b

Physics Letters B 598 (2004) 172–177

www.elsevier.com/locate/physlet

Search for the lepton flavor violation processesJ/ψ → µτ andeτ

BES Collaboration

M. Ablikim a, J.Z. Baia, Y. Banj, J.G. Biana, X. Caia, J.F. Changa, H.F. Chenp,H.S. Chena, H.X. Chena, J.C. Chena, Jin Chena, Jun Chenf, M.L. Chena, Y.B. Chena,

S.P. Chib, Y.P. Chua, X.Z. Cuia, H.L. Daia, Y.S. Dair, Z.Y. Denga, L.Y. Donga,S.X. Dua, Z.Z. Dua, J. Fanga, S.S. Fangb, C.D. Fua, H.Y. Fua, C.S. Gaoa, Y.N. Gaon,M.Y. Gonga, W.X. Gonga, S.D. Gua, Y.N. Guoa, Y.Q. Guoa, Z.J. Guoo, F.A. Harriso,K.L. Hea, M. Hek, X. Hea, Y.K. Henga, H.M. Hua, T. Hua, G.S. Huanga,2, L. Huangf,X.P. Huanga, X.B. Jia, Q.Y. Jiaj, C.H. Jianga, X.S. Jianga, D.P. Jina, S. Jina, Y. Jina,

Y.F. Lai a, F. Li a, G. Li a, H.H. Li a, J. Li a, J.C. Lia, Q.J. Lia, R.B. Li a, R.Y. Li a,S.M. Li a, W.G. Li a, X.L. Li g, X.Q. Li i, X.S. Li n, Y.F. Liangm, H.B. Liaoe, C.X. Liu a,F. Liu e, Fang Liup, H.M. Liu a, J.B. Liua, J.P. Liuq, R.G. Liua, Z.A. Liu a, Z.X. Liu a,F. Lua, G.R. Lud, J.G. Lua, C.L. Luoh, X.L. Luo a, F.C. Mag, J.M. Maa, L.L. Ma k,

Q.M. Maa, X.Y. Ma a, Z.P. Maoa, X.H. Mo a, J. Niea, Z.D. Niea, S.L. Olseno,H.P. Pengp, N.D. Qia, C.D. Qianl, H. Qinh, J.F. Qiua, Z.Y. Rena, G. Ronga,

L.Y. Shana, L. Shanga, D.L. Shena, X.Y. Shena, H.Y. Shenga, F. Shia, X. Shij,H.S. Suna, S.S. Sunp, Y.Z. Suna, Z.J. Suna, X. Tanga, N. Taop, Y.R. Tiann,

G.L. Tonga, G.S. Varnero, D.Y. Wanga, J.Z. Wanga, K. Wangp, L. Wanga, L.S. Wanga,M. Wanga, P. Wanga, P.L. Wanga, S.Z. Wanga, W.F. Wanga, Y.F. Wanga, Zhe Wanga,

Z. Wanga, Zheng Wanga, Z.Y. Wanga, C.L. Weia, D.H. Weic, N. Wua, Y.M. Wu a,X.M. Xia a, X.X. Xie a, B. Xin g, G.F. Xua, H. Xua, Y. Xu a, S.T. Xuea, M.L. Yanp,

F. Yangi, H.X. Yanga, J. Yangp, S.D. Yanga, Y.X. Yangc, M. Yea, M.H. Yeb, Y.X. Ye p,L.H. Yi f, Z.Y. Yi a, C.S. Yua, G.W. Yua, C.Z. Yuana, J.M. Yuana, Y. Yuana, Q. Yuea,

S.L. Zanga, Yu Zenga, Y. Zengf, B.X. Zhanga, B.Y. Zhanga, C.C. Zhanga,D.H. Zhanga, H.Y. Zhanga, J. Zhanga, J.Y. Zhanga, J.W. Zhanga, L.S. Zhanga,

Q.J. Zhanga, S.Q. Zhanga, X.M. Zhanga, X.Y. Zhangk, Y.J. Zhangj, Y.Y. Zhanga,Yiyun Zhangm, Z.P. Zhangp, Z.Q. Zhangd, D.X. Zhaoa, J.B. Zhaoa, J.W. Zhaoa,

