Physics Letters B 663 (2008) 297–301

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Physics Letters B

www.elsevier.com/locate/physletb

Search for the rare decays J/ψ → D−s π+, J/ψ → D−π+, and J/ψ → D̄0 K̄ 0

BES Collaboration

M. Ablikim a, J.Z. Bai a, Y. Ban m, X. Cai a, H.F. Chen r, H.S. Chen a, H.X. Chen a, J.C. Chen a, Jin Chen a, Y.B. Chen a,Y.P. Chu a, Y.S. Dai t, L.Y. Diao j, Z.Y. Deng a, Q.F. Dong p, S.X. Du a, J. Fang a, S.S. Fang a,1, C.D. Fu p, C.S. Gao a,Y.N. Gao p, K. Götzen c, S.D. Gu a, Y.T. Gu e, Y.N. Guo a, Z.J. Guo q,2, F.A. Harris q, K.L. He a, M. He n, Y.K. Heng a, J. Hou l,H.M. Hu a, J.H. Hu d, T. Hu a, G.S. Huang a,3, X.T. Huang n, X.B. Ji a, X.S. Jiang a, X.Y. Jiang f, J.B. Jiao n, D.P. Jin a, S. Jin a,Y.F. Lai a, G. Li a,4, H.B. Li a, J. Li a, R.Y. Li a, S.M. Li a, W.D. Li a, W.G. Li a, X.L. Li a, X.N. Li a, X.Q. Li l, Y.F. Liang o,H.B. Liao a, B.J. Liu a, C.X. Liu a, F. Liu g, Fang Liu a, H.H. Liu a, H.M. Liu a, J. Liu m,5, J.B. Liu a, J.P. Liu s, Jian Liu a,Q. Liu q, R.G. Liu a, Z.A. Liu a, Y.C. Lou f, F. Lu a, G.R. Lu f, J.G. Lu a, C.L. Luo k, F.C. Ma j, H.L. Ma b, L.L. Ma a,6, Q.M. Ma a,Z.P. Mao a, X.H. Mo a, J. Nie a, S.L. Olsen q, K.J. Peters c, R.G. Ping a, N.D. Qi a, H. Qin a, J.F. Qiu a, Z.Y. Ren a, G. Rong a,X.D. Ruan e, L.Y. Shan a, L. Shang a, C.P. Shen q, D.L. Shen a, X.Y. Shen a, H.Y. Sheng a, H.S. Sun a, S.S. Sun a, Y.Z. Sun a,Z.J. Sun a, X. Tang a, G.L. Tong a, G.S. Varner q, D.Y. Wang a,7, L. Wang a, L.L. Wang a, L.S. Wang a, M. Wang a, P. Wang a,P.L. Wang a, W.F. Wang a,8, Y.F. Wang a, Z. Wang a, Z.Y. Wang a, Zheng Wang a, C.L. Wei a, D.H. Wei a, Y. Weng a,N. Wu a, X.M. Xia a, X.X. Xie a, G.F. Xu a, X.P. Xu g, Y. Xu l, M.L. Yan r, H.X. Yang a, Y.X. Yang d, M.H. Ye b, Y.X. Ye r,G.W. Yu a, C.Z. Yuan a, Y. Yuan a, S.L. Zang a, Y. Zeng h, B.X. Zhang a, B.Y. Zhang a, C.C. Zhang a, D.H. Zhang a,H.Q. Zhang a, H.Y. Zhang a, J.W. Zhang a, J.Y. Zhang a,∗, S.H. Zhang a, X.Y. Zhang n, Yiyun Zhang o, Z.X. Zhang m,Z.P. Zhang r, D.X. Zhao a, J.W. Zhao a, M.G. Zhao a, P.P. Zhao a, W.R. Zhao a, Z.G. Zhao a,9, H.Q. Zheng m, J.P. Zheng a,Z.P. Zheng a, L. Zhou a, K.J. Zhu a, Q.M. Zhu a, Y.C. Zhu a, Y.S. Zhu a, Z.A. Zhu a, B.A. Zhuang a, X.A. Zhuang a,B.S. Zou a

