Measurements of psi(2S) decays to octet baryon-antibaryon pairs

0.22 MB·English

by  The BES Collaboration

1
Measurements of ψ(2S) decays to octet baryon-antibaryon pairs
7
0 M.Ablikim1,J.Z.Bai1,Y.Ban12,X.Cai1,H.F.Chen17,H.S.Chen1,H.X.Chen1,J.C.Chen1,JinChen1,
0 Y.B.Chen1,Y.P.Chu1,Y.S.Dai19,L.Y.Diao9,Z.Y.Deng1,Q.F.Dong15,S.X.Du1,J.Fang1,S.S.Fang1a,
2 C.D.Fu15,C.S.Gao1,Y.N.Gao15,S.D.Gu1,Y.T.Gu4,Y.N.Guo1,Z.J.Guo16b,F.A.Harris16,K.L.He1,M.He13,

n Y.K.Heng1,J.Hou11,H.M.Hu1,J.H.Hu3 T.Hu1,X.T.Huang13,X.B.Ji1,X.S.Jiang1,X.Y.Jiang5,J.B.Jiao13,
a D.P.Jin1,S.Jin1,Y.F.Lai1,G.Li1c,H.B.Li1,J.Li1,R.Y.Li1,S.M.Li1,W.D.Li1,W.G.Li1,X.L.Li1,X.N.Li1,
J X.Q.Li11,Y.F.Liang14,H.B.Liao1,B.J.Liu1,C.X.Liu1,F.Liu6,FangLiu1,H.H.Liu1,H.M.Liu1,J.Liu12d,

9 J.B.Liu1,J.P.Liu18,JianLiu1 Q.Liu1,R.G.Liu1,Z.A.Liu1,Y.C.Lou5,F.Lu1,G.R.Lu5,J.G.Lu1,C.L.Luo10,
1 F.C.Ma9,H.L.Ma2,L.L.Ma1e,Q.M.Ma1,Z.P.Mao1,X.H.Mo1,J.Nie1,S.L.Olsen16,R.G.Ping1,N.D.Qi1,
H.Qin1,J.F.Qiu1,Z.Y.Ren1,G.Rong1,X.D.Ruan4 L.Y.Shan1,L.Shang1,C.P.Shen1,D.L.Shen1,X.Y.Shen1,
2 H.Y.Sheng1,H.S.Sun1,S.S.Sun1,Y.Z.Sun1,Z.J.Sun1,X.Tang1,G.L.Tong1,G.S.Varner16,D.Y.Wang1f,
v
L.Wang1,L.L.Wang1,L.S.Wang1,M.Wang1,P.Wang1,P.L.Wang1,Y.F.Wang1,Z.Wang1,Z.Y.Wang1,
9
ZhengWang1,C.L.Wei1,D.H.Wei1,U.Wiedner20,Y.Weng1,N.Wu1,X.M.Xia1,X.X.Xie1,G.F.Xu1,X.P.Xu6,
7
0 Y.Xu11,M.L.Yan17,H.X.Yang1,Y.X.Yang3,M.H.Ye2,Y.X.Ye17,G.W.Yu1,C.Z.Yuan1,Y.Yuan1,
0 S.L.Zang1,Y.Zeng7,B.X.Zhang1,B.Y.Zhang1,C.C.Zhang1,D.H.Zhang1,H.Q.Zhang1,H.Y.Zhang1,
1 J.W.Zhang1,J.Y.Zhang1,S.H.Zhang1,X.Y.Zhang13,YiyunZhang14,Z.X.Zhang12,Z.P.Zhang17,D.X.Zhao1,
6 J.W.Zhao1,M.G.Zhao1,P.P.Zhao1,W.R.Zhao1,Z.G.Zhao1g,H.Q.Zheng12,J.P.Zheng1,Z.P.Zheng1,
0 L.Zhou1,K.J.Zhu1,Q.M.Zhu1,Y.C.Zhu1,Y.S.Zhu1,Z.A.Zhu1,B.A.Zhuang1,X.A.Zhuang1,B.S.Zou1
/
x (BESCollaboration)
e
- 1 Institute of HighEnergy Physics, Beijing 100049, People’s Republic of China
p 2 China Centerfor Advanced Science and Technology(CCAST), Beijing 100080, People’s Republic of China
e 3 Guangxi Normal University, Guilin 541004, People’s Republic of China
h
4 Guangxi University,Nanning 530004, People’s Republic of China
:
v 5 Henan Normal University,Xinxiang 453002, People’s Republic of China
Xi 6 Huazhong Normal University,Wuhan 430079, People’s Republic of China
7 Hunan University,Changsha 410082, People’s Republic of China
ar 8 Jinan University, Jinan 250022, People’s Republic of China
9 Liaoning University,Shenyang 110036, People’s Republic of China
10 Nanjing Normal University,Nanjing 210097, People’s Republic of China
11 Nankai University, Tianjin300071, People’s Republic of China
12 Peking University, Beijing100871, People’s Republic of China
13 Shandong University,Jinan 250100, People’s Republic of China
14 Sichuan University,Chengdu 610064, People’s Republic of China
