-meson signal contains 3 300 4- 170 events.

To select a clean sample of K+K - pairs, all candidates had to fulfil some ge- ometrical and kinematic selection criteria described in reference [1]. The K+K - effective mass spectra for the different targets show clear b-meson signals as illus- t ra ted for the hydrogen sample in fig. 2. Its width of 6.4 MeV/c 2 and its central mass value of 1019.6 4- 0.2 GeV/c 2 agree with the nominal values of 4.4 MeV/c 2 and 1019.4 MeV/c 2 [8], respectively.

The neutral strange particles were identified by their charged decays A --* p~r- and Ks ~ -* 7r+~r - without using information from the Cherenkov counters. The mass resolution is about 2 MeV/c 2 for A and about 3.5 MeV/c 2 for K ~ Examples of A and Ks ~ signals in the corresponding mass spectra can be found in our previous publications, e.g. [9].

Lego-plots of the invariant masses M ( K + K - ) versus M(lr+Tr - ) - M(K ~ and M ( K + K - ) versus M(p l r - ) - M(A) are shown in figs. 3a and 3b, respectively. Clear signals are visible in the CKs ~ and r overlapping regions. Subtraction of the background yields 94 4- 20 CK ~ and 144 4- 19 $A events. Here the C-meson mass range was defined from 1010 MeV/c 2 to 1030 MeV/c 2 and the K ~ and A were taken within the interval of 4-10 MeV/c 2 and of 4-6 MeV/c 2, respectively, around the nominal mass values.

In order to estimate the number of events with extra charged kaons in the final state the information from the Cherenkov counters was used. Since the kaon iden-

162 Czech. J. Phys. 42 (19g2)

r strange part ic les , and ~ r p roduc t ion . . .

tification is not unique, K+-candidates are contaminated by p/~ and misidentified pions. A method to estimate the true number of CK ~- events is described in [10]. The probability for interpreting Ir • and protons as K +, which was calculated from A and K ~ decays with known mass assignment was used as input. The overlap- ping of C-bands in K + K - K • -events was less than 5 %. We estimated to have 570 4- 120 CK + and 210 4- 45 CK- events in our data sample.

~ o) b) 60 60

off.lll'T'll I..>';. o~ 0o,~%1~ I ~ ~o- "~:>..,_ ~" .~-"~. oo _~,.~ ,u--- ~-~.~+r.o.o ~ ~.~," ~ ~1~

Fig. 3. T~e l~go-plots of the i,w,iaot m~ses M (K'K-) versus a) M(~-+~-)-- M(KO) ~how- ing the r ~ signal in the overlap region of r -- K ~ bands ~nd b) U(p~-)-- M(A) showing

the CA signal in the overlap region of the r -- A bands. Data from hydrogen and nuclear

targets were summarized.

The fraction of K+K - events with an additional A, K ~ K + or K- to all events containing K+K - , as a function of the K+K--mass, is shown in fig. 4. A definite increase of the strange particle rate can be observed in the C-meson mass region. The observed excess rates above the neighbouring background regions are deter- mined to be 0.40 % for A, 0.26 % for K ~ 1.41% for K + and 0.41% for K- . Since the observed rates are normalized to all K+K - pairs in the C-mass region, the actual rate for events containing a C-meson would have to be greater. After the correction for the background under the C-signal the actual rates of 2.2 4- 0.3 % for CA, 1.6 4- 0.4 % for CK ~ 8.6 4- 1.9 % for CK + and 3.2 4- 0.7 % for CK- were obtained [10]. In these rates no corrections for the limited spectrometer acceptance were yet applied at this stage. The C-events with two additional strange particles were not analysed because of small detection efficiency of such events.

In order to calculate the efficiency of the spectrometer and the losses due to selection criteria, a LUND Monte Carlo program was used [11]. It was checked that in our kinematic region the production properties of strange particles can be described satisfactorily by the Monte Carlo events. As an example a comparison

Czech. J. Phys. 42 (1992) 163

A. N. A l eev et al.

of the momentum spectra of the CA and CKs ~ systems with Monte Carlo spectra is shown in fig. 5. In the following we assume that the LUND-model gives a good description of the momentum spectra for associated strange particles in the full kinematic region. The ratios of detection efficiencies between CA, CKs ~ CK + and CK- systems and all C-mesons were found to be 24 %, 9 %, 25 % and 11%, respectively.

I o.8~

0.7

(::].~i

�9 o.~i ~ f l

0.4 u_ E: o.: <

o. 0.2

~z 0.1

$

^ ~ I K: ++ ~176 T,

~ T ' ' - _ -[--

4.~ . 2. K +

�9 + J r

!i '+1i: . : 0.2 I 0.2~ -

/ i i i I I / " I ~ I ? I

0.9 r 1.02 1.05 o.gg 1.02 1. ~5

EFFECTIVE MAS3 K~K - , Ge'v ':~

Fig. 4. The fraction of additional strange particles A, K 0, K + and K - versus the invariant mass

of the K+K - pairs. Clear enhancements in the C-mass region show the associate production of

C-mesons and strange particles.