a Institute of High Energy Physics, Beijing 100049, People’s Republic of Chinab China Center for Advanced Science and Technology (CCAST), Beijing 100080, People’s Republic of Chinac GSImbh Darmstadt, Darmstadt 64291, Germanyd Guangxi Normal University, Guilin 541004, People’s Republic of Chinae Guangxi University, Nanning 530004, People’s Republic of Chinaf Henan Normal University, Xinxiang 453002, People’s Republic of Chinag Huazhong Normal University, Wuhan 430079, People’s Republic of Chinah Hunan University, Changsha 410082, People’s Republic of Chinai Jinan University, Jinan 250022, People’s Republic of Chinaj Liaoning University, Shenyang 110036, People’s Republic of Chinak Nanjing Normal University, Nanjing 210097, People’s Republic of Chinal Nankai University, Tianjin 300071, People’s Republic of Chinam Peking University, Beijing 100871, People’s Republic of Chinan Shandong University, Jinan 250100, People’s Republic of Chinao Sichuan University, Chengdu 610064, People’s Republic of Chinap Tsinghua University, Beijing 100084, People’s Republic of Chinaq University of Hawaii, Honolulu, HI 96822, USAr University of Science and Technology of China, Hefei 230026, People’s Republic of Chinas Wuhan University, Wuhan 430072, People’s Republic of Chinat Zhejiang University, Hangzhou 310028, People’s Republic of China

* Corresponding author.E-mail address: jyzhang@ihep.ac.cn (J.Y. Zhang).

1 Current address: DESY, D-22607 Hamburg, Germany.2 Current address: Johns Hopkins University, Baltimore, MD 21218, USA.3 Current address: University of Oklahoma, Norman, OK 73019, USA.4 Current address: Universite Paris XI, LAL-Bat. 208-BP34, 91898 Orsay cedex, France.5 Current address: Max-Plank-Institut für Physik, Foehringer Ring 6, 80805 Munich, Germany.6 Current address: University of Toronto, Toronto M5S 1A7, Canada.7 Current address: CERN, CH-1211 Geneva 23, Switzerland.8 Current address: Laboratoire de l’Accélérateur Linéaire, Orsay F-91898, France.9 Current address: University of Michigan, Ann Arbor, MI 48109, USA.

0370-2693/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.physletb.2008.04.028

298 BES Collaboration / Physics Letters B 663 (2008) 297–301

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 February 2008Accepted 11 April 2008Available online 18 April 2008Editor: M. Doser

Rare decay modes J/ψ → D−s π+ + c.c., J/ψ → D−π+ + c.c., and J/ψ → D̄0 K̄ 0 + c.c. are searched for

using 5.77 × 107 J/ψ events collected with the BESII detector at the BEPC. No signal above backgroundis observed. We present upper limits on the branching fractions of B( J/ψ → D−

s π+) < 1.3 × 10−4,B( J/ψ → D−π+) < 7.5 × 10−5, and B( J/ψ → D̄0 K̄ 0) < 1.7 × 10−4 at the 90% confidence level.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Hadronic and electromagnetic decays of J/ψ have been widelystudied, but their weak decays have not been studied in detail [1].For the J/ψ , lying below D D̄ threshold, decays to D D̄ are forbid-den. However, weak decays to a single charm meson accompaniedby other non-charm mesons are kinematically allowed. The stan-dard model predicts that the branching fractions of flavor chang-ing processes via weak interactions, such as J/ψ → D X (whereD stands for Ds or D and X stands for π or K 0

S ) are at thelevel of 10−8 or below [2,3] and thus much smaller than thoseof strong and electromagnetic decays. This level is unobservablein current experiments. Extensions to the standard model, such asTop Color models [4], the minimal supersymmetric standard modelwith or without R-parity [5], and the two Higgs doublet model[6], can enhance the branching fraction to ∼10−5. The study ofJ/ψ → D−

s π+ , J/ψ → D−π+ , and J/ψ → D̄0 K̄ 0 can provide anexperimental check and serve as a probe of new physics [7,8].

