5 June 1997

PHYSICS LETTERS B

Physics Letters B 402 (1997) 207-212

Evidence for A, (2593) + production

ARGUS Collaboration

H. Albrecht a, T. Hamacher”, R.P. Hofmann”, T. Kirchhoffa, R. Mankel a~‘, A. Naua, S. Nowaka**, D. ReSing ‘, H. Schr6dera, H.D. Schulza, M. Waltera,‘, R. Wurtha, C. Hast b,

H. Kapitzab, H. Kolanoski bp2, A. Kosche b, A. Lange b, A. Lindner b, M. Schieberb, T. Siegmund b, H. Thurn b, D. Tijpfer b, D. Wegener b, C. Frankl’, J. Graft, M. Schmidtler”,

M. Schrammc, K.R. Schubert ‘, R. Schwierz c, B. Spaan ‘, R. Waldi ‘, K. Reim d, H. Wegener d, R. Eckmann e, H. Kuiperse, 0. Mai e, R. Mundt e, T. Oest e, R. Reiner e, A. Rohdee, W. Schmidt-Parzefalle, J. Stiewef, S. Werner f, K. Ehretg, W. Hofmanng,

A. Hiipper g, K.T. Knijpfle g, J. Spengler g, P. Krieger hv7, D.B. MacFarlane hv8, J.D. Prentice h*7, P.R.B. Saul1 h,8, K. Tzamariudaki h98, R.G. Van de Water h*7, T.-S. Yoon hv7, M. Schneider’, S. Weseler’, M. Brazkoj, G. Kernelj, P. Kri&nj, E. KriZniEj, G. Medinjv”, T. Podobnikj, T. $?ivkoj, V. Balagurak, S. Barsuk k, I. Belyaev k, R. Chistov k, M. Danilov k,

V. Eiges k, L. Gershtein k, Yu. Gershtein k, A. Golutvin k, 0. Igonkina k, I. Korolko k, G. Kostinak, D. Litvintsev k, P. Pakhlov k, S. Semenov k, A. Snizhko k, I. Tichomirov k,

Yu. Zaitsev k a DESK Hamburg, Germany

b Institutflr Physik, Universitiit Dortmund, Germany3

’ Institut fiir Kern- una’ Teilchexphysik, Technische Universitdt Dresden, Germany4 d Physikulisches Institut, Universitiit Erlangen-Niimberg. Germany5 e II. Institut fiir Experimentalphysik, Universitd[t Hamburg, Germany f lnstitut frir Hochenergiephysik. Universitdt Heidelberg, Germany6

g Max-Planck-lnstitutfr Kemphysik, Heidelberg, Germany h Institute of Particle Physics, Canada9

i Institut fiir Experimentelle Kemphysik, UniversW Karlsruhe, Germany ‘O j Institut J. Stefan and Oddelek za j.ziko, Univerza v rjubljani, Ljubljana, Slovenia I2

k Institute of Theoretical and Experimental Physics, Moscow, Russia I3

Received 13 February 1997 Editor: K. Winter

Abstract

Using the ARGUS detector at the e+e- storage ring DORIS II at DESY, we have found evidence for the production + + of the excited charmed batyon state &(2593)+ in the channel AC r 7~ -. Its mass was determined to be (2594.6 f 0.9 f

0.4) MeV/c’, and the natural width measured to be I? = (2.9’_~~~4) MeV. The production cross section times the branching ratios of v(e+e- + &(2593)+X) x Br(A,(2593)+ --) A:T’T-) x Br(A,f --$ pK-?r+) was measured to

0370-2693/97/$17.00 @ 1997 Published by Elsevier Science B.V. All rights reserved. PIISO370-2693(97)00503-O

208 ARGUS Colluborarion/ Physics Letters B 402 (1997) 207-212

be (0.25T’fi f 0.13) pb. Tbe fractions of A,(2593)+ decays proceeding through the Z~TT’ and H:+TT- channels were determined to be 0.29 f 0.10 f 0.11 and 0.37 4~ 0.12 f 0.13, respectively. @ 1997 Published by Elsevier Science B.V.

