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18 April 1996

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PHYSICS LETTERS B

Physics Letters B 373 (1996) 261-266

Observation of new decay modes of the charmed-strange baryon E,’

CLEO Collaboration

K.W. Edwards a, K.W. McLean a, M. Ogg ‘, A. Bellerive b, D.I. Britton b, E.R.F. Hyatt b, R. Janicek b, D.B. MacFarlane b, PM. Pate1 b, B. Spaan b, A.J. Sadoff ‘, R. Ammar d,

P. Baringerd, A. Bean d, D. Besson d, D. Coppaged, N. Copty d, R. Davis d, N. Hancock d, S. Kotov d, I. Kravchenko d, N. Kwak d, Y. Kubotae, M. Lattery e, J.K. Nelson e, S. Patton e,

R. Polinge, T. Riehlee, V. Savinove, R. Wang e, M.S. Alam f, I.J. Kim f, Z. Ling f, A.H. Mahmood f, J.J. O’Neill f, H. Severini f, C.R. Sun f, S. Timm f, F. Wappler f,

G. Crawford s, J.E. Duboscq s, R. Fulton s, D. Fujino s, K.K. Gan s, K. Honscheid s, H. Kagan s, R. Kass s, J. Lee s, M. Sung s, C. Whites, R. Wanke s, A. Wolfs,

M.M. Zoellers, X. Fu h, B. Nemati h, W.R. Ross h, P. Skubic h, M. Wood h, M. Bishai i, J. Fast i, E. Gerndt i, J.W. Hinson’, T. Miao i, D.H. Miller i, M. Modesitt i, E.I. Shibata’,

I.P.J. Shipsey i, P.N. Wang i, L. Gibbonsj, S.D. Johnsonj, Y. Kwonj, S. Robertsj, E.H. Thorndikej, T.E. Coan k, J. Dominickk, V. Fadeyev k, I. Korolkov k, M. Lambrecht k,

S. Sangherak, V. Shelkovk, R. Stroynowskik, I. Volobouevk, G. Wei k, M. Artuso ‘, M. Gao’, M. Goldberge, D. He!, N. Horwitz[, S. Koppe, G.C. Monetie, R. Mountaine, F. Muheime, Y. Mukhin e, S. Playfere, T. Skwamickie, S. Stone e, X. Xinge, J. Bartelt m,

S.E. Csornam, V. Jam”, S. Markam, D. Gibaut n, K. Kinoshita”, P. Pomianowski n, S. Schrenk”, B. Barish O, M. Chadha”, S. ChanO, D.F. Cowen’, G. Eigen O, J.S. Miller’, C. O’Grady O, J. Urheim’, A.J. Weinstein O, F. Wtirthwein O, D.M. Asner P, M. Athanas P,

D.W. Blissp, W.S. Browerr, G. Masekp, HP. Paarr, J. Gronbergq, C.M. Korteq, R. Kutschkeq, S. Menary 9, R.J. Morrisonq, S. Nakanishiq, H.N. Nelsonq, T.K. Nelsonq,

C. Qiaoq, J.D. Richmans, D. Robertsq, A. Rydq, H. Tajimaq, M.S. Witherellq, R. Balest r, K. Cho r, W.T. Ford ‘, M. Lohner r, H. Park r, I? Rankin r, J. Roy r, J.G. Smith’,

J.I? Alexander ‘, C. Bebek s, B.E. Berger s, K. BerkelmanS, K. Blooms, T.E. Browder”*‘, D.G. Cassel ‘, H.A. Cho ‘, D.M. Coffman ‘, D.S. Crowcroft ‘, M. Dickson s, P.S. Drell s,

D.J. Dumas ‘, R. Ehrlich ‘, R. Elia ‘, P Gaidarev ‘, B. Gittelman ‘, S.W. Gray s, D.L. Hartill s, B.K. Heltsley ‘, S. HendersonS, CD. Jones ‘, S.L. Jones ‘, J. Kandaswamy ‘, N. Katayama”,

P.C. Kim ‘, D.L. Kreinick ‘, T. Lee ‘, Y. Liu ‘, G.S. Ludwig ‘, J. Masui ‘, J. Mevissen ‘, N.B. Mistry ‘, C.R. Ng ‘, E. NordbergS, J.R. Patterson ‘, D. Peterson ‘, D. Riley ‘,

