arXiv:1308.2760v2 [hep-ex] 19 Feb 2014
Observation of a charged charmoniumlike structure in e
+e
−→ (D
∗D
¯
∗)
±π
∓at
√
s
= 4.26 GeV
M. Ablikim1, M. N. Achasov7,a, O. Albayrak4, D. J. Ambrose40, F. F. An1, Q. An41, J. Z. Bai1, R. Baldini Ferroli18A, Y. Ban27, J. Becker3, J. V. Bennett17, M. Bertani18A, J. M. Bian39, E. Boger20,b, O. Bondarenko21, I. Boyko20, S. Braun36,
R. A. Briere4, V. Bytev20, H. Cai45, X. Cai1, O. Cakir35A, A. Calcaterra18A, G. F. Cao1, S. A. Cetin35B, J. F. Chang1, G. Chelkov20,b, G. Chen1, H. S. Chen1, J. C. Chen1, M. L. Chen1, S. J. Chen25, X. R. Chen22, Y. B. Chen1, H. P. Cheng15,
Y. P. Chu1, D. Cronin-Hennessy39, H. L. Dai1, J. P. Dai1, D. Dedovich20, Z. Y. Deng1, A. Denig19, I. Denysenko20, M. Destefanis44A,44C, W. M. Ding29, Y. Ding23, L. Y. Dong1, M. Y. Dong1, S. X. Du47, J. Fang1, S. S. Fang1, L. Fava44B,44C,
C. Q. Feng41, P. Friedel3, C. D. Fu1, J. L. Fu25, O. Fuks20,b, Y. Gao34, C. Geng41, K. Goetzen8, W. X. Gong1, W. Gradl19, M. Greco44A,44C, M. H. Gu1, Y. T. Gu10, Y. H. Guan1,37, A. Q. Guo26, L. B. Guo24, T. Guo24, Y. P. Guo26, Y. L. Han1, F. A. Harris38, K. L. He1, M. He1, Z. Y. He26, T. Held3, Y. K. Heng1, Z. L. Hou1, C. Hu24, H. M. Hu1, J. F. Hu36, T. Hu1,
G. M. Huang5, G. S. Huang41, J. S. Huang13, L. Huang1, X. T. Huang29, Y. Huang25, T. Hussain43, C. S. Ji41, Q. Ji1, Q. P. Ji26, X. B. Ji1, X. L. Ji1, L. L. Jiang1, X. S. Jiang1, J. B. Jiao29, Z. Jiao15, D. P. Jin1, S. Jin1, F. F. Jing34, N. Kalantar-Nayestanaki21, M. Kavatsyuk21, B. Kloss19, B. Kopf3, M. Kornicer38, W. Kuehn36, W. Lai1, J. S. Lange36, M. Lara17, P. Larin12, M. Leyhe3, C. H. Li1, Cheng Li41, Cui Li41, D. M. Li47, F. Li1, G. Li1, H. B. Li1, J. C. Li1, K. Li11,
Lei Li1, P. R. Li37, Q. J. Li1, W. D. Li1, W. G. Li1, X. L. Li29, X. N. Li1, X. Q. Li26, X. R. Li28, Z. B. Li33, H. Liang41, Y. F. Liang31, Y. T. Liang36, G. R. Liao34, X. T. Liao1, D. X. Lin12, B. J. Liu1, C. L. Liu4, C. X. Liu1, F. H. Liu30,
Fang Liu1, Feng Liu5, H. Liu1, H. B. Liu10, H. H. Liu14, H. M. Liu1, H. W. Liu1, J. P. Liu45, K. Liu34, K. Y. Liu23, L. D. Liu27, P. L. Liu29, Q. Liu37, S. B. Liu41, X. Liu22, Y. B. Liu26, Z. A. Liu1, Zhiqiang Liu1, Zhiqing Liu1, H. Loehner21,
X. C. Lou1,c, G. R. Lu13, H. J. Lu15, J. G. Lu1, X. R. Lu37, Y. P. Lu1, C. L. Luo24, M. X. Luo46, T. Luo38, X. L. Luo1, M. Lv1, F. C. Ma23, H. L. Ma1, Q. M. Ma1, S. Ma1, T. Ma1, X. Y. Ma1, F. E. Maas12, M. Maggiora44A,44C, Q. A. Malik43,
Y. J. Mao27, Z. P. Mao1, J. G. Messchendorp21, J. Min1, T. J. Min1, R. E. Mitchell17, X. H. Mo1, H. Moeini21, C. Morales Morales12, K. Moriya17, N. Yu. Muchnoi7,a, H. Muramatsu40, Y. Nefedov20, I. B. Nikolaev7,a, Z. Ning1, S. L. Olsen28, Q. Ouyang1, S. Pacetti18B, J. W. Park38, M. Pelizaeus3, H. P. Peng41, K. Peters8, J. L. Ping24, R. G. Ping1, R. Poling39, E. Prencipe19, M. Qi25, S. Qian1, C. F. Qiao37, L. Q. Qin29, X. S. Qin1, Y. Qin27, Z. H. Qin1, J. F. Qiu1, K. H. Rashid43,
C. F. Redmer19, G. Rong1, X. D. Ruan10, A. Sarantsev20,d, M. Shao41, C. P. Shen2, X. Y. Shen1, H. Y. Sheng1, M. R. Shepherd17, W. M. Song1, X. Y. Song1, S. Spataro44A,44C, B. Spruck36, D. H. Sun1, G. X. Sun1, J. F. Sun13, S. S. Sun1, Y. J. Sun41, Y. Z. Sun1, Z. J. Sun1, Z. T. Sun41, C. J. Tang31, X. Tang1, I. Tapan35C, E. H. Thorndike40,
