Cross section measurements of e^{+}e^{-}\rightarrow\omega\chi_{c0} from \sqrt{s}=4.178 to 4.278 GeV

Cross section measurements of from to GeV

M. Ablikim, M. N. Achasov, P. Adlarson, S.  Ahmed, M. Albrecht, M. Alekseev, A. Amoroso, F. F. An, Q. An, Y. Bai, O. Bakina, R. Baldini Ferroli, I. Balossino, Y. Ban, K. Begzsuren, J. V. Bennett, N. Berger, M. Bertani, D. Bettoni, F. Bianchi, J Biernat, J. Bloms, I. Boyko, R. A. Briere, H. Cai, X. Cai, A. Calcaterra, G. F. Cao, N. Cao, S. A. Cetin, J. Chai, J. F. Chang, W. L. Chang, G. Chelkov, D. Y. Chen, G. Chen, H. S. Chen, J. C. Chen, M. L. Chen, S. J. Chen, Y. B. Chen, W. Cheng, G. Cibinetto, F. Cossio, X. F. Cui, H. L. Dai, J. P. Dai, X. C. Dai, A. Dbeyssi, D. Dedovich, Z. Y. Deng, A. Denig, I. Denysenko, M. Destefanis, F. De Mori, Y. Ding, C. Dong, J. Dong, L. Y. Dong, M. Y. Dong, Z. L. Dou, S. X. Du, J. Z. Fan, J. Fang, S. S. Fang, Y. Fang, R. Farinelli, L. Fava, F. Feldbauer, G. Felici, C. Q. Feng, M. Fritsch, C. D. Fu, Y. Fu, Q. Gao, X. L. Gao, Y. Gao, Y. Gao, Y. G. Gao, Z. Gao, B.  Garillon, I. Garzia, E. M. Gersabeck, A. Gilman, K. Goetzen, L. Gong, W. X. Gong, W. Gradl, M. Greco, L. M. Gu, M. H. Gu, S. Gu, Y. T. Gu, A. Q. Guo, L. B. Guo, R. P. Guo, Y. P. Guo, A. Guskov, S. Han, X. Q. Hao, F. A. Harris, K. L. He, F. H. Heinsius, T. Held, Y. K. Heng, Y. R. Hou, Z. L. Hou, H. M. Hu, J. F. Hu, T. Hu, Y. Hu, G. S. Huang, J. S. Huang, X. T. Huang, X. Z. Huang, N. Huesken, T. Hussain, W. Ikegami Andersson, W. Imoehl, M. Irshad, Q. Ji, Q. P. Ji, X. B. Ji, X. L. Ji, H. L. Jiang, X. S. Jiang, X. Y. Jiang, J. B. Jiao, Z. Jiao, D. P. Jin, S. Jin, Y. Jin, T. Johansson, N. Kalantar-Nayestanaki, X. S. Kang, R. Kappert, M. Kavatsyuk, B. C. Ke, I. K. Keshk, A. Khoukaz, P.  Kiese, R. Kiuchi, R. Kliemt, L. Koch, O. B. Kolcu, B. Kopf, M. Kuemmel, M. Kuessner, A. Kupsc, M. Kurth, M.  G. Kurth, W. Kühn, J. S. Lange, P.  Larin, L. Lavezzi, H. Leithoff, T. Lenz, C. Li, Cheng Li, D. M. Li, F. Li, F. Y. Li, G. Li, H. B. Li, H. J. Li, J. C. Li, J. W. Li, Ke Li, L. K. Li, Lei Li, P. L. Li, P. R. Li, Q. Y. Li, W. D. Li, W. G. Li, X. H. Li, X. L. Li, X. N. Li, X. Q. Li, Z. B. Li, Z. Y. Li, H. Liang, H. Liang, Y. F. Liang, Y. T. Liang, G. R. Liao, L. Z. Liao, J. Libby, C. X. Lin, D. X. Lin, Y. J. Lin, B. Liu, B. J. Liu, C. X. Liu, D. Liu, D. Y. Liu, F. H. Liu, Fang Liu, Feng Liu, H. B. Liu, H. M. Liu, Huanhuan Liu, Huihui Liu, J. B. Liu, J. Y. Liu, K. Y. Liu, Ke Liu, L. Y. Liu, Q. Liu, S. B. Liu, T. Liu, X. Liu, X. Y. Liu, Y. B. Liu, Z. A. Liu, Zhiqing Liu, Y.  F. Long, X. C. Lou, H. J. Lu, J. D. Lu, J. G. Lu, Y. Lu, Y. P. Lu, C. L. Luo, M. X. Luo, P. W. Luo, T. Luo, X. L. Luo, S. Lusso, X. R. Lyu, F. C. Ma, H. L. Ma, L. L.  Ma, M. M. Ma, Q. M. Ma, X. N. Ma, X. X. Ma, X. Y. Ma, Y. M. Ma, F. E. Maas, M. Maggiora, S. Maldaner, S. Malde, Q. A. Malik, A. Mangoni, Y. J. Mao, Z. P. Mao, S. Marcello, Z. X. Meng, J. G. Messchendorp, G. Mezzadri, J. Min, T. J. Min, R. E. Mitchell, X. H. Mo, Y. J. Mo, C. Morales Morales, N. Yu. Muchnoi, H. Muramatsu, A. Mustafa, S. Nakhoul, Y. Nefedov, F. Nerling, I. B. Nikolaev, Z. Ning, S. Nisar, S. L. Niu, S. L. Olsen, Q. Ouyang, S. Pacetti, Y. Pan, M. Papenbrock, P. Patteri, M. Pelizaeus, H. P. Peng, K. Peters, J. Pettersson, J. L. Ping, R. G. Ping, A. Pitka, R. Poling, V. Prasad, M. Qi, T. Y. Qi, S. Qian, C. F. Qiao, N. Qin, X. P. Qin, X. S. Qin, Z. H. Qin, J. F. Qiu, S. Q. Qu, K. H. Rashid, C. F. Redmer, M. Richter, A. Rivetti, V. Rodin, M. Rolo, G. Rong, Ch. Rosner, M. Rump, A. Sarantsev, M. Savrié, K. Schoenning, W. Shan, X. Y. Shan, M. Shao, C. P. Shen, P. X. Shen, X. Y. Shen, H. Y. Sheng, X. Shi, X. D Shi, J. J. Song, Q. Q. Song, X. Y. Song, S. Sosio, C. Sowa, S. Spataro, F. F.  