1. Introduction

Most ground state charmed baryons are now well established [ 11, and the measurements confirm their

mass predictions by theoretical models. As for the

mass predictions for the excited charmed baryons,

their experimental verification has been long over- due. New experimental observations would help to distinguish between different theoretical approaches [2]. The first observation of the excited charmed baryon t4 A, (2625)+ was made by the ARGUS Col- laboration [ 33. The E687 [ 41 and CLEO [ 51 Collab- orations subsequently confirmed this resonance. The CLEO Collaboration also reported the observation of another state decaying into A,+T+T- at a mass of about 2593 MeV/c2 [S] which was referred to as the A,(2593)+, and the E687 Collaboration pub- lished the confirmation of this state [6]. Recently, the CLEO Collaboration presented the observation of two new charmed baryon states at masses of about 25 18 MeV/c2 and 2520 MeV/c2 decaying into A,+z-- and A$& [ 71, respectively. In the charmed

I DESY, IfH Zeuthen. 2 Now at Institut fur Physik, Humboldt-Universit;it zu Berlin,

Germany. 3 Supported by the German Bundesministerium fiir Forschung

und Technologie, under contract number 054DO5 1 I? 4Supported by the German Bundesministerium fur Forschung

und Technologie, under contract number 056DDllP 5Supported by the German Bundesministerium fur Forschung

und Technologie, under contract number 054ER12P “Supported by the German Bundesministerium fiir Forschung

und Technologie, under contract number 055HD2 1 P ’ University of Toronto, Toronto, Ontario, Canada. * McGill University, Montreal, Quebec, Canada. 9 Supported by the Natural Sciences and Engineering Research

Council, Canada. lo Supported by the German Bundesministerium fib Forschung und Technologie, under contract number 055KAllP t t On leave from University of Montenegro, Yugoslavia. I2 Supported by the Ministry of Science and Technology of the

Republic of Slovenia and the Intemationales B&o KfA, Jiilich. t3 Partially supported by Grant MSB300 from the International Science Foundation. t4 All references to a specific charged state also imply the charge

conjugate state.

strange baryon sector, the CLEO Collaboration has evidence for excited states decaying into a,+n-- [ 81 and @T+ [ 91. In this paper we report on a study of the A,(2593)+ resonance in the decay channel AfT+T-. c

2. Selection criteria

The data used for this analysis was collected on the Y ( 1 S) , Y (2s) and Y (4s) resonances, and in the nearby continuum using the ARGUS detector at the e+e- storage ring DORIS II at DESY. The integrated luminosity of the data sample is equal to 476 pb-’ . The ARGUS detector is a 4~ solenoidal magnetic spec- trometer. Its components are a vertex detector, a main drift chamber which measures both the spatial position of tracks and their specific ionization, time-of-flight counters, an electromagnetic calorimeter, and a muon detector system. A more detailed description of the ARGUS detector, its trigger requirements and particle identification capabilities have been given elsewhere [ lo]. Charged tracks from the main vertex were re- quired to have a polar angle, 0, in the range 1 cos( /3) 1 < 0.92. Charged particle identification was based on a likelihood ratio calculated from measurements of spe- cific ionization and time-of-flight for the allowed mass hypotheses (e, CL, T, K, and p) [ lo]. Each particle was used as a pion, kaon, or proton if the correspond- ing likelihood ratio exceeded l%, 5%, or 15%, re- spectively. A0 (e) candidates were defined as pr-

( T+T- ) pairs forming a secondary vertex and having an invariant mass within f10 (f30) MeV/c2 of the nominal A0 (e) mass. In addition it was required that cos( cu) > 0.995(0.9), where (Y is the angle be- tween the A0 (e) momentum and the vector pointing from the main vertex to the decay vertex. All A0 and @ candidates were kinematically constrained to their nominal masses [ I]. For the AZ reconstruction five decay modes were used - pK-rrf, pe, pe?r’r-, Ao& and AOn-+?r-&. All combinations having an invariant mass within f25 MeV/c2 (for the pK-TT+, AOrr+~-& and pedr- modes), f30 MeV/c2 (for the pe mode) and f40 MeV/c2 (for the Aor’

