Download pdf - Observation of a doubly charged charmed baryon in the reaction υp→μ−Σ++c

Volume 109B, number 3 PHYSICS LETTERS 18 February 1982

OBSERVATION OF A DOUBLY CHARGED CHARMED BARYON

IN THE REACTION up ~ ~ t - ~ c ++

A a c h e n - B o n n - C E R N - M u n i c h (MPl)-Oxford Collaboration

P.C. BOSETTI, H. GRASSLER, D. LANSKE and R. SCHULTE I11. Physikalisches lnstitut der Technischen Hochschule, Aachen, Germany

K. BOCKMANN, B. ECKART, W. GEBEL and C. GEICH-GIMBEL Physikalisches Institut der Universitiit Bonn, Bonn, Germany

V.T. COCCONI, A. GRANT, P.O. HULTH, H. KLEIN, D.R.O. MORRISON, Ch. PEYROU, P. SCHMID, E. SIMOPOULOU l and H. WACHSMUTH CERN, European Organization for Nuclear Research, Geneva, Switzerland

M. ADERHOLZ, N. SCHMITZ, R. SETTLES, K.L. WERNHARD and W. WITTEK Max-Planck-Institut fffr Physik und Astrophysik, Munich, Germany

R. BATLEY, R. GILES, G. MYATT, D. RADOJICIC and B. SAITTA Department of Nuclear Physics, Oxford, UK

Received 11 November 1981

Evidence for the quasi-elastic production of the doubly charged charmed baryon I ~ has been observed in a up interaction in BEBC. The mass of the l~c + and of its decay product, the Ac, are 2454 _+ 5 MeV and 2288 ± 5 MeV, respectively. The mass difference M(Nc +) - M(Ac) is 166 ± 1 MeV. Combining the A c mass value obtained here with the values obtained in the two A c events previously observed in this experiment, the value M(Ac) = 2283 + 3 MeV is deduced.

This letter reports the observation of an interaction _ _ + +

up -~/a pK 7rlTr 2 in which the pK-zr~ system has an + +

invariant mass consistent with a A S and the Acn 2 invariant mass yields 2454 MeV, consistent with the

+ +

Y'c mass, Thus the event is a candidate for the quasi- elastic reaction

up . - X c ++ . ( 1 )

Only one candidate of reaction (1) was reported previously in the literature [1], It was found at BNL in 1975 and it represented the first observation of a charmed baryon.

1 Now at Democritos, Athens, Greece.

234

The data used here come from an exposure of the Big European Bubble Chamber (BEBC) filled with hy- drogen to the CERN wideband neutrino beam. The sample of up interactions so far analyzed contains a little over 10 000 charged current (CC) events with

Evi s > 5 GeV and an EMI identified negative muon of momentum greater than 3 GeV/c. In a subsample of ~ 7 7 0 0 CC interactions which was searched for events with a 3-constraint fit involving only one strange particle of negative strangeness, four events were foun. with uniquely identified charmed particles, namely two D *+ mesons [2] and two Ac + baryons [3]. The event reported here was selected in a similar search performed in a recently acquired additional subsample of 4 2 4 6 0 CC events.

0 031-9163/82/0000-0000/$ 02.75 © 1982 North-Holland


..',, " - , _ | %

t \ \

v p - - - p. ~.. c + + +

~p K-TT, +

NEUTRINO

BEAM

Fig. 1.

A reduced fiducial volume has been used such that the distance between the interaction point and the chamber wall was at least 1 m, to ensure momentum measurements of good accuracy. If the strange particle was a K - , it was required to have a momentum of less than 5 GeV/c to give good particle identification either

by ionization (up to momenta of ~ 8 0 0 MeV/c) or by energy and momentum conservation. If any other hy- pothesis fitted the event with a ×2 probability > 10 -4 , the event was rejected.

One event (shown in fig. 1) has been found fulfilling these criteria and fitting uniquely the reaction

/a pK 7rlTr 2 . (2) / ) 1 9 - - - > - - - - + +

Repeated measurements of this event gave consistent results and an average fit probability of 35%.

The most energetic negative particle is identified as a ~ - by hits in both planes of the External Muon Identifier. One of the positive tracks is identifiod as 7r + by ionization. The squared four-momentum transfer between the v and the/~ is Q2 = 2.5 GeV 2 and the val-

ues o fx =Q2 /2M(Ev - E , ) a n d y = (E v - E , ) / E v are 0.33 and 0.18, respectively. Table 1 summarizes the kinematic characteristics of the five tracks.

