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Physics Letters B 610 (2005) 192–198
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Measurements ofJ/ψ decays into 2(π+π−)η and 3(π+π−)η
BES Collaboration
M. Ablikim a, J.Z. Baia, Y. Bank, J.G. Biana, X. Caia, J.F. Changa, H.F. Chenq,H.S. Chena, H.X. Chena, J.C. Chena, Jin Chena, Jun Cheng, M.L. Chena, Y.B. Chena,
S.P. Chib, Y.P. Chua, X.Z. Cuia, H.L. Daia, Y.S. Dais, Z.Y. Denga, L.Y. Donga,1,Q.F. Dongo, S.X. Dua, Z.Z. Dua, J. Fanga, S.S. Fangb, C.D. Fua, H.Y. Fua, C.S. Gaoa,
Y.N. Gaoo, M.Y. Gonga, W.X. Gonga, S.D. Gua, Y.N. Guoa, Y.Q. Guoa, Z.J. Guop,F.A. Harrisp, K.L. Hea, M. Hel, X. Hea, Y.K. Henga, H.M. Hua, T. Hua,
G.S. Huanga,2, X.P. Huanga, X.T. Huangl, X.B. Jia, C.H. Jianga, X.S. Jianga,D.P. Jina, S. Jina, Y. Jina, Yi Jin a, Y.F. Laia, F. Li a, G. Li b, H.H. Li a, J. Li a, J.C. Lia,
Q.J. Lia, R.Y. Li a, S.M. Li a, W.D. Li a, W.G. Li a, X.L. Li h, X.Q. Li j, Y.L. Li d,Y.F. Liangn, H.B. Liaof, C.X. Liu a, F. Liu f, Fang Liuq, H.H. Liu a, H.M. Liu a, J. Liuk,
J.B. Liua, J.P. Liur, R.G. Liua, Z.A. Liu a, Z.X. Liu a, F. Lua, G.R. Lue, H.J. Luq,J.G. Lua, C.L. Luoi, L.X. Luo d, X.L. Luo a, F.C. Mah, H.L. Maa, J.M. Maa, L.L. Ma a,
Q.M. Maa, X.B. Mae, X.Y. Ma a, Z.P. Maoa, X.H. Mo a, J. Niea, Z.D. Niea,S.L. Olsenp, H.P. Pengq, N.D. Qia, C.D. Qianm, H. Qini, 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. Shik,3, H.S. Suna, J.F. Suna, S.S. Suna, Y.Z. Suna, Z.J. Suna, X. Tanga, N. Taoq,
Y.R. Tiano, G.L. Tonga, G.S. Varnerp, D.Y. Wanga, J.Z. Wanga, K. Wangq, L. Wanga,L.S. Wanga, M. Wanga, P. Wanga, P.L. Wanga, S.Z. Wanga, W.F. Wanga,4, Y.F. Wanga,
Z. Wanga, Z.Y. Wanga, Zhe Wanga, Zheng Wangb, C.L. Weia, D.H. Weia, N. Wua,Y.M. Wu a, X.M. Xia a, X.X. Xie a, B. Xin h,2, G.F. Xua, H. Xua, S.T. Xuea, M.L. Yanq,F. Yangj, H.X. Yanga, J. Yangq, Y.X. Yangc, M. Yea, M.H. Yeb, Y.X. Ye q, L.H. Yi g,
Z.Y. Yi a, C.S. Yua, G.W. Yua, C.Z. Yuana, J.M. Yuana, Y. Yuana, S.L. Zanga,Y. Zengg, Yu Zenga, B.X. Zhanga, B.Y. Zhanga, C.C. Zhanga, D.H. Zhanga,
H.Y. Zhanga, J. Zhanga, J.W. Zhanga, J.Y. Zhanga, Q.J. Zhanga, S.Q. Zhanga,X.M. Zhanga, X.Y. Zhangl, Y.Y. Zhanga, Yiyun Zhangn, Z.P. Zhangq, Z.Q. Zhange,
D.X. Zhaoa, J.B. Zhaoa, J.W. Zhaoa, M.G. Zhaoj, P.P. Zhaoa, W.R. Zhaoa, X.J. Zhaoa,Y.B. Zhaoa, Z.G. Zhaoa,5, H.Q. Zhengk, J.P. Zhenga, L.S. Zhenga, Z.P. Zhenga,
0370-2693/$ – see front matter 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.physletb.2005.02.004
BES Collaboration / Physics Letters B 610 (2005) 192–198 193
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,6, Z.A. Zhua, B.A. Zhuanga, X.A. Zhuanga,
B.S. Zoua
a Institute of High Energy Physics, Beijing 100049, People’s Republic of Chinab China Center for Advanced Science and Technology, Beijing 100080, People’s Republic of China
c Guangxi Normal University, Guilin 541004, People’s Republic of Chinad Guangxi University, Nanning 530004, People’s Republic of China