Fig. 1 shows the Feynman diagrams for these decay modes inthe framework of the standard model. Charge conjugate states areimplicitly included. In this Letter, we search for J/ψ → D−

s π+ ,J/ψ → D−π+ , and J/ψ → D̄0 K̄ 0 in a sample of 5.77 × 107 J/ψevents collected with the Beijing Spectrometer (BESII) [9] detectorat the Beijing Electron–Positron Collider (BEPC) [10].

2. BESII detector

BES is a conventional solenoidal magnetic detector that is de-scribed in detail in Ref. [11]. BESII is the upgraded version ofthe BES detector [9]. A 12-layer Vertex Chamber (VC) surround-ing the beryllium beam pipe provides track and trigger informa-tion. A forty-layer main drift chamber (MDC) located just out-side the VC provides measurements of charged particle trajecto-ries covering 85% of 4π ; it also provides ionization energy loss(dE/dx) measurements which are used for particle identification(PID). A momentum resolution of 0.017

√1 + p2 (p in GeV/c) and

a dE/dx resolution for hadronic tracks of ∼ 8% are obtained. Timeof flight (TOF) of charged particles is measured with an array of48 scintillation counters surrounding the MDC. The resolution is

Fig. 1. Leading order electroweak Feynman diagrams for (a) J/ψ → D−s π+ ,

(b) J/ψ → D−π+ , and (c) J/ψ → D̄0 K̄ 0 in the standard model.

about 200 ps for hadrons. Outside the TOF counters, a 12 radiationlength, lead-gas barrel shower counter (BSC), measures energiesand positions of electrons and photons. The solid angle covered isover 80%, and resolutions of σE/E = 0.22/

√E (E in GeV), σφ =

7.9 mrad, and σz = 2.3 cm are obtained. Outside the solenoidalcoil, which provides a 0.4 T magnetic field over the tracking vol-ume, three double-layer muon counters instrumented in the fluxreturn identify muons with momenta greater than 500 MeV/c.

Monte Carlo simulations are performed using a GEANT3 basedprogram (SIMBES) with detailed consideration of the detector ge-ometry and response. The consistency between data and MonteCarlo has been checked in many J/ψ and ψ(2S) decays with rea-sonable agreement. Details are described in Ref. [12].

3. Event selection

Due to the large J/ψ hadronic decay background, non-leptonicdecay modes of Ds and D mesons do not offer good sensitivity.Therefore, in this analysis, they are reconstructed via semileptonicdecay modes: D−

s → φe−νe , D− → K ∗0e−νe , D− → K 0e−νe , andD̄0 → K +e−νe . The neutrino is undetectable in the detector, butcarries energy and momentum. Ds and D mesons cannot be iden-tified by their invariant mass. However, two-body constraints canbe applied in the mode J/ψ → D X . Thus they are identified usingthe X meson momentum information.

Four charged tracks are required in all selected decay modes,and the total charge must be equal to zero. In order to ensurewell-measured momenta and reliable particle identification, all thetracks are required to be reconstructed in the main drift chamberwith a good helix fit. Each track is required to satisfy |cos θ | < 0.8,where θ is the polar angle, and must originate from the beaminteraction region (except the decay daughters of K 0

S ), which isdefined by Rxy < 2.0 cm and |z| < 20.0 cm, where Rxy and |z|are the distances in the xy plane and z direction of the point ofclosest approach of the charged track to the beamline.

A kaon or pion candidate is required to satisfy W K ,π > 0.1%,where W K ,π is the weighted likelihood of the kaon or pion hy-pothesis, which combines the TOF and dE/dx information. To re-duce misidentification, the likelihood ratio R K ,π = W K ,π /(W K +Wπ ) for the kaon or pion is required to be greater than 0.7. Theratio of the energy deposit in the BSC to the momentum is used toconstruct the likelihood for electron identification, and We > 1%and Re = We/(Wπ + W K + We) > 0.85 are required for elec-tron candidates. The angle between the identified electron and thenearest charged track is required to be greater than 12◦ to removebackgrounds from photon conversion. Low momentum electronsand pions cannot be unambiguously identified, so Pe > 0.25 GeV/cis required for electron identification.

