b

Physics Letters B 633 (2006) 19–24

www.elsevier.com/locate/physlet

First measurements ofηc decaying intoK+K−2(π+π−) and 3(π+π−)

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, 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

0370-2693/$ – see front matter 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.physletb.2005.10.078

20 BES Collaboration / Physics Letters B 633 (2006) 19–24

e

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, Hawaii 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 28 May 2005; accepted 27 October 2005

Available online 9 November 2005

Editor: M. Doser

Abstract

The decays ofηc to K+K−2(π+π−) and 3(π+π−) are observed for the first time using a sample of 5.8 × 107 J/ψ events collected by thBESII detector. The product branching fractions are determined to beB(J/ψ → γ ηc) ·B(ηc → K+K−π+π−π+π−) = (1.21± 0.32± 0.24)×10−4, B(J/ψ → γ ηc) ·B(ηc → K∗0K̄∗0π+π−) = (1.91±0.64±0.48)×10−4, andB(J/ψ → γ ηc) ·B(ηc → π+π−π+π−π+π−) = (2.59±0.32± 0.47) × 10−4. The upper limit forηc → φπ+π−π+π− is also obtained asB(J/ψ → γ ηc) · B(ηc → φπ+π−π+π−) < 6.03× 10−5 atthe 90% confidence level. 2005 Elsevier B.V. All rights reserved.

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

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1. Introduction

The ηc, a 1S0 state in the charmonium family, was founin the inclusive photon spectra fromJ/ψ andψ(2S) [1] de-cays, as well as in hadronic decays[2]. A number of decaymodes ofηc were then measured[3]. More recent measurements of hadronic decays ofηc are listed in Ref.[4]. Accordingto Ref. [5], theηc is expected to have numerous decay mointo hadronic final states. In comparison, the number of expmentally reconstructed exclusive decay channels is rather sThis means that many decay modes ofηc are unknown. The 58million, (57.70± 2.72) × 106 [6], J/ψ events taken at BESIprovide a chance to observe new decays. In this analysis,ηc de-caying intoK+K−π+π−π+π− andπ+π−π+π−π+π− arestudied usingJ/ψ → γ ηc.

The upgraded Beijing Spectrometer detector, located aBeijing Electron–Positron Collider (BEPC), is a large solangle magnetic spectrometer which is described in detaRef. [7]. The momentum of the charged particle is demined by a 40-layer cylindrical main drift chamber (MDCwhich has a momentum resolution ofσp/p = 1.78%

√1+ p2

(p in GeV/c). Particle identification is accomplished by spcific ionization (dE/dx) measurements in the drift chamband time-of-flight (TOF) information in a barrel-like array of 4

* Corresponding author.E-mail address: fangss@ihep.ac.cn(S.S. Fang).

1 Current address: Iowa State University, Ames, IA 50011-3160, USA.2 Current address: Purdue University, West Lafayette, IN 47907, USA.3 Current address: Cornell University, Ithaca, NY 14853, USA.4 Current address: Laboratoire de l’Accélératear Linéaire, F-91898 O

France.5 Current address: University of Michigan, Ann Arbor, MI 48109, USA.6 Current address: DESY, D-22607 Hamburg, Germany.

si-ll.

e

n

y,

scintillation counters. ThedE/dx resolution isσdE/dx = 8.0%;the TOF resolution for Bhabha events isσTOF = 180 ps. Ra-dially outside of the time-of-flight counters is a 12-radiatiolength barrel shower counter (BSC) comprised of gas tuinterleaved with lead sheets. The BSC measures the enand direction of photons with resolutions ofσE/E � 21%

√E

(E in GeV),σφ = 7.9 mrad, andσz = 2.3 cm. The iron flux re-turn of the magnet is instrumented with three double layercounters that are used to identify muons.

A GEANT3 based Monte Carlo package (SIMBES) wdetailed consideration of the detector performance is useobtain the detection efficiency. The consistency betweenand Monte Carlo has been carefully checked in many highrity physics channels, and the agreement is reasonable[8].