e Henan Normal University, Xinxiang 453002, People’s Republic of Chinaf Huazhong Normal University, Wuhan 430079, People’s Republic of China
g Hunan University, Changsha 410082, People’s Republic of Chinah Liaoning University, Shenyang 110036, People’s Republic of China
i Nanjing Normal University, Nanjing 210097, People’s Republic of Chinaj Nankai University, Tianjin 300071, People’s Republic of Chinak Peking University, Beijing 100871, People’s Republic of Chinal Shandong University, Jinan 250100, People’s Republic of China
m Shanghai Jiaotong University, Shanghai 200030, People’s Republic of Chinan Sichuan University, Chengdu 610064, People’s Republic of Chinao Tsinghua University, Beijing 100084, People’s Republic of China
p University of Hawaii, Honolulu, HI 96822, USAq University of Science and Technology of China, Hefei 230026, People’s Republic of China
r Wuhan University, Wuhan 430072, People’s Republic of Chinas Zhejiang University, Hangzhou 310028, People’s Republic of China
Received 25 November 2004; accepted 1 February 2005
Editor: M. Doser
Abstract
Based on a sample of 5.8 × 107 J/ψ events taken with the BESII detector, the branching fractions ofJ/ψ → 2(π+π−)η
andJ/ψ → 3(π+π−)η are measured for the first time to be(2.26± 0.08± 0.27) × 10−3 and(7.24± 0.96± 1.11) × 10−4,respectively. 2005 Elsevier B.V. All rights reserved.
PACS: 13.25.Gv; 14.40.Gx; 13.40.Hq
60,
07,
A.ire,
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s ofat
las-ays,
too-
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after
E-mail address: [email protected](S.S. Fang).1 Current address: Iowa State University, Ames, IA 50011-31
USA.2 Current address: Purdue University, West Lafayette, IN 479
USA.3 Current address: Cornell University, Ithaca, NY 14853, US4 Current address: Laboratoire de l’Accélératear Linéa
F-91898 Orsay, France.5 Current address: University of Michigan, Ann Arbor, M
48109, USA.6 Current address: DESY, D-22607 Hamburg, Germany.
1. Introduction
More than one hundred exclusive decay modethe J/ψ have been reported since its discoveryBrookhaven[1] and SLAC[2] in 1974. TheJ/ψ de-cays into hadrons in the lowest order of QCD are csified into hadronic decays, electromagnetic decradiative decays into light hadrons, and transitiontheηc. In Ref.[3], the rates of direct hadronic, electrmagnetic and radiative decays to the totalJ/ψ decayare estimated to be 69.2%, 13.4%, and 4.3%, restively. Up to now, only about half of all hadronic decays have been measured in exclusive reactions
194 BES Collaboration / Physics Letters B 610 (2005) 192–198
I,singrst
ec-ereter
40-s
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nel
taking into account the isospin corrections[4]. Thesample of 58 millionJ/ψ events, taken at BESIprovides a chance to measure some of the mishadronic decays. In this analysis, we report the fimeasurements ofJ/ψ → 2(π+π−)η and J/ψ →3(π+π−)η.