0370-2693/96/$12.00 @ 1996 Elsevier Science B.V. All rights reserved

PII SO370-2693(96)00111-6

262 CLEO Collaboration/ Physics Letters B 373 (1996) 261-266

A. SofferS, C. Ward ‘, P. Avery t, A. Freyberger t, K. Lingel t, C. Prescott t, J. Rodriguez t, S. Yang’, J. Yelton t, G. BrandenburgU, D. Cinabro”, T. Liu”, M. Saulnier”, R. Wilson”, H. Yamamoto”, T. Bergfeld “, B.I. Eisenstein “, J. Ernst “, G.E. Gladding”, G.D. Gollin “,

M. Palmer”, M. SelenV, J.J. Thaler” ’ Carleton University, Ottawa, Ont. KIS 586, and the Institute of Particle Physics, Canada

’ McGill University, MontrPal, Que. H3A 2T8, and the Insrirute of Particle Physics, Canada ’ Ithaca College, Ithaca, NY 14850, USA

d University of Kansas, Lawrence. KS 66045, USA e University of Minnesota, Minneapolis, MN 55455, USA

’ State University of New York at Albany, Albany, NY 12222, USA g Ohio State University, Columbus, OH, 43210, USA h University of Oklahoma. Norman, OK 73019, USA ’ Purdue University, West Lafayette. IN 47907, USA j University of Rochester Rocheste,: NY 14627, USA

k Southern Methodist University* Dallas, TX 75275, USA e Syracuse University, Syracuse, NY 13244. USA

m Vanderbilt Universiq, Nashville. TN 37235, USA n Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA

’ California Institute of Technology, Pasadena, CA 91125, USA P University of California. San Diego. La Jolla, CA 92093, USA

9 University of California, Santa Barbara, CA 93106, USA ’ University of Colorado, Boulder; CO 80309-0390, USA

s Cornell University_ Ithaca, NY 14853, USA t Universip of Florida, Gainesville, FL 32611, USA u Harvard University, Cambridge, MA 02138, USA

v University of Illinois, Champaign-Urbana, IL 61801, USA

Received 23 January 1996 Editor: L. Montanet

Abstract

Using the CLEO 11 detector operating at the e + - e Cornell Electron Storage Ring (CESR), we present evidence for new decay modes of the e,’ into sow+, B”~‘?ro, and -’ E V+T-?T+. The branching ratios of these decay modes, relative to -+ + ’ 5,+5-?l IT ) have been measured to be 0.55 f 0.13 & 0.09, 2.34 f. 0.57 f 0.37, and 1.74 f 0.42 f 0.27, respectively.

PACS: 13.3O.Eg; 14.2O.Q Keyw0rd.F: Charmed-strange baryon decay ES

WA62 [ 1 ] at CERN, CLEO [ 2-41 and other experimental groups [5-71 have reported the observation of the charmed-strange baryons 3: and E:. Using the excellent photon detection capability of the CLEO II detector, we have searched for new decay modes of the E:, which involve the reconstruction of a 8’ hy- peron in the final state. In this report, we present the observation of new decay modes of the Zz into Z’?r+. &r+& and @r+T-qr+ , and measure the branch-

ing ratios of these modes relative to the previously reported decay mode Zz -+ Z-&T+. The hyperons are reconstructed using the decay modes, 5’ -t AT’, B- --) AT-, and A -+ pr-. Charge conjugate modes are implied throughout the entire discussion.

The data sample used in this analysis corresponds to an integrated luminosity of 3.2 fb-’ from the Y (3s) and Y (4s) energy regions, and energies just below and above the Y (4s) resonance, collected with the CLEO II detector operating at CESR.

The CLEO II detector is described in detail else- ’ Permanent address: University of Hawaii at Manoa.

CLEO Collaboration/Physics Letters B 373 (1996) 261-266 263

where [ 81. The detector consists of a charged particle tracking system surrounded by a scintillation counter based time-of-flight system and an electromagnetic shower detector consisting of 7800 thallium-doped ce- sium iodide crystals.

Identification of hadrons is achieved by using time- of-flight (TOF) and specific ionization (dE/dx) measurements. Using the two measurements, we form a combined probability for the particle to be a pion, a kaon or a proton. A charged track is defined to be consistent with a pion (proton) hypothesis, if its probability to be a pion (proton) is greater than 0.003.

At CESR energies (around 10.5 GeV), charmed baryons can be produced from B meson decays or directly from e+e- annihilations into ci? jets. In this analysis, we have used the fragmentation variable x,,, defined as x,, = p/p-, where p is the momentum of the a:, p,& = EL,, - IV?.:, Ebem is the beam

energy, and M=+ is the mass of the E:,‘. In order to

reduce combinz&ial background, which tends to have low xp, we limit our search to xP greater than 0.4. This cut eliminates charmed baryons from B meson decay.