15 Tsinghua University,Beijing 100084, People’s Republic of China
16 Universityof Hawaii, Honolulu, HI 96822, USA
17 University of Science and Technology of China, Hefei 230026, People’s Republic of China
18 Wuhan University,Wuhan 430072, People’s Republic of China
19 Zhejiang University, Hangzhou 310028, People’s Republic of China
20 Bochum University,Inst. f. Experimentalphysik I, D-44780 Bochum, Germany
a Current address: DESY, D-22607, Hamburg, Germany
b Current address: Johns Hopkins University,Baltimore, MD 21218, USA
c Current address: UniversiteParis XI, LAL-Bat. 208– -BP34, 91898- ORSAYCedex, France
d Current address: Max-Plank-Institut fuerPhysik, Foehringer Ring 6, 80805 Munich, Germany
e Current address: University of Toronto, Toronto M5S1A7, Canada
f Current address: CERN, CH-1211 Geneva23, Switzerland
g Current address: Universityof Michigan, AnnArbor, MI 48109, USA
With a sample of 14×106 ψ(2S) events collected by the BESII detector at the Beijing Electron Positron
Collider (BEPC), thedecay channelsψ(2S)→B8B¯8 (pp¯, ΛΛ¯, Σ0Σ¯0, Ξ−Ξ¯+) are measured,and theirbranching
ratios are determined to be(3.36±0.09±0.25)×10−4, (3.39±0.20±0.32)×10−4, (2.35±0.36±0.32)×10−4,
(3.03±0.40±0.32)×10−4, respectively. In the decay ψ(2S) → pp¯, the angular distribution parameter α is
1. Introduction are several measurements for α for J/ψ pp¯,
→
andthe recentresultofα=0.676 0.036 0.042
Thebranchingratiosofψ(2S)decaysintooctet ± ±
givenby BES[6]is quite closeto Carimalo’spre-
baryon-antibaryon pairs were measured by the
diction α = 0.69 [5]. However, there is only one
BES-IandCLEOccollaborations,andtheresults
measurement for ψ(2S) pp¯, made by E835 [7].
differ significantly, as shown in Table 1. It is →
Results for ψ(2S) pp¯ are summarized in Ta-
therefore important to make new measurements →
ble 2. The ψ(2S) pp¯ events in BESII allow
to help clarify these differences using the sample →
the measurement of α, which can be compared
of14 106ψ(2S)eventscollectedbyBESII,which
× with the existing result and used to test hadron
is the world’s largest e+e− ψ(2S) sample.
helicity conservation.
Table 1
Table 2
Branching ratios of ψ(2S) B B¯ ( 10−4).
→ 8 8 × Predicted and measuredvalues of α for ψ(2S)
→
Channel BES-I [1] CLEO-c [2] pp¯.
pp¯ 2.16 0.15 0.36 2.87 0.12 0.15
± ± ± ± α value Source
ΛΛ¯ 1.81 0.20 0.27 3.28 0.23 0.25
± ± ± ± Predicted value:
Σ0Σ¯0 1.2±0.4±0.4 2.63±0.35±0.21 α=0.58 Claudson et al. [4]
Ξ−Ξ¯+ 0.94 0.27 0.15 2.38 0.30 0.21
± ± ± ± α=0.80 Carimalo [5]
Measured value:
α=0.67 0.15 0.04 M. Ambrogiani et al. [7]
± ±
According to the hadron helicity conservation,
theangulardistributionofψ(2S) B B¯ canbe
8 8
→
expressed as:
BESII is a large solid-angle magnetic spec-
dN 1+αcos2θ, (1) trometer which is described in detail in Ref. [8].