D. Toth39, M. Ullrich36, I. Uman35B, G. S. Varner38, B. Wang1, D. Wang27, D. Y. Wang27, K. Wang1, L. L. Wang1, L. S. Wang1, M. Wang29, P. Wang1, P. L. Wang1, Q. J. Wang1, S. G. Wang27, X. F. Wang34, X. L. Wang41, Y. D. Wang18A,
Y. F. Wang1, Y. Q. Wang19, Z. Wang1, Z. G. Wang1, Z. Y. Wang1, D. H. Wei9, J. B. Wei27, P. Weidenkaff19, Q. G. Wen41, S. P. Wen1, M. Werner36, U. Wiedner3, L. H. Wu1, N. Wu1, S. X. Wu41, W. Wu26, Z. Wu1, L. G. Xia34, Y. X Xia16,
Z. J. Xiao24, Y. G. Xie1, Q. L. Xiu1, G. F. Xu1, Q. J. Xu11, Q. N. Xu37, X. P. Xu32, Z. R. Xu41, Z. Xue1, L. Yan41, W. B. Yan41, Y. H. Yan16, H. X. Yang1, Y. Yang5, Y. X. Yang9, H. Ye1, M. Ye1, M. H. Ye6, B. X. Yu1, C. X. Yu26, H. W. Yu27, J. S. Yu22, S. P. Yu29, C. Z. Yuan1, Y. Yuan1, A. A. Zafar43, A. Zallo18A, S. L. Zang25, Y. Zeng16, B. X. Zhang1,
B. Y. Zhang1, C. Zhang25, C. C. Zhang1, D. H. Zhang1, H. H. Zhang33, H. Y. Zhang1, L. Zhang1, J. Q. Zhang1, J. W. Zhang1, J. Y. Zhang1, J. Z. Zhang1, R. Zhang37, S. H. Zhang1, X. J. Zhang1, X. Y. Zhang29, Y. Zhang1, Y. H. Zhang1,
Z. H. Zhang5, Z. P. Zhang41, Z. Y. Zhang45, G. Zhao1, H. S. Zhao1, J. W. Zhao1, Lei Zhao41, Ling Zhao1, M. G. Zhao26, Q. Zhao1, S. J. Zhao47, T. C. Zhao1, X. H. Zhao25, Y. B. Zhao1, Z. G. Zhao41, A. Zhemchugov20,b, B. Zheng42, J. P. Zheng1,
Y. H. Zheng37, B. Zhong24, L. Zhou1, X. Zhou45, X. K. Zhou37, X. R. Zhou41, C. Zhu1, K. Zhu1, K. J. Zhu1, S. H. Zhu1, X. L. Zhu34, Y. C. Zhu41, Y. S. Zhu1, Z. A. Zhu1, J. Zhuang1, B. S. Zou1, J. H. Zou1
(BESIII Collaboration)
1 Institute of High Energy Physics, Beijing 100049, People’s Republic of China 2 Beihang University, Beijing 100191, People’s Republic of China
3 Bochum Ruhr-University, D-44780 Bochum, Germany 4 Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA 5 Central China Normal University, Wuhan 430079, People’s Republic of China
6 China Center of Advanced Science and Technology, Beijing 100190, People’s Republic of China 7 G.I. Budker Institute of Nuclear Physics SB RAS (BINP), Novosibirsk 630090, Russia
8 GSI Helmholtzcentre for Heavy Ion Research GmbH, D-64291 Darmstadt, Germany 9 Guangxi Normal University, Guilin 541004, People’s Republic of China
10 GuangXi University, Nanning 530004, People’s Republic of China 11 Hangzhou Normal University, Hangzhou 310036, People’s Republic of China 12 Helmholtz Institute Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz, Germany
13 Henan Normal University, Xinxiang 453007, People’s Republic of China
14 Henan University of Science and Technology, Luoyang 471003, People’s Republic of China 15Huangshan College, Huangshan 245000, People’s Republic of China
16Hunan University, Changsha 410082, People’s Republic of China 17 Indiana University, Bloomington, Indiana 47405, USA
Italy
19Johannes Gutenberg University of Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz, Germany 20 Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia
21 KVI, University of Groningen, NL-9747 AA Groningen, The Netherlands 22Lanzhou University, Lanzhou 730000, People’s Republic of China 23Liaoning University, Shenyang 110036, People’s Republic of China 24 Nanjing Normal University, Nanjing 210023, People’s Republic of China
25 Nanjing University, Nanjing 210093, People’s Republic of China 26 Nankai university, Tianjin 300071, People’s Republic of China 27 Peking University, Beijing 100871, People’s Republic of China