To calculate the acceptance-corrected fraction of C-mesons accompanied by an extra strange particle only the hydrogen data were used which correspond roughly to one half of the quoted event number. The data from nuclear targets were ex- cluded because the A-dependence for C-meson production with additional strange particles is unknown. Taking into account the detection efficiencies and the branch- ing ratios, we conclude that 20 4- 4 %, 55 t: 17 %, 33 :i: 11% and 26 + 10 % of the inclusive C-production is accompanied by an extra A,K~176 + or K- , respec- tively. These values hold for C-mesons with longitudinal momenta PL greater than 8 GeV/c and transverse momentapw smaller than 1 GeV/c. The lower limit of the longitudinal momentum corresponds to a mean XF value of 0.1. For the additional strange particles the corrections include the full kinematic range.

The rate of the OZI-allowed processes can be approximated as half the sum of the rates determined above taking into account double counting of some exclusive channels of associated production of r and two strange particles. This

164 Czech. J. Phys. 42 (1992)

C-meson~ strange particles, and CO-meson production . . .

gives the result that 67 =k 12 % of the C-mesons axe accompanied by extra strange particles. This value should be considered as a lower limit because some channels, e.g. ~]~• are not taken into account.

z 50

b 4o

z 20

z

:D z

J i r - i i i I

<p~(~.):>. 37.7 GeV/c EXPER. ,57.9 GeV/r LUND

~o 2'5 3'0 & 20 ;5 5'o & ~o p~(a,^) , OeV/c

4.0 <edeK~>= .~&9 GeV/c ~PE:R.- ,35 32.9 GeV/c LUNO

25 2O ~5

+ o ~'o 2'~ ~ o % 2o & 5'0

P,..(r , GeV/c

Fig. 5. The momentum spectra of CA and 0 CKs-systems of the hydrogen sample in comparison with the corresponding spectra of the LUND Monte Carlo events,

Our data thus show that the main fraction of the inclusive ~meson production proceeds via OZI-aUowed processes with extra strange particles. Hence the diagram of the OZLallowed fusion of strange sea quarks (fig. la) dominates in the C-meson production by neutrons.

u'3

O

~ q 7-" ~ 14 44 23 Y O

~ . ~ 12 15 16 i

099 1,0t 1.03 1.05

MIK'K-), O~v/e

Fig. 6. The table of M1 (K + K - ) versus M2 (K + K - ) indicating double C-meson production in the overlap region of the two C bands.

To search for double C-meson production we have selected events with two can- didates of K+K - pairs. The scatter plot of both K+K - masses is shown in fig. 6. Within the r162 box from 1010 MeV/c 2 to 1030 MeV/c 2 there are 44 entries.

Czech. J, Phys, 42 (1992) 16.5

A. N. Aleev et M.

Although two entries per event enter the plot, we found only one entry per event within the CC box. The background has been estimated from the eight neighbouring regions. After subtraction of the background a signal of 21 4- 8 events remains.

The invariant mass spectrum of all candidates inside the CC box is shown in fig. 7. Despite the low statistics presented here, an accumulation of events near threshold without structure can be observed. The general features of the CC mass spectrum are similar to those obtained in other experiments [12].

Finally, the ratio of double b-meson production to single C-meson production has been estimated. Again the hydrogen data only have been used for this purpose. After unfolding the detection efficiency the ra~e of the inclusive double C-meson production was found to be 1.6 4- 0.9 %. This value corresponds to the kinematic range PL > 8 GeV/c and PT < 1 GeV/c for both b-mesons. Using the inclusive b-meson cross section of 53 4- 9/~b in this region [1], we obtain for the cross section of double b-meson production the value of 0.85 4- 0.50 stb. The model dependent extrapolation to the full kinematic region gives a rate of 7 4- 4 % which corresponds to an inclusive r cross section of 15 4- 9 #b. The extrapolation is based on the assumption that the momentum spectra of the CO:events in the unobserved kinematic region can be described by those of the inclusive C-meson production [1].

%

',d" o. 0

Z w

n,,. bJ (]3

z

10

8

6

4

2

,I-l, , I1 �9 . 2 . 6 2 . 8

I'~(K*K-K*K-) , GeV/c 2

Fig. 7. The invariant mass spectrum M(K + K - K+K - ) for 44 candidates of the C~ box of fig. 6.

From the production rates on CK + and c K - being 33 4-11% and 264-10 % one expects an inclusive CK+K - rate between 10 % and 20 %. Compared to this value, the rate of double C-production is surprisingly high, pointing to a possible violation of the OZI-rule in such processes. Our data confirm the results from experiments at higher energies which have found large r rates in the r - events samples [13].

The authors are grateful to A. M. Baldin, E. I. Maltsev, I. A. Savin for supporting these inves-

tigations; H. Bottcher, Ch. Spiering and L. V. Schreiber for useful discussions; E. M. Likhacheva,

L. V. Silvestrov, V. E. Simonov, G. G. Takhtamyshev, N. V. Vlasov for their participation in the

experiment.

166 Czech. J. Phys. 42 ( lgg2)

r strange part icles , and r162 produc t ion . . .

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