IL NUOVO CIMENTO VOL. 107A, N. 10 Ottobre 1994

New Results on Charmed Baryons from ARGUS (*).

ARGUS COLLABORATION

presented by J. SPENGLER

Max-Planck-Institut ffer Kernphysik - Heidelberg, Germany

(ricevuto il 14 Aprile 1994)

Summary. - - In the following, new results on charmed-baryon decays are presented by the ARGUS Collaboration. The experimental data have been recorded with the ARGUS detector operating at the e § e- storage ring DORIS II at DESY, Hamburg. In these data, the excited charmed baryon A* § decaying into A~ + ~§ ~- is observed for the first time. The results of a study of the decay h i -~ pK~ ~ are described. In addition, the observation of semi-leptonic decays of the ~_0 and ~]c baryons are presented.

PACS 13.25 - Hadronic decays of mesons. PACS 14.20.Kp - Charmed and other heavy baryons and baryon resonances. PACS 01.30.Cc - Conference proceedings.

1. - I n t r o d u c t i o n .

Since the observation of the Ac by the MARK II Collaboration [1] in 1978, a lot of experimental information has been added to the field of charmed baryon physics. The Quark Model predicts twelve charmed-baryon states with spin 1/2, nine of which contain one charmed quark. Up to now, seven of the J = 1/2, C = 1 ground states have been observed experimentally[2,3], whereas none of the spin-3/2 states has been seen.

In the following, recent results on charmed-baryon physics obtained by the ARGUS Collaboration will be presented: a preliminary study of the decay Ac + --. -o p~0 ~+ 7:- [4], the observation of a new charmed baryon A* + [5], the observation of ~c~~ semi-leptonic decays [6], and evidence for semi-leptonic decays of the ~2c [7].

The data were collected using the ARGUS detector[8] at the DORIS II e + e- storage ring at DESY, Hamburg, in the energy range of the Y resonances and the nearby continuum. The data samples used for the analysis presented here comprise integrated luminosities of 400 to 500 events /pb.

(*) Paper presented at the conference HADRON '93, Como, June 21-25, 1993.

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2046

2. - Study of decay A: - -~pK~ + ~- .

J. SPENGLER

In the first section the preliminary analysis of the decay A [ - - ~ p K ~ § ~- and a search for intermediate resonant states is discussed. The K ~ is reconstructed by its decay into two pions. Charmed baryons are expected to be products of the initial charmed-quark fragmentation. Therefore they should have rather large momenta in contrast to the combinatorial background. The invariant mass of p K ~ + ~ - combinations which have passed a cut in the scaled momentum xp > 0.5 with xp =

= P/Pm~, Pm~ = ~/E~ear~- m2(pK~ ~ - ) is plotted in fig. 1. The mass spectrum shows a clear peak at the Ar + mass. Fit t ing a Gaussian with a fixed width of 10 MeV/c 2 and a third-order polynomial to describe the background yields a signal of (83.7-+ 17.7) events at a mass of (2284.3_ 3.6)MeV/c 2. In order to extract the momentum distribution, the number of events is determined for five different xp intervals starting from xp = 0.5. The xp spectrum is corrected for the acceptance of the detector and then fitted with the fragmentation function of Peterson et al. [9]

[ ]2 dN 1 1 1

dxp Xp Xp 1 - xp

For the fragmentation parameter s we obtain (0.19 _+ 0.09). Using this result to extrapolate to the full momentum range, the production cross-section times branching ratio is

z.BR(Ac + __~ p~O~+ u- ) = (7.1 _+ 1.5 _+ 2.0) pb.

oo[ t " ' �9 , t,. ,,,j,.t.tl .t, ll, ,l,f..

t!!ttt'N t''' " "

25 I S

0 ~ . . ~ . . " : , ~ , i ~ , . . . . , . . . . , . . . . , . . . .

1.8 2.0 2.2 2.4 2.6 2.8 0 + - M (K s p~ rc ) (GeV/c 2)

Fig. 1. - Invariant-mass distribution of all pK~:: + ::- combinations with Xp > 0.5. The structure around 2.2 MeV/c 2 is a reflection from the decay D'--~ K ~ 3:: and is excluded from the fit.

NEW RESULTS ON CHARMED BARYONS FROM ARGUS 2047

30

20

10

' ' ' ' I . . . . I ' ' ' ' I ' ' ' ' i ' ' '

I . . . . . . 0 . . . . , . . ,

1.8 2.0 2.2 2.4 2.6 2.8 M [pK* (892) ~+] (GeV/c 2)

Fig. 2. - Invariant-mass distribution of pK*- ~ § combinations with x, > 0.5.