Sui, G. X. Sun, J. F. Sun, L. Sun, S. S. Sun, X. H. Sun, Y. J. Sun, Y. K Sun, Y. Z. Sun, Z. J. Sun, Z. T. Sun, Y. T Tan, C. J. Tang, G. Y. Tang, X. Tang, V. Thoren, B. Tsednee, I. Uman, B. Wang, B. L. Wang, C. W. Wang, D. Y. Wang, H. H. Wang, K. Wang, L. L. Wang, L. S. Wang, M. Wang, M. Z. Wang, Meng Wang, P. L. Wang, R. M. Wang, W. P. Wang, X. Wang, X. F. Wang, X. L. Wang, Y. Wang, Y. Wang, Y. F. Wang, Z. Wang, Z. G. Wang, Z. Y. Wang, Zongyuan Wang, T. Weber, D. H. Wei, P. Weidenkaff, H. W. Wen, S. P. Wen, U. Wiedner, G. Wilkinson, M. Wolke, L. H. Wu, L. J. Wu, Z. Wu, L. Xia, Y. Xia, S. Y. Xiao, Y. J. Xiao, Z. J. Xiao, Y. G. Xie, Y. H. Xie, T. Y. Xing, X. A. Xiong, Q. L. Xiu, G. F. Xu, L. Xu, Q. J. Xu, W. Xu, X. P. Xu, F. Yan, L. Yan, W. B. Yan, W. C. Yan, Y. H. Yan, H. J. Yang, H. X. Yang, L. Yang, R. X. Yang, S. L. Yang, Y. H. Yang, Y. X. Yang, Yifan Yang, Z. Q. Yang, M. Ye, M. H. Ye, J. H. Yin, Z. Y. You, B. X. Yu, C. X. Yu, J. S. Yu, C. Z. Yuan, X. Q. Yuan, Y. Yuan, A. Yuncu, A. A. Zafar, Y. Zeng, B. X. Zhang, B. Y. Zhang, C. C. Zhang, D. H. Zhang, H. H. Zhang, H. Y. Zhang, J. Zhang, J. L. Zhang, J. Q. Zhang, J. W. Zhang, J. Y. Zhang, J. Z. Zhang, K. Zhang, L. Zhang, S. F. Zhang, T. J. Zhang, X. Y. Zhang, Y. Zhang, Y. H. Zhang, Y. T. Zhang, Yang Zhang, Yao Zhang, Yi Zhang, Yu Zhang, Z. H. Zhang, Z. P. Zhang, Z. Y. Zhang, G. Zhao, J. W. Zhao, J. Y. Zhao, J. Z. Zhao, Lei Zhao, Ling Zhao, M. G. Zhao, Q. Zhao, S. J. Zhao, T. C. Zhao, Y. B. Zhao, Z. G. Zhao, A. Zhemchugov, B. Zheng, J. P. Zheng, Y. Zheng, Y. H. Zheng, B. Zhong, L. Zhou, L. P. Zhou, Q. Zhou, X. Zhou, X. K. Zhou, X. R. Zhou, Xiaoyu Zhou, Xu Zhou, A. N. Zhu, J. Zhu, J.  Zhu, K. Zhu, K. J. Zhu, S. H. Zhu, W. J. Zhu, X. L. Zhu, Y. C. Zhu, Y. S. Zhu, Z. A. Zhu, J. Zhuang, B. S. Zou, J. H. Zou (BESIII Collaboration) Institute of High Energy Physics, Beijing 100049, People’s Republic of China Beihang University, Beijing 100191, People’s Republic of China Beijing Institute of Petrochemical Technology, Beijing 102617, People’s Republic of China Bochum Ruhr-University, D-44780 Bochum, Germany Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA Central China Normal University, Wuhan 430079, People’s Republic of China China Center of Advanced Science and Technology, Beijing 100190, People’s Republic of China COMSATS University Islamabad, Lahore Campus, Defence Road, Off Raiwind Road, 54000 Lahore, Pakistan Fudan University, Shanghai 200443, People’s Republic of China G.I. Budker Institute of Nuclear Physics SB RAS (BINP), Novosibirsk 630090, Russia GSI Helmholtzcentre for Heavy Ion Research GmbH, D-64291 Darmstadt, Germany Guangxi Normal University, Guilin 541004, People’s Republic of China Guangxi University, Nanning 530004, People’s Republic of China Hangzhou Normal University, Hangzhou 310036, People’s Republic of China Helmholtz Institute Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz, Germany Henan Normal University, Xinxiang 453007, People’s Republic of China Henan University of Science and Technology, Luoyang 471003, People’s Republic of China Huangshan College, Huangshan 245000, People’s Republic of China Hunan Normal University, Changsha 410081, People’s Republic of China Hunan University, Changsha 410082, People’s Republic of China Indian Institute of Technology Madras, Chennai 600036, India Indiana University, Bloomington, Indiana 47405, USA (A)INFN Laboratori Nazionali di Frascati, I-00044, Frascati, Italy; (B)INFN and University of Perugia, I-06100, Perugia, Italy (A)INFN Sezione di Ferrara, I-44122, Ferrara, Italy; (B)University of Ferrara, I-44122, Ferrara, Italy Institute of Physics and Technology, Peace Ave. 54B, Ulaanbaatar 13330, Mongolia Johannes Gutenberg University of Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz, Germany Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia Justus-Liebig-Universitaet Giessen, II. Physikalisches Institut, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany KVI-CART, University of Groningen, NL-9747 AA Groningen, The Netherlands Lanzhou University, Lanzhou 730000, People’s Republic of China Liaoning University, Shenyang 110036, People’s Republic of China Nanjing Normal University, Nanjing 210023, People’s Republic of China Nanjing University, Nanjing 210093, People’s Republic of China Nankai University, Tianjin 300071, People’s Republic of China Peking University, Beijing 100871, People’s Republic of China Shandong Normal University, Jinan 250014, People’s Republic of China Shandong University, Jinan 250100, People’s Republic of China Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China Shanxi University, Taiyuan 030006, People’s Republic of China Sichuan University, Chengdu 610064, People’s Republic of China Soochow University, Suzhou 215006, People’s Republic of China Southeast University, Nanjing 211100, People’s Republic of China State Key Laboratory of Particle Detection and Electronics, Beijing 100049, Hefei 230026, People’s Republic of China Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China Tsinghua University, Beijing 100084, People’s Republic of China (A)Ankara University, 06100 Tandogan, Ankara, Turkey; (B)Istanbul Bilgi University, 34060 Eyup, Istanbul, Turkey; (C)Uludag University, 16059 Bursa, Turkey; (D)Near East University, Nicosia, North Cyprus, Mersin 10, Turkey University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China University of Hawaii, Honolulu, Hawaii 96822, USA University of Jinan, Jinan 250022, People’s Republic of China University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom University of Minnesota, Minneapolis, Minnesota 55455, USA University of Muenster, Wilhelm-Klemm-Str. 9, 48149 Muenster, Germany University of Oxford, Keble Rd, Oxford, UK OX13RH University of Science and Technology Liaoning, Anshan 114051, People’s Republic of China University of Science and Technology of China, Hefei 230026, People’s Republic of China University of South China, Hengyang 421001, People’s Republic of China University of the Punjab, Lahore-54590, Pakistan (A)University of Turin, I-10125, Turin, Italy; (B)University of Eastern Piedmont, I-15121, Alessandria, Italy; (C)INFN, I-10125, Turin, Italy Uppsala University, Box 516, SE-75120 Uppsala, Sweden Wuhan University, Wuhan 430072, People’s Republic of China Xinyang Normal University, Xinyang 464000, People’s Republic of China Zhejiang University, Hangzhou 310027, People’s Republic of China Zhengzhou University, Zhengzhou 450001, People’s Republic of China Also at Bogazici University, 34342 Istanbul, Turkey Also at the Moscow Institute of Physics and Technology, Moscow 141700, Russia Also at the Functional Electronics Laboratory, Tomsk State University, Tomsk, 634050, Russia Also at the Novosibirsk State University, Novosibirsk, 630090, Russia Also at the NRC ”Kurchatov Institute”, PNPI, 188300, Gatchina, Russia Also at Istanbul Arel University, 34295 Istanbul, Turkey Also at Goethe University Frankfurt, 60323 Frankfurt am Main, Germany Also at Key Laboratory for Particle Physics, Astrophysics and Cosmology, Ministry of Education; Shanghai Key Laboratory for Particle Physics and Cosmology; Institute of Nuclear and Particle Physics, Shanghai 200240, People’s Republic of China Also at Government College Women University, Sialkot - 51310. Punjab, Pakistan. Also at Key Laboratory of Nuclear Physics and Ion-beam Application (MOE) and Institute of Modern Physics, Fudan University, Shanghai 200443, People’s Republic of China Also at Harvard University, Department of Physics, Cambridge, MA, 02138, USA
Abstract

