ISSN 0253-2778

CN 34-1054/N

2016 Vol. 46, No. 7

Display Method:
Theoretical review of XYZ
LIU Xiang
2016, 46(7): 533-540. doi: 10.3969/j.issn.0253-2778.2016.07.001
In the past 12 years, dozens of charmonium-like states have been reported in experiment. Facing so abundant novel phenomena, theorists have been hard at work to reveal the underlying mechanism relevant to these XYZ states. A concise review of the observed XYZ states is first given. Then, an introduction is presented to the theoretical progress made by our group in the study of XYZ states, which includes ① the hadronic molecular state explanations to Y(3940), Y(4140) and Y(4274); ② the non-resonant explanation to Y(4260) and Y(4360); ③ the P-wave charmonium assignment to Y(3915), Z(3930) and X(4350); ④ the initial single pion emission mechanism and the observation of Zc(3900).
ZHANG Jingzhi, YUAN Changzheng (for the BESIII Collaboration)
2016, 46(7): 541-548. doi: 10.3969/j.issn.0253-2778.2016.07.002
With 5.1 fb-1 of data taken at s=3.8~4.6 GeV, BESIII made a significant contribution to the study of charmonium-like states, i.e., the XYZ states. We review the results of observations of the Zc states, the X(3872) in e+e- annihilation, and charmonium ψ(13D2) state, as well as measurements of the cross-sections of ωχcJ and ηJ/ψ.
XYZ states at Belle
SHEN Chengping, LI Zhuang (for the Belle Collaboration)
2016, 46(7): 549-556. doi: 10.3969/j.issn.0253-2778.2016.07.003
Exotic hadronic states beyond the conventional quark model (called charmoniumlike/bottomoniumlike states or XYZ particles) have been searched for and many candidates have been proposed including glueballs, hybrids, multi-quark states, hadron molecules, etc. Dramatic progress was made in the study of them after the running of the B-factories. The most recent results on the XYZ states at Belle are presented, including ① X states: the first observation of B0→X(3872)K+π- and evidence for B+→X(3872)K0π+; search for the Xb state; ② Y states: the updated results for the Y(4360) and Y(4660) and cross section measurements of e+e-→K+K-J/ψ and γχcJ; ③ Z states: the evidence for the Zc(4050)±→π± ψ(2S); search for the Zcs in e+e-→K+K-J/ψ.
New results on exotic baryon resonances at LHCb
ZHANG Liming (for the LHCb Collaboration)
2016, 46(7): 557-566. doi: 10.3969/j.issn.0253-2778.2016.07.004
Observation of exotic resonant structures decaying into J/ψp found in the LHCb experiment is discussed. Examination of the J/ψp system in Λ0b→J/ψK-p decays shows two states, each of which must be composed of at least cuud quarks, and are thus consistent with pentaquarks. The significance of each of these resonances is more than 9 standard deviations. Their masses are (4 380±8±29)MeV and (4 4498±17±25)MeV, and their corresponding widths are (205±18±86)MeV, and (39±5±19)MeV. The preferred JP assignments are of opposite parity, with one state having spin 3/2 and the other 5/2.
Light hadron spectroscopy at BESIII
FANG Shuangshi (for the BESIII Collaboration)
2016, 46(7): 567-573. doi: 10.3969/j.issn.0253-2778.2016.07.005
Based on the samples of 1.3×109 J/ψ events and 1.06×108 ψ(3686) events taken at the BESIII detector, the recent progress on the light meson spectroscopy, baryon spectroscopy and light meson decays is presented.
Towards the nature of X(3872) resonance
2016, 46(7): 574-579. doi: 10.3969/j.issn.0253-2778.2016.07.006
The spectra of decays of resonance X(3872) with good analytical and unitary properties are constructed, which allows to define the branching ratio of the X(3872)→D*0D0+c.c. decay by studying only one more decay, for example, the X(3872)→π+π-J/ψ(1S) decay, and it is shown that the spectra are an effective means of selection of models for the resonance X(3872). Then the scenario is discussed where the X(3872) resonance is the c=χc1(2P) charmonium which “sits on” the D*0D0 threshold. An explanation is given of the shift of the mass of the X(3872) resonance with respect to the prediction of a potential model for the mass of the χc1(2P) charmonium by the contribution of the virtual D*D+c.c. intermediate states into the self energy of the X(3872) resonance. This allows us to estimate the coupling constant of the X(3872) resonance with the D*0D0 channel, the branching ratio of the X(3872)→D*0D0+c.c. decay, and the branching ratio of the X(3872) decay into all non-D*0D0+c.c. states. A significant number of unknown decays of X(3872) via two gluons: X(3872)→gluon gluon→hadrons are predicted.
Status of the Monte Carlo generators for low energy e+e- scattering