ARGUS Colluboration/ Physics Letters B 402 (1997) 207-212 209

M(A$r+lr-) [GeV/c*]

Fig. 1. The h:?r+?r- invariant mass distribution. The solid his- togram represents the same distribution but using the A,f side- bands.

mode) of the A,’ nominal mass [ l] were considered as AZ candidates and kinematically constraint to its mass.

Each selected A,’ candidate was combined with all r+K pairs in an event. The momentum spectra of charmed hadrons from the continuum in efe- anni- hilation have proven to be hard compared to the com- binatorial background, and therefore the requirement of a large scaled momentum xP, where

xp = p@r+~- )/Pnlax

and

Pmax= 4 E&__ - M*(A$r+p-),

is a good tool to improve the signal to background ratio. In this analysis the scaled momentum x,, of each A,frr+lr- combination was required to be greater than 0.7 if not stated otherwise.

3. Data analysis

The invariant mass spectrum of all accepted AZ&K combinations is shown in Fig. 1. Two nar- row peaks are seen at masses of about 2593 MeV/c* and 2625 MeV/c*. The right peak corresponds to the previously observed Ac( 2625)+ resonance [ I]. The solid histogram in Fig. 1 represents the invariant mass spectrum for artificial A,+&r- combinations with the AZ candidates taken from the sidebands. It has also been checked that the invariant mass spec- tra for wrong-charge combinations (A,+lr-lr- and

A,+&&) have a smooth behavior in the signal regions.

The lower-mass peak proved to be significantly wider than the expected detector resolution of 1.8 MeV/c* found from a Monte Carlo simulation. The spectrum presented in Fig. 1 was fitted with the sum of a non-relativistic Breit-Wigner convoluted with a Gaussian resolution function parametrizing the lower-mass peak plus a pure Gaussian to de- scribe the higher-mass signal [3,5,6]. The widths of the Gaussians were fixed to the expected val- ues of 1.8 MeV/c* and 3.0 MeV/c*, respectively. A background parametrization consisting of a sec- ond order polynomial, multiplied by a three-body threshold factor completed the fit function. Using a Monte Carlo simulation procedure it was found that the mass resolutions and efficiencies do not sub- stantially depend upon whether the baryons under study decay through a resonant Z.,r* channel or not. The fit yields 18.8 f 5.9 and 51.3 f 8.2 events for the A, (2593) + and A, (2625) + states respectively. The masses M(A,(2593)+), M(A,(2625)+), and the mass difference of SM = M(A,(2625)+) - M(A,(2593)+) were measured to be (2594.6 f 0.9 f 0.4) MeV/c*, (2627.0 f 0.5 f 0.5) MeV/*, and (32.4 f 1.0 f 0.7) MeV/c*, respectively. The natural width for the A,(2593)+ state was obtained to be I = (2.9:;;?$ MeV. We also measured the mass differences ‘of h(A,(2593)+) - M(Az) and M(A,(2625)+) - M(Az) to be (309.7 f 0.9 f 0.4) MeV/c* and (342.1 f 0.5 f 0.5) MeV/c*, re- spectively. The systematic errors come from varying the cuts, signal widths and background parametriza- tion. An additional systematic error of 0.6 MeV/c* should be added to the mass measurements as the er- ror in the world average A,f mass [ 11. The obtained AC (2593) + parameters are in a good agreement with the results of the CLEO [ 51 and E687 [ 61 Collabo- rations (see Table 1) .

