Observation of a broad structure in the pi+ pi- J/psi mass spectrum around 4.26 GeV/c2

First Observation of a Three-Resonance Structure in e^{+}e^{-}→Nonopen Charm Hadrons

We report the measurement of the inclusive cross sections for e^{+}e^{-}→nOCH (where nOCH denotes non-open charm hadrons) with improved precision at center-of-mass (c.m.) energies from 3.645 to 3.871 GeV. We observe three resonances: R(3760), R(3780), and R(3810) with significances of 8.1σ, 13.7σ, and 8.8σ, respectively. The R(3810) state is observed for the first time, while the R(3760) and R(3780) states are observed for the first time in the nOCH cross sections. Two sets of resonance parameters describe the energy-dependent line shape of the cross sections well. In set I [set II], the R(3810) state has mass (3805.7±1.1±2.7) [(3805.7±1.1±2.7)]  MeV/c^{2}, total width (11.6±2.9±1.9) [(11.5±2.8±1.9)]  MeV, and an electronic width multiplied by the nOCH decay branching fraction of (10.9±3.8±2.5) [(11.0±3.4±2.5)]  eV. In addition, we measure the branching fractions B[R(3760)→nOCH]=(25.2±16.1±30.4)%[(6.4±4.8±7.7)%] and B[R(3780)→nOCH]=(12.3±6.6±8.3)%[(10.4±4.8±7.0)%] for the first time. The R(3760) state can be interpreted as an open-charm (OC) molecular state, but containing a simple four-quark state component. The R(3810) state can be interpreted as a hadrocharmonium state.

Observation of Structures in the Processes e^{+}e^{-}→ωχ_{c1} and ωχ_{c2}

We present measurements of the Born cross sections for the processes e^{+}e^{-}→ωχ_{c1} and ωχ_{c2} at center-of-mass energies sqrt[s] from 4.308 to 4.951 GeV. The measurements are performed with data samples corresponding to an integrated luminosity of 11.0  fb^{-1} collected with the BESIII detector operating at the Beijing Electron Positron Collider storage ring. Assuming the e^{+}e^{-}→ωχ_{c2} signals come from a single resonance, the mass and width are determined to be M=(4413.6±9.0±0.8)  MeV/c^{2} and Γ=(110.5±15.0±2.9)  MeV, respectively, which is consistent with the parameters of the well-established resonance ψ(4415). In addition, we also use one single resonance to describe the e^{+}e^{-}→ωχ_{c1} line shape and determine the mass and width to be M=(4544.2±18.7±1.7)  MeV/c^{2} and Γ=(116.1±33.5±1.7)  MeV, respectively. The structure of this line shape, observed for the first time, requires further understanding.

Precise Measurement of the e^{+}e^{-}→D_{s}^{*+}D_{s}^{*-} Cross Sections at Center-of-Mass Energies from Threshold to 4.95 GeV

The process e^{+}e^{-}→D_{s}^{*+}D_{s}^{*-} is studied with a semi-inclusive method using data samples at center-of-mass energies from threshold to 4.95 GeV collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross sections of the process are measured for the first time with high precision in this energy region. Two resonance structures are observed in the energy-dependent cross sections around 4.2 and 4.4 GeV. By fitting the cross sections with a coherent sum of three Breit-Wigner amplitudes and one phase-space amplitude, the two significant structures are assigned masses of (4186.8±8.7±30) and (4414.6±3.4±6.1)  MeV/c^{2}, widths of (55±15±53) and (122.5±7.5±8.1)  MeV, where the first errors are statistical and the second ones are systematic. The inclusion of a third Breit-Wigner amplitude is necessary to describe a structure around 4.79 GeV.

Observation of Three Charmoniumlike States with J^{PC}=1^{--} in e^{+}e^{-}→D^{*0}D^{*-}π^{+}

The Born cross sections of the process e^{+}e^{-}→D^{*0}D^{*-}π^{+} at center-of-mass energies from 4.189 to 4.951 GeV are measured for the first time. The data samples used correspond to an integrated luminosity of 17.9  fb^{-1} and were collected by the BESIII detector operating at the BEPCII storage ring. Three enhancements around 4.20, 4.47, and 4.67 GeV are visible. The resonances have masses of 4209.6±4.7±5.9  MeV/c^{2}, 4469.1±26.2±3.6  MeV/c^{2}, and 4675.3±29.5±3.5  MeV/c^{2} and widths of 81.6±17.8±9.0  MeV, 246.3±36.7±9.4  MeV, and 218.3±72.9±9.3  MeV, respectively, where the first uncertainties are statistical and the second systematic. The first and third resonances are consistent with the ψ(4230) and ψ(4660) states, respectively, while the second one is compatible with the ψ(4500) observed in the e^{+}e^{-}→K^{+}K^{-}J/ψ process. These three charmoniumlike ψ states are observed in the e^{+}e^{-}→D^{*0}D^{*-}π^{+} process for the first time.