2. Analysis of J/ψ → γ ηc,ηc → K+K−π+π−π+π−

These events are observed in the topologyγK+K−π+π−π−π−. Events with six good charged tracks and at leastisolated photon are selected. The selection criteria for gcharged tracks and isolated photons are described in detRef. [9]. Each charged track must be well fitted to a heoriginating from the interaction region ofRxy < 2 cm and|z| < 20 cm, and have a polar angleθ in the range|cosθ | < 0.8.Here Rxy is the distance from the beam axis, andz is alongthe beam axis. Isolated photons are those that have energposited in the BSC greater than 60 MeV, an angle betwthe direction at the first layer of the BSC and the developdirection of the cluster less than 30◦, and an angle betweethe photon and any charged tracks larger than 5◦. Two of thecharged tracks should be identified as kaons by combinedanddE/dx information.

A four-constraint (4C) kinematic fit is performed undthe hypothesis ofJ/ψ → γK+K−π+π−π+π−. For the

BES Collaboration / Physics Letters B 633 (2006) 19–24 21

-matfit-

ied

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era.ne

to

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dith

is theg

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Fig. 1. The distribution ofmK+K−π+π−π+π− for selected events. The histogram with error bars is from data, the shaded part is the background estifrom J/ψ → anything Monte Carlo simulation, and the curve represents theting results described in the text.

events with more than one photon, all combinations are trand the combination with the smallestχ2 is retained. Theχ2

γK+K−π+π−π+π− is required to be less than 10. T

reject background fromJ/ψ → γ γK+K−π+π−π+π−,χ2

γK+K−π+π−π+π− is required to be less tha

χ2γ γK+K−π+π−π+π− . Background events fromJ/ψ →

K+K−π+π−π+π− are eliminated by requiringχ2

γK+K−π+π−π+π− < χ2K+K−π+π−π+π− and Pmiss >

55 MeV/c, wherePmiss is the missing momentum of chargetracks.

After the above selection, theK+K−π+π−π+π− invari-ant mass,mK+K−π+π−π+π− , distribution is shown inFig. 1.A peak at theηc mass is observed. The shaded histogramthe background estimated from 58 millionJ/ψ → anythingMonte Carlo events generated with the Lund-charm gentor [10]; no prominent signal in theηc mass region is seenAlso, 100 000 events for the two possible background chanJ/ψ → K+K−2(π+π−) and J/ψ → γ 3(π+π−) are simu-lated. After final selection, no events remain in theηc massregion. A Breit–Wigner folded with a Gaussian to take inaccount the mass resolution of 12.3 MeV/c2 at theηc and asecond order polynomial background are used in the fit.fit gives 100± 26ηc events with a statistical significance4.0σ , where the mass and width ofηc are fixed to the PDGvalues[11].

Using this sample, we search for the decay modeηc →K∗0K̄∗0π+π−. To selectK∗0K̄∗0π+π− events, we requirethat the invariant masses ofK+π− and K−π+ must satisfy|mKπ − 0.896| < 0.05 GeV/c2. After theK∗0 andK̄∗0 selec-tion, theK+K−2(π+π−) invariant mass is shown inFig. 2.A small peak at theηc mass is observed. The backgrouevents corresponding to the shaded histogram inFig. 2are esti-mated fromK∗0 andK̄∗0 sidebands (0.1< |mK+π− −0.896| <0.15 GeV/c2 and 0.1 < |mK−π+ − 0.896| < 0.15 GeV/c2),and there is no evidentηc signal. 45± 15 events are obtaineby fitting the mass spectrum with a Breit–Wigner folded w

ed

,

s

-

ls

e

Fig. 2. The distribution ofmK+K−π+π−π+π− for ηc → K∗0K̄∗0π+π− can-didate events. The histogram with error bars is for data, the shaded partbackground estimated fromK∗0(K̄∗0) sidebands, and the curve is the fittinresults described in the text.