The cross section of the process is measured at center-of-mass energies from 4.178 to 4.278 GeV using a data sample of 7 fb collected with the BESIII detector operating at the BEPCII storage ring. The dependence of the cross section on shows a resonant structure with mass of  MeV/ and width of  MeV, respectively. This observation confirms and improves upon the result of a previous study. The angular distribution of the process is extracted for the first time.

pacs:
14.40.Rt, 13.25.Gv, 13.66.Bc, 14.40.Pq

I I. Introduction

BESIII recently investigated the process using data collected at = 4.23 and 4.26 GeV combined with smaller data samples at nearby energies omegachic (). An enhancement was found in the cross section around = 4.22 GeV, referred to as the . Resonance signals were not observed in a subsequent study above 4.4 GeV omegachic2 (). Various models the1 (); the2 (); the3 (); the4 (); the5 (); the6 (); the7 () are proposed to explain the observed line-shape. Possible scenarios include a missing state the3 (), a contribution from the state the4 (), a tetraquark state the5 (), or a molecule state the6 (); the7 (). Intriguingly, similar and possibly related structures are also observed in the same energy region for other processes, such as  pipijpsi-bes (),  pipipsip-bes (),  pipihc-bes (), and  piDDstar ().

In this paper, we report a study of the reaction based on the most recent annihilation data collected with the BESIII detector besiii () at nine energy points in the range 4.178 4.278 GeV, with a total integrated luminosity of about 7 fb. The state is detected via , and the is reconstructed via the decay.

Ii Ii. Besiii Detector and Monte Carlo Simulation

The BESIII detector is a magnetic spectrometer besiii () located at the Beijing Electron Positron Collider (BEPCII) bepcii (). The cylindrical core of the BESIII detector consists of a helium-based multilayer drift chamber (MDC), a plastic scintillator time-of-flight system (TOF), and a CsI(Tl) electromagnetic calorimeter (EMC), which are all enclosed in a superconducting solenoidal magnet providing a 1.0 T magnetic field. The solenoid is supported by an octagonal flux-return yoke with resistive plate counter muon identifier modules interleaved with steel. The acceptance of charged particles and photons is 93% over the solid angle. The charged-particle momentum resolution at is , and the resolution is for the electrons from Bhabha scattering. The EMC measures photon energies with a resolution of () at  GeV in the barrel (end cap) region. The time resolution of the TOF barrel part is 68 ps, while that of the end cap part is 110 ps. The end cap TOF system was upgraded in 2015 with multi-gap resistive plate chamber technology, providing a time resolution of 60 ps etof ().

Simulated data samples produced with the geant4-based geant4 () Monte Carlo (MC) package, which includes the geometric description of the BESIII detector and the detector response, are used to determine the detection efficiency and to estimate the background contributions. The simulation models the beam energy spread and initial state radiation (ISR) in the annihilations using the generator kkmc KKMC (). For the signal we use a MC sample of the process generated according to the measured angular distribution, which is introduced in section VI. The inclusive MC samples consist of the production of open charm processes, the ISR production of vector charmonium(-like) states, and the continuum processes incorporated in kkmc KKMC (). The known decay modes are modelled with evtgen ref:evtgen () using branching fractions taken from the Particle Data Group (PDG) pdg (), and the remaining unknown decays from the charmonium states are generated with lundcharm ref:lundcharm (). Final state radiation (FSR) effects from charged final state particles are incorporated the photos package photos ().

Iii Iii. Event Selection

For each charged track, the distance of closest approach to the interaction point (IP) is required to be within cm in the beam direction and within 1 cm in the plane perpendicular to the beam direction. The polar angle () of the tracks must be within the fiducial volume of the MDC . Photons are reconstructed from isolated showers in the EMC, which are at least away from the nearest charged track. The photon energy is required to be at least 25 MeV in the barrel region or 50 MeV in the end cap region . To suppress electronic noise and energy depositions unrelated to the event, the time after the collision at which the photon is recorded in the EMC is required to satisfy ns.