F rom this, we obtain the following result for the ratio of branching ratios:

BR(A+ __) p~O~+ ~z- )

BR(Ac + --) pK + ,~+ ) = (0.59 + 0.16 -- 0.18).

In the following we search for intermediate K*- resonances. Plotting the invariant mass of pK~247 combinations with xp > 0.5 and the additional requirement, that I M ( K ~ I < 50MeV/c 2, we obtain clear evidence for the decay Ac + --. pK* 7: § , as can be seen from fig. 2. A fit to the mass spectrum yields a signal of (44.6 __ 8.9) events. As a crosscheck, we fit the K~ spectrum for those p K ~ § ~- combinations, whose mass lies within 50 MeV/c 2 around the Ae mass. The result of (39.1---8.6) events is in good agreement with the above-mentioned number.

For the resonant decay mode, the ratio of branching ratios is

BR(Ar + --) pK*- 7: + )

BR(Ac + ---) pK- ~+ ) = (0.68 _+ 0.18 _+ 0.18).

In the complementary data sample, where the K*- region is excluded, we find evidence for the non-resonant decay with (29.6 _+ 13.4) events. The ratio of branching ratios is

BR(Ac + ~ [p~0~+ 7: ]NR)

BR(A + ~ p K ~+) = (0.31 __+ 0.15 _+ 0.17).

3. - Sea rch for the A *+ .

The search for an excited charmed baryon Ac *§ in the final state Ac + 7:§ is motivated by the fact that an excited A *§ state would decay strongly in ei ther Ar + ~ + ~ - or in Ec~, but not into A[ n ~ which is forbidden by isospin.

2048 J. SPENGLER

30

20

i0

' I . . . . I . . . . I ' ' ' ' I . . . . I '

2.56 2.60 2.64 2.68 2.72

M (A~ + n+n -) ( G e V / c 2)

Fig. 3. - I n v a r i a n t - m a s s d i s t r ibu t ion of h~ + ~+ ~ - combina t ions wi th x > 0.5. The solid h i s t o g r a m resu l t s f rom us ing the h~ + s idebands .

The A~ + is reconstructed in four different decay modes p K - n § , pK ~ A:: + and An§ - . All combinations with an invariant mass lying within _+25MeV/c 2 for p K - ~+ of the nominal Ac + mass were subjected to a mass constraint fit to improve the momentum resolution. The invariant mass of all Ac + ~§ n - combinations with a scaled momentum Xp above 0.5 are plotted in fig. 3. A nar row peak at a mass of about 2627 MeV/c 2 is observed, whereas the distribution build up f rom the Ar + sidebands shows a smooth behaviour. A fit to the mass spectrum with a Gaussian with free width

. . . . l . . . . i . . . . i . . . . I . . . .

0.3

~i~

0.1

0 0.2 0.4 0.6 0.8 xp 1.0

A c . The solid curve is the r e su l t of a fit wi th the P e t e r s o n Fig. 4. - The xp spec t rum of the * + function.

NEW RESULTS ON CHARMED BARYONS FROM A R G U S 2049

2O

~! lO

0 , . F ] . 2 . 5 6 2 . 6 0 2 . 6 4 2 . 6 8 2 . 7 2

M (A + ~+~-) ( G e V / c 2)

Fig. 5. - Invariant-mass distribution of A+~+~ - combinations with Xp > 0.5. A~+~ +/ combinations with I M ( A + ~ + ) - M ( X : + ) I ~<5.1MeV/c 2 and IM(A+~ ) -M(X~ ~< ~< 5.1 MeV/c z are excluded.

and position and a second-order polynomial to describe the background results in (42.4 _+ 8.8) events at a mass of (2626.6 _+ 0.5 -+ 1.5) MeV/c 2 and a width of ~ = (2.2 -- _+ 0.5)MeV/c 2. The width of the signal is consistent with the expected detector resolution. To obtain an upper limit for the natural wid th , / ' , of the A* +, the signal shape is parametr ized using a nonrelativistic BReit-Wigner convoluted with a Gaussian. We obtain an upper limit of F < 3.2 MeV/c 2 at 90% confidence level.