dcosθ ∝ The momentum of charged particles is deter-
where θ is the angle between B and the beam mined by a forty-layer cylindrical main drift
8
direction of the positron in the center-of-mass chamber (MDC) which has a resolution of
(CM) system. In the limit of infinitely heavy σp/p=1.78%p1+p2(pinGeV/c). Particleiden-
charmmass,hadronhelicity conservationimplies tification is accomplished using specific ioniza-
α=1[3]forbothJ/ψ andψ(2S)decaystooctet tion (dE/dx) measurements in the drift cham-
baryonanti-baryon pairs. ber and time-of-flight (TOF) information in a
Values of α for J/ψ, ψ(2S) pp¯ have been barrel-like array of forty-eight scintillation coun-
→
predictedtheoreticallybasedonfirstorderQCD. ters. The dE/dx resolution is σ 8.0%;
dE/dx
≃
In the prediction of Claudson, Glashow, and the TOFresolutionforBhabhaeventsisσ =
TOF
Wise[4],themassofthefinalbaryonistakeninto 180 ps. Radially outside of the time-of-flight
account as a whole, while the constituent quarks counters is a 12-radiation-length barrel shower
insidethebaryonaretakenasmasslesswhencom- counter(BSC)comprisedofgastubesinterleaved
puting the decay amplitude. Inthe predictionby with lead sheets. The BSC measures the en-
Carimalo [5], mass effects at the quark level are ergy and direction of photons with resolutions of
taken into consideration. Experimentally there σ /E 21%/√E (E in GeV), σ = 7.9 mrad,
E φ
≃
2
3
and σ = 2.3 cm. The iron flux return of the is defined to be 20 cm longitudinally and 2 cm
z
±
magnet is instrumented with three double layers radially.
ofproportionalcountersthat areusedto identify In order to remove cosmic rays, the difference
muons. between the time-of-flights of the positive and
Monte Carlo (MC) simulation is used for mass negative tracks, t+ t− , is required to be less
| − |
resolutionanddetectionefficiencydetermination. than4.0ns. Protonsandantiprotonsarerequired
Inthisanalysis,aGEANT3[9]basedMCpackage to be identified by the TOF; the measured time-
(SIMBES) with detailed consideration of the de- of-flightofthetrackmustbeclosesttothepredic-
tectorperformance(suchasdeadelectronicchan- tionfortheproton/anti-protonhypothesis. Since
nels) is used. The consistency between data and ψ(2S) pp¯isaback-to-backtwo-bodydecay,we
→
MC has been carefully checked in many high pu- requirethe acollinearityangle oftwotracksto be
rity physics channels, and the agreement is rea- less than 5o. The deposited energy in the BSC
sonable [10]. of the positive particle is requiredto be less than
The data samples used for this analysis con- 0.75GeVtoremovepossiblee+e− finalstatecon-
sist of 14.0 106(1 4%) ψ(2S) events [11] and tamination. Finally, the energy sum (calculated
× ±
6.42(1 4%) pb−1 of continuum data at √s = from the track’s momentum) of the two tracks is
±
3.65 GeV [12]. The decay channels investigated required to be within 130 MeV of the expected
areψ(2S) pp¯, ΛΛ¯, Σ0Σ¯0, andΞ−Ξ¯+,where Λ sum, 3.686 GeV, and the momentum of the neg-
→
decays to pπ−(63.9%), Σ0 decays to Λγ(100%), ative trackis requiredtobe within150MeV/cof
− −
and Ξ decays to Λπ (99.9%). the expected momentum 1.586 GeV/c.
Events surviving the selection criteria are
shown in Fig. 1 as dots with error bars. The
2. Event selectionand branching ratio de-
sameselectioncriteriahavebeenappliedtoback-
termination
ground events generated by the MC and nor-
2.1. ψ(2S) → pp¯ malized according to branching ratios listed in
The experimental signature for the decay PDG(2006)[13],and38.1backgroundeventssur-
ψ(2S) pp¯ is two back-to-back, oppositely- vive and are shown as the dashed line in Fig. 1.