The upgraded Beijing Spectrometer (BESII) dettor located at the Beijing Electron–Positron Collid(BEPC) is a large solid-angle magnetic spectromwhich is described in detail in Ref.[5]. The momen-tum of the charged particle is determined by alayer cylindrical main drift chamber (MDC) which haa momentum resolution ofσp/p = 1.78%
√1+ p2
(p in GeV/c). Particle identification is accomplisheby specific ionization (dE/dx) measurements in thdrift chamber and time-of-flight (TOF) information ia barrel-like array of 48 scintillation counters. TdE/dx resolution isσdE/dx = 8.0%; the TOF resolution for Bhabha events isσTOF = 180 ps. Radiallyoutside of the time-of-flight counters is a 12-radiatiolength barrel shower counter (BSC) comprised oftubes interleaved with lead sheets. The BSC measthe energy and direction of photons with resolutioof σE/E � 21%
√E (E in GeV),σφ = 7.9 mrad, and
σz = 2.3 cm. The iron flux return of the magnet is istrumented with three double layers of countersare used to identify muons.
A GEANT3 based Monte Carlo package (SIMBEwith detailed consideration of the detector perfmance is used. The consistency between dataMonte Carlo has been carefully checked in many hpurity physics channels, and the agreement is reaable.
2. Analysis of J/ψ → 2(π+π−)η
This decay is observed in the topoloπ+π−π−π−γ γ . Events with four charged tracks anat least two isolated photons are selected. The setion criteria for charged tracks and photons arescribed in detail in Ref.[6]. Each charged track mube well fitted to a helix, originating from the interation region ofRxy < 0.02 m and|z| < 0.2 m, and havea polar angle,θ , in the range|cosθ | < 0.8.
Isolated photons are those that have energyposited in the BSC greater than 60 MeV, the anbetween the direction at the first layer of the BS
Fig. 1. The distribution ofχ2π+π−π+π−γ γ
where the crosses ar
data, the shaded histogram is from Monte Carlo simulationJ/ψ → 2(π+π−)η, the dots are from theη sidebands, and the fuhistogram is the sum of sideband background and Monte Carloulation.
and the developing direction of the cluster less th30◦, and the angle between photons and any chatracks larger than 10◦. To eliminate tracks fromγ
conversions, the minimum angle between anyoppositely-charged tracks is required to be greater10◦.
A 4C kinematic fit is performed under the hypothsisπ+π−π+π−γ γ . The distribution ofχ2
π+π−π+π−γ γ
is shown inFig. 1 and the chi-square is requiredbe less than 15.χ2
π+π−π+π−γ γis also required to be
less than the chi-squares for theK+K−π+π−γ γ andπ+π−π+π−γ γ γ hypotheses.
After the above selection, themγγ distribution isshown inFig. 2, where a clearη signal is observedMonte Carlo simulation is used to estimate the baground, and backgrounds from simulated channelslisted inTable 1, whereNMC
sel is the number of eventafter event selection andNnorm is the background normalized to 58 millionJ/ψ events. The sum of background events is 18 events, which can be ignoAnother possible background channel is fromJ/ψ →γ 2(π+π−)η. No obviousη signal is seen in themγγ
distribution fromJ/ψ → γ 2(π+π−)η, as shown inFig. 3(a). Therefore the background from this chancan also be ignored.
BES Collaboration / Physics Letters B 610 (2005) 192–198 195
h
astbe-to
sis
nt
nd
Fig. 2. The distribution ofmγγ for candidateJ/ψ → 2(π+π−)η
events. Dots with error bars are data, the histogram is the shape oftheη from Monte Carlo simulation, and the curve is background.