A candidates are formed from pairs of oppositely charged tracks, assuming the higher momentum track to be a proton and requiring it to be consistent with the proton hypothesis. To reduce the background from spurious vertices, we require the radial distance of the A vertex from the beam line to be greater than 2 mm. We require the invariant mass of A candidates to be within 5 MeV/c2 (N 2.5~) of the known A mass.

8- candidates are formed by combining each A candidate with each remaining negatively charged track in the event, assuming that the additional track is a pion. A vertex is formed from the intersection of the A and the negatively charged track. We require the radial distance of the a- vertex from the beam line to be at least 2 mm and to be less than that of the daughter A vertex. We also require the impact parameter of the negatively charged track from the E- with respect to the run-averaged beam position to be greater than 0.3 mm. Combinations of AT- with an invariant mass within 6 MeV/c2 (- 3a) of the measured a- mass are accepted as 8- candidates.

8’ candidates are found by forming all A$ combinations in the event. Candidates for g’s are formed from pairs of photon candidates, of which at least one is detected in the barrel CsI crystals, i.e. with 1 cos 81 <

0.707, where 8 is the angle between the photon and beam line. All photon candidates are required to de- posit at least 50 MeV in the Cd crystals. Showers that are matched to charged tracks projected into the calorimeter are excluded. The Z” finding is done using a E”~A# reconstruction algorithm used in previous CLEO analyses. [9] We also require the impact parameter of the A from a0 with respect to the averaged beam position to be greater than 0.3 mm. a0 candidates are defined as A$ combinations which have an invariant mass within 8 MeV/c* (N 3a) of the known 8’ mass.

-0 + For the a,+ + E”$, o r r 0, and &rf,r-,r+

decay modes, we require the primary pions to be in the forward hemisphere as defined by the direction of the 8: in the laboratory system. For the a$ -+ Z”&?ro mode, which has higher backgrounds due to the large number of $’ candidates in each event, we further require the momentum of the 9’s from the primary vertex to be greater than 150 MeV/c. Charged pions from the primary vertex used in constructing 8: candidates are required to have impact parameters in the r-4 plane, relative to the primary vertex, of less than 2 mm and impact parameters along the beam axis coordinate of less than 3 cm. Further, we require the tracks to be consistent with the pion hypothesis.

In Fig. 1, we present the invariant mass distributions corresponding to the decays of the E,’ to Z”7r+, iZ’&&‘, iZ’&rr-& and Z-P+&, respectively. To measure the signal yield, we fit each invariant mass distribution to the sum of a Gaussian function with a fixed width and a second order polynomial background. The fixed widths were determined, separately for each mode, using a GEANT [lo] based Monte Carlo simulation of the detector. We obtain signal yieldsof39f9,81&17,57&12and 147f 16events corresponding to the above decay modes. The results of the fits are listed in Table 1.

We also measure the product of the Zz production cross-section and the branching fraction for each decay mode; the results are shown in Table 1. The reconstruction efficiency in each mode is found as a function of momentum using Monte Carlo simulations. We present the cross-section times the branching fraction for each decay mode and also present branching fractions of the first three modes relative to that of 8,+ + E_7r+7r+. The main sources of systematic error are due to uncertainties in the reconstruction efficiencies

264 CLEO Collaboration/Physics Letters B 373 119%) 261-266

50 ri- 80 2 40

2 z

80 30

0

2 20 40

ii 'Z E 10 20 w

0 0 2.10 2.30 2.50 2.70 2.10 2.30 2.50 2.70

M go + (GeV / c*) MO 7r 8 n+xJGeV ’ c2)

60 120

NF 50 100 > Q) I

40 80

8 30 60

G .s! 20 40 i= c W

10 20

0 0 2.10 2.30 2.50 2.70 2.10 2.30 2.50 2.70

M son+ =- =+

(GeV I c*) M - = T+T+ (GeV ’ c2)

Fig. I. hvariant mass distributions of EZ$ decays to the (a) E”?ri, (b) B”w+?r”, (cf E”~+n--~+ and (d) H-&r+ modes, with

x,, > 0.4 for all modes.

Table I Summary of the measurements of Hz yields and relative branching fractions for various decay modes. The first error is statistical and the second is systematic.

Yield V. B (x, > 0.4) (pb) Relative branching ratios

39.4 f 8.5 0.34 f 0.07 f 0.06 0.55 f 0.13 f 0.09 81.2% 17.1 1.45 f 0.31 f 0.22 2.34 f 0.57 f 0.37 57.0 f 12.0 1.08zk0.23zk0.18 I .74 f 0.42 f 0.27

147.2 f 16.2 0.62 f 0.07 f 0.05 1.0

for A, 8 - and E” (3-7%)) charged particle tracking (Z%per track), 7r” finding efficiency (5-IO%), uncertainties in the width of the signals (l-10%) and variations in the selection criteria (3-l 1%) .