Since the final states of the signal are or , candidate events must have four tracks with zero net charge and at least two photons. The tracks with a momentum larger than 1 GeV/ are identified as from the decay of the , whereas lower momentum tracks are considered as pions from decays. To reduce the background contributions and to improve the mass resolution, a 5C-kinematic fit is performed to both constrain the total four momentum of the final state partices to the total initial four momentum of the colliding beams and to constrain the invariant mass of the two photons from the decay of the to its nominal mass pdg (). If there is more than one candidate in an event, the one with the smallest of the kinematic fit is selected. The two track candidates of the decay of the are considered to be either a or a pair depending on the of the 5C-kinematic fit. If , the two tracks are identified as a pair, otherwise, as a pair. The of the candidate events is required to be less than 100.

Iv Iv. Born Cross Section Measurement

The correlation between the invariant mass, , and the mass, , is shown in the top panel in Fig. 1 for data taken at GeV. A high density area can be observed that originates from the channel. The mass range [0.75, 0.81] GeV/ in is defined as the signal region and is indicated by horizontal dashed lines. A sideband in the range [0.60, 0.72] GeV is used to study the non-resonant background. The signal region is indicated by the vertical dashed lines and is defined as [3.38, 3.45] GeV/. The bottom panel of Fig. 1 shows the distribution of for data in the signal region. The shaded (green) histogram corresponds to normalized events in the sideband region.

Figure 1: (top) The distribution of versus for data at GeV. The blue dashed lines denote the and mass bands. (bottom) The invariant mass distribution for the data at GeV. The red solid line is the fit to the data and the blue dashed line is a fit of the background. The green shaded histogram corresponds to the normalized background events from the sideband region.

An unbinned maximum likelihood fit is performed to obtain the signal yields. In the fit, we use the MC-determined shape to describe the signal. The background is described with a generalized ARGUS function argus ()

(1)

where is fixed to GeV), with 0.75 GeV being the lower limit of , and , are free parameters. The red solid curve in the bottom panel of Fig. 1 shows the fit result. The data taken at the other center-of-mass energies are analyzed using the same method and the fit results are summarized in Table 1.

     (GeV)      (pb)          1 + (s)                (pb)
4.178 3194.5 0.63 1.055 24.71
4.189 524.6 0.64 1.056 24.59
4.199 526.0 0.66 1.057 25.68
4.209 518.0 0.68 1.057 25.70
4.219 514.6 0.71 1.057 25.52
4.236 530.3 0.80 1.056 25.92
4.244 538.1 0.86 1.055 25.51
4.267 531.1 1.44 1.053 22.10
4.278 175.7 2.68 1.053 17.11
Table 1: Born cross sections (or upper limits at 90% C.L. ) for the reaction at the different center-of-mass energies , together with integrated luminosities , number of signal events , radiative correction factor 1 + (s), vacuum polarization factor , and efficiency . The first uncertainties are statistical, and the second systematic.

The Born cross section is calculated with

(2)

where is the number of signal events, is the integrated luminosity obtained using the same method in Ref. luminosity (), 1 + is the radiative correction factor obtained from a Quantum Electrodynamics (QED) calculation QED (); KKMC () using the obtained preliminary cross section as input and iterating it until the results converge, is the correction factor for vacuum polarization vacuum (), is the product of branching fractions , and is the event selection efficiency. The Born cross sections (or upper limits at 90% C.L.) at each energy point for are listed in Table 1.

  Source / (GeV)   4.178   4.189   4.199   4.209   4.219   4.236   4.244   4.267   4.278
Luminosity 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
Tracking efficiency 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0
Photon detection 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0
Kinematic fit 0.6 0.8 0.6 0.6 0.5 0.6 0.8 2.4 3.8
Radiative correction 14.0 11.2 4.5 3.6 0.2 0.9 1.8 19.8 45.9
Fit range 9.3 2.3 2.9 2.5 2.0 2.3 8.7
Signal shape 1.9 5.1 9.4 0.2 1.5 11.4 1.6
Background shape 1.9 3.7 6.5 2.8 1.5 2.3 6.3
Angular distribution 0.1 0.1 0.3 0.5 0.5 0.8 0.6 0.7 0.9
Branching fraction 6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9
Sum 16.3 17.0 11.6 14.9 9.2 8.9 14.6 24.2 46.8
Table 2: Relative systematic uncertainties (in ) from the different sources. Sources marked with an asterisk have common relative systematic uncertainties for the different center-of-mass energies. Dashes mean that the results are not applicable.