Table 1Background estimates for theJ/ψ → 2(π+π−)η case.NMC
sel isthe number of events passing selection criteria;Nnorm is the back-ground normalized to 58 millionJ/ψ events
Channel MC sample NMCsel Nnorm
J/ψ → φη 40000 0 0J/ψ → ρη 40000 0 0J/ψ → γ ηπ+π− 70000 2 3J/ψ → ωη(η → γ γ ) 40000 11 8J/ψ → ωη′(η′ → π+π−η) 100000 135 2J/ψ → φη′(η′ → π+π−η) 40000 2 0J/ψ → γ η′(η′ → π+π−η) 50000 6 5
Background from events with aK0S in the fi-
nal state are estimated fromη sidebands.Fig. 3(b)shows the mass distribution of allπ+π− pairs forevents withmγγ in the η region (|mγγ − 0.55| <
0.05 GeV/c2), and the shaded histogram is forη
sidebands (0.45 GeV/c2 < mγγ < 0.50 GeV/c2 and0.60 GeV/c2 < mγγ < 0.65 GeV/c2). From Fig. 3,we conclude that theK0
S signals are consistent witcoming from background, as estimated fromη side-band events.
Themγγ distribution, shown inFig. 2, is fitted witha Monte Carlo determined shape for theη and a sec-ond order polynomial and yields 4839± 158J/ψ →2(π+π−)η, η → γ γ events.
3. Analysis of J/ψ → 3(π+π−)η
Events with six good charged tracks and at letwo isolated photons are selected. The angletween two oppositely charged tracks is requiredbe greater than 10◦ to removeγ conversions. A fourconstraint kinematic fit is made to the hypotheJ/ψ → 3(π+π−)γ γ , andχ2
π+π−π+π−π+π−γ γis re-
quired to be less than 15.After the above selection, the two photon invaria
mass distribution is shown inFig. 4; anη signal is ev-ident.
Monte Carlo simulation indicates that backgroufrom the decay modes listed inTable 1 can be ig-nored. Other possible backgrounds are fromJ/ψ →γ 2(π+π−)η and J/ψ → γ 3(π+π−)η events. Asdescribed in Section2, the m distribution from
γ γFig. 3. (a) Distribution ofmγγ from Monte Carlo simulation ofJ/ψ → γ 2(π+π−)η. (b) Distribution ofmπ+π− from theη signal region(blank histogram) andη sidebands (shaded histogram) forJ/ψ → γ γ 2(π+π−) events.
196 BES Collaboration / Physics Letters B 610 (2005) 192–198
n
e
.
e
nd.
d by
-
ni-
hstri-
dding
.
ostin
cay
J/ψ → γ 2(π+π−)η shows no clearη peak. ForJ/ψ → γ 3(π+π−)η, the mγγ distribution fromMonte Carlo simulation, shown inFig. 5(a), also doesnot show a peak in theη region, so its contribution caalso be ignored.
The background withK0S final states can also b
estimated, as was done previously, using theη side-bands.Fig. 5(b) shows theπ+π− mass distributionThe full histogram is themπ+π− distribution for eventsin theη signal region (|mγγ − 0.55| < 0.05 GeV/c2),and the shaded histogram is for events from thη
Fig. 4. Distribution ofmγγ for J/ψ → 3(π+π−)γ γ candidateevents. Dots with error bars are data, the histogram is theη shape de-termined by Monte Carlo simulation, and the curve is backgrou
sidebands (0.45 < mγγ < 0.50 GeV/c2 and 0.60 <
mγγ < 0.65 GeV/c2). As in Section2, we can con-clude from Fig. 5(b) that most events withK0
S areassociated with backgrounds which are measureη sidebands.
Fitting theγ γ mass distribution inFig. 4 with theMonte Carlo shape forη and a second order polynomial, as described in Section2, yields 616±82 events.