28Seoul National University, Seoul, 151-747 Korea 29Shandong University, Jinan 250100, People’s Republic of China 30 Shanxi University, Taiyuan 030006, People’s Republic of China 31 Sichuan University, Chengdu 610064, People’s Republic of China
32 Soochow University, Suzhou 215006, People’s Republic of China 33Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China
34Tsinghua University, Beijing 100084, People’s Republic of China
35 (A)Ankara University, Dogol Caddesi, 06100 Tandogan, Ankara, Turkey; (B)Dogus University, 34722 Istanbul, Turkey; (C)Uludag University, 16059 Bursa, Turkey
36 Universitaet Giessen, D-35392 Giessen, Germany
37 University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China 38 University of Hawaii, Honolulu, Hawaii 96822, USA
39University of Minnesota, Minneapolis, Minnesota 55455, USA 40University of Rochester, Rochester, New York 14627, USA
41 University of Science and Technology of China, Hefei 230026, People’s Republic of China 42 University of South China, Hengyang 421001, People’s Republic of China
43 University of the Punjab, Lahore-54590, Pakistan
44 (A)University of Turin, I-10125, Turin, Italy; (B)University of Eastern Piedmont, I-15121, Alessandria, Italy; (C)INFN, I-10125, Turin, Italy
45Wuhan University, Wuhan 430072, People’s Republic of China 46Zhejiang University, Hangzhou 310027, People’s Republic of China 47Zhengzhou University, Zhengzhou 450001, People’s Republic of China
a Also at the Novosibirsk State University, Novosibirsk, 630090, Russia b Also at the Moscow Institute of Physics and Technology, Moscow 141700, Russia
c Also at University of Texas at Dallas, Richardson, Texas 75083, USA dAlso at the PNPI, Gatchina 188300, Russia
We study the process e+e− → (D∗D¯∗)±π∓ at a center-of-mass energy of 4.26 GeV using a 827 pb−1 data sample obtained with the BESIII detector at the Beijing Electron Positron Collider. Based on a partial reconstruction technique, the Born cross section is measured to be (137±9±15) pb. We observe a structure near the (D∗D¯∗)±threshold in the π∓recoil mass spectrum, which we denote as the Z±
c (4025). The measured mass and width of the structure are (4026.3 ± 2.6 ± 3.7) MeV/c2 and (24.8 ± 5.6 ± 7.7) MeV, respectively. Its production ratio σ(e+e−→Zc±(4025)π
∓→(D∗D¯∗)±π∓) σ(e+e−→(D∗D¯∗)±π∓) is determined to be 0.65 ± 0.09 ± 0.06. The first uncertainties are statistical and the second are systematic.
PACS numbers: 14.40.Rt, 13.66.Bc, 13.25.Gv
Two charged bottomoniumlike particles, dubbed the Zb(10610) and Zb(10650), have been observed in the
π±Υ(nS) and π±h
b(mS) mass spectra at the Belle
ex-periment in the decays of Υ(10860) to π+π−Υ(nS) (n =
1, 2, 3) and to π+π−h
b(mP ) (m = 1, 2) [1]. Unlike a
conventional meson, the two states must involve at least four constituent quarks to produce a non-zero electric charge. The masses of the Zb(10610) and Zb(10650)
are close to the B ¯B∗ and B∗B¯∗ thresholds,
respective-ly, which supports a molecular interpretation of Zb’s as
B ¯B∗ and B∗B¯∗ bound states [2]. In addition, this
sce-nario is supported by the subsequent observations of the decays Zb(10610) → B ¯B∗ and Zb(10650) → B∗B¯∗ from
the Belle experiment [3].