Neutral kaons are identified via their decay to π+π− . All pairsof oppositely charged tracks are assumed to be π+π− . The dis-tance between the K 0

S decay vertex and the beam axis is requiredto be greater than 5 mm in the xy plane. If there is more than oneπ+π− combination, the one with mass closest to that of the K 0

S ischosen. Background from J/ψ decays to states with extra neutralparticles is removed by requiring the number of isolated photonsto be zero. An isolated photon is a photon having the angle with

BES Collaboration / Physics Letters B 663 (2008) 297–301 299

Fig. 2. The invariant mass distributions of resonance candidates: (a) φ from J/ψ → D−s π+ , D−

s → φe−νe , (b) K 0S from J/ψ → D̄0 K̄ 0, D̄0 → K +e−νe , (c) K ∗0 from

J/ψ → D−π+ , D− → K 0∗e−νe , and (d) K 0S from J/ψ → D−π+ , D− → K 0e−νe . Data are shown as dots with error bars; the expected signal shapes from Monte Carlo

simulated signal events are shown as histograms. The mass requirements are illustrated by arrows and are discussed in the text. In this figure, all requirements are usedexcept for the Pe requirement and the requirement that the missing particle be in the sensitive region of the detector.

the nearest charged track of at least 22◦ , the difference betweenthe angle of the cluster development direction in the BSC and thereconstructed photon emission direction of less than 60◦ , and theenergy deposit in the BSC larger than 0.1 GeV.

The neutrino in the semileptonic decay of the D is unde-tected. A kinematic quantity Umiss = Emiss − c Pmiss is used toidentify missing neutrinos, where Emiss and Pmiss are the energyand momentum of the neutrino. Ideally, Umiss should be consis-tent with zero. A requirement |Umiss| < 0.1 GeV helps to removebackgrounds from J/ψ decaying to K 0

L , η, and partial π0 finalstates which are not rejected through the initial selection. Eventswith missing energy due to misidentified pions are rejected byPmiss > 0.2 GeV/c. The missing neutrino is required to be in thesensitive region of the detector to further suppress hadronic back-ground.

In the decay mode J/ψ → D−s π+ , D−

s mesons are recon-structed through their decay to D−

s → φe−νe , and φ candidates

are reconstructed from two oppositely charged kaons, namely:φ → K +K − . The invariant mass of φ candidates is required tobe within 0.015 GeV/c2 of the nominal φ mass. In the decaymode J/ψ → D−π+ , D− mesons are reconstructed through theirdecays to D− → K ∗0e−νe and D− → K 0e−νe . The K ∗0 candi-dates are formed from K − and π+ candidates. If there is morethan one K −π+ combination, the one with mass closest to theK ∗0 mass is chosen. Their invariant mass is required to satisfy|MKπ − MK ∗0 | < 0.060 GeV/c2. The K 0

S candidates are formed fromπ+ and π− candidate tracks, and they must satisfy |Mπ+π− −MK 0

S| < 0.020 GeV/c2. For J/ψ → D̄0 K̄ 0, D̄0 mesons are recon-

structed through their decay to K +e−νe . Fig. 2 shows the K K ,Kπ , and ππ invariant mass distributions from the various decaymodes. Fig. 3 shows the momentum distribution of π or K 0

S tracks,which recoil against Ds or D mesons for the three decay modes af-ter all selection criteria. The two decay modes D− → K 0∗e−νe andD− → K 0e−νe are combined in Fig. 3(b).