Fig. 3. The distribution ofmK+K−π+π−π+π− for ηc → φπ+π−π+π− can-didate events.

a Gaussian to account for theηc mass resolution plus a seond order polynomial background. The corresponding masswidth of theηc are fixed to PDG values[11]. Since the signif-icance of the peak is only 3σ , we also give the upper limit foηc → K∗0K̄∗0π+π−. With the Bayes method, the fit of thdistribution yields an upper limit of 65 events at the 90% codence level.

TheJ/ψ → γK+K−π+π−π+π− sample can also be useto search forηc → φπ+π−π+π−. For selecting aφ signal,the K+K− mass,mK+K− , is required to be in the regio|mK+K− −1.02| < 0.015 GeV/c2. After this selection, no cleaηc signal is found in the distribution ofmK+K−π+π−π+π− ,as shown inFig. 3. Using Bayes method, a fit toηc →K+K−π+π−π+π− with a Breit–Wigner folded with a Gaussian and a second order polynomial background gives an ulimit of 13.5ηc events at the 90% confidence level.

From Monte Carlo simulation, in which the angle (θ ) be-tween the direction of thee+ andηc in the laboratory frameis generated according to a 1+ cos2 θ distribution and uniform

22 BES Collaboration / Physics Letters B 633 (2006) 19–24

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phase-space is used forηc decaying intoK+K−2(π+π−) andφ2(π+π−), the detection efficiencies ofJ/ψ → γ ηc(ηc →K+K−π+π−π+π−), J/ψ → γ ηc(ηc → K∗0K̄∗0π+π−),and J/ψ → γ ηc(ηc → φπ+π−π+π−) are determined a(1.43± 0.04)%, (0.92± 0.03)%, and(1.01± 0.02)%, respec-tively. Therefore, the branching fractions obtained are

B(J/ψ → γ ηc) · B(ηc → K+K−π+π−π+π−)

= (1.21± 0.32) × 10−4,

B(J/ψ → γ ηc) · B(ηc → K∗0K̄∗0π+π−)

= (1.91± 0.64) × 10−4,

B(J/ψ → γ ηc) · B(ηc → K∗0K̄∗0π+π−)

< 2.76× 10−4 at 90% C.L.,

and

B(J/ψ → γ ηc) · B(ηc → φπ+π−π+π−)

< 4.72× 10−5 at 90% C.L.

3. Analysis of J/ψ → γ ηc, ηc → π+π−π+π−π+π−

These events are observed in the topologyJ/ψ →γπ+π−π+π−π+π−. Events with six good charged tracks aat least one isolated photon are selected. No particle idecation is required. A 4C kinematic fit is performed underhypothesisγπ+π−π+π−π+π−, and theχ2 is required to beless than 10. To reject background fromJ/ψ → 3(π+π−) andJ/ψ → 3(π+π−)π0, we requireχ2

γπ+π−π+π−π+π− to be less

thanχ2π+π−π+π−π+π− andχ2

π+π−π+π−π+π−π0.Fig. 4 shows theπ+π−π+π−π+π− invariant mass spec

trum after the above selection. A clearηc peak is observedThe shaded histogram inFig. 4 corresponds to background etimated from 58 millionJ/ψ → anything Monte Carlo eventsgenerated using the Lund-Charm generator[10], where noηc

signal is evident. A fit of themπ+π−π+π−π+π− distribution,

Fig. 4. The distribution ofmπ+π−π+π−π+π− for selected events. The histogram with error bars is data, the shaded part is the background estimfrom Monte Carlo simulation, and the curve is the fitting result described intext.

-

ed

which is shown as the solid curve inFig. 4, using a Breit–Wigner convoluted with a Gaussian to represent the signal asecond order polynomial for the background, yields 479±59ηc

events with a statistical significance of 8.4σ . In the fit, the massand width ofηc are again fixed to PDG values[11].