The momentum distribution as plotted in fig. 4 has been obtained by fitting the number of events in five different xp intervals and correcting for acceptance effects. The overlaid curve corresponds to the fit of the Peterson fragmentat ion function. The value found for the fragmentat ion parameter is s = (0.044_+0.018). Thus the momentum distribution of the Ac* + is considerably harder than the distribution of the A:.

The fitted fragmentat ion function is used to extrapolate the number of events into the full momentum range. Using the ARGUS measurement of BR(Ac + --+ p K - ~+ ) = = (4.0 _+ 0.3 -+ 0.8)% [10], the production rate times branching ratio is determined to be

a .BR(A *+ --+ Ae + 7: + ~ - ) = (11.5 ___ 2.5 _+ 3.0) pb.

Three possible decay channels can contribute to the observed signal:

non-resonant decay

resonant decay

resonant decay

Ac*+ --+ i + ~+ ~- ,

Ac *+ --+~+ + ~- __+ A+ ~+ ~ -

Ac,+ __+ ~o~+ __+ A + ~ ~+ .

The nonresonant contribution is estimated by excluding those A[ 7: + / - combinations with an invariant mass within 51 MeV/c 2 of the Ec mass. The resulting mass spectrum is shown in fig. 5. Fitting the signal with a Gaussian, we obtain (16.2 _+ 6.1) events.

In order to determine the resonant contributions, the invariant mass spectra of all A~ + r combinations with I M ( A c + T: + 7~- ) - 26271 ~< 6 MeV/c ~ are plotted in fig. 6. Enhancements at the position of the Z~ + § mass can be seen. The shape of the mass

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a)

2 H 0 . . . . ~ . , I ] . . . 2.40 2.42 2.44 2.46

b)

2.48 2.50 2.40 2.42

M (A + ~+) (GeV/c 2)

2.44 2.46 2.48 2.50

Fig. 6. - Invariant-mass distribution of A~ + +/ combinations from the signal region with [M(,% + ~+ ~- ) - 26271 ~< 6 MeV/c 2 for a) A~ + ~+ and b) ,%+ n - . The solid curves correspond to the fits described in the text.

distribution is complicated by additional substructures , which are due to the limited phase space of the decays A* § --~ Er + +/0 : :- / + _+ A+ 7: + ~ - . Fo r example consider the decay Ac *+ ~ Ec + + 7:- ~ Ac + n+ 7:- (fig. 6a)). Ac+'S picking up a p r imary : :- produce a bump in the A~ + ~ - distribution (fig. 6b)). According to our Monte Carlo studies, this bump is expected to have a mass around 2458 MeV/c 2 and a resolution of ~ = = 3 MeV/c ~ . Therefore both spectra shown in fig. 6 are fitted to a sum of two Gaussians and a polynomial multiplied by a threshold factor. The fit yields (11.4 _+ 5.4)Zr + § and (10.2 -+ 5.3)E ~ in the A~ + ~+ distribution and (10.4 +_ 5.6)E ~ and (13.7 __ 5.6)Zc + § in the A~ + ~+ distribution. F r o m these numbers we conclude tha t the branching rat ios for the E~++~ and o + Ec ~ channels are approximately equal, as one would expect for an excited Ac *§ baryon.

F rom these the following ratios of branching ratios were obtained:

BR(A,+ _+Ec + + / o ~ - / + ) = 0.51 + 0.21,

BR(A *+ --, A + ~+ ~ - )

B R ( A *+ - ~ [Ac + ~+ ~- ]NR) = 0.53 --+ 0.23. BR(A *+ --+ Ac+ ~+ n - )

4. - Obse rva t ion of semi - lep ton ic E ~ decays.

In this section the first observation of the semi-leptonic decay of the -cV~ baryon is discussed. The decay mode studied is ~o __~ E- C X, where e § is ei ther an electron or a muon. Because the neutrino escapes the detector, the existence of such a decay channel can only be proven by studying correlations between the E - and the lepton.

Recent theoretical work has es t imated the branching ratio of this semi-leptonic decay to be a few percent. F o r example both Singleton [11] and Hussain, KSrner [12] calculated a value of 3%. In addition, the measured branching rat ios for the semi-leptonic decays of the Ac + are BR(Ar + - - , C X) -- 4.5% and BR(Ac + --+M + X) -- 2.3%. F r o m these numbers we conclude tha t our da ta sample should allow for the detection of this decay mode.

The E is identified by its decay into A~ . The electrons and muons are identified by combining the information of all detector components into a likelihood ratio.

