arXiv:1303.3108v3 [hep-ex] 13 Jun 2013
Study of
J/ψ → ωp ¯
p at BESIII
1 M. Ablikim1 , M. N. Achasov6 , O. Albayrak3 , D. J. Ambrose39 , F. F. An1 , Q. An40 , J. Z. Bai1 , R. Baldini Ferroli17A, 2 Y. Ban26 , J. Becker2 , J. V. Bennett16 , M. Bertani17A, J. M. Bian38 , E. Boger19,a, O. Bondarenko20 , I. Boyko19 , R. A. Briere3 , 3 V. Bytev19 , H. Cai44 , X. Cai1, O. Cakir34A, A. Calcaterra17A, G. F. Cao1
, S. A. Cetin34B, J. F. Chang1 , G. Chelkov19,a, 4 G. Chen1 , H. S. Chen1 , J. C. Chen1 , M. L. Chen1 , S. J. Chen24 , X. Chen26 , Y. Chen1 , Y. B. Chen1 , H. P. Cheng14 , 5 Y. P. Chu1 , D. Cronin-Hennessy38 , H. L. Dai1 , J. P. Dai1 , D. Dedovich19 , Z. Y. Deng1 , A. Denig18 , I. Denysenko19,b, 6
M. Destefanis43A,43C, W. M. Ding28
, Y. Ding22 , L. Y. Dong1 , M. Y. Dong1 , S. X. Du46 , J. Fang1 , S. S. Fang1 , L. Fava43B,43C, 7 C. Q. Feng40 , P. Friedel2 , C. D. Fu1 , J. L. Fu24 , O. Fuks19,a, Y. Gao33 , C. Geng40 , K. Goetzen7 , W. X. Gong1 , W. Gradl18 , 8 M. Greco43A,43C, M. H. Gu1 , Y. T. Gu9 , Y. H. Guan36 , A. Q. Guo25 , L. B. Guo23 , T. Guo23 , Y. P. Guo25 , Y. L. Han1 , 9 F. A. Harris37 , K. L. He1 , M. He1 , Z. Y. He25 , T. Held2 , Y. K. Heng1 , Z. L. Hou1 , C. Hu23 , H. M. Hu1 , J. F. Hu35 , T. Hu1 , 10 G. M. Huang4 , G. S. Huang40 , J. S. Huang12 , L. Huang1 , X. T. Huang28 , Y. Huang24 , Y. P. Huang1 , T. Hussain42 , C. S. Ji40 , 11 Q. Ji1 , Q. P. Ji25 , X. B. Ji1 , X. L. Ji1 , L. L. Jiang1 , X. S. Jiang1 , J. B. Jiao28 , Z. Jiao14 , D. P. Jin1 , S. Jin1 , F. F. Jing33 , 12 N. Kalantar-Nayestanaki20 , M. Kavatsyuk20 , B. Kopf2 , M. Kornicer37 , W. Kuehn35 , W. Lai1 , J. S. Lange35 , P. Larin11 , 13 M. Leyhe2 , C. H. Li1 , Cheng Li40 , Cui Li40 , D. M. Li46 , F. Li1 , G. Li1 , H. B. Li1 , J. C. Li1 , K. Li10 , Lei Li1 , Q. J. Li1 , 14 S. L. Li1 , W. D. Li1 , W. G. Li1 , X. L. Li28 , X. N. Li1 , X. Q. Li25 , X. R. Li27 , Z. B. Li32 , H. Liang40 , Y. F. Liang30 , 15 Y. T. Liang35 , G. R. Liao33 , X. T. Liao1 , D. Lin11 , B. J. Liu1 , C. L. Liu3 , C. X. Liu1 , F. H. Liu29 , Fang Liu1 , Feng Liu4 , 16 H. Liu1 , H. B. Liu9 , H. H. Liu13 , H. M. Liu1 , H. W. Liu1 , J. P. Liu44 , K. Liu33 , K. Y. Liu22 , Kai Liu36 , P. L. Liu28 , Q. Liu36 , 17 S. B. Liu40 , X. Liu21 , Y. B. Liu25 , Z. A. Liu1 , Zhiqiang Liu1 , Zhiqing Liu1 , H. Loehner20 , G. R. Lu12 , H. J. Lu14 , J. G. Lu1 , 18 Q. W. Lu29 , X. R. Lu36 , Y. P. Lu1 , C. L. Luo23 , M. X. Luo45 , T. Luo37 , X. L. Luo1 , M. Lv1 , C. L. Ma36 , F. C. Ma22 , 19 H. L. Ma1 , Q. M. Ma1 , S. Ma1 , T. Ma1 , X. Y. Ma1 , F. E. Maas11
, M. Maggiora43A,43C, Q. A. Malik42
, Y. J. Mao26 , 20 Z. P. Mao1 , J. G. Messchendorp20 , J. Min1 , T. J. Min1 , R. E. Mitchell16 , X. H. Mo1 , H. Moeini20 , C. Morales Morales11 , 21 K. Moriya16 , N. Yu. Muchnoi6 , H. Muramatsu39 , Y. Nefedov19 , C. Nicholson36 , I. B. Nikolaev6 , Z. Ning1 , S. L. Olsen27 , 22 Q. Ouyang1 , S. Pacetti17B, J. W. Park27 , M. Pelizaeus2 , H. P. Peng40 , K. Peters7 , J. L. Ping23 , R. G. Ping1 , R. Poling38 , 23 E. Prencipe18 , M. Qi24 , S. Qian1 , C. F. Qiao36 , L. Q. Qin28 , X. S. Qin1 , Y. Qin26 , Z. H. Qin1 , J. F. Qiu1 , K. H. Rashid42 , 24 G. Rong1 , X. D. Ruan9 , A. Sarantsev19,c, B. D. Schaefer16 , M. Shao40 , C. P. Shen37,d, X. Y. Shen1 , H. Y. Sheng1 , 25 M. R. Shepherd16 , W. M. Song1 , X. Y. Song1