The detection efficiency for J/ψ → γ ηc, ηc →π+π−π+π−π+π− is determined to be(3.21 ± 0.04)%, byMonte Carlo simulation with the distribution ofθ , the angle be-tween the directions ofe+ andηc in the laboratory frame, beingenerated with a 1+cos2 θ and withηc decaying into 3(π+π−)

being generated with a uniform phase-space distribution.branching ratio is then found to be

B(J/ψ → γ ηc) · B(ηc → π+π−π+π−π+π−)

= (2.59± 0.32) × 10−4.

4. Systematic errors

The systematic errors mainly come from the followisources:

(1) MDC tracking efficiency. This has been measured wclean channels likeJ/ψ → ΛΛ̄ and ψ(2S) → π+π−J/ψ ,J/ψ → µ+µ−. It is found that the Monte Carlo simulatioagrees with data within 1–2% for each charged track. Thfore, 12% is conservatively taken as the systematic error intracking efficiencies for the 6-prong final states analyzed he

(2) Photon detection efficiency. This has been studied udifferent methods withJ/ψ → ρ0π0 events[8]. The differencebetween data and Monte Carlo simulation is about 2% for ephoton in the photon energy region from 0.1 to 0.8 GeV/c2, and2% is taken as the systematic error for the photon efficiencthis analysis.

(3) Particle identification (PID). The PID for pions has bestudied withJ/ψ → ρπ [9]. The efficiency of PID from datais in good agreement with that from Monte Carlo simulatand the average difference is less than 1% for each track. Inanalysis, the systematic error from PID forJ/ψ → γ ηc(ηc →π+π−π+π−π+π−) is not considered since no PID requirment is applied in the events selection. With the same metthe PID for kaons has been studied withJ/ψ → K+K−π0.The PID efficiency difference between Monte Carlo simulatand data is within 2% for each track. Therefore, forJ/ψ →γK+K−π+π−π+π− decay, 4% is taken as the systematicror from PID.

(4) Kinematic fit. The kinematic fit is useful to reduce bacground. Using the same method for estimating the systemerror as in Ref.[9], the decay modeJ/ψ → 3(π+π−)π0 isalso analyzed. The efficiency difference of the kinematic fitdata and Monte Carlo is 7.7%. Since the decay ofJ/ψ →3(π+π−)π0 is similar to the two channels analyzed in thLetter, 7.7% is also taken here as the systematic error okinematic fit.

(5) ηc parameters. Although theηc signal is clear, the number of events is not large enough to determine the Breit–Wigparameters and the background shape well. The variation ofit solution due to changes of theηc mass and width correspon

BES Collaboration / Physics Letters B 633 (2006) 19–24 23

Table 1Systematic error sources and contributions (%)

Sources K+K−2(π+π−) K∗0K̄∗0π+π− φ2(π+π−) 3(π+π−)

MDC tracking 12 12 12 12Particle ID 4 4 4 negligiblePhoton efficiency 2 2 2 2Kinematic fit 7.7 7.7 7.7 7.7ηc parameters 9.9 18.6 14.7 7.4MC statistics 2.6 3.3 2.9 1.1Background uncertainty 5.1 5.1 6.9Efficiency uncertainty 3.6B(φ → K+K−) 1.4Number ofJ/ψ events 4.7 4.7 4.7 4.7

Total 19.7 25.0 21.7 18.2

Table 2Numbers used in the calculations of branching fractions and upper limits

Decay mode Nobs ε (%) Branching fraction

J/ψ → γ ηc, ηc → K+K−2(π+π−) 100±26 1.43± 0.04 (1.21±0.32±0.24)×10−4

J/ψ → γ ηc, ηc → K∗0K̄∗0π+π− 45±15 0.92± 0.03 (1.91±0.64±0.48)×10−4

J/ψ → γ ηc, ηc → K∗0K̄∗0π+π− < 65 0.92± 0.03 < 3.68× 10−4 (90% C.L.)J/ψ → γ ηc, ηc → φ2(π+π−) < 13.5 1.01± 0.02 < 6.03× 10−5 (90% C.L.)J/ψ → γ ηc, ηc → 3(π+π−) 479±59 3.21± 0.04 (2.59±0.32±0.47)×10−4

thlist

omisiat

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rumck

iesuni

f

n-rro

error

sum

irs oftppercy-

thetedinaChi-, the

ing to the uncertainties in the PDG, as well as changes infitting mass region used, is taken as a systematic error andin Table 1.