CZYZ· Henryk
2016, 46(7): 580-586. doi: 10.3969/j.issn.0253-2778.2016.07.007
A short review of the existing Monte Carlo generators used for luminosity measurements at meson factories and simulating reactions e+e-→hadrons, e+e-→hadrons+photons or e+e-→e+e-+hadrons is presented, and the physical accuracy of the codes with emphasis on QED radiative corrections is discussed.
Dark mediator searches at KLOE/KLOE-2
PREZ DEL RO Elena (for the KLOE- Collaboration)
2016, 46(7): 587-593. doi: 10.3969/j.issn.0253-2778.2016.07.008
The existence of a new vector boson has been postulated in different scenarios where the coupling to the SM can be achieved either via a kinetic mixing term, the U boson, or by coupling to the baryon number, the B boson. Direct searches for these dark matter mediators are performed at accelerator facilities. The KLOE detector at the DANE Φ-factory has been prolific in searches for the U boson in both Dalitz decays of the  meson, →ηU with U→e+e-, and continuum events, e+e-→Uγ. For all of these processes, an upper limit for the U boson coupling ε2 of 10-7 to 10-5 has been established in the mass range 4 MeV·c-2
Search for dark photons in π0 decays at NA48 and NA62
GONNELLA Francesco (for the NA/ Collaboration)
2016, 46(7): 594-600. doi: 10.3969/j.issn.0253-2778.2016.07.009
The NA48/2 experiment at CERN collected, between 2003 and 2004, 1.69×107 fully reconstructed π0→γe+e- in the kinematic range mee>10 MeV/c2, with a negligible background contamination. The sample is analysed to search for the presence of dark photons (A′) via the decay chain π0→γA′, A′→e+e-. No signal has been observed, and the exclusion limits on space of the dark photon mass mA′ and the mixing parameter 2 are reported.
Search for dark photon and long-lived particles at BaBar
LUSIANI A. (for the BaBar Collaboration)
2016, 46(7): 601-607. doi: 10.3969/j.issn.0253-2778.2016.07.010
The complete large and clean sample of e+e- collisions recorded by the BaBar detector at PEP-II at the SLAC National Laboratory was used to search for a photon-like particle with mass and decaying into an e+e- or μ+μ- pair, which is proposed in some Dark Matter theory models, and to search for a long-lived particle that decays into an oppositely charged fermion pair, predicted in a number of New Physics models. We do not observe a significant signal and we set 90% confidence level upper limits of several production rates and on the parameters of some proposed New Physics models.
Precision hadron physis at the Mainz Microtron MAMI and MESA
2016, 46(7): 608-616. doi: 10.3969/j.issn.0253-2778.2016.07.011
The Mainz Microtron MAMI is a high-intensity electron accelerator for fixed-target experiments in the fields of hadron and low-energy particle physics. It provides a polarized beam of up to 1.6 GeV beam energy. Two major installations are currently in operation at MAMI: the high-resolution spectrometer setup A1 as well as the A2 detector setup, which consists of the Crystal Ball detector in conjunction with the TAPS calorimeter. Highlights of the research program at MAMI are measurements of the electromagnetic form factors and polarizabilities of the proton, which are related to the proton radius puzzle, the measurement of the transition form factors and slope parameters of the eta meson, as well as the search for hypothetical gauge bosons of the dark sector, also denoted as dark photons. Currently, the new electron accelerator MESA is in preparation at Mainz, which will allow for a precision measurement of the electroweak mixing angle at low momentum transfer as well as measurements of low-energy electron-nucleon/nucleus scattering for various applications in nuclear, hadron, and particle physics.
The Belle II experiment and SuperKEKB upgrade
WANG Boqun (for the Belle II Collaboration)
2016, 46(7): 617-624. doi: 10.3969/j.issn.0253-2778.2016.07.012
The Belle II/SuperKEKB experiment is an e+e- collider running at the Υ(4S) resonance energy to produce B meson pairs. As an upgrade of the Belle/KEKB experiment, it will start physics data taking from 2018 and with ~40 times luminosity. Its goal is to accumulate 50 ab-1 of e+e- collision data. Now the upgrade of the sub-detector systems is on-going in KEK. The physics program has a wide range of areas, including searches for direct CP violation (CPV), lepton flavour violation and dark matter. The current upgrade status of Belle II and SuperKEKB is reviewed and some physics opportunities at this facility are introduced.