, S. Spataro43A,43C, B. Spruck35
, D. H. Sun1 , G. X. Sun1 , J. F. Sun12 , S. S. Sun1 , 26 Y. J. Sun40 , Y. Z. Sun1 , Z. J. Sun1 , Z. T. Sun40 , C. J. Tang30 , X. Tang1 , I. Tapan34C, E. H. Thorndike39 , D. Toth38 , 27 M. Ullrich35 , I. Uman34B, G. S. Varner37 , B. Q. Wang26 , D. Wang26 , D. Y. Wang26 , K. Wang1 , L. L. Wang1 , L. S. Wang1 , 28 M. Wang28 , P. Wang1 , P. L. Wang1 , Q. J. Wang1 , S. G. Wang26 , X. F. Wang33 , X. L. Wang40 , Y. D. Wang17A, Y. F. Wang1 , 29 Y. Q. Wang18 , Z. Wang1 , Z. G. Wang1 , Z. Y. Wang1 , D. H. Wei8 , J. B. Wei26 , P. Weidenkaff18 , Q. G. Wen40 , S. P. Wen1 , 30 M. Werner35 , U. Wiedner2 , L. H. Wu1 , N. Wu1 , S. X. Wu40 , W. Wu25 , Z. Wu1 , L. G. Xia33 , Y. X Xia15 , Z. J. Xiao23 , 31 Y. G. Xie1 , Q. L. Xiu1 , G. F. Xu1 , G. M. Xu26 , Q. J. Xu10 , Q. N. Xu36 , X. P. Xu31 , Z. R. Xu40 , F. Xue4 , Z. Xue1 , L. Yan40 , 32 W. B. Yan40 , Y. H. Yan15 , H. X. Yang1 , Y. Yang4 , Y. X. Yang8 , H. Ye1 , M. Ye1 , M. H. Ye5 , B. X. Yu1 , C. X. Yu25 , 33 H. W. Yu26 , J. S. Yu21 , S. P. Yu28 , C. Z. Yuan1 , Y. Yuan1 , A. A. Zafar42 , A. Zallo17A, S. L. Zang24 , Y. Zeng15 , B. X. Zhang1 , 34 B. Y. Zhang1 , C. Zhang24 , C. C. Zhang1 , D. H. Zhang1 , H. H. Zhang32 , H. Y. Zhang1 , J. Q. Zhang1 , J. W. Zhang1 , 35 J. Y. Zhang1 , J. Z. Zhang1 , LiLi Zhang15 , R. Zhang36 , S. H. Zhang1 , X. J. Zhang1 , X. Y. Zhang28 , Y. Zhang1 , Y. H. Zhang1 , 36 Z. P. Zhang40 , Z. Y. Zhang44 , Zhenghao Zhang4 , G. Zhao1 , H. S. Zhao1 , J. W. Zhao1 , K. X. Zhao23 , Lei Zhao40 , Ling Zhao1 , 37 M. G. Zhao25 , Q. Zhao1 , S. J. Zhao46 , T. C. Zhao1 , X. H. Zhao24 , Y. B. Zhao1 , Z. G. Zhao40 , A. Zhemchugov19,a, B. Zheng41 , 38 J. P. Zheng1 , Y. H. Zheng36 , B. Zhong23 , L. Zhou1 , X. Zhou44 , X. K. Zhou36 , X. R. Zhou40 , C. Zhu1 , K. Zhu1 , K. J. Zhu1 , 39 S. H. Zhu1 , X. L. Zhu33 , Y. C. Zhu40 , Y. M. Zhu25 , Y. S. Zhu1 , Z. A. Zhu1 , J. Zhuang1 , B. S. Zou1 , J. H. Zou1 40 (BESIII Collaboration) 41 1
Institute of High Energy Physics, Beijing 100049, People’s Republic of China
42
2
Bochum Ruhr-University, D-44780 Bochum, Germany
43
3
Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
44
4
Central China Normal University, Wuhan 430079, People’s Republic of China
45
5
China Center of Advanced Science and Technology, Beijing 100190, People’s Republic of China
46
6
G.I. Budker Institute of Nuclear Physics SB RAS (BINP), Novosibirsk 630090, Russia
47
7
GSI Helmholtzcentre for Heavy Ion Research GmbH, D-64291 Darmstadt, Germany
48
8
Guangxi Normal University, Guilin 541004, People’s Republic of China
49
9
GuangXi University, Nanning 530004, People’s Republic of China
50
10
Hangzhou Normal University, Hangzhou 310036, People’s Republic of China
51
11
Helmholtz Institute Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz, Germany
52
12
Henan Normal University, Xinxiang 453007, People’s Republic of China
53
13
Henan University of Science and Technology, Luoyang 471003, People’s Republic of China
54
14
Huangshan College, Huangshan 245000, People’s Republic of China
55
15