(6) Background. Forηc → K+K−π+π−π+π−, the big-gest background comes fromηc → K0

SK0SK+K−. When the

invariant mass ofπ+π− is required to be within theK0S mass

region (|mπ+π− − 0.497| < 0.02 GeV/c2), five events remainin theηc mass region. If all of them are regarded as signal frηc → K0

SK0SK+K−, the background from this decay mode

about 5.1%, and this is taken as the systematic error assocwith background for this channel. No events remain forηc →K∗0K̄∗0π+π− and the upper limit is 2.3 events at 90% condence level. Then the uncertainty caused byηc → K0

SK0Sπ+π−

is 5.1%.For the ηc → 3(π+π−), Monte Carlo simulation is useto estimate the background fromηc → K0

SK0Sπ+π−. Using

the branching fraction forηc → K0K̄0π+π−, obtained fromB(ηc → K+K−π+π−) [11], Monte Carlo simulation indicatethat 33 background events contribute to theηc signal. Com-pared to the 479 signal events from fitting the mass spectthe background fraction is 6.9% which is taken as the baground systematic error for this channel.

(7) Efficiency uncertainty. In the analysis, the efficiencare estimated from Monte Carlo simulation by assumingform phase-space forηc decaying intoK+K−π+π−π+π−and K∗0K̄∗0π+π−. However, in the signals oK+K−π+π−π+π−, part of them may come fromηc →K∗0K̄∗0π+π−. Therefore, the difference between the efficiecies of two decay modes leads to another systematic ewhich is about 3.6%.

(8) The total number ofJ/ψ events. The number ofJ/ψ

events is(57.70± 2.72)× 106, determined fromJ/ψ inclusive

eed

ed

,-

-

r,

four-prong events. The uncertainty is taken as a systematicin the branching ratio measurement.Table 1lists the systematicerrors from all sources, and the total systematic error is theof them added in quadrature.

5. Results

The decays ofηc → K+K−π+π−π+π− and ηc →π+π−π+π−π+π− are observed for the first time, and thedecay branching ratios are measured. The upper limitηc → φπ+π−π+π− andηc → K∗0K̄∗0π+π− are also set athe 90% confidence level. To conservatively estimate the ulimit, the systematic error is included by lowering the efficienby one standard deviation.Table 2shows the branching ratio results including systematic errors.

Using the branching fraction ofJ/ψ → γ ηc asB(J/ψ →γ ηc) = (1.3± 0.4)% from the PDG[11], we obtain

B(ηc → K+K−π+π−π+π−)

= (0.93± 0.25± 0.34) × 10−2,

B(ηc → K∗0K̄∗0π+π−) = (1.47± 0.49± 0.58) × 10−2,

B(ηc → K∗0K̄∗0π+π−)

< 3.51× 10−2 (90% C.L.),

B(ηc → φπ+π−π+π−)

< 5.81× 10−3 (90% C.L.),

B(ηc → π+π−π+π−π+π−) = (1.99± 0.25± 0.71) × 10−2.

Acknowledgements

The BES Collaboration thanks the staff of BEPC andcomputing center for their hard efforts. This work is supporin part by the National Natural Science Foundation of Chunder contracts Nos. 19991480, 10225524, 10225525, thenese Academy of Sciences under contract No. KJ 95T-03

24 BES Collaboration / Physics Letters B 633 (2006) 19–24

Udeat-06by02

Chi-

100 Talents Program of CAS under Contract Nos. U-11,24, U-25, and the Knowledge Innovation Project of CAS unContract Nos. U-602, U-34 (IHEP); and by the National Nural Science Foundation of China under Contract No. 10175(USTC), and No. 10225522 (Tsinghua University); andthe US Department of Energy under Contract No. DE-FG04ER41291.

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