M.G. Zhaoi, P.P. Zhaoa, W.R. Zhaoa, X.J. Zhaoa, Y.B. Zhaoa, Z.G. Zhaoa,1,

0370-2693/$ – see front matter 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.physletb.2004.08.005

BES Collaboration / Physics Letters B 598 (2004) 172–177 173

nto be

H.Q. Zhengj, J.P. Zhenga, L.S. Zhenga, Z.P. Zhenga, X.C. Zhonga, B.Q. Zhoua,G.M. Zhoua, L. Zhoua, N.F. Zhoua, K.J. Zhua, Q.M. Zhua, Y.C. Zhua, Y.S. Zhua,

Yingchun Zhua, Z.A. Zhua, B.A. Zhuanga, B.S. Zoua

a Institute of High Energy Physics, Beijing 100039, PR Chinab China Center for Advanced Science and Technology (CCAST), Beijing 100080, PR China

c Guangxi Normal University, Guilin 541004, PR Chinad Henan Normal University, Xinxiang 453002, PR China

e Huazhong Normal University, Wuhan 430079, PR Chinaf Hunan University, Changsha 410082, PR China

g Liaoning University, Shenyang 110036, PR Chinah Nanjing Normal University, Nanjing 210097, PR China

i Nankai University, Tianjin 300071, PR Chinaj Peking University, Beijing 100871, PR China

k Shandong University, Jinan 250100, PR Chinal Shanghai Jiaotong University, Shanghai 200030, PR China

m Sichuan University, Chengdu 610064, PR Chinan Tsinghua University, Beijing 100084, PR Chinao University of Hawaii, Honolulu, HI 96822, USA

p University of Science and Technology of China, Hefei 230026, PR Chinaq Wuhan University, Wuhan 430072, PR China

r Zhejiang University, Hangzhou 310028, PR China

Received 4 June 2004; accepted 9 August 2004

Available online 20 August 2004

Editor: W.-D. Schlatter

Abstract

The lepton flavor violation processesJ/ψ → µτ andeτ are searched for using a sample of 5.8×107 J/ψ events collectedwith the BESII detector. Zero and one candidate events, consistent with the estimated background, are observed iJ/ψ →µτ, τ → eν̄eντ andJ/ψ → eτ, τ → µν̄µντ decays, respectively. Upper limits on the branching ratios are determinedBr(J/ψ → µτ) < 2.0× 10−6 and Br(J/ψ → eτ) < 8.3× 10−6 at the 90% confidence level (C.L.). 2004 Elsevier B.V. All rights reserved.

PACS: 13.25.Gv; 14.40.Gx; 13.40.Hq

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1. Introduction

In the Standard Model (SM), lepton flavor is coserved, but it is expected to be violated in maextensions of the SM, such as grand unified mels [1], supersymmetric models[2], left-right sym-metric models[3], and models where electrowe

E-mail address: yuanjm@ihep.ac.cn(J.M. Yuan).1 Visiting professor to University of Michigan, Ann Arbor, M

48109, USA.2 Current address: Purdue University, West Lafayette, IN 479

USA.

symmetry is broken dynamically[4]. Recent experimental results from Super-Kamiokande[5], SNO[6],and KamLAND [7] indicate strongly that neutrinohave masses and can mix with each other. Coquently, lepton flavor symmetry is a broken symmeThere have been many studies both experimentallytheoretically on searching for lepton flavor violatin(LFV) processes[8], mainly from µ, τ and Z de-cays[9]. Theoretical predictions of LFV in decayscharmonium and bottomonium systems are discusin Refs. [10–12], and the search for theJ/ψ → eµ

LFV process at BESII is presented in Ref.[13]. In thisLetter, we report on a search for LFV via the deca

174 BES Collaboration / Physics Letters B 598 (2004) 172–177

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J/ψ → µτ, τ → eν̄eντ andJ/ψ → eτ, τ → µν̄µντ

using 5.8×107J/ψ events taken at the center-of-maenergy of 3.097 GeV with the BESII detector duri1999–2001.