300 BES Collaboration / Physics Letters B 663 (2008) 297–301

Table 1Detection efficiencies and D branching fractions [13]

Decay mode Intermediate decay ε D branching fractions

J/ψ → D−s π+ D−

s → φe−νe , φ → K − K + 2.47% (2.49 ± 0.28)% × (49.3 ± 0.6)%J/ψ → D−π+ D− → K ∗0e−νe , K ∗0 → K −π+ 1.90% (3.71 ± 0.21)%J/ψ → D−π+ D− → K 0e−νe , K 0

S → π+π− 3.76% (8.6 ± 0.5)% × 1/2 × (69.20 ± 0.05)%J/ψ → D̄0 K̄ 0 D̄0 → K +e−νe , K 0

S → π+π− 4.82% (3.50 ± 0.09)% × 1/2 × (69.20 ± 0.05)%

Fig. 3. Momentum distributions of π or K 0S recoiling against Ds or D mesons for

(a) J/ψ → D−s π+ , (b) J/ψ → D−π+ , and (c) J/ψ → D̄0 K̄ 0. Data is shown as

dots with error bars. The expected signal shapes from Monte Carlo simulated sig-nal events are shown as histograms. The dashed vertical lines denote the signalregion.

4. Monte Carlo simulation

Detection efficiencies are determined by Monte Carlo simula-tions of J/ψ → D−

s π+ , J/ψ → D−π+ , and J/ψ → D̄0 K̄ 0, and50 000 Monte Carlo events are generated for each decay mode.For the branching fractions of D−

s , D− , and D̄0 decays, world av-erage values are taken from Ref. [13]. Detection efficiencies andD branching fractions are listed in Table 1.

5. Systematic errors

The systematic errors on the branching fractions are dominatedby uncertainties of the MDC simulation (including systematic un-certainties of the tracking efficiency and other requirements). Thisuncertainty varies between 14.2% and 19.9%. Using J/ψ → ρ0π0

decays, we estimate a systematic error of 2% for the requirementthat the number of isolated photons be equal to zero [14]. Theelectron identification systematic uncertainty is estimated to be 5%.Differences in the pion and kaon identification between data andMonte Carlo simulation determine a systematic error of 1.5% foreach track. The errors on the intermediate decay branching frac-tions of D−

s , D− , D̄0, φ, and K ∗0 are taken from the PDG [13].The statistical error of the Monte Carlo sample is also taken intoaccount. The number of J/ψ events is (57.7 ± 2.7) × 106 [15], de-termined from inclusive 4-prong hadronic final states, and 4.7%is taken as a systematic uncertainty. The total systematic errors,determined by adding the systematic errors in quadrature, rangebetween 18.2% to 21.4% for the three decay modes. All the system-atic errors are summarized in Table 2.

Table 2Summary of the systematic errors.

J/ψ → D−s π+ J/ψ → D−π+ J/ψ → D̄0 K̄ 0

D−s → φe−νe D− → K ∗0e−νe D− → K 0e−νe D̄0 → K +e−νe

MDC Simulation 15.6% 14.2% 19.9% 15.5%Photon veto 2.0% 2.0% 2.0% 2.0%e PID 5.0% 5.0% 5.0% 5.0%π , K PID 4.5% 4.5% 4.5% 4.5%B(Ds, D) 11.2% 5.7% 5.8% 2.6%MC Statistics 4.0% 3.2% 3.9% 3.4%Number of J/ψ 4.7% 4.7% 4.7% 4.7%Total 21.4% 17.8% 22.7% 18.2%

Table 3Comparison of different methods to calculate the upper limits on branching fraction