The systematic uncertainty of the Born cross section measurement originates mainly from the luminosity determination, the tracking efficiency, photon detection efficiency, kinematic fit, radiative correction, fit range, signal and background shapes, angular distribution, and the branching fractions for .

The luminosity is measured with a precision of about using the well-known Bhabha scattering process luminosity (). The uncertainty in the tracking efficiency is obtained as per track using the process  omegachic2 (). The uncertainty in photon reconstruction is per photon, obtained by studying the decay photon ().

The systematic uncertainty due to the kinematic fit is estimated by correcting the helix parameters of charged tracks according to the method described in Ref. helix (). The difference between detection efficiencies obtained from MC samples with and without correction is taken as the uncertainty.

The line-shape of the cross section will affect the radiative correction factor and the efficiency. In the nominal results, we use a piecewise Gaussian function as the line-shape to describe the cross section. The shape is used as input and is iterated until the results converge. To estimate the uncertainty from the radiative correction, we change the line-shape to the published Breit-Wigner (BW) function of the  omegachic2 (). The difference between the results is taken as a systematic uncertainty.

The uncertainty from the fit range is obtained by varying the limits of the fit range by 0.01 GeV/. We take the largest difference of the corresponding cross section measurement with respect to the nominal one as the systematic uncertainty. We use the MC-determined shape convolved with a Gaussian function to fit the data as input to get the uncertainty of the signal shape. The difference in the results with respect to the nominal one is taken as the systematic uncertainty. To estimate the systematic uncertainty caused by the background shape, we vary by GeV/ in the ARGUS function, and take the largest difference in the results as the uncertainty.

The measured angular distribution is used as a model to generate signal events in the MC simulations. The detection efficiency of the reaction will depend upon its angular distribution. We obtained an angular distribution parameter, defined in section VI, of . The systematic uncertainty of the efficiency due to uncertainties in the angular distribution is estimated by varying the value by one standard deviation, the total uncertainty on .

The uncertainty in the product of the branching fractions is taken from the uncertainties quoted by the PDG pdg ().

Table 2 summarizes all the systematic uncertainties related to the cross section measurements of the process for each center-of-mass energy. The overall systematic uncertainties are obtained by adding all the sources of systematic uncertainties in quadrature assuming they are uncorrelated.

V V. resonant parameter measurement

Figure 2 shows the dressed cross sections () for the reaction as a function of center-of-mass energy. The black square points are taken from Refs. omegachic (); omegachic2 (), and the blue circular points are from this work. We observe an enhancement in the cross section around 4.22 GeV. By assuming that the signals all come from a single resonance, which we label as the , with mass and width , we fit the cross section data with the following formula convolved with a Gaussian function for the energy spread:

(3)

where is the two-body phase space factor and is the electronic width. The fit to all the data in Fig. 2 gives eV,  MeV/, MeV, where the uncertainties are statistical only. In the fit, the cross sections’ statistical uncertainties are used only. The goodness of fit is , where ndf is the number of degrees of freedom.

Figure 2: The cross section as a function of the center-of-mass energy. The blue points are from this work, the black square points are from Refs. omegachic (); omegachic2 () and the red solid line is the fit result.

The systematic uncertainties on the resonant parameters mainly arise from uncertainties in the absolute beam energy, the parametrization of the BW function, and the cross section measurement. Because the energy spread effect has been considered in the fit, we ignore the systematic uncertainty from energy spread.

Since the uncertainty of the beam energy is about 0.8 MeV, which is obtained using the same method in Ref. ecms (), the uncertainty of the resonant parameters caused by the beam energy is estimated by varying within 0.8 MeV.

The cross section has been fitted with a BW function having the energy-dependent width in the denominator, where is the nominal width of the resonance, to estimate the uncertainty from parametrization of the BW function. The difference between this fit result and the nominal result is taken as the uncertainty from the parametrization of the BW function.

The systematic uncertainty of the cross section measurement will affect the resonant parameters in the fit and can be divided into two parts. One part comes from the uncorrelated uncertainty among the different center-of-mass energies, and the other part is a common uncertainty. The first part has been considered by including the systematic uncertainty of the cross section in the fit. The difference between the parameters obtained in this fit to those from the nominal fit is taken as the uncertainty. We vary the cross section within the systematic uncertainty coherently for the second part and take the difference between this fit result and the nominal result as the uncertainty. We add the two parts in quadrature assuming they are uncorrelated.

Table 3 summarizes all the systematic uncertainties of the resonant parameters. The total systematic uncertainty is obtained by summing all the sources of systematic uncertainties in quadrature by assuming they are uncorrelated.

        
Absolute beam energy 0.1 0.9 0.2
Resonance parametrization 0.1 3.9 1.1
Cross section measurement 0.2 0.3 1.1
Sum 0.3 4.0 1.6
Table 3: Summary of systematic uncertainties on the resonant parameters. The units for , , and are eV, MeV/, and MeV, respectively.