→
charged tracks, each with a momentum of ThedataarefittedbyaMChistogramforthesig-
1.586 GeV/c. The main backgrounds are: nalplusabackgroundfunctionwhichcorresponds
Bhabha and dimuon (e+e− µ+µ−) events, to the 38.1 simulated background events and a
→
ψ(2S) e+e−, µ+µ−, π+π−, K+K−,ψ(2S) flat distribution to describe the remaining back-
→ →
γχCJ(J=0,1,2) γπ+π−(K+K−, pp¯), ψ(2S) ground. From the fit, the number of pp¯events is
→ →
π0π0J/ψ π0π0e+e−(µ+µ−), ψ(2S) π0pp¯, determined to be 1618.2 43.4, where the error
→ → ±
etc. is statistical.
The event selection requires two well recon-
structed and oppositely charged tracks. Each 2.1.1. Angular distribution of pp¯
track is required to be well fitted to a three Toobtaintheparameterαforψ(2S) pp¯,the
→
dimensional helix, be in the polar angle region cosθ dependence of the event selection efficiency
cosθ <0.8,andhaveamomentumgreaterthan must be taken into account, which is determined
| |
70 MeV/c in the xy-plane. The point of clos- usingaflatangulardistribution(α=0)intheMC
est approach of each track to the beamline is re- simulation; see Fig. 2(a). However, there are im-
quired to be within the interaction region which perfections in the MC simulation, which will dis-
4
the energy and momentum for ψ(2S) pp¯ are
→
160 scaled to the reference channel J/ψ pp¯. Then
→
following the re-weighting procedures in Ref. [6]
for our selection criteria, the correction function
V/c) 120 fc(cosθ) is obtained and is shown in Fig. 2(c).
e Withε (cosθ)denotingtheefficiencyobtained
M MC
nts/(10 80 ffruonmctiψo(n2fSo)r→thepep¯ffiMcCienacnyd,wfce(cfiotstθh)etmheeacsourrreedctaionn-
e
v gular distribution of ψ(2S) pp¯ data with the
E →
function N(cosθ),
40
N(cosθ)=N (1+αcos2θ) ε (cosθ) f (cosθ),
0 MC c
× × ×
as shown in Fig. 2(d). The fit uses a binned
0
1.340 1.440 1.540 1.640 1.740 χ2 minimization method in the angular range
Pproton (GeV/c) cosθ [-0.7,0.7] and gives χ2 = 10.88 for 12
∈ min
degrees of freedom. The fitted value of the pa-
rameter α is 0.85 0.24, where the error is sta-
±
Figure1. Thefittedprotonmomentumspectrum. tistical.
The dots with error bars are data, the histogram As a consistency check, we also obtained
is the fit to the data including the signal shape f (cosθ) directly from the ψ(2S) pp¯ sample,
c
→
from MC and all backgrounds, and the dashed and the fitted result obtained using this correc-
line is the background. tion yields α = 0.83 0.24, but its systematic
±
uncertaintyis0.14,mainlyduetothelowerstatis-
tics of the ψ(2S) pp¯sample, and much larger
tort the efficiencies determined by the MC as a →
than the systematic error of 0.04 determined us-
function of cosθ. In order to reduce this system-
ing f (cosθ) obtained from the J/ψ pp¯ sam-
c
atic error, a correction to the MC efficiency is →
ple (see section 3.1). This demonstrates that
made [6]. The correction factor f (cosθ) is de-
c f (cosθ) determined from the J/ψ pp¯ sam-
c
fined as: →
ple improves the systematic error on α without
ε ε (i)
fc(cosθ)= Data(cosθ)=Y Data (cosθ), changing its central value and statistical error.
ε ε (i)
MC MC
i
2.1.2. Branching ratio of ψ(2S) → pp¯
where i denotes the selection criterion, ε (i)
Data The selection efficiency is determined using
is the efficiency determined for data for criterion
1 105 ψ(2S) pp¯ MC events. The MC-
i, and ε (i) is the efficiency from the MC for × →
MC determinedefficiencyisǫ =(34.4 0.2)%,and
MC
criterion i. The corrected MC efficiency is then: ±
the branching ratio is determined to be:
′
εMC(cosθ)=εMC(cosθ)×fc(cosθ). Br(ψ(2S) pp¯)=(3.36 0.09) 10−4,
→ ± ×
Due to the limitation on the number of
where the error is statistical.