NEW RESULTS ON CHARMED BARYONS FROM ARGUS 2051

24 a )

12

1.25 1.30 1.35

I b)

1.40 1.45 1.25 1.30 1.35 1.40 1.45 mass (An ) (GeV/c 2)

Fig. 7. - Invariant-mass distribution of A=- combinations for events with: a) a right-sign lepton and b) a wrong-sign lepton.

Electrons are measured for momenta above 0.4 GeV/c with a fake rate of (0.5 _+ _+ 0.2)%. Muons must have momenta above 0.9 GeV/c to hit the outer muon chambers. Their fake rate is (2.0 _+ 0.6)%. In order to suppress combinatorial background, only A~- combinations with a scaled momentum xp > 0.45 and an invariant mass MA~ e + < 2.473 GeV/c 2 are accepted. The resulting A= mass distribution is plotted separately in fig. 7 for combinations with a C (right-sign lepton) and a C- (wrong- sign lepton).

Fit t ing fig. 7a) with a Gaussian and a third-order polynomial including a threshold factor yields (22.6 _+ 5.8)A~-U events at a mass of (1322.2 _+ 0.7)MeV/c ~ and a width of v = (2.2 _+ 0.6)MeV/c 2. Fixing the fit to the same mass and width for the wrong-sign distribution (fig. 7b)) yields a signal of (2.1 +_2.3)A~-e events. A~ combinations within 6.6 MeV/c 2 of the E mass are accepted as E- candidates. The resulting mass distribution of the ~- ( system is shown in fig. 8 for both right-sign and wrong-sign signal for Xp > 0 and xp > 0.45.

The following background sources have to be considered and are discussed as follows:

a) 2 - and C come from different sources and are combined by chance,

b) a fake :=- is combined with a lepton,

30 t ~ 20 -

~ 1 0

15 a) b)

5

r n I [ I , , , , [ ,

1 2 3 4 50 1 mass (E + ( ) (GeV/c ~)

4

Fig. 8. - Invariant mass of the ~- {' system for the right-sign (histogram) and the wrong-sign (solid squares) distributions with a) xp > 0 and b) Xp > 0.45.

2052 J. SPENGLER

c) a real 2 - is combined with a faked lepton,

d) feeddown from other charmed-baryon decays.

To estimate the size of the first source of background, continuum events have been generated using the Lund Monte Carlo [13] and processed in the some way as the real data. Scaled to our data sample, we find 1.1 _+ 0.2 right-sign background events with xp > 0.45.

According to our Monte Carlo studies, randomly correlated wrong-sign events tend to have a 2 - and a C- in different jets which results in a low Xp and a high invariant mass. This is conf~med by our data. In fig. 8a) the wrong-sign mass distribution is larger than the right-sign above the ~o ~c mass. This background is removed to a large fraction by the xp-cut, as can be seen from fig. 8b).

The second and largest background source comes from the combination of a fake 2- with a lepton. It can be estimated by determining the amount of An events in fig. 7a) below the 2 signal. Fitting the An mass distribution, we find (14.9 +_ 1.4) events within 6.6 MeV/c 2 of the 2 mass. The background due to faked leptons is calculated by multiplying the lepton-hadron fake rate with the charged hadron multiplicity in 7: events. This yields (3.0 + 0.9) right-sign and (1.7 -+ 0.5) wrong-sign events. As the final background source we consider contributions from the decay 2c + ---)2(1530) 0 e § This background is examined by combining the selected Y.- with additional ~§ Fitting the 2- ~+ mass distribution, we obtain a signal of (1.7 _+ + 1.7)2(1530) 0 events. This background is accounted for by excluding all 2 - ~§ events from the analysis which are within 3.0~ of the 2(1530) ~

The amount of semi-leptonic ~c~~ decays is finally determined by subtracting the back- ground sources a) and b). There remains a net signal of (18.1 _+ 5.9) events. Correcting for the detector efficiency the production cross-section times branching ratio is found to be

~. BR(' ; ~ -~ 2 - e + X) = (0.74 + 0.24 + 0.09) pb.

Because the branching ratio of the above decay is unknown, we compare this result with a previous ARGUS measurement of the decay 2 ~ -~ 2- ~+ [14]

B R ( 2 ~ 2 e ' X ) = 0.96 _+ 0.47.

BR(2 ~ --, 2 - n § )

5. - Search for the decay []c---) ~ -C+ X.