As discussed in ref. [3], the probability for an

event of this type to be due to associated strange-particle production, where the second strange particle is either not detected or not identified, is extremely low.

The masses of the (pK-zr~-) and (pK-Tr2 +) combina- tions are 2288 -+ 5 MeV and 1774 + 5 MeV, respectively. The former value is in very good agreement

with the value for the Ac + mass obtained from the two A + events found in this experiment [3]. The mass of

Table 1 Description of the charmed baryon event a)

up ~ t~- 2 ++ + + C 7r2 _ +

pK 7r 1

Particle Momentum Identification (GeV/c)

18.363 _+ 0.044 EMI, FIT K- 1.137 _~ 0.005 FIT p 2.965 -+ 0.010 FIT

+

~rl 0.377 _+ 0.002 FIT zr + 0.214 + 0.001 IONIZATION, FIT 2

a)Ev= 22.43 _+ 0.05 GeV, W= 2.454 _+ 0.005 GeV, M(pK-rr~) = 2.288 ± 0.005 GeV.

235


the total hadronic system in this event is 2454 +- 5 MeV and this is close to the value of the Zc ++ mass predicted theoretically [4]. Thus the event is a candidate for the quasi-elastic reaction

up - + / a - ~ -~+

ac+Tr~ (3)

LpK- ; Assuming reaction (3), the mass difference M(Gc ++)

- M ( A ¢ +) is then found to be 166 +- 1 MeV .1 . This value is in good agreement with that found in ref. [1] of 166 + 15 MeV. Both events are thus consistent with the theoretical prediction of &M ~ 160 MeV between

the A c and the lowest lying (cuu) state of spin 1/2 [4]. The higher (cuu) state of spin 3/2, Y,*++ is predicted m G }

to be ~ 7 0 MeV higher. Thus, the two events observed can be interpreted as evidence for the Zc ++ state, which corresponds to the C~ "+ baryon of the 20' representa- t ion of SU(4), in the notations of ref. [4].

In ref. [ 1], the values reported for the masses of the Go++ and A + are 2426 + 12 MeV and 2260 ± 12 MeV, respectively, both ~ 2 8 MeV lower than the values determined here. Though several other experiments [5] have reported values for M ( A +) of "-'2.26 OeV, recent more accurate determinations point to a higher value. In e+e - annihilations, SLAC reports M ( A +) = 2285 + 5 MeV [6]. The value obtained in this up experiment by averaging the value determined here with the two values reported in ref. [3] for the two other events of A + production, is M(A +) = 2283 -+ 3

.1 Note that the error on the.mass difference is not determined by the errors on the two masses (which axe strongly correlated), but rather by the error on the angle between A~ and 7r~, which is very well measured.

MeV, in good agreement with the SLAC result. The error of 3 MeV given above consists of a statistical error of 2.5 MeV and a systematic error of 1 MeV, derived mainly from the uncertainties in the optical reconstruction of the tracks (the absolute measurements of the magnetic field of BEBC make the uncertainties due to the field negligible). The higher value of M(A +) supports the ~++ mass value obtained here, higher than that reported in ref. [1].

It is interesting to note that the mass difference between the doubly charged Zc ++ and the A + obtained here, 166 ± 1 MeV, is the same within errors as that reported in ref. [7], namely 168 ± 3 MeV, for one

+ 0 case of singly charged Y~+ decaying into Acn . In conclusion, evidence is presented for the quasi-

elastic production of the doubly charged charmed baryon Zc ++ in neu t r ino -p ro ton interactions. The Ec ++ and A~ masses are determined with small errors.

It is a pleasure to acknowledge the work of the scanning and measuring teams of our laboratories, and we thank the staff at CERN for the operat ion of the CPS and SPS accelerators, neutrino beam, BEBC chamber and EMI.

References

[11 E.G. Cazzoli et al., Phys. Rev. Lett. 34 (1975) 1125. [2] ABCMO Collab., J. Bhetschau et al., Phys. Lett. 86B

(1979) 108. [3] ABCMO Collab., H. Gr/issler et al., Phys. Lett. 99B (1981)

159. [4] A. de Rfijula et al., Phys. Rev. D12 (1975) 147;

I.G. K6rner et al., Z. Phys. C2 (1979) 117. [5] C. Baltay et al., Phys. Rev. Lett. 42 (1979) 1721;

K.L. Giboni et al., Phys. Lett. 85B (1979) 437. [6] G.S. Abrams et al., Phys. Rev. Lett. 44 (1980) 10. [71 M. Calicchio et al., Phys. Lett. 93B (1980) 521.

236