ψ(2S) pp¯ events, the ”reference” channel
→
J/ψ pp¯ is chosen to determine the correc- 2.2. ψ(2S) → ΛΛ¯
→
tion factor due to its higher statistics and sim- Candidate events require four well recon-
ilar kinematics. The selection criteria related to structed charged tracks. The positive (negative)
5
ncy 0.6 (a) Bin 6 (b) 35
Efficie 0.5 ntries/ 5
E 30
0.4 4
0.3 3
25
0.2 2
m)
0.1 1 01 20
0 0 0.
-0.5 0 0.5cosq -0.5 0 0.5cosq nts/
qf(cos)c0.18 (c) Entries/Bin111468000 (d) (Eve 1105
120
0.6 100
80
0.4 5
60
40
0.2
20 0
0 0.1 0.2 0.3 0.4 0.5
0 -0.5 0 0.5 0 -0.5 0 0.5 L Sum of L and L– (m)
cosq cosq xy
Figure 2. (a) The selection efficiency versus cosθ Figure 3. The sum ofthe Λ and Λ¯ decaylengths.
obtained from MC; (b) angular distribution of The histogram is the signal shape from the MC
background events from MC, which survive the plus simulated background, the dots with error
same selection criteria as used for data; (c) the bars are data, and the shaded histogram is the
correction obtained from data (f (cosθ)) to the background.
c
MC efficiency; and (d) the angular distribution
of candidate ψ(2S) pp¯events.
→
MC plus a background function which describes
charged track with the higher momentum is as- thesimulatedbackgroundsandaflatdistribution
sumed to be the proton (antiproton); the other todescribeanyremainingsources. Thesimulated
two are assumed to be the π+ and π−. The two backgroundsaremainlyfromψ(2S) Σ0Σ¯0 and
→
pπ pairs are required to pass the Λ’s vertex find- ψ(2S) ΛΣ¯0+c.c.andnormalizingaccordingto
→
ing algorithm successfully, and the sum of the Λ branching ratios from PDG(2006), a total of 32
and Λ¯ decay lengths must be greater than 0.02 background events are obtained. The final num-
m (see Fig. 3). The sum of the Λ and Λ¯ ener- ber of signal events from the fit is 337.2 19.9.
±
gies must be in the regionfrom 3.60 GeV to 3.81 The ψ(2S) ΛΛ¯ pp¯π+π− efficiency is de-
→ →
GeV (see Fig. 4). The missing momentum of the terminedtobeǫ =(17.4 0.2)%using2 105
MC
± ×
events should be less than 0.18 GeV/c, and the MC-simulatedsignalevents. Thebranchingratio
difference between the measured mass of Mp¯π+ is then:
and its expected value, M , should be less than
Λ
12 Me/c2 (three times the resolutionof the M ).
Λ Br(ψ(2S) ΛΛ¯)=(3.39 0.20) 10−4,
Theeventsthatsatisfytheselectioncriteriaare → ± ×
shown in Fig. 5 as dots with error bars; they are
fitted by a histogram of the signal shape from where the error is statistical.
6
100
60
80
MeV) cut cut 2eV/c) 40
0 60 M
2 2
(Events/ 40 Events/(
20
20
0
0
3 3.2 3.4 3.6 3.8 4 4.2 4.4 1.075 1.095 1.115 1.135 1.155
ELL – (GeV) Mpp - (GeV/c2)
Figure 4. The sum of the Λ and Λ¯ energies. The Figure 5. The fitted Λ mass spectrum. The dots
histogram is the signal shape from the MC plus with error bars are data, the histogram is the fit
backgrounds. The dots with error bars are data, to data whichincludes the signalshape fromMC
andthe dashedline is the mainbackgroundfrom plus all backgrounds, and the dashed line is the
ψ(2S) Σ0Σ¯0. The peaks at 3.1 and 4.2 GeV background.