4. Detection efficiency
Initially events were generated according to uform phase space. However, the cosθ distribution ofcharged tracks inJ/ψ → 2(π+π−)η was inconsis-tent with that from Monte Carlo simulation. Mucbetter agreement is obtained when the angular dibution is generated according to 1+ α cos2 θ , whereα = 0.65± 0.03 is obtained by fitting the cosθ dis-tribution of charged tracks.Fig. 6(a) and (b) showthe comparison of the cosθ distributions for chargedtracks andη, respectively, with Monte Carlo simulateevents with the charged tracks generated accorto this distribution. Including the contribution fromηsidebands, the angular distributions are consistent
For J/ψ → 3(π+π−)η, the detection efficiencyis obtained from phase space events since the cθ
distributions of charged tracks andη are consistenwith those from Monte Carlo simulation, as shownFig. 7(a) and (b), respectively. For the above two demodes, the detection efficiencies are(9.43± 0.10)%and(3.74± 0.06)%.
Fig. 5. (a) Themγγ distribution from Monte Carlo simulation ofJ/ψ → γ 3(π+π−)η; (b) The distribution ofmπ+π− from η signal events(histogram) andη sidebands (shaded histogram) forJ/ψ → γ γ 3(π+π−) events.
BES Collaboration / Physics Letters B 610 (2005) 192–198 197
onte
onte
ol-
a-
orkenfor.ud-
im-is,
k-fit
fu-
cay
ne-nd
lsas
yheom-ape
byck-
Fig. 6. The cosθ distribution of (a) charged tracks and (b)η parti-cles, where the crosses are data, the shaded histogram is from MCarlo simulation ofJ/ψ → 2(π+π−)η, and the full histogram isthe sum of sideband background and Monte Carlo simulation.
Fig. 7. The cosθ distribution of (a) charged tracks and (b)η parti-cles, where the crosses are data, the shaded histogram is from MCarlo simulation ofJ/ψ → 3(π+π−)η, and the full histogram isthe sum of sideband background and Monte Carlo simulation.
5. Systematic errors
The systematic errors mainly come from the flowing sources:
Table 2Summary of systematic errors (%)
Source 2(π+π−)η 3(π+π−)η
MDC tracking 8 12Photon efficiency 4 4Kinematic fit 4.3 5.5Background 5 5B(η → γ γ ) 0.7 0.7Number ofJ/ψ events 4.7 4.7
Total 12.1 15.4
(1) The MDC tracking efficiency has been mesured using channels likeJ/ψ → ΛΛ̄ andψ(2S) →π+π−J/ψ,J/ψ → µ+µ−. It is found that the MonteCarlo simulation agrees with data within 1–2% feach charged track. Therefore, 8% and 12% are taas the systematic errors in the tracking efficienciesthe 4-prong and 6-prong final states analyzed here
(2) The photon detection efficiency has been stied with different methods usingJ/ψ → ρ0π0 events[7]; the difference between data and Monte Carlo sulation is about 2% for each photon. In this analys4% is taken as the systematic error for theη decayinginto two photons.
(3) The kinematic fit is useful to reduce bacground. The systematic error from the kinematicis studied using the clean channelJ/ψ → π+π−π0,as described in Ref.[6]. The efficiency difference othe kinematic fit between data and Monte Carlo simlation is about 4%. With the same method, the demodesJ/ψ → 2(π+π−)π0 andJ/ψ → 3(π+π−)π0
are also analyzed. The efficiency difference of kimatic fit between data and Monte Carlo is 4.3% a5.5% respectively. SinceJ/ψ → 2(π+π−)π0 andJ/ψ → 3(π+π−)π0 are similar to the two channeanalyzed in this Letter, 4.3% and 5.5% are takenthe systematic error of the kinematic fit.