A number of theoretical interpretations have been pro-posed to describe the nature of the Zb’s [4–7]. One
in-triguing suggestion is to look for corresponding particles in the charmonium sector [5]. As anticipated, a charged charmoniumlike structure, Zc(3900), was observed in the
π±J/ψ mass spectrum in e+e− → π+π−J/ψ by the
BESIII experiment [8], by the Belle experiment [9] and using data from the CLEO-c experiment [10]. More recently, BESIII has observed another charged state in the π±h
c mass spectrum in e+e− → π+π−hc, the
Zc(4020) [11]. The masses of these states are
3 Therefore, a search of Zc candidates via their direct
de-cays into D∗D¯∗ pairs is strongly motivated.
In this Letter, we report on a study of the process e+e− → (D∗D¯∗)±π∓ at a center-of-mass energy √s =
(4.260 ± 0.001) GeV, where (D∗D¯∗)± refers to the sum
of the D∗+D¯∗0 and its charge conjugate D∗−D∗0 final
states. In the following, we use the notation of D∗+D¯∗0
and the inclusion of the charge conjugate mode is al-ways implied, unless explicitly stated. We use a partial reconstruction technique to identify the D∗+D¯∗0π−final
states. This technique requires that only the π−from the
primary decay (denoted as the bachelor π−), the D+
de-caying from D∗+→ D+π0 and at least one soft π0 from
D∗+ → D+π0 or ¯D∗0 → ¯D0π0 decay are
reconstruct-ed. By reconstructing the D+ particle, the charges of
its mother particle D∗+ and the bachelor π− can be
un-ambiguously identified. Therefore, possible combinatoric backgrounds are suppressed with respect to the signals. We observe a charged charmoniumlike structure, denot-ed as Z+
c (4025), in the π− recoil mass spectrum. The
data presented in this Letter correspond to an integrat-ed luminosity of 827 pb−1, which were accumulated with
the BESIII detector [12] viewing e+e− collisions at the
BEPCII collider [13].
The BESIII detector is an approximately cylindrically symmetric detector with 93% coverage of the solid angle around the e+e−collision point. The apparatus relevant
to this work includes, from inside to outside, a 43-layer main wire drift chamber (MDC), a time-of-flight (TOF) system with two layers in the barrel region and one layer for each end-cap, and a 6240 cell CsI(Tl) crystal electro-magnetic calorimeter (EMC) with both barrel and end-cap sections. The barrel components reside within a su-perconducting solenoid magnet providing a 1 T magnetic field aligned with the beam axis. The momentum resolu-tion for charged tracks in the MDC is 0.5% for transverse momenta of 1 GeV/c. The energy resolution for showers in the EMC is 2.5% for 1 GeV photons. For charged tracks, particle identification is accomplished by combin-ing the measurements of the energy deposit registered in MDC, dE/dx, and the flight time obtained from TOF to determine a probability L(h) (h = π, K) for each hadron (h) hypothesis. More details about the BESIII spectrom-eter are described elsewhere [12].
Simulated data produced by the geant4-based [14] Monte Carlo (MC) package, which includes the geomet-ric description of the BESIII detector and the detector response, is used to optimize the event selection crite-ria, to determine the detection efficiency and to esti-mate backgrounds. The simulation includes the beam energy spread and initial-state radiation (ISR) modeled with kkmc [15]. The inclusive MC sample consists of the production of the Y (4260) state and its exclusive de-cays, e+e− → D(∗)D¯(∗)(π), the production of ISR
pho-tons to low mass ψ states and QED processes. Specific decays that are tabulated in the Particle Data Group (PDG) [16] are modeled with evtgen [17] and the un-known decay modes with lundcharm [18]. For the
pro-) 2 ) (GeV/c + π + π -M(K 1.84 1.86 1.88 1.9 2 Events / 0.8 MeV/c 5000 10000 Data inclusive MC (a) ) 2 )(GeV/c + )-m(D + )+M(D + RM(D 1.8 2 2.2 2.4 2 Events / 8 MeV/c 5000 10000 15000 Data PHSP MC signal (b)
FIG. 1. (a): a comparison of invariant mass M (K−π+π+) between data and MC simulation. The MC component is normalized to the area of the histogram of the data. Arrows indicate the mass region requirement. (b): a comparison of D+recoil mass distributions between data and MC simulated three-body process e+e−→D∗+D¯∗0π−(PHSP signal). The level of the PHSP MC sample is scaled arbitrarily. The arrows show the position of the requirement RM (D+) + M (D+) − m(D+) > 2.3 GeV/c2. See the text for a detailed description.