2. BES detector

The Beijing Spectrometer (BES)[14,15] is a con-ventional solenoidal magnetic detector at the BeijElectron Positron Collider (BEPC). The upgraded vsion of the BES detector, BESII, includes a 12-lavertex chamber (VC), surrounding the beam pipeproviding trigger information; a forty-layer main drichamber (MDC), located radially outside the VC aproviding trajectory and energy loss (dE/dx) in-formation for charged tracks over 85% of the tosolid angle; and an array of 48 scintillation countsurrounding the MDC to measure the time-of-flig(TOF) of charged tracks with a resolution of∼ 200 psfor hadrons. The momentum resolution of the MDCσp/p = 1.78%

√1+ p2 (p in GeV/c), and thedE/dx

resolution for hadron tracks is about 8%. Radially oside the TOF system is a 12 radiation length, lead-sandwich barrel shower counter (BSC). This measuthe energies of electrons and photons with an eneresolution ofσE/E = 21%/

√E (E in GeV). Outside

the solenoidal coil, which provides a 0.4 Tesla mnetic field over the tracking volume, is an iron flureturn that is instrumented with three double layerscounters to identify muons of momentum greater th0.5 GeV/c. It provides coordinate measurements wresolutions in the outermost layer of 10 and 12 cmrφ and z. The solid angle coverage of the layers67%, 67%, and 63% of 4π , respectively.

In the analysis, a GEANT3 based Monte Carlo pgram (SIMBES) with detailed consideration of detetor performance (such as dead electronic channelused. The consistency between data and Monte Chas been checked in many high purity physics chnels, and the agreement is reasonable[16].

3. Event selection

We require candidate events forJ/ψ → µτ, τ →eν̄eντ andJ/ψ → eτ, τ → µν̄µντ to have two wellreconstructed and oppositely charged tracks, eac

which is well fitted to a helix originating from the interaction region of|x| <0.015 m,|y| <0.015 m, and|z| < 0.15 m and with a polar angle,θ , satisfying|cosθ | < 0.8. To reject cosmic rays, the time of fligdifference of the two charged tracks should be lthan 4 ns.

Isolated photons are defined as those photonsing an energy deposit in the BSC greater than 50 Man angle with any charged track greater than 15◦, andan angle between the direction defined by the filayer hit in the BSC and the developing directionthe cluster in thexy-plane less than 18◦. There mustbe no isolated photon in the selected event.

Information from the BSC, TOF, and MDC (dE/

dx) is used to select electrons.Fig. 1(a) shows theratio of the energy deposited by the electron inBSC to its momentum (E/P ) for Monte Carlo simu-lated events, andFig. 1(b) shows the energy depositeby the muon in the BSC for Monte Carlo simulatevents. To be an electron, the charged track shhave no hits in the muon counter, and theE/P ratioshould be larger than 0.7. To further distinguishelectron from hadrons, it is required that℘e

dE/dx >

℘πdE/dx , ℘e

dE/dx > ℘KdE/dx and℘e

TOF > ℘p

TOF, where

℘idE/dx and℘i

TOF are the particle identification confidence levels for thedE/dx and TOF measuremenandi denotese, π , K or p.

To select muon tracks, the differences,δi (i =rφ, z), between the closest muon hit and the projecMDC track in each layer are used. A good hit in theµ

counter requires|δi | < 2σi for i = rφ andz. The to-tal number of goodµ hits in theµ counter,µgood

hit , canrange from 0 to 3. A track is considered as a muothe deposited energy in the BSC, shown inFig. 1(b),is less than 0.3 GeV andµgood

hit is equal to 3.For the decay ofJ/ψ → eτ , τ → µν̄µντ , the

momentum of the electron is monochromatic, whthat of the muon is broad. The main backgroufor this channel comes fromJ/ψ → (γ )µ+µ− ande+e− → (γ )µ+µ−, which can be rejected by requiing that the momentum of the electronPe be in theregion from 1.00 to 1.08 GeV/c and the momentumof the muon be less than 1.4 GeV/c. Similar require-mentsPe < 1.4 GeV/c and 1.00< Pµ < 1.08 GeV/care applied toJ/ψ → µτ , τ → eν̄eντ candidates tosuppress the background fromJ/ψ → (γ )e+e− ande+e− → (γ )e+e−. Fig. 2 shows the momentum dis

BES Collaboration / Physics Letters B 598 (2004) 172–177 175

he

ntum

Fig. 1. (a) Distribution ofE/P for electrons (MC simulation). (b) Distribution of energy deposited by muons in the BSC (MC simulation). Tsolid histogram representsJ/ψ → eτ , τ → µν̄µντ channel, and the dashed one is forJ/ψ → µτ , τ → eν̄eντ channel.