→
are from ψ(2S) π0π0J/ψ, J/ψ ΛΛ¯ and
→ →
ψ(2S) π+π−J/ψ, J/ψ e+e− (or µ+µ−),
→ →
respectively.
plus a background function which describes the
simulated backgrounds and a flat distribution to
2.3. ψ(2S) → Σ0Σ¯0 describe the remaining background. The main
backgrounds are from ψ(2S) ΛΛ¯, ψ(2S)
Candidate events are required to have four → →
γχ γΛΛ¯, ψ(2S) Ξ0Ξ¯0, ψ(2S)
well reconstructed charged tracks plus at least CJ(J=0,1,2) → → →
two good photons. The Λ and Λ¯ are selected ΛΣ¯0+c.c.andψ(2S)→Σ0Ξ¯0+c.c.,andnormaliz-
ing using branching ratiosfrom PDG(2006),16.5
using the method described in Section B. The
background events are obtained. The final num-
missing momentum of the events is required to
ber of signal events from the fit is 59.1 9.1.
be less than 0.25 GeV/c. The χ2 of the four ±
The ψ(2S) Σ0Σ¯0 ΛΛ¯γγ pp¯π+π−γγ
constraint (4C) kinematic fit to the hypothesis → → →
ψ(2S) pp¯π+π−γγ must be less than 20. The efficiencyisdeterminedtobeǫMC =(4.4±0.1)%
→ using 2 105 MC generated signal events. The
difference between the measured mass of Mp¯π+γ ×
branching ratio of signal channel is then:
and its expected value, MΣ¯0, should be less than
36 MeV/c2 (three times the MΣ¯0 resolution).
The events that survive selection are shown Br(ψ(2S) Σ0Σ¯0)=(2.35 0.36) 10−4,
→ ± ×
in Fig. 6 as dots with error bars; they are fit-
ted by a histogram of the signal shape from MC where the error is statistical.
7
30
16
25
2V/c) 12 eV) 20
e G
M 2
0 0
nts/(1 8 nts/0. 15 cut cut
Eve (Eve 10
4
5
0
1.10 1.20 1.30 1.40 03.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2
Mpp -g (GeV/c2) EXX - (GeV)
Figure6. ThefittedΣ0 massspectrum. Thedots Figure 7. The Ξ− and Ξ¯+ energy sum. The his-
with error bars are data, the histogram is the fit togram is the signal shape from MC plus back-
to data which includes the signal shape from the ground. The dots with error bars are data, and
MC and all backgrounds, and the dashed line is the shaded area is the sum of simulated back-
the background. grounds.
2.4. ψ(2S) → Ξ−Ξ¯+ Fig. 8 as dots with error bars, and they are fit-
ted by a histogram of the signal shape from MC
Candidateeventsrequiresixwellreconstructed
plus a background function which describes the
charged tracks. The positive (negative) charged
simulated backgrounds and a flat distribution to
track with highest momentum is assumed to
describe remaining background. The main back-
be the proton (antiproton); the other four are
ground is from ψ(2S) π+π−J/ψ π+π−ΛΛ¯,
assumed to be πs. Looping over all possi-
→ →
ble pπ−, p¯π+ combinations in an event, the and normalizing by PDG(2006) branching frac-
tions, 11.3 background events are obtained. The
one which successfully passes the vertex find-
final number of signal events from the fit is
ing algorithm and has the smallest value of
67.4 8.9.
p(Mpπ− −MΛ)2+(Mp¯π+ −MΛ¯)2 isselect−edfor Th±e ψ(2S) Ξ−Ξ¯+ ΛΛ¯π+π−
further analysis. The energy sum of the Ξ and
→ → →
Ξ¯+ should be between 3.593 and 3.779 GeV (see pp¯π+π−π+π− efficiency is determined to be
ǫ = (3.9 0.1)% using 2 105 signal events
Fig.7), andthe missingmomentum ofthe events MC
± ×
generatedbyMC.The branchingratioofthe sig-
should be less than 0.15 GeV/c. The difference
nal channel is then:
between the measured mass of M and its
p¯π+π+
expected value, MΞ¯+, should be less than 18 Br(ψ(2S) Ξ−Ξ¯+)=(3.03 0.40) 10−4,
MeV/c2 (three times the MΞ− resolution). → ± ×
The events surviving selection are shown in where the error is statistical.
8
24 quadraturegivesatotalsystematicerrorof4.2%.
20 3.2. Branching ratios
The systematic errors on the branching ratios
2) 16 are mainly from the uncertainties in the MDC
c
V/ tracking,α,thehadronicinteractionmodel,back-
e
M
5 12 groundestimations,anddifferencesbetweendata
nts/( and MC for the Λ vertex finding, decay length
e
v requirement, and kinematic fitting.