Hunan University, Changsha 410082, People’s Republic of China
56
16
Indiana University, Bloomington, Indiana 47405, USA
57
17
(A)INFN Laboratori Nazionali di Frascati, I-00044, Frascati,
58
Italy; (B)INFN and University of Perugia, I-06100, Perugia, Italy
59
18
Johannes Gutenberg University of Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz, Germany
60
19
Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia
20
KVI, University of Groningen, NL-9747 AA Groningen, The Netherlands
62
21
Lanzhou University, Lanzhou 730000, People’s Republic of China
63
22
Liaoning University, Shenyang 110036, People’s Republic of China
64
23
Nanjing Normal University, Nanjing 210023, People’s Republic of China
65
24
Nanjing University, Nanjing 210093, People’s Republic of China
66
25
Nankai University, Tianjin 300071, People’s Republic of China
67
26
Peking University, Beijing 100871, People’s Republic of China
68
27
Seoul National University, Seoul, 151-747 Korea
69
28
Shandong University, Jinan 250100, People’s Republic of China
70
29
Shanxi University, Taiyuan 030006, People’s Republic of China
71
30
Sichuan University, Chengdu 610064, People’s Republic of China
72
31
Soochow University, Suzhou 215006, People’s Republic of China
73
32
Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China
74
33
Tsinghua University, Beijing 100084, People’s Republic of China
75
34
(A)Ankara University, Dogol Caddesi, 06100 Tandogan, Ankara, Turkey; (B)Dogus
76
University, 34722 Istanbul, Turkey; (C)Uludag University, 16059 Bursa, Turkey
77
35
Universitaet Giessen, D-35392 Giessen, Germany
78
36
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
79
37
University of Hawaii, Honolulu, Hawaii 96822, USA
80
38
University of Minnesota, Minneapolis, Minnesota 55455, USA
81
39
University of Rochester, Rochester, New York 14627, USA
82
40
University of Science and Technology of China, Hefei 230026, People’s Republic of China
83
41
University of South China, Hengyang 421001, People’s Republic of China
84
42
University of the Punjab, Lahore-54590, Pakistan
85
43
(A)University of Turin, I-10125, Turin, Italy; (B)University of Eastern
86
Piedmont, I-15121, Alessandria, Italy; (C)INFN, I-10125, Turin, Italy
87
44
Wuhan University, Wuhan 430072, People’s Republic of China
88
45
Zhejiang University, Hangzhou 310027, People’s Republic of China
89
46
Zhengzhou University, Zhengzhou 450001, People’s Republic of China
90
a Also at the Moscow Institute of Physics and Technology, Moscow 141700, Russia
91
b On leave from the Bogolyubov Institute for Theoretical Physics, Kiev 03680, Ukraine
92
c Also at the PNPI, Gatchina 188300, Russia
93
d Present address: Nagoya University, Nagoya 464-8601, Japan
94
(Dated: June 15, 2013)
95
The decay J/ψ → ωp¯p has been studied, using 225.3 × 106
J/ψ events accumulated at BESIII. No significant enhancement near the p¯p invariant-mass threshold (denoted as X(p¯p)) is observed. The upper limit of the branching fraction B(J/ψ → ωX(p¯p) → ωp¯p) is determined to be 3.9 × 10−6
at the 95% confidence level. The branching fraction of J/ψ → ωp¯p is measured to be B(J/ψ →
ωp¯p) = (9.0 ± 0.2 (stat.) ± 0.9 (syst.)) × 10−4.