Counting Bayesian

Signal Background Upper limit N0.9 Upper limit

J/ψ → D−s π+ 0 0.3 1.4 × 10−4 1.70 1.3 × 10−4

J/ψ → D−π+ 1 1.2 3.0 × 10−5 6.21 7.5 × 10−5

J/ψ → D̄0 K̄ 0 1 0.4 1.2 × 10−4 4.61 1.7 × 10−4

6. Results

Since the numbers of events seen for the reactions J/ψ →D−

s π+ , J/ψ → D−π+ , and J/ψ → D̄0 K̄ 0 are consistent withbackground, upper limits for the three decays are calculated. Us-ing a Bayesian method [13], a Gaussian and a linear function arechosen to describe the signal (S) and background (B). Scanning andfitting the ratio of S/(S+B), one can obtain the upper limits on thenumber of events. During the fitting, the peak and resolution forthe signal are fixed to the values obtained from Monte Carlo sim-ulation. For J/ψ → D−

s π+ , J/ψ → D−π+ , and J/ψ → D̄0 K̄ 0, theupper limits on the observed number of events at the 90% confi-dence level are 1.70, 6.21, and 4.61 respectively, as shown in Fig. 4.

The upper limits on the branching fractions are calculated using

Bup = nobsUL

N J/ψεB(1 − σ sys), (1)

where nobsUL is the upper limit of the observed number of events

at the 90% confidence level, N J/ψ is the number of J/ψ events,ε is the detection efficiency, B is the D(Ds) branching fraction, andσ sys is the systematic error. For the decay mode J/ψ → D−π+ ,εB is the sum of the products obtained from decay modes D− →K 0∗e−νe and D− → K 0e−νe , and weighting by the branching frac-tions of the two decays, the combined systematic uncertainty is21.2%.

For comparison, a counting method is also used to calculatethe upper limits. The signal and background for J/ψ → D−

s π+ ,J/ψ → D−π+ , and J/ψ → D̄0 K̄ 0 are estimated by the numberof events in the signal and sideband regions. The signal regionsare shown with dashed lines in Fig. 3, 3σ momentum of π orK 0

S . The sideband regions are defined from 0.6 to 1.2 GeV/c ex-cluding the signal windows. The background events are estimatedaccording to the ratio between the signal and the sideband regions.The numbers of signal and background events are listed in Table 3.The upper limits are calculated by incorporating systematic uncer-

BES Collaboration / Physics Letters B 663 (2008) 297–301 301

Fig. 4. Likelihood distributions for the observed numbers of events for (a) J/ψ →D−

s π+ , (b) J/ψ → D−π+ , and (c) J/ψ → D̄0 K̄ 0. The observed number of eventsat a Bayesian 90% confidence level for the three channels are indicated by arrowsin the plots.

tainties into the confidence interval [16]. These results are morestringent than those obtained using the Bayesian method.

In summary, we have searched for the first time for J/ψ →D−

s π+ , J/ψ → D−π+ , and J/ψ → D̄0 K̄ 0 decays using 5.77 ×107 J/ψ events taken by the BESII detector at the BEPC e+e− col-lider. No evidence for any of these decays is found. The final resultsat the 90% confidence level upper limit on the branching fractionsare listed in Table 4. These upper limits on the branching fractionare not inconsistent with the standard model.

Table 4Numbers used in the calculation of upper limits on the J/ψ → D−

s π+ , J/ψ →D−π+ , and J/ψ → D̄0 K̄ 0 branching fractions

J/ψ → D−s π+ J/ψ → D−π+ J/ψ → D̄0 K̄ 0

nobsUL 1.70 6.21 4.61

εB 3.03 × 10−4 1.82 × 10−3 5.84 × 10−4

Sys. Error 21.4% 21.2% 18.2%B (90% C.L.) < 1.3 × 10−4 < 7.5 × 10−5 < 1.7 × 10−4

Acknowledgements

The BES Collaboration thanks the staff of BEPC and computingcenter for their hard efforts. This work is supported in part by theNational Natural Science Foundation of China under contracts Nos.10491300, 10225524, 10225525, 10425523, 10625524, 10521003,the Chinese Academy of Sciences under contract No. KJ 95T-03,the 100 Talents Program of CAS under Contract Nos. U-11, U-24,U-25, and the Knowledge Innovation Project of CAS under ContractNos. U-602, U-34 (IHEP), the National Natural Science Foundationof China under Contract No. 10225522 (Tsinghua University), andthe Department of Energy under Contract No. DE-FG02-04ER41291(U. Hawaii).

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