Observation of Resonance Structures in e^{+}e^{-}→π^{+}π^{-}ψ_{2}(3823) and Mass Measurement of ψ_{2}(3823)

Using a data sample corresponding to an integrated luminosity of 11.3  fb^{-1} collected at center-of-mass energies from 4.23 to 4.70 GeV with the BESIII detector, we measure the product of the e^{+}e^{-}→π^{+}π^{-}ψ_{2}(3823) cross section and the branching fraction B[ψ_{2}(3823)→γχ_{c1}]. For the first time, resonance structure is observed in the cross section line shape of e^{+}e^{-}→π^{+}π^{-}ψ_{2}(3823) with significances exceeding 5σ. A fit to data with two coherent Breit-Wigner resonances modeling the sqrt[s]-dependent cross section yields M(R_{1})=4406.9±17.2±4.5  MeV/c^{2}, Γ(R_{1})=128.1±37.2±2.3  MeV, and M(R_{2})=4647.9±8.6±0.8  MeV/c^{2}, Γ(R_{2})=33.1±18.6±4.1  MeV. Though weakly disfavored by the data, a single resonance with M(R)=4417.5±26.2±3.5  MeV/c^{2}, Γ(R)=245±48±13  MeV is also possible to interpret data. This observation deepens our understanding of the nature of the vector charmoniumlike states. The mass of the ψ_{2}(3823) state is measured as (3823.12±0.43±0.13)  MeV/c^{2}, which is the most precise measurement to date.

Prediction of a Narrow Exotic Hadronic State with Quantum Numbers J^{PC}=0^{--}

Lots of charmonium-like structures have been observed in the last two decades. Most of them have quantum numbers that can be formed by a pair of charm and anticharm quarks, thus it is difficult to unambiguously identify the exotic ones among them. In this Letter, by exploiting heavy quark spin symmetry, we present a robust prediction of the hadronic molecular scenario, where the ψ(4230), ψ(4360) and ψ(4415) are identified as DD[over ¯]_{1}, D^{*}D[over ¯]_{1}, and D^{*}D[over ¯]_{2}^{*} bound states, respectively. We show that a flavor-neutral charmonium-like exotic state with quantum numbers J^{PC}=0^{--}, denoted as ψ_{0}(4360), should exist as a D^{*}D[over ¯]_{1} bound state. The mass and width of the ψ_{0}(4360) are predicted to be (4366±18)  MeV and less than 10 MeV, respectively. The ψ_{0}(4360) is significant in two folds: no 0^{--} hadron has been observed so far, and a study of this state will enlighten the understanding of the mysterious vector mesons between 4.2 and 4.5 GeV, as well as the nature of previously observed exotic Z_{c} and P_{c} states. We propose that such an exotic state can be searched for in e^{+}e^{-}→ηψ_{0}(4360) and uniquely identified by measuring the angular distribution of the outgoing η meson.

Observation of B 0 → ψ (2S)K S 0 π + π - and B s 0 → ψ (2S)K S 0 decays

Using a data sample of s = 13 TeV proton-proton collisions collected by the CMS experiment at the LHC in 2017 and 2018 with an integrated luminosity of 103 fb - 1 , the B s 0 → ψ ( 2S ) K S 0 and B 0 → ψ ( 2S ) K S 0 π + π - decays are observed with significances exceeding 5 standard deviations. The resulting branching fraction ratios, measured for the first time, correspond to B ( B s 0 → ψ ( 2S ) K S 0 ) / B ( B 0 → ψ ( 2S ) K S 0 ) = ( 3.33 ± 0.69 ( stat ) ± 0.11 ( syst ) ± 0.34 ( f s / f d ) ) × 10 - 2 and B ( B 0 → ψ ( 2S ) K S 0 π + π - ) / B ( B 0 → ψ ( 2S ) K S 0 ) = 0.480 ± 0.013 ( stat ) ± 0.032 ( syst ) , where the last uncertainty in the first ratio is related to the uncertainty in the ratio of production cross sections of B s 0 and B 0 mesons, f s / f d .