PACS numbers: 13.25.Gv, 12.39.Mk, 13.75.Cs
96
I. INTRODUCTION
97
An anomalous enhancement near the threshold of the
98
p¯p system, namely X(p¯p), was first observed by the
BE-99
SII experiment in the radiative decay J/ψ → γp¯p [1],
100
and it was recently confirmed by the CLEO and BESIII
101
experiments [2–4]. In the BESII experiment, its mass is
102 measured to be 1859+3−10 (stat.) +5 −25 (syst.) MeV/c 2 and 103
the total width is Γ < 30 MeV/c2 at the 90%
confi-104
dence level (C.L.). While in the BESIII experiment, a
105
partial wave analysis (PWA) with a correction for the
106
final-state interaction (FSI) is performed, and the
spin-107
parity of X(p¯p) is determined to be 0−+, its mass is
108
1832+19−5 (stat.) +18
−17 (syst.) MeV/c
2 and the total width
109
is Γ < 76 MeV/c2 at the 90% C.L. [3].
110
The discovery of X(p¯p) stimulated a number of
the-111
oretical interpretations and experimental studies [5–16].
112
There is no experimental evidence of such an
enhance-113
ment in other quarkonium decays, e.g. J/ψ → π0p¯p [1]
114
or Υ(2S) → γp¯p [5]. In ψ(2S) → γp¯p, the recent BESIII
115
measurement shows a relative production rate to that of
116
J/ψ decays of R = 5.08% [3]. A number of theoretical
117
speculations have been proposed to interpret the nature
118
of this structure, including baryonium [9–11], a
multi-119
quark state [12] or mainly a pure FSI [13, 14]. It was
120
proposed to associate this enhancement with a broad
en-121
hancement observed in B meson decays [17, 18] or a new
122 resonance X(1835) in J/ψ → γπ+π− η′ decay at BE-123 SII [19]. 124
The investigation of the near-threshold p¯p invariant
125
mass spectrum in other J/ψ decay modes will be helpful
126
in understanding the nature of the observed structure.
127
The decay J/ψ → ωp¯p restricts the isospin of the p¯p
128
system, and it is helpful to clarify the role of the p¯p
129
FSI. The BESII collaboration studied J/ψ → ωp¯p via ω
130
decaying to π0π+π−
with a data sample of 5.8 × 107J/ψ
events [6]. No significant signal near the threshold of the
132
p¯p invariant-mass spectrum was observed and an upper
133
limit on the branching fraction of J/ψ → ωX(p¯p) → ωp¯p
134
was determined to be 1.5 × 10−5at the 90% C.L., which
135
disfavored the interpretation of a pure FSI effect giving
136
rise to the X(p¯p). In this paper, the analysis of J/ψ →
137
ωp¯p via the decay channel ω → γπ0 is presented, based
138
on a data sample of (225.3 ± 2.8) × 106 J/ψ events [20]
139
accumulated with the BESIII detector. Searching for the
140
X(p¯p) in the decay mode J/ψ → ωp¯p → γπ0p¯p has a
141
particular advantage: a low irreducible background from
142
N∗
is expected. The channel J/ψ → ωp¯p → πππ0p¯p has
143
irreducible background from various N∗
decays and ∆
144
decays, where interferences may have a large impact on
145
the uncertainty of the measurements.
146
BESIII/BEPCII [21] is a major upgrade of the BESII
147
experiment at the BEPC accelerator [22] for studies of
148
hadron spectroscopy and τ -charm physics [23]. The
de-149
sign peak luminosity of the double-ring e+e−
collider,
150
BEPCII, is 1033cm−2s−1at beam currents of 0.93 A. The 151
BESIII detector with a geometrical acceptance of 93%
152
of 4π, consists of the following main components: 1) a
153
small-celled, helium-based main drift chamber (MDC)
154
with 43 layers. The average single wire resolution is
155
135 µm, and the momentum resolution for 1 GeV/c2
156
charged particles in a 1 T magnetic field is 0.5%; 2)
157
an electromagnetic calorimeter (EMC) made of 6240
158
CsI (Tl) crystals arranged in a cylindrical shape (barrel)
159
plus two end-caps. For 1.0 GeV photons, the energy
res-160
olution is 2.5% in the barrel and 5% in the end-caps, and
161
the position resolution is 6 mm in the barrel and 9 mm in
162
the end-caps; 3) a Time-Of-Flight system (TOF) for
par-163
ticle identification (PID) composed of a barrel part made
164
of two layers with 88 pieces of 5 cm thick, 2.4 m long
plas-165
tic scintillators in each layer, and two end-caps with 48
166
fan-shaped, 5 cm thick, plastic scintillators in each
end-167
cap. The time resolution is 80 ps in the barrel, and 110
168
ps in the end-caps, corresponding to a K/π separation
169
by more than 2σ for momenta below about 1 GeV/c2;
170
4) a muon chamber system (MUC) made of 1000 m2 of
171
Resistive Plate Chambers (RPC) arranged in 9 layers in
172
the barrel and 8 layers in the end-caps and incorporated
173
in the return iron yoke of the superconducting magnet.
174
The position resolution is about 2 cm.