E 8
The MDC tracking gives a systematic error of
about2%foraprotonoranti-proton[10]and1%
4
for a low-energyπ, which is determined from the
channel ψ(2S) π+π−J/ψ π+π−µ+µ−. The
0 → →
1.25 1.30 1.35 1.40 1.45 detection efficiency depends on the angular dis-
Mpp - p - (GeV/c2) tribution of the baryon pair. For pp¯decay, when
changing the α value by 1σ, the branching ra-
tio changes by 2.4%; in the other three channels,
−
Figure 8. The fitted Ξ mass spectrum. Dots α=0.5isusedasanominalvalue,themaximum
with error bars are data, the histogram is the fit differences for ǫ ǫ and ǫ ǫ
α=0.5 α=0 α=0.5 α=1
| − | | − |
to data which includes the signal shape from the are taken as systematic errors, they are 6.5%,
MC and all backgrounds, and the dashed line is 7.6%,6.8%,respectively. Theuncertaintiesofthe
the sum of the backgrounds. detection efficiencies caused by assumed flat an-
gulardistributionsforsecondarydecayofbaryons
are much smaller than those from angular distri-
butions of ψ(2S) to baryon pair primary decays,
3. Systematic error
and are therefore neglected here [14]. Different
3.1. ψ(2S) → pp¯ angular distribution simulation models for the hadronic interaction
Thesystematicerroronαinψ(2S) pp¯decay (GCALOR/Geant-FLUKA) [15,16]give different
→
from the tracking reconstruction is determined efficiencies, giving systematic errors of 2.18%,
using different MDC wire resolution models in 0.46%,0.00%,1.08%forthe studiedchannels,re-
the MC simulations, which changes α by 2.7%. spectively. The background uncertainty is stud-
When the fit parameter of the efficiency correc- ied by changing the nominal branching ratios of
tion curve f (cosθ) is changed by 1σ, α changes the backgrounds which have large statistical er-
c
by 2.3%. The performance of the BES detector rors. If the branching ratios of the background
has small differences between the time when the channels are changed by 100% in the pp¯, ΛΛ¯,
58 106 J/ψ events were obtained and when the and Ξ−Ξ¯+ channels, the changes in the branch-
×
14 106 ψ(2S) events were obtained. Using pa- ing ratios in the signal channels are 0.1%, 1.0%
×
rameter files describing the performance of BES and 0.2%, respectively. For the Σ0Σ¯0 channel,
detector atthese two data taking periods, the ef- where the shape of the simulated backgrounds is
fect of this variation on α is determined to be in good agreement with the data in the Λγ in-
2.2%. The effect of the background uncertainty variantmassdistribution, the branchingratiosof
on α is negligible. Adding these contributions in backgrounds are only changed by 20%, resulting
9
in a change of the branching ratios of the signal ψ(2S) pp¯is measuredto be 0.85 0.24 0.04,
→ ± ±
channelof2.3%. Accordingtothereferencechan- which is in agreement within 1σ with the E835
nel J/ψ ΛΛ¯ [17], the secondary vertex finding result [7], and close to Carimalo’s prediction [5].
→
of Λ gives a systematic error of 0.7% for each Λ
vertex, and the requirement on the sum of the
Table 3
decay length contributes 1.4%.
Systematicerrorsinthebranchingratiomeasure-
In the branching ratio determination of four
ments (%).
channels, the continuum contribution must be
Source pp¯ ΛΛ¯ Σ0Σ¯0 Ξ−Ξ¯+
subtracted. Thecontinuumdataarealsoselected
with the same criteria as for the ψ(2S) decay MDC tracking 4 4.5 4.5 5.7
signal channels, and the number of the surviv- PID 2.4
ing events times a luminosity normalization fac- Cosmic Ray Exc. 0.9
tor is takenas a systematic error. The kinematic Deposit Energy 0.9
fit of pp¯π+π−γγ in ψ(2S) Σ0Σ¯0 gives a sys- Acol. angle 0.9
→
tematic error of 7.6% from the reference channel Vtx. finding 1.4 1.4 1.4
ψ(2S) π+π−J/ψ π+π−π+π−π0 [18]. The Decay length 1.0 1.0
→ →
uncertainty on the total number of ψ(2S) events EBB¯, MB (or PB) 0.8 0.6 1.6 1.6
is 4%. The systematic errors of the acollinearity Pmiss 1.6 0.5 1.7
angle,EBB¯ region,baryonmass(ormomentum), γ tracking 4
and P requirements are studied with corre- Kinematic fit 7.6
miss
sponding J/ψ pp¯decays. Bg. Esti. 1.0 2.3 0.2
→
Intheψ(2S) pp¯selection,thesystematicer- Continuum data 0.8 1.0
→
rors due to the uncertainties from particle iden- α value 2.4 6.5 7.6 6.8
tification, the cosmic ray veto, and the deposited Hadronic Interaction 2.2 0.5 1.1
energycriterionarestudiedbythischannelitself. Nψ(2S) 4 4 4 4
All the systematic errors in the branching ratio Total error 7.3 9.4 13.4 10.3
measurements are summarized in Table 3.