The CGEM-IT: An Upgrade for the BESIII Experiment

The BESIII experiment has been collecting data since 2009 at the e+e− collider BEPCII in Beijing, a charm-τ factory characterized by high statistics and high precision. The discovery of exotic charmonium-like states and the still open questions in low-energy QCD led to an extension of the experimental program, with several upgrades. This review focuses on the CGEM-IT, the innovative solution proposed to replace the current inner tracker, which is aging. It consists of three, co-axial, cylindrical triple-GEM detectors and will be the first cylindrical GEM operating inside a 1 T magnetic field with analogue readout. For this purpose, a dedicated mixed-signal ASIC for the readout of CGEM-IT signals and FPGA-based electronics for data processing have been developed. The simultaneous measurement of both ionization charge and time distribution enables three reconstruction algorithms, to cope with the asymmetry of the electron avalanche in the magnetic field and with non-orthogonal incident tracks. The CGEM-IT will not only restore the design efficiency but also improve the secondary vertex reconstruction and the radiation tolerance. The gas mixture and gain settings were chosen to optimize the position resolution to ∼130 µm in the transverse plane and better than 350 µm along the beam direction. This paper addresses the innovative aspects in terms of construction, readout, and software, employed to achieve the design goals as well as the experimental measurements performed during the development and commissioning of the CGEM-IT.

The Interplay between Compact and Molecular Structures in Tetraquarks

Due to the cluster reducibility of multiquark operators, a strong interplay exists in tetraquarks between the compact structures, resulting from the direct confining forces acting on quarks and gluons, and the molecular structure, dominated by the mesonic clusters. This issue is studied within an effective field theory approach, where the compact tetraquark is treated as an elementary particle. The key ingredient of the analysis is provided by the primary coupling constant of the compact tetraquark to the two mesonic clusters, considered here in the framework of a scalar interaction. Under the influence of this coupling, an initially formed compact tetraquark bound state evolves towards a new structure, where a molecular configuration is also present. In the strong-coupling limit, the evolution may end with a shallow bound state of the molecular type. The strong-coupling regime is also favored by the large Nc properties of QCD. The interplay between compact and molecular structures may provide a natural explanation of the existence of many shallow bound states.

Electromagnetic Form Factor of Doubly-Strange Hyperon

The standard model of particle physics is a well-tested theoretical framework, but there are still some issues that deserve experimental and theoretical investigation. The Ξ resonances with strangeness S=−2, the so-called doubly-strange hyperon, can provide important information to further test the standard model by studying their electromagnetic form factors, such as probing the limitation of the quark models and spotting unrevealed aspects of the QCD description of the structure of hadron resonances. In this work, we review some recent studies of the electromagnetic form factors on doubly-strange hyperons in pair production from positron–electron annihilation experiment.

Impact of clustering inside compact tetraquarks

Due to the reducibility of tetraquark operators into mesonic clusters, a strong interplay exists in tetraquarks between compact and molecular structures. This issue is studied within an effective field theory approach, where the compact tetraquark is treated as an elementary particle. Under the influence of the coupling to the mesonic clusters, an initially formed compact tetraquark bound state is deformed towards a new structure of the molecular type, having the attributes of a shallow bound state.

Charmonium and charmoniumlike states at the BESIII experiment

Charmonium is a bound state of a charmed quark and a charmed antiquark, and a charmoniumlike state is a resonant structure that contains a charmed quark and antiquark pair but has properties that are incompatible with a conventional charmonium state. While operating at center-of-mass energies from 2 to 5 GeV, the BESIII experiment can access a wide mass range of charmonium and charmoniumlike states, and has contributed significantly in this field. We review BESIII results involving conventional charmonium states, including the first observation of the M1 transition ψ(2S) → γη c (2S) and the discovery of the ψ2(3823) state; and report on studies of charmoniumlike states, including the discoveries of the Z c (3900) and Z c (4020) tetraquark candidates, the resolution of the fine structure of the Y(4260) state, the discovery of the new production process e + e - → γX(3872) and the uncovering of strong evidence for the commonality among the X(3872), Y(4260) and Z c (3900) states. The prospects for further research at BESIII and proposed future facilities are also presented.