175
The optimization of the event selection and the
es-176
timate of physics backgrounds are performed through
177
Monte Carlo (MC) simulations. The GEANT4-based
178
simulation software BOOST [24] includes the geometric
179
and material description of the BESIII detectors and the
180
detector response and digitization models, as well as the
181
tracking of the detector running conditions and
perfor-182
mance. The production of the J/ψ resonance is
simu-183
lated by the MC event generator KKMC [25], while the
184
decays are generated by EVTGEN [26] for known
de-185
cay modes with branching ratios being set to PDG [27]
186
world average values, and by LUNDCHARM [28] for the
187
remaining unknown decays. The analysis is performed in
188
the framework of the BESIII offline software system [29]
189
which takes care of the detector calibration, event
recon-190
struction and data storage.
191
II. EVENT SELECTION
192
Signal J/ψ → ωp¯p events with ω → γπ0 final states
193
have the topology γγγp¯p. The event candidates are
re-194
quired to have two well reconstructed charged tracks with
195
net charge zero, and at least three photons.
196
Charged-particle tracks in the polar angle range
197
| cos θ| < 0.93 are reconstructed from the MDC hits, only
198
tracks in barrel region (| cos θ| < 0.8) are used to reduce
199
systematic uncertainties in tracking and particle
identi-200
fication. Tracks with their points of closest approach to
201
the beamline within ±10 cm of the interaction point in
202
the beam direction, and within 1 cm in the plane
perpen-203
dicular to the beam are selected. TOF and dE/dx
infor-204
mation are combined to determine particle identification
205
confidence levels for π, K and p(¯p) hypotheses; and the
206
particle type with highest confidence level is assigned to
207
each track. A proton and an anti-proton are required.
208
To reduce the systematic error due to differences of the
209
tracking efficiency at low momentum between data and
210
MC, the momentum of the proton or anti-proton is
fur-211
ther required to be larger than 300 MeV/c.
212
Photon candidates are reconstructed by clustering
sig-213
nals in EMC crystals. The photon candidates are
re-214
quired to be in the barrel region (| cos θ| < 0.8) of the
215
EMC with at least 25 MeV energy deposition, or in the
216
end-caps region (0.86 < | cos θ| < 0.92) with at least
217
50 MeV energy deposition, where θ is the polar angle of
218
the shower. Timing information from the EMC is used to
219
suppress electronic noise and energy depositions that are
220
unrelated to the event. To suppress showers generated by
221
charged particles, the photon candidates are furthermore
222
required to be separated by an angle larger than 10◦
and
223
larger than 30◦
from the proton and anti-proton,
respec-224
tively.
225
A four-constraint (4C) energy-momentum conserving
226
kinematic fit is performed to the γγγp¯p hypothesis. For
227
events with more than three photon candidates, the
com-228
bination with the minimum χ2
4Cis selected, and χ24C< 30 229
is required. The π0candidates are reconstructed from the
230
two of the three selected photons with an invariant mass
231
closest to the π0 mass, and |M
γγ− Mπ0| < 15 MeV/c2is 232
required.
233
III. BRANCHING FRACTION AND YIELD
234
MEASUREMENTS
235
236
Figure 1 shows the γπ0 invariant mass spectrum for
237
candidate J/ψ → γπ0p¯p events, where a distinctive ω
238
signal is seen. An unbinned maximum likelihood fit is
239
performed to the γπ0 invariant mass with the ω signal
) 2 ) (GeV/c 0 π γ M( 0.7 0.75 0.8 0.85 0.9 ) 2 Events/ (0.003 GeV/c 0 50 100 150 200 250 300 350 400 450 ) 2 ) (GeV/c 0 π γ M( 0.7 0.75 0.8 0.85 0.9 ) 2 Events/ (0.003 GeV/c 0 50 100 150 200 250 300 350 400 450 FIG. 1. γπ0
invariant mass distribution of J/ψ → γπ0
p¯p
candidates. The dashed line is the signal shape which is
parametrized by a Breit-Wigner function convoluted with the detector resolution described by the Novosibirsk func-tion; the dashed-dotted line is the background shape which is described by a second order Chebychev polynomial; and the solid line is the total contribution of the two compo-nents. The solid arrows indicate the ω signal region (0.753 < M (γπ0
) < 0.813 GeV/c2
) and the two pairs of dashed arrows indicate the ω sidebands (0.663 < M (γπ0
) < 0.693 GeV/c2 and 0.873 < M (γπ0
) < 0.903 GeV/c2 ).
parametrized by a Breit-Wigner function convoluted with
241
the Novosibirsk function [30] which describes the
detec-242
tor resolution. The background shape is described by a
243
second-order Chebychev polynomial function. The mass
244
and width of the ω peak are fixed to the values published
245
by the Particle Data Group (PDG) [27], and the yield of
246
the ω signal obtained from the fit is Nobs= 2670 ± 69.