4. Summary and discussion
5. Acknowledgement
Based on 14 106 ψ(2S) events, the branch-
×
ing ratios of ψ(2S) pp¯, ΛΛ¯, Σ0Σ¯0, and Ξ−Ξ¯+ The BES collaboration thanks the staff of
→
are measured, the results are listed in Table 4, BEPC for their hard efforts. This work is sup-
together with the ratios of ψ(2S) BB¯ to ported in part by the National Natural Sci-
→
J/ψ BB¯. They are in agreement with the ence Foundation of China under contracts Nos.
→
results published by the CLEO collaboration [2] 10491300, 10225524, 10225525, 10425523, the
within2σforpp¯andwithin1σfortheotherthree Chinese Academy of Sciences under contractNo.
channels. The differences of the branching frac- KJ 95T-03, the 100 Talents Program of CAS
tionsbetweencurrentmeasurementsandthoseof under Contract Nos. U-11, U-24, U-25, and
BESIare2.5σ,3.1σ,1.5σ,3.5σ forthe fourchan- the Knowledge Innovation Project of CAS un-
nels, respectively. der Contract Nos. U-602, U-34 (IHEP), the Na-
The angular distribution parameter α for tionalNaturalScienceFoundationofChinaunder
10
Table 4 10. M. Ablikim et al. (BES Collab.), Nucl. Inst.
Branching ratios of ψ(2S) decays into baryon and Meths. A 552, 344 (2005).
anti-baryon pairs. The first error is statisti- 11. X. H. Mo et al., HEP & NP 28, 455 (2004).
cal and the second systematic. The value Q is
12. S. P. Chi, X. H. Mo and Y. S. Zhu, Measure-
BR(ψ(2S) BB¯)/BR(J/ψ BB¯). The J/ψ ment of the Integrated Luminosity at √s =
→ →
branching ratios are taken from Ref. [6] for pp¯,
3.65,3.686GeV.
Ref. [17] for ΛΛ¯ and Σ0Σ¯0, and Ref. [13] for 13. W. M. Yao et al., J. Phys. G 33, 1 (2006).
Ξ−Ξ¯+.
14. R.G.Ping,H.B.Li,HEP&NP,30(09),819
modes BRs ( 10−4) Q (%) (2006).
×
pp¯ 3.36 0.09 0.25 14.9 1.4 15. C.ZeitnitzandT.A.Gabriel,Nucl.Inst.and
± ± ±
ΛΛ¯ 3.39 0.20 0.32 16.7 2.1 Meths. A 349, 106 (1994).
± ± ±
Σ0Σ¯0 2.35 0.36 0.32 16.8 3.6 16. K. Hanssgen, H. J. Mohring and J. Ranft,
± ± ±
Ξ−Ξ¯+ 3.03 0.40 0.32 16.8 4.7 Nucl. Sci. Eng. 88, 551 (1984); J. Ranft and
± ± ±
S. Ritter, Zeit. Physik C 20, 347 (1983);
A. Fasso et al., FLUKA 92, Proceedings of
the Workshop on Simulating Accelerator Ra-
Contract No. 10225522 (Tsinghua University), diation Environment, Santa Fe(1993).
and the Department of Energy under Contract 17. M. Ablikim et al. (BES Collab), Phys. Lett.
No.DE-FG02-04ER41291(U Hawaii). B 632, 181 (2006).
18. M. Ablikim et al. (BES Collab), Phys. Lett.
B 619,247(2005);J.Z.Baiet al. (BESCol-
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