Direct Measurement of the Branching Fractions B(ψ(3686)→J/ψX) and B(ψ(3770)→J/ψX), and Observation of the State R(3760) in e^{+}e^{-}→J/ψX

We report a measurement of the observed cross sections of e^{+}e^{-}→J/ψX based on 3.21  fb^{-1} of data accumulated at energies from 3.645 to 3.891 GeV with the BESIII detector operated at the BEPCII collider. In analysis of the cross sections, we measured the decay branching fractions of B(ψ(3686)→J/ψX)=(64.4±0.6±1.6)% and B(ψ(3770)→J/ψX)=(0.5±0.2±0.1)% for the first time. The energy-dependent line shape of these cross sections cannot be well described by two Breit-Wigner (BW) amplitudes of the expected decays ψ(3686)→J/ψX and ψ(3770)→J/ψX. Instead, it can be better described with one more BW amplitude of the decay R(3760)→J/ψX. Under this assumption, we extracted the R(3760) mass M_{R(3760)}=3766.2±3.8±0.4  MeV/c^{2} , total width Γ_{R(3760)}^{tot}=22.2±5.9±1.4  MeV, and product of leptonic width and decay branching fraction Γ_{R(3760)}^{ee}B[R(3760)→J/ψX]=(79.4±85.5±11.7)  eV. The significance of the R(3760) is 5.3σ. The first uncertainties of these measured quantities are from fits to the cross sections and second systematic.

Radiative and Meson Decays of Y(4230) in Flavor SU(3)

The charmonium-like exotic states Y(4230) and the less known Y(4320), produced in e+e− collisions, are sources of positive parity exotic hadrons in association with photons or pseudoscalar mesons. We analyze the radiative and pion decay channels in the compact tetraquark scheme, with a method that proves to work equally well in the most studied D∗→γ/π+D decays. The decay of the vector Y into a pion and a Zc state requires a flip of charge conjugation and isospin that is described appropriately in the formalism used. Rates are found to depend on the fifth power of pion momentum, which would make the final states πZc(4020) strongly suppressed with respect to πZc(3900). The agreement with BES III data would be improved considering the πZc(4020) events to be fed by the tail of the Y(4320) resonance under the Y(4230). These results should renovate the interest in further clarifying the emerging experimental picture in this mass region.

Tetra- and Penta-Quark Structures in the Constituent Quark Model

With the development of high energy physics experiments, a large amount of exotic states in the hadronic sector have been observed. In order to shed some light on the nature of the tetraquark and pentaquark candidates, a constituent quark model, along with the Gaussian expansion method, has been employed systematically in real- and complex-range investigations. We review herein the double- and fully-heavy tetraquarks, but also the hidden-charm, hidden-bottom and doubly charmed pentaquarks. Several exotic hadrons observed experimentally were well reproduced within our approach; moreover, their possible compositeness and other properties, such as their decay widths and general patterns in the spectrum, are analyzed. Besides, we report also some theoretical predictions of tetra- and penta-quark states which have not seen by experiment yet.

Dynamical Approach to Decays of XYZ States

We review the existing results on the exotic XYZ states and their decays obtained within the confined covariant quark model. This dynamical approach is based on a non-local Lagrangian of hadrons with quarks, has built-in quark confinement, and is suited well for the description of different multiquark states, including the four quark ones. We focus our analysis on the various decay modes of five exotic states, X ( 3872 ) , Z c ( 3900 ) , Y ( 4260 ) , Z b ( 10610 ) , and Z b ′ ( 10650 ) , aiming to clarify their internal quark structures. By considering mostly branching fractions and decay widths using the molecular-type or the tetraquark-type interpolating currents, conclusions about the nature of these particles are drawn: the molecular structure is favored for Z c ( 3900 ) , Z b ( 10610 ) , and Z b ′ ( 10650 ) and the tetraquark for X ( 3872 ) and Y ( 4260 ) .