As the last topic the first experimental evidence for semi-leptonic decays of the ~)~c in the decay mode f~c--~ ~ - U X is discussed. In the same way as described in the previous section, the presence of this decay is inferred from studying correlations between the ~ - and the lepton. However, in this analysis only electrons are used, because the detection efficiency for muons in this decay is too small due to the high momentum cut-off.

The ~ - is reconstructed by its decay ~2---~ AK-. The invariant (AK-) mass is plotted in fig. 9. Fitting a Gaussian to the peak gives a mass of (1672.0 _ 1.0) MeV/c 2 in good agreement with the table value and a width of ~ = (4.0 _+ 0.8)MeV/c e.

Several cuts have to be applied in order to suppress the combinatorial background. We restrict the (Q- e + X) mass to be less than 2.720 MeV/c 2 which is the

NEW RESULTS ON CHARMED BARYONS FROM ARGUS 2053

60

50

~40 30 20

11 1.60 1.65 1.70 1.75 1.80 1.85 1.90

(GeV)

Fig. 9. - Invariant-mass distribution of the AK system.

mass of the ~c as measured by ARGUS. The cosine of the angle between the momentum direction of the AK- e § system and the event thrus t axis has to be larger than 0.7. The next cut is motivated by the observation that charmed baryons have a hard fragmentat ion function. Therefore, the cosine of the opening angle between the A and the K - is required to be above 0.7. This cut removes low-momentum random combinations. The last cut makes use of the quite large lifetime of hyperons. The cz-value of the ~ - is 2.46 cm, that of the A is 7.89 cm. Thus only A hyperons with a

' ' ' ' l ; ' " ' l - - ; ' ' ' ' ' i ' ' l i i ' ' ' | ' " ' '

3~ I 2.5

2.0

~1.5 1.0 ,~ 0.5

0.0 1.60 1.65

[7 ! ! 1.70 1.75 1.80 1.85 1.90

(GeV)

Fig. 10. - Invariant-mass distribution of the AK- system in events with an additional e + .

2054 J. SPENGLER

decay length above 3 cm are taken into account. After application of the above- mentioned cuts, a signal at the position of the ~ - mass is visible in fig. 10.

Fit t ing this signal with a Gaussian and a flat background yields a signal of (7.6 _+ _+ 3.1) events at a mass of (1673 _+ 4 )MeV/c 2 for z = (7.3 -+ 2.9) MeV. Using Poisson statistics we calculate a probability of 1.4.10 -3 for the peak to be due to a fluctuation of the background, which corresponds to a significance of 3.2 standard deviations.

No signal is observed in either of the two wrong-charge combinations AK § e § and A K - e - . We therefore in terpre t the correlation observed as evidence for the semi-leptonic decay of the fie. The production cross-section times branching ratio derived is

z .BR(~r --~ ~ - e + X) =(0.52 +_ 0.23 +__ 0.13) pb.

R E F E R E N C E S

[1] MARK II (G. S. ABRAMS et al.): Phys. Rev. Lett., 44, 10 (1980). [2] PARTICLE DATA GROUP: Review of Particle Properties, Phys. Rev. D, 45, 1 (1992). [3] Z. STIEWE: XXVI International Conference on High Energy Physics, Dallas, 1992,

p. 1076. [4] ARGUS COLLABORATION (H. ALBRECHT et al.): to be published. [5] ARGUS COLLABORATION (H. ALBRECHT et al.): submitted to Phys. Lett. [6] ARGUS COLLABORATION (H. ALBRECHT et al.): Phys. Lett. B, 303, 368 (1993). [7] ARGUS COLLABORATION (H. ALBRECHT et al.): contributed paper to the Europhysics

Conference, Marseille, 1993. [8] ARGUS COLLABORATION (U. ALBRECHT et al.): NucL Instruna Methods A, 275, 1 (1989). [9] C. FETERSON et al.: Phys. Rev. D, 27, 105 (1983).

[10] ARGUS COLLABORATION (H. ALBRECHT et al.): Z. Phys. C, 56, 1 (1992). [11] R. SINGLETON jr.: Phys. Rev. D, 43, 2939 (1991). [12] F. HUSSAIN and J. G. KORNER: Z. Phys. C, 51, 607 (1991). [13] T. SJhSTRAND and M. BENGTSSON: Comput. Phys. Commun., 43, 367 (1987). [14] ARGUS COLLABORATION (U. ALBRECHT et al.): Phys. Lett. B, 283, 161 (1992).