To obtain the A,(2593)+ momentum distribution we fitted the invariant Azlr+rr- mass spectra in five xP bins starting from xP = 0.5. The mass and width of the A, (2593) + state were fixed to the values deter- mined from the overall fit. Gaussian resolutions were fixed in each xP bin to the appropriate values obtained from a Monte Carlo simulation. The resulting effi- ciency corrected A,( 2593)+ momentum spectrum is shown in Fig. 2. The efficiency in each xP bin is the

210

Table 1

ARGUS CoIlrrborution/ Physics Letters B 402 (1997) 207-212

The extracted parameters of the h+(2593) baryon along with measurements of other groups

&(2593)+ parameter [ MeV/c*]

ARGUS CLEO 151 E687 [6]

WAC”+) 2594.6 f 0.9 z!z 0.4

M(A;+) - M(lq) 309.7 f 0.9 f 0.4 307.5 It 0.4 rfr 1 .o 309.2 i 0.7 zt 0.3 SM 32.4 f I .O f 0.7 34.7 f 0.5 l I .2

VAT+) 2 ’ 9+*.9+ul -2.1-1.4 3,9+1.4+2.0 -1.2-1.0

Fig. 2. The xP spectrum for A,(2593)+. The superimposed curve represents the fit with a Peterson et al. fragmentation function.

sum of the products of the reconstruction efficiency for the corresponding AZ mode found from a Monte Carlo simulation and the ratio of BP-(&! -t X)/Br(A: + pK-vr+) for this mode [ 11. The Peterson et al. frag- mentation function [ 141

da

dx, c( xp -W-;-&]-2

was used to fit this xp spectrum and to obtain the total number of A, (2593)+ baryons produced in the entire momentum interval, and the fragmentation parameter E. The fit results in E = 0.069~~,~$~ f 0.040, and the production cross section times the branching ratios in e+e- annihilation at fi = 10.4 GeV:

a(A,(2593)+) x Br(A,(2593)+ j A;r+lr-)

x Br(A,+ --) pK-n-+)

= (0.25?;,;; f 0.13) pb.

The cross section in the momentum interval of xp > 0.7 was derived from the fitted number of A,( 2593) i events in the A,+IT+~~- spectrum and equals to

a(A,(2593)+jxp > 0.7)

x Br(A,(2593)+ * A,+&r-)

x Br(A: --) pK-nf)

= (0.14 f 0.04 f 0.03) pb.

This value is free of the extrapolation uncertainty. The quoted systematic errors include the uncertainty on variation of the fit parameters in each xp bin, the errors on the Monte Carlo reconstruction efficiency, and the errors on Br(Rz -+ X)/Br(Az --) pK-T+) [I]. Using the ARGUS measurement of a(A,+) x Br(A: + pK-?r+) = (12.0f 1.0f 1.3) pb [ 131 the fraction of A,’ baryons produced from A, (2593) + ---) A,+&lr- decays was found to be (2.1”_:.,: * 1.1) %, in agreement with the analogous measurement of the CLEO Collaboration [ 53 of ( 1.44 f 0.24 f 0.30) %.

An HQET [ 111 and some other models [ 121 predict that the A,(2593)+, being interpreted as a Jp = (i)- state, should have resonant 2,~ S-wave decay channels. Therefore the experimental resonant structure measurements in the decay A,(2593) + ---f A:&r- can help to establish the identity of this state. For the A,(2593)+ state the CLEO [ 51 and E687 [6] Collaborations have found that substantial fractions of A,(2593)+ 4 Afn-+r- decays pro- c teed through the &7r* resonant channels. Here we present our investigation of the resonant structure in the A,(2593)+ ---j 12:7.r+rr- decay. To get more statistics in this part of the analysis the xp cut was relaxed to the value of 0.5.