Measurement of the Cross Section for e^{+}e^{-}→Ξ^{-}Ξ[over ¯]^{+} and Observation of an Excited Ξ Baryon

Using a total of 11.0  fb^{-1} of e^{+}e^{-} collision data with center-of-mass energies between 4.009 and 4.6 GeV and collected with the BESIII detector at BEPCII, we measure fifteen exclusive cross sections and effective form factors for the process e^{+}e^{-}→Ξ^{-}Ξ[over ¯]^{+} by means of a single baryon-tag method. After performing a fit to the dressed cross section of e^{+}e^{-}→Ξ^{-}Ξ[over ¯]^{+}, no significant ψ(4230) or ψ(4260) resonance is observed in the Ξ^{-}Ξ[over ¯]^{+} final states, and upper limits at the 90% confidence level on Γ_{ee}B for the processes ψ(4230)/ψ(4260)→Ξ^{-}Ξ[over ¯]^{+} are determined. In addition, an excited Ξ baryon at 1820  MeV/c^{2} is observed with a statistical significance of 6.2-6.5σ by including the systematic uncertainty, and the mass and width are measured to be M=(1825.5±4.7±4.7)  MeV/c^{2} and Γ=(17.0±15.0±7.9)  MeV, which confirms the existence of the J^{P}=3/2^{-} state Ξ(1820).

The XYZ mesons: what they aren’t

I discuss the properties of some representative XYZ mesons in the context of the most commonly proposed models for their underlying nature.

X, Y, Z Search at Belle II

Search for exotics has increased importance since the observation of the X(3872), 13 years ago, announced by the Belle Collaboration. The observation of pentaquark states by LHCb, and the Z-charged states observed at Belle and BES III have raised even more the attention to the field. Presently several states are observed that do not fit potential models, and looking for them in different production mechanisms and search for their decay modes it is important, as well as to do precise measurement of their mass, width, lineshape. We shortly report in this note about the plan in searching for exotics at Belle II at KEK (Tsukuba, Japan), that just ended the Phase-II running period, and show the first re-discovery results using 5 pb−1 integrated luminosity.

Exotics at BESIII

With a collected integrated luminosity of about 12 fb−1, useful data set for XYZ new resonances, BESIII continues the exploration of these exotic charmonium-like states. In this proceeding, recent results on the measurements of the line-shape of e+e− → π+π−ψ(3868) and π+D0D*−, as well as the JP determination of ZC (3900) will be presented. An abundance of Y will be reported in several decay channels. Finally the present BESIII data above the ${e^ + }{e^ - }\, \to \,{\Lambda _C}{\bar \Lambda _C}$ threshold will be shortly discussed because they are barely in agreement with the current Belle data on the candidate charmed baryonium state Y(4660). A BEPCII upgrade, that will allow to settle this open question, has been funded.

XYZ states as hadronic molecules

In the past decade, a lot of new hadrons containing heavy quarks were discovered which do not fit into the scheme provided by the traditional quark models. Such states are known as the XYZ states and they are conventionally referred to as exotic ones. At present, there is no consensus on their nature, and different models and approaches have been suggested to explain their unusual properties. The talk is devoted to a brief overview of the molecule model for such exotic states.

Exclusive open-charm near-threshold cross sections in a coupled-channel approach

Data on 7 open-charm channels collected by the Belle Collaboration are analyzed simultaneously using a unitary approach based on a coupled channel model in a wide energy range $ \sqrt s $ = 3.7 − 4.7 GeV. The resulting fit provides a remarkably good overall description of the line shapes in all studied channels. Parameters of 5 vector charmonium resonances are extracted from the fit. It is demonstrated, that this approach could be used account for all exclusive channels and thus solve the long-term problem of the charmonium spectra near threshold.

X, Y, Z search at Belle II

The Belle II experiment has successfully concluded Phase-2 of data taking in July 2018, and just started Phase-3, with the complete detector setup. Great perspectives and unique physics cases are enhanced in the Belle II physics program. In the sector of charmonium and spectroscopy, Belle II will investigate several physics processes: ISR physics for the vector states, bottomonium search and the study of the X(3872) line shape are some examples where the contribution of the Belle II experiment will help in better understanding QCD, for which several open issues still remain unsolved. First simulations and data analyses will be shown, with the first available dataset (2018), and the plan for spectroscopy search at Belle II will be discussed.