247
The branching fraction of J/ψ → ωp¯p is calculated
248 according to : 249 B(J/ψ → ωp¯p) = Nobs NJ/ψ× B(ω → γπ0) × B(π0→ γγ) × εrec . (1)
where Nobs is the number of signal events determined
250
from the fit to the γπ0invariant mass; N
J/ψ is the
num-251
ber of J/ψ events [20]; B(ω → γπ0) and B(π0 → γγ)
252
are branching fractions of ω → γπ0 and π0 → γγ,
re-253
spectively, as from the PDG [27]; and the detection
effi-254
ciency εrec is (16.1 ± 1.7)% obtained from a MC sample
255
for J/ψ → ωp¯p events generated according to a
phase-256
space distribution. The measured branching fraction is
257
B(J/ψ → ωp¯p) = (9.0 ± 0.2 (stat.)) × 10−4.
258
Candidate J/ψ → ωp¯p events are selected with the
259
mass window requirement 0.753 GeV/c2 < M (γπ0) <
260
0.813 GeV/c2, and the Dalitz plot of these events is shown
261
in Fig. 2. There are no obvious structures in the Dalitz
262
plot, though the distribution is different from the pure
263
ωp¯p phase space distribution. The corresponding p¯p,
264
ωp and ω ¯p invariant-mass spectra are also presented in
265
Fig. 2. The data points with error bars are from signal
266
region and the hatched area are from the sideband region.
267
the mass threshold is shown in Fig. 3.
268
To obtain the number of J/ψ → ωX(p¯p) → ωp¯p
269
events, an unbinned maximum likelihood fit is performed
270
to the p¯p invariant mass around the mass threshold. In
271
the fit, the spin-parity of X(p¯p) is assumed to be 0−
,
272
and the signal of X(p¯p) in the J/ψ → ωX(p¯p) → ωp¯p
273
decay is parametrized by an acceptance-weighted S-wave
274 Breit-Wigner function : 275 BW (M ) ≃ q 2L+1k3 (M2− M2 0)2+ M02Γ2 × εrec(M ) . (2)
Here, q is the momentum of the proton in the p¯p rest
276
frame; k is the the momentum of the ω meson; L = 0
277
is the relative orbital angular momentum; M is the
in-278
variant mass of p¯p; M0 and Γ are the mass and width
279
of the X(p¯p), respectively, which are taken from
BESI-280
II results [3]; εrec is the detection efficiency. The
non-281
ω background is presented by a function of the form
282
f (δ) = N (δ1/2+ a
1δ3/2+ a2δ5/2) with δ = Mp ¯p− 2mp 283
where mp is the proton mass. The normalization and
284
shape parameters a1and a2are determined by a
simulta-285
neous fit to the M (p¯p) in ω signal region and ω sideband
286
region 0.09 GeV/c2 < |M (γπ0) − 0.783| < 0.12 GeV/c2. 287
The non-resonant J/ψ → ωp¯p events are also described
288
by the function f (δ), where the normalization and shape
289
parameters are allowed to float. The fit results are shown
290
in Fig. 3, and the number of X(p¯p) events is 0 ± 1.6.
291
A Bayesian approach [27] estimate the upper limit of
292
B(J/ψ → ωX(p¯p) → ωp¯p), and Nobs < 9 at 95% C.
293
L. is determined by finding the value NUP
obs with 294 RNUP obs 0 LdNobs R∞ 0 LdNobs = 0.95, (3)
where Nobs is the number of signal events, and L is the
295
value of the likelihood function with the Nobsvalue fixed
296
in the fit. The upper limit on the product of branching
297
fractions is calculated with
298 B(J/ψ → ωX(p¯p) → ωp¯p) < N UL obs NJ/ψ× (1 − σsys.) × B(ω → γπ0) × B(π0→ γγ) × εrec , (4)
) 4 /c 2 ) (GeV p ω ( 2 M 3 3.5 4 4.5 ) 4 /c 2 p) (GeV ω ( 2 M 3 3.5 4 4.5 ) 2 ) (GeV/c p M(p 1.9 2 2.1 2.2 2.3 ) 2 Events / (0.01 GeV/c 0 50 100 150 ) 2 p) (GeV/c ω M( 1.7 1.8 1.9 2 2.1 ) 2 Events / (0.01 GeV/c 0 50 100 150 ) 2 ) (GeV/c p ω M( 1.7 1.8 1.9 2 2.1 ) 2 Events / (0.01 GeV/c 0 50 100 150
FIG. 2. Dalitz plot and p¯p, ωp, ω ¯p invariant-mass spectra of J/ψ → ωp¯p candidates. The data points with error bars are from signal region and the hatched areas are from the sideband region.
where σsys.is the total systematic uncertainty which will
299
be described in the next section. The upper limit on the
300
product of branching fractions is B(J/ψ → ωX(p¯p) →
301
ωp¯p) < 3.9 × 10−6 at the 95% C.L.. 302
An alternative fit with a Breit-Wigner function
includ-303
ing the J¨ulich FSI
304 BW (M ) ≃ fFSI× q 2L+1k3 (M2− M2 0)2+ M02Γ2 × εrec(M ), (5)
for X(p¯p) is performed. Here, fFSI is the J¨ulich FSI cor-305
rection factor [14]. The mass and width of X(p¯p) are
306
taken from the previous BESIII PWA results [3]. The
307
upper limit on the product of branching fractions is
de-308
termined to be B(J/ψ → ωX(p¯p) → ωp¯p) < 3.7 × 10−6
309
at the 95% C.L..