Vi VI. angular distribution measurement

Both and -wave contributions are possible in the process . A measurement of their strengths can be helpful to extract information about the underlying dynamics of the decay process. We therefore performed an angular analysis of the relatively high-statistics data samples taken at = 4.219, 4.226, and 4.236 GeV (selection of = 4.226 GeV data was reported in Ref. omegachic ()). The helicity angle, , defined by the scattering angle of the with respect to the electron beam in the center-of-mass frame was reconstructed for each event. Figure 3 shows the bin-by-bin efficiency-corrected events as a function of for the three center-of-mass energies. We performed a simultaneous fit using the function cos, assuming is common to the three energy points. The red line in Fig. 3 shows the best fit result with , where the first uncertainty is statistical and the second systematic. The goodness of the fit is ndf. The fit result indicates evidence for a combination of and wave contributions in the process, although the statistical significance of this conclusion is only compared with a pure wave contribution of .

\setkeys

Ginwidth=0.23\OVP@calc(a) \setkeysGinwidth=0.23\OVP@calc(b) \setkeysGinwidth=0.23\OVP@calc(c) \setkeysGinwidth=0.23\OVP@calc(d)

Figure 3: Simultaneous fit to the angular distributions for data taken at = 4.219 (a), 4.226 (b) and 4.236 (c) GeV. (d) shows the summed result of the three center-of-mass energies.

The systematic uncertainty of has been estimated by varying the fit range (0.02), the signal (0.01) and background shapes (0.03), and the radiative correction factor (0.03). The uncertainties are indicated in brackets and are determined with the same method described earlier for the cross section measurements. In addition, we estimate an additional source of systematic uncertainty by varying the number of bins. For this, we change the number of bins from 10 to 8, and repeat the process. The difference in is found to be 0.01. The overall systematic uncertainty (0.05) is obtained by summing all the items of systematic uncertainties in quadrature by assuming they are uncorrelated.

Vii VII. Summary

The process has been studied using 9 data samples collected at center-of-mass energies from 4.178 to 4.278 GeV. The -dependence of the cross section has been measured and the results are listed in Table 1 and are shown in Fig. 2. A clear enhancement is seen around 4.22 GeV which confirms, and statistically improves upon, an earlier observation omegachic (). By fitting the cross section with a single resonance, the mass and width for the structure are determined to be MeV/ and MeV. The obtained resonance parameters are not compatible with the vector charmonium state , ruling out its possible contribution to the structure the4 (). Moreover, we studied the angular distribution of the process . We measured , which indicates a combination of and wave contributions in the decay.

Figure 4 shows the measured mass and width of the from the different processes. The masses are consistent with each other, while the widths are not. The widths from the processes , , and are larger than those from the processes and . From these inconsistencies in the width, we cannot draw a conclusion on whether the structure observed in these processes is the same state or whether the inconsistencies are caused by the BW parameterization. Further experimental studies with higher statistics are needed to draw a more reliable conclusion on the nature of this structure.

Figure 4: Mass and width of the obtained from the processes , , , and

Viii Acknowledgments

The BESIII collaboration thanks the staff of BEPCII and the IHEP computing center for their strong support. This work is supported in part by National Key Basic Research Program of China under Contract No. 2015CB856700; National Natural Science Foundation of China (NSFC) under Contracts Nos. 11625523, 11635010, 11735014; National Natural Science Foundation of China (NSFC) under Contract No. 11835012; National Natural Science Foundation of China (NSFC) under Contract No. 11847028; the Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program; Joint Large-Scale Scientific Facility Funds of the NSFC and CAS under Contracts Nos. U1532257, U1532258, U1732263, U1832207; CAS Key Research Program of Frontier Sciences under Contracts Nos. QYZDJ-SSW-SLH003, QYZDJ-SSW-SLH040; 100 Talents Program of CAS; INPAC and Shanghai Key Laboratory for Particle Physics and Cosmology; Foundation of Henan Educational Committee (No. 19A140015); Nanhu Scholars Program for Young Scholars of Xinyang Normal University; German Research Foundation DFG under Contract No. Collaborative Research Center CRC 1044; Istituto Nazionale di Fisica Nucleare, Italy; Koninklijke Nederlandse Akademie van Wetenschappen (KNAW) under Contract No. 530-4CDP03; Ministry of Development of Turkey under Contract No. DPT2006K-120470; National Science and Technology fund; The Swedish Research Council; U. S. Department of Energy under Contracts Nos. DE-FG02-05ER41374, DE-SC-0010118, DE-SC-0012069; University of Groningen (RuG) and the Helmholtzzentrum fuer Schwerionenforschung GmbH (GSI), Darmstadt.