cess e+e− → D∗+D¯∗0π−, ISR is included in the
simula-tion, which requires as input the cross section dependence on the center-of-mass energy. For this, the observed cross sections for the process e+e− → D∗+D¯∗0π− at a
sequence of energy values around 4.260 GeV at BESIII are used. The maximum energy of the ISR photon in the simulation is 89 MeV, corresponding to a D∗+D¯∗0π−
mass of 4.17 GeV/c2. For the resonant signal process
e+e− → Z+
c (4025)π− → D∗+D¯∗0π−, we assume that
the Z+
c (4025) state has spin-parity of 1+and we simulate
the cascade decays with angular distributions calculated from the corresponding matrix element. This assump-tion is consistent with our observaassump-tion in this analysis. However, other spin-parity assignments are not ruled out. As discussed above, the reconstruction of the combina-tions of the D+ and the bachelor π− is used to identify
e+e−→ D∗+D¯∗0π− final states. For the D+
reconstruc-tion, we only use the D+ → K−π+π+ decay, because it
has dominant yields and the cleanest backgrounds com-pared to other D+ deay modes. We first select events
with at least four charged tracks. For each track, the po-lar angle in the MDC must satisfy | cos θ| < 0.93 and the point of closest approach to the e+e− interaction point
must be within ±10 cm in the beam direction and within 1 cm in the plane perpendicular to the beam direction. A K(π) meson is identified by requiring L(K) > L(π) (L(π) > L(K)). Among the identified tracks, at least one K−, two π+’s and one π−are required in each event.
For the D+ → K−π+π+ selection, a vertex fit is
imple-mented that constrains the K−π+π+tracks to a common
vertex; a fit quality requirement is applied to suppress non-D+decays.
Figure 1(a) shows the M (K−π+π+) distribution where
a D+ peak is clearly evident. All combinations with
in-variant mass in the region (1.854, 1.884) GeV/c2are
iden-tified as candidate D+ mesons. The three peaks in the
D+ recoil mass spectrum in Fig. 1(b) correspond, from
left to right, to the two-body processes e+e−→ D+D−,
D+D∗− and D∗+D∗−, respectively. The D∗+D∗− peak
position corresponds to the sum of the D∗− and π0
miss-2.01 2.02 ) (GeV/c) 0π ( * P 0.02 0.04 0.06 0.08 data )+m 0 π )-M( + )-M(D 0 π + M(D 2.01 2.02 0.02 0.04 0.06 0.08 PHSP signal ) 2 ) (GeV/c 0 π )+m( + (D
FIG. 2. Scatter plot of P∗(π0) versus invariant mass of D+π0 in data (left) and in PHSP signal MC (right).
ing. The signal events lie at the rightmost side of the plot. To improve the mass resolution, we exploit the correlations between RM (D+) and M (D+) and use
RM (D+) + M (D+) −m(D+) instead of RM (D+). Here, RM (D+) is the recoil mass of the D+candidate, M (D+)
is the reconstructed mass of D+candidate and m(D+) is
the world average D+ mass [16]. The recoil mass of X is
determined from RM (X) = |pe+e−− pX|/c, where pe+e−
and pX are the four-momenta of the initial e+e−
sys-tems and X in the laboratory frame, respectively. This technique is also used in plotting other mass distribu-tions presented in this paper. Backgrounds from the two-body process e+e−→ D(∗)D(∗) are reduced by requiring
RM (D+) + M (D+) − m(D+) > 2.3 GeV/c2.
Additional background suppression is provided by re-quiring that at least one π0 is reconstructed in the
final states. A π0 candidate is selected by
requir-ing at least two photon candidates reconstructed from EMC showers [19] have an invariant mass in the range (0.120, 0.145) GeV/c2. This π0 can be either from the
D∗+→ D+π0 or ¯D∗0 → ¯D0π0 decay. In the case where
the π0 is from D∗+ → D+π0, the D+π0 invariant mass
peaks at the D∗+ mass and a mass region requirement
2.008 GeV/c2< M (D+π0)−M(D+)+m(D+)−M(π0)+
m(π0) < 2.013 GeV/c2is used, corresponding to the
ver-tical band in Fig. 2. In the case where the π0 is from
¯
D∗0→ ¯D0π0, its momentum in the D+π− recoil system,
P∗(π0), peaks at 43 MeV/c and a momentum
require-ment in the range (0.03, 0.05) GeV/c is applied, corre-sponding to the horizontal band in Fig. 2. As verified by MC simulations, the D+π−recoil mass is nearly the same
as that of the D∗+π−recoil system, but is slightly
broad-ened due to the neglect of the soft π0in the D∗+→ D+π0
process. Events with at least one π0 candidate, the one
that fulfills either of the above requirements, are retained. Figure 3(a) shows the D+π− recoil mass spectrum,
where a peak corresponding to the D∗+D¯∗0π− signal
channel is evident. The peak position roughly corre-sponds to the sum of the mass of ¯D∗0 and the mass
of a π0, since the soft π0 that originates from the
D∗+ is not used in the computation of the recoil mass.