310
IV. SYSTEMATIC UNCERTAINTIES
311
Several sources of systematic uncertainties are
con-312
sidered in the measurement of the branching fractions.
313
These include differences between data and the MC
sim-314
ulation for the tracking algorithm, the PID, photon
de-315
tection, the kinematic fit, as well as the fitting procedure,
316
the branching fraction of the intermediate states and the
317
total number of J/ψ events.
318
The systematic uncertainties associated with the
track-319
ing efficiency and PID efficiency have been studied with
320
J/ψ → p¯pπ+π−
using a technique similar to that
dis-321
cussed in Ref. [31]. The difference of tracking efficiencies
322
between data and MC simulation is 2% per charged track.
323
The systematic uncertainty from PID is 2% per proton
324
(anti-proton).
325
The photon detection systematic uncertainty is studied
326
by comparing the photon efficiency between MC
simula-327
tion and the control sample J/ψ → ρπ. The relative
328
efficiency difference is about 1% for each photon [32, 33].
) 2 (GeV/c p )-2m p M(p 0 0.05 0.1 0.15 0.2 ) 2 Events/ (0.01 GeV/c 0 10 20 30 40 50 60 70 80 total fit data ψ J/ )) p X(p ω → ψ (J/ obs UP 5xN non-resonant contribution sideband fit ω sideband data ω
FIG. 3. Near-threshold p¯p invariant-mass spectrum. The
sig-nal J/ψ → ωX(p¯p) → ωp¯p is described by an
acceptance-weighted Breit-Wigner function, and and signal yield is con-sistent with zero. The dotted line is the shape of the signal which is normalized to five times the estimated upper limit. The dashed line is the non-resonant contribution described by the function f (δ) and the dashed-dotted line is the non ωp¯p contribution which is estimated from ω sidebands. The sol-id line is the total contribution of the two components. The hatched area is from the sideband region.
Here, 3% is taken as the systematic error for the
efficien-330
cy of detecting three photons. The uncertainty due to π0
331
reconstruction efficiency is taken as 1% [32, 33].
332
To estimate the uncertainty associated with the
kine-333
matic fit, selected samples of J/ψ → Σ+Σ¯−
→ pπ0pπ¯ 0 334
events are used. The kinematic fit efficiency is defined as
335
the ratio between the signal yield of Σ+ with or without
336
the kinematic fit. The difference of kinematic fit
effi-337
ciency between data and MC is 3%, and is taken as the
338
systematic uncertainty caused by the kinematic fit.
339
As described above, the yield of J/ψ → ωp¯p is
de-340
rived from a fit to the invariant-mass spectrum of γπ0
341
pairs. To evaluate the systematic uncertainty
associ-342
ated with the fitting procedure, the following two
as-343
pects are studied (i) Fitting region: In the nominal fit,
344
the mass spectrum of γπ0 is fitted in the range from
345
0.663 GeV/c2 to 0.903 GeV/c2. Alternative fits within
346
ranges 0.653 GeV/c2 to 0.913 GeV/c2 and 0.673 GeV/c2
347
to 0.893 GeV/c2 are performed, and the difference in
348
the signal yield of 2% is taken as the systematic
un-349
certainty associated with the fit interval. (ii)
Back-350
ground shape: To estimate the uncertainty due to the
351
background parametrization for the branching fraction
352
B(J/ψ → ωp¯p), a first or third order instead of a
second-353
order Chebychev polynomial is used in the fitting. The
354
difference of 1.2% is used as an estimate of the systematic
355
uncertainty.
356
For the upper limit on the branching fraction B(J/ψ →
357
ωX(p¯p) → ωp¯p), the systematic uncertainty
associat-358
ed with the fitting procedure is estimated by fixing
359
the shape of the non-resonant contribution to a phase
360
space MC simulation of J/ψ → ωp¯p, which is
pre-361 ) 2 (GeV/c p )-2m p M(p 0 0.05 0.1 0.15 0.2 ) 2 Events/ (0.01 GeV/c 0 10 20 30 40 50 60 70 80 90 total fit data ψ J/ )) p X(p ω → ψ (J/ obs UP 5xN non-resonant contribution sideband fit ω phsp mc p p ω → ψ J/
FIG. 4. Near-threshold p¯p invariant-mass spectrum. The
sig-nal J/ψ → ωX(p¯p) → ωp¯p is described by an
acceptance-weighted Breit-Wigner function, and and signal yield is con-sistent with zero. The dashed line is the non-resonant
contri-bution fixed to a phase space MC simulation of J/ψ → ωp¯p
and the dashed-dotted line is the non ωp¯p contribution which is estimated from ω sidebands. The solid line is the total con-tribution of the two components. The hatched area is from a
phase space MC simulation of J/ψ → ωp¯p.