In order to obtain the resonant contribution to the decay A, (2593)+ j A,fn-+r- we performed a max- imum likelihood fit to the Azlr+?r- invariant mass spectrum in bins of the A$& invariant mass. The A, (2593) + parameters were fixed to the values found from the overall fit, and the mass resolution was fixed

ARGUS Collaboration/ Physics Letters B 402 f 1997) 207-212 211

Table 2 Summary of measurements for the A+(2593) decay fractions

&(2593)+

decay fractions ARGUS cm [51 E687 [6]

fg 0.29fO.lOf0.11 0.42 f 0.09 f 0.09

fz:+ 0.37 f 0.12f 0.13 0.36 f 0.09 f 0.09

f o++ PC’

0.66+“‘3 -0.16 f 0 ’ 07 > 0.51 @ 90% CL

I 1 I 2.41 2.42 2.43 2.44 2.45 2.46 2.47

M(A+r+) c [GeV/“]

Fig. 3. AZ& invariant mass spectrum for A,(2593)+ decays. The overlayed curve is the fit described in the text.

to the value found from a Monte Carlo simulation (g = 1.9 MeV/c*). The spectrum obtained is presented in Fig. 3, where two peaks on a small non-resonant background are clearly seen. The peak at a mass of 2453 MeV/c* is attributed to the Xz+ baryon, the peak at 2428 MeV/c* corresponds to a reflection of the 2:. This reflection arises because of the small phase space in the &(2593)+ + Z$r+(Sz -+ A,+K) de- cay. The A,+& mass distribution was fitted using two Gaussians to describe the 2:+ signal and ZF reflec- tion plus a constant term times square-root threshold factor to describe the non-resonant contribution. The Gaussian widths and central values were fixed from a Monte Carlo simulation. The fit results in the follow- ing decay fractions for the A, (2593) + state:

fq = Br(A,(2593)+ -+ IZzd)

Br(A,(2593)+ -+ Afn-f?r-) c =0.29fO.lOf0.11.

fx:+ = Br(A,(2593)+ + Z,++K)

Br(A,(2593)+ --+ A+&r-) c = 0.37 f 0.12 f 0.13,

f Br(A,(2593)+ + (A,(2593)+~+~).,)

nr = Br(A,(2593)+ + A++w-) c

= 0.34:;,:6, f 0.07.

Using the constraint fsf + fg+ + fnr = 1, the total

fraction of Zc7r* decays becomes 0.66::,{36 f 0.07. The systematic errors on the extracted fractions were evaluated by varying the fit parameters and the shape of the non-resonant function. Our measurement of the resonant decay fractions is in agreement with the re- sults of the CLEO [ 51 and E687 [ 61 Collaborations (see Table 2).

4. Conclusions

In summary, we confirmed the existence of the A,( 2593)+ resonance and studied its production and decay in eie- annihilation at 10.4 GeV center-of- mass energy.

The mass and the natural width of the AC (2593) + state were measured to be (2594.6 f 0.9 f 0.4 f 0.6)MeV/c2 and (2.9T$.:+_‘;;)MeV, respectively. The mass differences of M( A,( 2593)+) - M( A,‘) and M(A,(2625)+) - M(A,(2593)+) were deter- mined to be (309.7 f 0.9 f 0.4) MeV/c* and ( 32.4 f 1.0 f 0.7) MeV/c*, respectively. The production cross section of a(A,(2593)+) x Br(A,(2593)+ -+ Az7r+7rP) x Br(Az --f pK-r+) was found to be (0.25~~,~~f0.13) pb. The fractions of A,(2593)+ +

A,f&r- decays going through the 8:~~ and X,“7rs channels were determined to be 0.29 f 0.10 f 0.11 and 0.37 f 0.12 f 0.13, respectively. Our A, (2593)+ mass, natural width and resonant decay fraction mea- surements are in agreement with the CLEO [ 51 and E687 [ 61 Collaborations results.

212 ARGUS ColIoboration/Physics Letters B 402 (1997) 207-212

Acknowledgements

It is a pleasure to thank U. Djuanda, E. Konrad, E. Michel and W. Reinsch for their competent techni- cal help in running the experiment and processing the data. We thank Dr. H. Nesemann, B. Sarau, and the DORIS group for the excellent operation of the stor- age ring. The visiting groups wish to thank DESY di- rectorate for the support and kind hospitality extended to them.

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