Fig. 2. (a) Muon momentum distribution afterall requirements except the momentum requirements. The solid histogram is residualJ/ψ data,the dashed one (MC simulation) isJ/ψ → µτ , τ → eν̄eντ . (b) Electron momentum distribution after all requirements except the momerequirements. The solid histogram is residualJ/ψ data, the dashed one (MC simulation) isJ/ψ → eτ , τ → µν̄µντ .

umi-e

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of

tribution after all requirements except the momentrequirements. Applying these requirements, no canddates forJ/ψ → µτ , τ → eν̄eντ and one candidatfor J/ψ → eτ , τ → µν̄µντ survive.

4. Efficiencies and backgrounds

In this analysis, theµ particle identification effi-ciency εµPID in the µ counter is determined usinreal µ tracks. All other efficiencies, including thgeometric acceptance, momentum requirement efficiency, electron particle identification efficiency, etc.

are combined into one term,εMC, which is determinedby Monte Carlo simulation. The overall efficiencycalculated asεtotal = εµPID × εMC.

Theµ track sample selected from 5.8×107 J/ψ →(γ )µ+µ− decays, as described in Ref.[13], is used todetermine theµ particle identification efficiencies iboth channels. Theµ particle identification efficiencyis a function of the transverse momentum,Pxy , of themuon. Therefore,εµPID is determined from

∑i εi�i ,

whereεi is theµ particle identification efficiency inthe ith Pxy bin determined from theµ track sample,and�i is the weight corresponding to the numberevents in the bin determined from the signal MC.Ta-

176 BES Collaboration / Physics Letters B 598 (2004) 172–177

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Table 1Theεi and�i values in differentPxy regions in theJ/ψ → µτ andeτ channels

Pxy (GeV/c) εi (%) �i

J/ψ → µτ ,τ → eν̄eντ

J/ψ → eτ ,τ → µν̄µντ

0.5 < Pxy < 0.6 0.0 0.0 13.70.6 < Pxy < 0.7 0.0 3.1 12.30.7 < Pxy < 0.8 8.1 11.3 10.90.8 < Pxy < 0.9 40.6 17.8 9.70.9 < Pxy < 1.0 52.2 33.9 7.91.0 < Pxy < 1.1 53.4 33.9 6.31.1 < Pxy < 1.2 56.2 0.0 4.51.2 < Pxy < 1.3 57.7 0.0 2.81.3 < Pxy < 1.4 53.6 0.0 1.0

Table 2Efficiency summary

J/ψ → µτ ,τ → eν̄eντ (%)

J/ψ → eτ ,τ → µν̄µντ (%)

εµPID 43.9 17.0εMC 26.2 28.1

εtotal 11.5 4.8

ble 1 lists theεi and�i in the differentPxy regions,andTable 2lists the selection efficiencies.

The remaining background in both theJ/ψ → µτ ,τ → eν̄eντ and J/ψ → eτ , τ → µν̄µντ processesare studied through Monte Carlo simulation. Amost all two-prong decay modes are generated w5 to 10 times the number of events expected fr5.8 × 107 J/ψ events. ForJ/ψ → eτ , τ → µν̄µντ ,the estimated background is about 0.4 events fJ/ψ → K̄∗(892)−K+(+c.c.). For the decayJ/ψ →µτ, τ → eν̄eντ , no simulated events survive.