For other non-signal processes that have the same fi-nal state, such as e+e− → D+π0D¯∗0π−, D∗+D¯0π0π−
and D+π0D¯0π0π−, MC simulations of the phase space
(PHSP) model do not produce narrow structures. The distribution of combinatorial backgrounds is estimated
) 2 ) (GeV/c + )-m(D + )+M(D -π + RM(D 2.05 2.1 2.15 2.2 ) 2 Events / ( 7.5 MeV/c 100 200 300 400 (4025) c Z PHSP
Argus fit to sidebands
data WS (a) ) 2 ) (GeV/c -π RM( 3.95 4 4.05 4.1 ) 2 Events /(10 MeV/c 50 100 (b) ) 2 ) (GeV/c -π RM( 3.95 4 4.05 4.1 ) 2 Events /(10 MeV/c 100 200 300 (c)
FIG. 3. (a): spectra of recoil mass of D+π−with the exclu-sion of events, for which RM (π−) > 4.1 GeV/c2. Horizontal dotted-line arrows indicate the sidebands and vertical arrows indicate the signal region. The histogram of WS events is scaled by a factor of 1.9 to match the sideband data. (b) and (c): comparisons of the π−recoil mass distributions between data and the WS events corresponding to the sideband and full regions as indicated in plot (a), respectively.
by combining a reconstructed D+ with a pion of the
wrong charge, referred to as wrong-sign (WS) events. The D+π− recoil mass distribution for the WS events,
shown in Fig. 3(a), is compatible with an ARGUS-function [20] shape fit to the sidebands of the signal peak in the data. As shown in Fig. 3(b) and (c), the WS events with a scaling factor of 1.9 well represent the combinato-rial backgrounds in the recoil mass spectra of the bachelor π−. This scaling is verified by an analysis of the
inclu-sive MC data. Backgrounds from the soft π− from D∗−
decays in the e+e− → D∗+D∗−(π0, γ
ISR) processes are
not well described by the WS background; its RM (π−)
distribution peaks in the region above 4.1 GeV/c2, which is excluded in this analysis.
In Fig. 3(c), a clear enhancement above the WS back-ground is evident. To study the enhancement, the events of the D∗+D¯∗0π− final states within the signal region
(2.135, 2.175) GeV/c2 in Fig. 3(a) are selected and
dis-played in Fig. 4. The enhancement cannot be attributed to the PHSP e+e− → D∗+D¯∗0π− process. We
simu-late the processes of e+e−→ D∗∗D¯(∗), D∗∗→ D(∗)π(π),
where D∗∗ denotes neutral and charged highly
excit-ed D states, such as D∗
0(2400), D1(2420), D1(2430)
and D∗
2(2460). Among these processes, only those with
D∗+D¯∗0π− final states, which are not components of the
WS backgrounds, would contribute to the difference be-tween data and the WS backgrounds. No peaking struc-ture in the π− recoil mass spectra for these simulated
events is seen in Fig. 4. Since the energy√s = 4.26 GeV is much lower than the production thresholds of D∗∗D¯∗,
we neglect the possibility of backgrounds relevant to D∗∗D¯∗ processes.
The observed enhancement is very close to the m(D∗+) + m( ¯D∗0) mass threshold. We assume that the enhancement is due to a particle, labeled as Z+
c(4025),
and parameterize its line shape by the product of an S-wave Breit-Wigner (BW) shape and a phase space factor
5 ) 2 ) (GeV/c -π RM( 4.02 4.04 4.06 4.08 ) 2 Events / ( 2.5 MeV/c 20 40 60 80 comb. BKG D*D** data total fit (4025) c Z PHSP signal WS
FIG. 4. Unbinned maximum likelihood fit to the π− recoil mass spectrum in data. See the text for a detailed description of the various components that are used in the fit. The scale of the D∗D∗∗ shape is arbitrary.
p · q 1 M2− m2+ imΓ/c2 2 · p · q. (1) Here, M is the reconstructed mass; m is the resonance mass; Γ is the width; p(q) is the D∗+(π−) momentum in
the rest frame of the D∗+D¯∗0 system (the initial e+e−
system).