(4) Other possibleJ/ψ decay modes which macontribute toη signals have been studied, and tbackground from them can be ignored. The error frthe background under theη peak is included in the fitting error. The uncertainties of the background shin the two channels are estimated to be about 3.4%changing the order of the polynomial. Possible baground from the continuum events[8] is estimatedusing data at
√s = 3.07 GeV/c. After applying the
same selection criteria as above, no significantη sig-
198 BES Collaboration / Physics Letters B 610 (2005) 192–198
Table 3Numbers used and branching fractions measured
Decay modes Nobs ε (%) Branching fraction
J/ψ → 2(π+π−)η 4839± 158 9.43± 0.10 (2.26± 0.08± 0.27) × 10−3
J/ψ → 3(π+π−)η 616± 82 3.74± 0.06 (7.24± 0.96± 1.11) × 10−4
thistheis
sys-
er-
ol-
f
al-al
ns
of
Chisralos.ad-theU-
ect);of
No.e-2-
14.
59
.92
277
nal is observed. Therefore, the background fromsource is also negligible. From the above analysis,background uncertainty for the two decay modesless than 5%, which is taken as the backgroundtematic error.
(5) The branching fraction ofη → γ γ is (39.43±0.26)% [9]. The error is also taken as a systematicror.
(6) The number ofJ/ψ events is(57.70± 2.72)×106, determined from inclusive 4-prong hadrons[10].The uncertainty, 4.7%, is also a systematic error.Ta-ble 2lists the systematic errors from all sources.
6. Results
The branching fractions are calculated with the flowing relation:
(1)
B(J/ψ → n
(π+π−)
η) = Nobs
ε · B(η → γ γ ) · NJ/ψ
,
wheren is 2 or 3,Nobs is the observed events,ε isthe detection efficiency,B(η → γ γ ) is the branchingfraction ofη → γ γ , andNJ/ψ is the total number oJ/ψ events.
Table 3summarizes the quantities used in the cculation of the two branching fractions and the finresults, including systematic errors.
7. Summary
In this Letter, the decays ofJ/ψ → 2(π+π−)η andJ/ψ → 3(π+π−)η are studied with the BESII 5.8 ×107 J/ψ event sample and their branching fractioare measured for the first time to be
B(J/ψ → 2
(π+π−)
η)
= (2.26± 0.08± 0.27) × 10−3,
B(J/ψ → 3
(π+π−)
η)
= (7.24± 0.96± 1.11) × 10−4.
Comparing with other branching fractions ofJ/ψ de-caying into stable hadrons, the branching fractionsJ/ψ → 2(π+π−)η and J/ψ → 3(π+π−)η are notlarge.
Acknowledgements
The BES Collaboration thanks the staff of BEPand the computing center for their hard efforts. Twork is supported in part by the National NatuScience Foundation of China under contracts N19991480, 10225524, 10225525, the Chinese Acemy of Sciences under contract No. KJ 95T-03,100 Talents Program of CAS under Contract Nos.11, U-24, U-25, and the Knowledge Innovation Projof CAS under Contract Nos. U-602, U-34 (IHEPand by the National Natural Science FoundationChina under Contract No. 10175060 (USTC), and10225522 (Tsinghua University); and by the US Dpartment of Energy under Contract No. DE-FG004ER41291.
References
[1] J.J. Aubert, et al., Phys. Rev. Lett. 33 (1974) 1404.[2] J.E. Augustin, et al., Phys. Rev. Lett. 33 (1974) 1406.[3] P. Wang, C.Z. Yuan, X.H. Mo, Phys. Rev. D 70 (2004) 1140[4] L. Kopke, A. Wermes, Phys. Rep. 174 (1989) 67.[5] J.Z. Bai, et al., Nucl. Instrum. Methods A 458 (2001) 627.[6] J.Z. Bai, et al., Phys. Rev. D 70 (2004) 012005.[7] S.M. Li, et al., High Energy Phys. Nucl. Phys. 28 (2004) 8
(in Chinese).[8] P. Wang, X.H. Mo, C.Z. Yuan, Phys. Lett. B 557 (2003) 192[9] Particle Data Group, S. Eidelman, et al., Phys. Lett. B 5
(2004) 1, and references therein.[10] S.S. Fang, et al., High Energy Phys. Nucl. Phys. 27 (2003)
(in Chinese).