sented by Figure. 4; enlarging/reducing the
normaliza-362
tion of the non-ω contribution by 7% (the difference of
363
the estimation of non-ω background level between
da-364
ta and inclusive MC); and varying the sideband region
365
to 0.095 GeV/c2 < |M (γπ0) − 0.783| < 0.115 GeV/c2 366
and 0.085 GeV/c2 < |M (γπ0) − 0.783| < 0.125 GeV/c2. 367
When fitting with or without the FSI effect, the signal
368
yields for the alternative fits are lower or equal to the
369
nominal fit, therefore the conservative upper limit from
370
the fit without FSI correction is reported.
371
Various distributions obtained with data and the
372
phase-space MC sample have been compared and some
373
discrepancies are observed. To determine the
systemat-374
ic error on the detection efficiency associated with these
375
discrepancies, an alternative detection efficiency is
esti-376
mated by the re-weighting phase-space MC samples. The
377
difference in detection efficiency compared to the
nomi-378
nal one is 7% and taken as a systematic uncertainty. The
379
number of J/ψ events is determined from an inclusive
380
analysis of J/ψ hadronic events and an uncertainty of
381
1.24% is associated to it [20]. The uncertainties due to
382
the branching fractions of ω → γπ0 and π0 → γγ are
383
taken from the PDG [27].
384
TABLE I. Summary of systematic uncertainties. ’-’ means the corresponding systematic uncertainty is negligible.
Upper limit of Upper limit of
Source B(J/ψ → ωp¯p) B(J/ψ → ωX(p¯p) → ωp¯p) B(J/ψ → ωX(p¯p) → ωp¯p) with FSI
Tracking 4% 4% 4% PID 4% 4% 4% Photon 3% 3% 3% Kinematic Fit 3% 3% 3% π0 reconstruction 1% 1% 1% Fitting region 2% − − Background Shape 1% − −
Branching fraction of intermediate state 3% 3% 3%
Total J/ψ numbers 1.24% 1.24% 1.24%
MC Generator 7% − −
Total uncertainty 10.3% 7.8% 7.8%
V. SUMMARY
386
In summary, using (225.3 ± 2.8) × 106 J/ψ events
col-387
lected with the BESIII detector, the decay of J/ψ →
388
ωp¯p in the decay mode ω → γπ0 is studied. The
389
branching fraction B(J/ψ → ωp¯p) is measured to be
390
(9.0 ± 0.2 (stat.) ± 0.9 (syst.)) × 10−4. No obvious
en-391
hancement around the p¯p invariant-mass threshold is
ob-392
served. At the 95% C.L., the upper limits on the
prod-393
uct of branching fractions B(J/ψ → ωX(p¯p) → ωp¯p) are
394
measured to be 3.7 × 10−6and 3.9 × 10−6with and
with-395
out accounting for the J¨ulich FSI effect, respectively. As
396
isospin for J/ψ → γp¯p and ωp¯p should both favor I = 0
397
(I = 1 should be suppressed in J/ψ → γp¯p as in
oth-398
er J/ψ radiative decays), the non-observation of X(p¯p)
399
in ωp¯p disfavors the pure FSI interpretation for the p¯p
400
threshold enhancement in the decay J/ψ → γp¯p.
401
VI. ACKNOWLEDGMENT
402
The BESIII collaboration thanks the staff of BEPCII
403
and the computing center for their hard efforts. This
404
work is supported in part by the Ministry of Science and
405
Technology of China under Contract No. 2009CB825200;
406
National Natural Science Foundation of China (NSFC)
407
under Contracts Nos. 10625524, 10821063, 10825524,
408
10835001, 10935007, 11125525, 11235011; Joint Funds
409
of the National Natural Science Foundation of China
410
under Contracts Nos. 11079008, 11179007; the
Chi-411
nese Academy of Sciences (CAS) Large-Scale Scientific
412
Facility Program; CAS under Contracts Nos.
KJCX2-413
YW-N29, KJCX2-YW-N45; 100 Talents Program of
414
CAS; German Research Foundation DFG under
Con-415
tract No. Collaborative Research Center CRC-1044;
Is-416
tituto Nazionale di Fisica Nucleare, Italy; Ministry of
417
Development of Turkey under Contract No.
DPT2006K-418
120470; U. S. Department of Energy under Contracts
419
Nos. FG02-04ER41291, FG02-05ER41374,
DE-420
FG02-94ER40823; U.S. National Science Foundation;
421
University of Groningen (RuG) and the
Helmholtzzen-422
trum fuer Schwerionenforschung GmbH (GSI),
Darm-423
stadt; WCU Program of National Research Foundation
424
of Korea under Contract No. R32-2008-000-10155-0.
425
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