The signal yield of the Z+
c(4025) is estimated by an
unbinned maximum likelihood fit to the spectrum of RM (π−). The fit results are shown in Fig. 4. Possible
interference between the Z+
c (4025) signals and the PHSP
processes is neglected. The Z+
c (4025) signal shape is
tak-en as an efficitak-ency-weighted BW shape convoluted with a detector resolution function, which is obtained from MC simulation. The detector resolution is about 2 MeV/c2
and is asymmetric due to the effects of ISR. The shape of the combinatorial backgrounds is taken from the kernel-estimate [21] of the WS events and its magnitude is fixed to the number of the fitted background events within the signal window in Fig. 3(a). The shape of the PHSP sig-nal is taken from the MC simulation and its amplitude is taken as a free parameter in the fit. By using the MC shape, the smearing due to effects of ISR and the detec-tor resolution are taken into account. From the fit, the parameters of m and Γ in Eq. (1) are determined to be
m(Zc+(4025)) = (4026.3 ± 2.6) MeV/c2,
Γ(Zc+(4025)) = (24.8 ± 5.6) MeV.
A goodness-of-fit test gives a χ2/d.o.f.= 30.4/33 = 0.92.
The Z+
c (4025) signal is observed with a statistical
signifi-cance of 13 σ, as determined by the ratio of the maximum likelihood value and the likelihood value for a fit with a null-signal hypothesis. When the systematic uncertain-ties are taken into account, the significance is evaluated to be 10 σ.
The Born cross section is determined from σ =
nsig
L(1+δ)εB, where nsig is the number of observed signal
events, L is the integrated luminosity, ε is the detec-tion efficiency, 1 + δ is the radiative correcdetec-tion factor
Source m(MeV/c2) Γ(MeV) σtot(%) R(%)
Tracking 4 Particle ID 5 Tagging π0 4 Mass scale 1.8 Signal shape 1.4 7.3 1 5 Backgrounds 1.5 0.6 5 5 Efficiencies 0.9 2.2 1 5 D∗∗ states 2.2 0.7 5 2 Fit range 0.9 0.9 1 1 D∗+D¯∗0π−line shape 4 PHSP model 2 2 Luminosity 1.0 Branching fractions 2.6 total 3.7 7.7 11 9
TABLE I. A summary of the systematic uncertainties on the measurements of the Z+
c(4025) resonance parameters and cross sections. We denote σtot = σ(e+e− →(D∗D¯∗)±π∓). The total systematic uncertainty is taken as the square root of the quadratic sum of the individual uncertainties.
and B is the branching fraction of D∗+ → D+(π0, γ),
D+ → K−π+π+. From the fit results, we obtain
560.1 ± 30.6 D∗+D¯∗0π− events, among which 400.9 ±
47.3 events are Z+
c (4025) candidates. With the
in-put of the observed center-of-mass energy dependence of σ(D∗+D¯∗0π−), the radiative correction factor is
cal-culated to second-order in QED [22] to be 0.78 ± 0.03. The efficiency for the Z+
c (4025) signal process is
deter-mined to be 23.5%, while the efficiency of the PHSP sig-nal process is 17.4%. The total cross section σ(e+e− →
(D∗D¯∗)∓π±) is measured to be (137 ± 9) pb, and the
ra-tio R =σ(e+e−→Z±c(4025)π
∓→(D∗D¯∗)±π∓)
σ(e+e−→(D∗D¯∗)±π∓) is determined to
be 0.65 ± 0.09.
Sources of systematic error on the measurement of the Z+
c (4025) resonance parameters and the cross section are
listed in Table I. The main sources of systematic un-certainties relevant for determining the Zc+(4025)
reso-nance parameters and the ratio R include the mass scale, the signal shape, background models and potential D∗∗
backgrounds. We use the process e+e− → D+D¯∗0π−
to study the mass scale of the recoil mass of the low momentum bachelor π−. By fitting the peak of ¯D∗0 in
the D+π− recoil mass spectrum, we obtain a mass of
2008.6 ± 0.1 MeV/c2. This deviates from the PDG
ref-erence value by 1.6 ± 0.2 MeV/c2. Since the fitted
vari-able RM (D+π−) + M (D+) − m(D+) removes the
corre-lation with M (D+), the shift mostly is due to the
mo-mentum measurement of the bachelor π−. Hence, we
take the mass shift of 1.8 MeV/c2 as a systematic
un-certainty on RM (π−) due to the mass scale. If one
as-sumes Z+
c (4025) also decays to other final states such
as π+(ψ(2S), J/ψ, h
c), variations of their relative
cou-pling strengths would affect the measurements of the Zc+(4025) mass and width. The Flatt´e formula [23] is
used to take into account possible multiple channels, and the maximum changes on the mass and the width are 0.4 MeV/c2 and 0.1 MeV, respectively. When we
as-sume that the relative momentum between the π− and
Z+
c (4025) in the rest frame of the e+e− system is a P
-wave, the mass and width change from the nominal re-sults by 1.4 MeV/c2and 7.3 MeV, respectively. The
max-imum variations are taken as systematic uncertainties. Variations in the unbinned and non-parametric kernel-estimate of the WS events and fluctuations of the esti-mated numbers of combinatorial backgrounds give maxi-mum changes of 1.5 MeV/c2in the mass, 0.6 MeV in the
width, 5% in the total cross section and 5% in the ratio R. We vary the parameters of the BW shape used to model the Z+
c (4025) signals in the MC simulation; the
mass is changed in the range of (4.02, 4.04) GeV/c2 and
the width is changed in the range of (20, 45) MeV. All these variations would influence the efficiency curves and thereby, affect the cross section results. The maximum changes are taken into account as systematic uncertain-ties. We performed a fit with the inclusion of the possible backgrounds due to the e+e−→ D∗∗D∗processes in the
RM (π−) spectrum. The resultant changes are taken as
a systematic uncertainty.