5. Systematic errors

The systematic errors in the branching ratio msurements come from the uncertainty of the MDtracking efficiency for charged tracks, the differencein the efficiencies between data and Monte Carlo sulation for some selection criteria, such as the elecand muon identification criteria, the momentum cthe uncertainty inτ decay branching ratio, as well athe error from the total number ofJ/ψ events. The to-tal number ofJ/ψ events is determined fromJ/ψ →

Table 3Summary of systematic errors

Source J/ψ → µτ ,τ → eν̄eντ (%)

J/ψ → eτ ,τ → µν̄µντ (%)

ePID 3.5 3.3µPID 15.4 13.7Br(τ decay) 0.3 0.4MDC tracking 4.0 4.0Momentum resolution 4.3 5.4Number ofJ/ψ events[17] 4.7 4.7

Sum 17.5 16.3

four-prong events and its main error comes fromuncertainty of the background. The systematic erfrom each source are listed inTable 3; the dominanterror is from muon identification. Adding all the sytematic errors in quadrature, the total systematic erare 17.5% and 16.3% forJ/ψ → µτ , τ → eν̄eντ andJ/ψ → eτ , τ → µν̄µντ , respectively.

6. Results and discussion

No J/ψ → µτ, τ → eν̄eντ candidate and onJ/ψ → eτ, τ → µν̄µντ candidate are observed froa sample of 5.8 × 107 J/ψ events, where the estmated background events in the two channels ar0 and 0.4 events, respectively. For a conservativetimate, we setNBG = 0 in the following estimationUpper limits on the branching ratios ofJ/ψ → µτ

andJ/ψ → eτ are calculated with:

Br(J/ψ → X)

<λ(NSignal,NBG)

NJ/ψ × Br(X → Y ) × εJ/ψ→X→Y

,

whereX and Y stand for the intermediate and finstates,λ is the upper limit on the number of observevents at the 90% C.L.,NSignal andNBG are the num-bers of observed signal and background eventsspectively,NJ/ψ represents the total number ofJ/ψ

events, andε is the detection efficiency. The valuof λ (NSignal and NBG) can be calculated using thmethod described in Refs.[18,19].

With the numbers summarized inTable 4, the upperlimits on the branching ratios, after incorporating tsystematic errors, are

Br(J/ψ → µτ) < 2.0× 10−6,

BES Collaboration / Physics Letters B 598 (2004) 172–177 177

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Table 4Numbers and efficiencies used in the calculation of the upper lim

J/ψ → µτ ,τ → eν̄eντ

J/ψ → eτ ,τ → µν̄µντ

Nbg 0 0ε (%) 11.5 4.8NJ/ψ 5.8× 107 5.8× 107

NSignal 0 1λ(NSignal,Nbg) 2.4 4.0Br(τ decay) (%) 17.84 17.36

Upper limit of Br 2.0× 10−6 8.3× 10−6

Br(J/ψ → eτ) < 8.3× 10−6

at the 90% C.L.Previously BES reported an upper limit o

Br(J/ψ → eµ) to be 1.1×10−6 at the 90% C.L.[13].In summary, the LFV processesJ/ψ → µτ and

eτ are searched for using a sample of 5.8× 107 J/ψ

events. No candidate forJ/ψ → µτ , τ → eν̄eντ andone candidate forJ/ψ → eτ , τ → µν̄µντ , consistentwith the estimated background, are observed. Theper limits on the branching ratios at the 90% C.L.determined to be Br(J/ψ → µτ) < 2.0 × 10−6 andBr(J/ψ → eτ) < 8.3× 10−6.

Acknowledgements

The BES Collaboration thanks the staff of BEPand the computing center of IHEP for their hardforts. We also thank Profs. Xinmin Zhang and Jianiong Wang for helpful discussions. This worksupported in part by the National Natural ScienFoundation of China under contracts Nos. 19991410225524, 10225525, the Chinese Academy of Sences under contract No. KJ 95T-03, the 100 TaleProgram of CAS under Contract Nos. U-11, U-2U-25, and the Knowledge Innovation Project of CAunder Contract Nos. U-602, U-34 (IHEP); and by t

National Natural Science Foundation of China unContract No.10175060 (USTC), and No. 102255(Tsinghua University) and by the Department of Eergy under Contract No. DE-FG03-94ER40833 (Uversity of Hawaii).

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