References

  • (1) M. Ablikim et al. [BESIII Collaboration], Phys. Rev. Lett. 114, 092003 (2015).
  • (2) M. Ablikim et al. [BESIII Collaboration], Phys. Rev. D 93, 011102 (2016).
  • (3) L. Y. Dai, M. Shi, G. Y. Tang and H. Q. Zheng, Phys. Rev. D 92, 014020 (2015).
  • (4) L. Ma, X. H. Liu, X. Liu and S. L. Zhu, Phys. Rev. D 91, 034032 (2015).
  • (5) D. Y. Chen, X. Liu and T. Matsuki, Phys. Rev. D 91, 094023 (2015).
  • (6) X. Li and M. B. Voloshin, Phys. Rev. D 91, 034004 (2015).
  • (7) R. Faccini, G. Filaci, A. L. Guerrieri, A. Pilloni and A. D. Polosa, Phys. Rev. D 91, 117501 (2015).
  • (8) M. Cleven and Q. Zhao, Phys. Lett. B 768, 52 (2017).
  • (9) Z. G. Wang, Chin. Phys. C 41, 083103 (2017).
  • (10) M. Ablikim et al. [BESIII Collaboration], Phys. Rev. Lett. 118, 092001 (2017).
  • (11) M. Ablikim et al. [BESIII Collaboration], Phys. Rev. D 96, 032004 (2017).
  • (12) M. Ablikim et al. [BESIII Collaboration], Phys. Rev. Lett. 118, 092002 (2017).
  • (13) M.  Ablikim et al. [BESIII Collaboration], arXiv:1808.02847.
  • (14) M. Ablikim et al. [BESIII Collaboration], Nucl. Instrum. Meth. A 614, 345 (2010).
  • (15) C. H. Yu et al., Proceedings of IPAC2016, Busan, Korea, 2016, doi:10.18429/JACoW-IPAC2016-TUYA01.
  • (16) X. Li et al., Radiat. Detect. Technol. Methods 1, 13 (2017); Y. X. Guo et al., Radiat. Detect. Technol. Methods 1, 15 (2017).
  • (17) S. Agostinelli et al. [GEANT4 Collaboration], Nucl. Instrum. Meth. A 506, 250 (2003).
  • (18) S. Jadach, B. F. L. Ward and Z. Was, Phys. Rev. D 63, 113009 (2001); Comput. Phys. Commun. 130, 260 (2000).
  • (19) D. J. Lange, Nucl. Instrum. Meth. A 462, 152 (2001); R. G. Ping, Chin. Phys. C 32, 599 (2008).
  • (20) M. Tanabashi et al. [Particle Data Group], Phys. Rev. D 98, 030001 (2018).
  • (21) J. C. Chen, G. S. Huang, X. R. Qi, D. H. Zhang and Y. S. Zhu, Phys. Rev. D 62, 034003 (2000); R. L. Yang, R. G. Ping and H. Chen, Chin. Phys. Lett. 31, 061301 (2014).
  • (22) E. Richter-Was, Phys. Lett. B 303, 163 (1993).
  • (23) H. Albrecht et al. [ARGUS Collaboration], Phys. Lett. B 241, 278 (1990).
  • (24) M. Ablikim et al. [BESIII Collaboration], Chin. Phys. C 39, 093001 (2015).
  • (25) E. A. Kuraev and V. S. Fadin, Sov. J. Nucl. Phys. 41, 466 (1985).
  • (26) S. Actis et al. [Working Group on Radiative Corrections and Monte Carlo Generators for Low Energies Collaboration], Eur. Phys. J. C 66, 585 (2010).
  • (27) M. Ablikim et al. [BESIII Collaboration], Phys. Rev. Lett. 118, 221802 (2017).
  • (28) M. Ablikim et al. [BESIII Collaboration], Phys. Rev. D 87, 012002 (2013).
  • (29) M. Ablikim et al. [BESIII Collaboration], Chin. Phys. C 40, 063001 (2016).
Comments 0
Request Comment
You are adding the first comment!
How to quickly get a good reply:
  • Give credit where it’s due by listing out the positive aspects of a paper before getting into which changes should be made.
  • Be specific in your critique, and provide supporting evidence with appropriate references to substantiate general statements.
  • Your comment should inspire ideas to flow and help the author improves the paper.

The better we are at sharing our knowledge with each other, the faster we move forward.
""
The feedback must be of minimum 40 characters and the title a minimum of 5 characters
   
Add comment
Cancel
Loading ...
352773
This is a comment super asjknd jkasnjk adsnkj
Upvote
Downvote
""
The feedback must be of minumum 40 characters
The feedback must be of minumum 40 characters
Submit
Cancel

You are asking your first question!
How to quickly get a good answer:
  • Keep your question short and to the point
  • Check for grammar or spelling errors.
  • Phrase it like a question
Test
Test description