The spin-dependence of the non-resonant process is studied by changing the orientation of the decay plane and by changing the relative angular distributions among the final states of D∗+D¯∗0π−. The influences on the
measurements of the cross section and the ratio R are at the 2% level. Other items in Table I mostly influence the measurement of the total cross section. The efficien-cies of the soft π± are well understood in MC
simula-tion [24]. Uncertainties associated with the efficiencies of the tracking and the identification of the four final charged track are estimated to be 4% and 5%, respec-tively. A possible bias in the efficiency determination for tagging the π0 is estimated to be 4% by comparing the
measurements of σ(e+e−→ D∗+D¯∗0π−) with and
with-out detecting the π0. The line shape of the D∗+D¯∗0π−
cross sections affects the radiative correction factor and the detection efficiency simultaneously. This uncertainty is estimated to be 4% by changing the input of the ob-served line-shape within errors. The uncertainty of the integrated luminosity, measured with large angle Bhabha events, is determined to be 1%. Branching fractions for D∗+ → D+(π0, γ), D+ → K−π+π+ are used in
calcu-lating the cross section and their uncertainty taken from the PDG [16] is included as a systematic uncertainty.
To summarize, we observe an enhancement near the
threshold of m(D∗+) + m( ¯D∗0) in the π∓ recoil mass
spectrum in the process e+e− → (D∗D¯∗)±π∓ at √s =
4.260 GeV. If the enhancement is due to a charmoni-umlike particle, namely Z±
c (4025), its mass and width
are measured to be (4026.3 ± 2.6 ± 3.7) MeV/c2 and
(24.8 ± 5.6 ± 7.7) MeV, respectively. To validate the establishment of the Zc(4025), a rigorous spin analysis
is required based on a larger data sample. Since the Zc(4025) couples to (D∗D¯∗)± and has electric charge,
the observation suggests that the Zc(4025) may be a
virtual D∗D¯∗ resonant system [5]. The resonance
pa-rameters of the Zc(4025) agree with the Zc(4020) within
1.5 σ [11]. To identify whether they are same particle, one needs a further sophisticated analysis with a coupled channel technique. The Born cross section σ(e+e− →
(D∗D¯∗)±π∓) is measured to be (137 ± 9 ± 15) pb, based
on a second-order QED calculation, which is compat-ible with CLEO-c’s result [25], assuming that isospin symmetry is not largely broken. The first uncertainties are statistical and the second are systematic. The ratio R = σ(e+e−→Zc±(4025)π
∓→(D∗D¯∗)±π∓)
σ(e+e−→(D∗D¯∗)±π∓) is determined to be
0.65 ± 0.09 ± 0.06.
The BESIII collaboration thanks the staff of BEPCII and the computing center for their strong support. This work is supported in part by the Ministry of Science and Technology of China under Contract No. 2009CB825200; Joint Funds of the National Natural Science Foundation of China under Contracts Nos. 11079008, 11179007, U1332201; National Natural Science Foundation of China (NSFC) under Contracts Nos. 10625524, 10821063, 10825524, 10835001, 10935007, 11125525, 11235011, 1127526; the Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program; CAS under Contracts Nos. KJCX2-YW-N29, KJCX2-YW-N45; 100 Talents Program of CAS; German Research Foundation DFG under Contract No. Collaborative Research Center CRC-1044; Istituto Nazionale di Fisica Nucleare, Italy; Ministry of Development of Turkey under Contract No. DPT2006K-120470; U. S. Department of Energy under Contracts Nos. FG02-04ER41291, FG02-05ER41374, DE-FG02-94ER40823, DESC0010118; U.S. National Science Foundation; University of Groningen (RuG) and the Helmholtzzentrum fuer Schwerionenforschung GmbH (GSI), Darmstadt; WCU Program of National Research Foundation of Korea under Contract No. R32-2008-000-10155-0.
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