In Fig. 2, we show the xp distribution of the Z:,’

for the highest statistics decay mode Z-a+&. Fit- ting this distribution to Peterson’s parameterization of the fragmentation function [ 111, we obtain EQ =

0.23::,:: f 0.03, where the first error is statistical and

the second error is systematic. By comparison, CLEO measures EQ = 0.29 & 0.05 [ 121 for continuum A,+ production and ARGUS measures EQ = 0.24 f 0.08 for Z, [7].

We have investigated possible B* ( 1530) substructure in the decay modes E”&ti, E’T+T-T+, and ~-T+w+. K6mer [ 131 predicts zero decay ampli- tudes for Zz --+ Z**O?T+ and Z*‘p+. We do not see

any evidence for substructure in these decay modes.

CLEO Collaboration/Physics Letters B 373 (19%) 261-266 265

Table 2 Comparison between theoretical predictions for two body decay modes of E$ into E”?r+ and E”p+ with the results of this experiment. For the 23( Fop+) experimental limit, we use the f3( E”a+rro) result as an upper limit. The absolute scale of these branching ratios has been set from our previous measurement [ 141.

Theoretical models u(aolr+) (a) f3(@pf) 1%)

Xu and Kamal [ 16) 0.3 Uppal,Varma and Kbanna [ 171 1.6 Zenczykowski [ IS] 1.6 5.5 Khmer and Kramer [ I31 0.3 7.6 Cheng and Tseng [ 191 0.3 Datta [20] 0.3 0.3

This experiment 1.2f0.5f0.3 <4.9f2.2fl.2

0 j 1. 1 a 1 " 3 ' 0.40 0.50 0.60 0.70 0.90 0.90 1.00

xP

Fig. 2. Fit of the Peterson function to the Hz xp distribution, using E:f - H-rr+rr+ decays.

To set the scale, we estimate an upper limit of Z?( Zz + Z:*%rf) I a(=,+ -+ Z”&$) < 30% at the 90% confidence level.

In Table 2, we compare our experimental results for the two body decay modes Z”& and B”pf with a variety of predictions. Since we are not very sensitive to possible p contributions, we assume that the 8: -+ Zap+ decay rate is at most equal to the Z,+ + B”,rr+7ro decay rate. We convert each relative branching fraction to an absolute branching fraction using CLEO’s estimate [ 141 of f?(Z,f + E-r+&) = (2.1 f 0.8 f 0.4) x lo-* and rz,+ = (0.35$%) x

lo-‘* s for the lifetime of =+ MC [151. We have measured the mass of the 8:, using only

the high statistics mode 8: + Z-?r+&. To calibrate our absolute mass scale, we start with the PDG mass for the D+, add the observed mass difference between the D+ + K-T+& and A,’ --+ pK-TT+ decay modes, and finally add the observed mass difference between the A,’ + E-K+nf and a: + Z-r’?r+ modes. By determining mass differences of modes with similar decay products, we minimize the systematic errors due to the energy loss correction and to 8- reconstruction. We measure the mass of the a$ to be 2467.0f 1.6f2.0 MeV/c*. The first error is statistical and the second error is systematic. For comparison, the world average valueofthemassofZ1,+is2465.1&11.6MeVlc2. [15]

In summary, three new E’,’ decays into Z”&, Z’QT+~, and 80&r-& have been observed and their branching fractions, relative to that into a-&&, have been measured to be 0.55 f 0.13 f 0.09, 2.34 f 0.57 f 0.37, and 1.74 f 0.42 f 0.27, respectively. The mass of E:,’ is measured to be 2467.0 f 1.6 f 2.0 MeV/c2.

We gratefully acknowledge the effort of the CESR staff in providing us with excellent luminosity and running conditions. J.P.A., J.R.P., and I.P.J.S. thank the NYI program of the NSF, G.E. thanks the Heisen- berg Foundation, K.K.G., M.S., H.N.N., T.S., and H.Y. thank the OJI program of DOE, J.R.P thanks the AP Sloan Foundation, and A.W. thanks the Alexander von Humboldt Stiftung for support. This work was sup- ported by the National Science Foundation, the US Department of Energy, and the Natural Sciences and Engineering Research Council of Canada.

266 CLEO Collaboration/Physics Letters B 373 (19%) 261-266

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