Exotic hadrons from BESIII

We report the progress in the study of the exotic hadrons, also called XYZ states, at BESIII experiment with its unique data samples in e+ e− annihilation at center-of-mass energies of 3.8–4.6 GeV. The talk covers the observation of the Zc(3900) and Zc(4020) and new measurements related to them; updated measurement of the resonant parameters of the Y(4260) in e+ e−→ π+π− J/ψ and discovery of its new decay modes π+π−ψ(2S), ωχc0, π+π−hc, and D0D∗−π++c.c.; and observation of X(3872) in e+e− annihilation.

XYZ at Belle

Recent results of exotic states with heavy quarks, denoted as XYZ states, are presented. The results include searches for the Y(4260) in B meson decays, a spin partner of Y(4630), and the strange Pentaquark $ P_s^ + $. An addition, the measurements of the absolute branching fraction of X(3872) and search for double Zc production is dicussed.

Precise Predictions of Charmed-Bottom Hadrons from Lattice QCD

We report the ground state masses of hadrons containing at least one charm and one bottom quark using lattice quantum chromodynamics. These include mesons with spin (J) and parity (P), (J^{P}): 0^{-}, 1^{-}, 1^{+}, and 0^{+} and the spin 1/2 and 3/2 baryons. Among these hadrons, only the ground state of 0^{-} is known experimentally, and therefore our predictions provide important information for the experimental discovery of all other hadrons with these quark contents.

Precise Measurement of the e^{+}e^{-}→π^{+}π^{-}J/ψ Cross Section at Center-of-Mass Energies from 3.77 to 4.60 GeV

The cross section for the process e^{+}e^{-}→π^{+}π^{-}J/ψ is measured precisely at center-of-mass energies from 3.77 to 4.60 GeV using 9  fb^{-1} of data collected with the BESIII detector operating at the BEPCII storage ring. Two resonant structures are observed in a fit to the cross section. The first resonance has a mass of (4222.0±3.1±1.4)  MeV/c^{2} and a width of (44.1±4.3±2.0)  MeV, while the second one has a mass of (4320.0±10.4±7.0)  MeV/c^{2} and a width of (101.4_{-19.7}^{+25.3}±10.2)  MeV, where the first errors are statistical and second ones are systematic. The first resonance agrees with the Y(4260) resonance reported by previous experiments. The precision of its resonant parameters is improved significantly. The second resonance is observed in e^{+}e^{-}→π^{+}π^{-}J/ψ for the first time. The statistical significance of this resonance is estimated to be larger than 7.6σ. The mass and width of the second resonance agree with the Y(4360) resonance reported by the BABAR and Belle experiments within errors. Finally, the Y(4008) resonance previously observed by the Belle experiment is not confirmed in the description of the BESIII data.

Evidence of Two Resonant Structures in e^{+}e^{-}→π^{+}π^{-}h_{c}

The cross sections of e^{+}e^{-}→π^{+}π^{-}h_{c} at center-of-mass energies from 3.896 to 4.600 GeV are measured using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The cross sections are found to be of the same order of magnitude as those of e^{+}e^{-}→π^{+}π^{-}J/ψ and e^{+}e^{-}→π^{+}π^{-}ψ(2S), but the line shape is inconsistent with the Y states observed in the latter two modes. Two structures are observed in the e^{+}e^{-}→π^{+}π^{-}h_{c} cross sections around 4.22 and 4.39  GeV/c^{2}, which we call Y(4220) and Y(4390), respectively. A fit with a coherent sum of two Breit-Wigner functions results in a mass of (4218.4_{-4.5}^{+5.5}±0.9)  MeV/c^{2} and a width of (66.0_{-8.3}^{+12.3}±0.4)  MeV for the Y(4220), and a mass of (4391.5_{-6.8}^{+6.3}±1.0)  MeV/c^{2} and a width of (139.5_{-20.6}^{+16.2}±0.6)  MeV for the Y(4390), where the first uncertainties are statistical and the second ones systematic. The statistical significance of Y(4220) and Y(4390) is 10σ over one structure assumption.

Observation of the ψ(1 3D2) State in e+e-→π+π-γχc1 at BESIII

We report the observation of the X(3823) state in the process e+e-→π+π-X(3823)→π+π-γχc1 with a statistical significance of 6.2σ, in data samples at center-of-mass energies sqrt[s]=4.230, 4.260, 4.360, 4.420, and 4.600 GeV collected with the BESIII detector at the BEPCII electron positron collider. The measured mass of the X(3823) state is (3821.7±1.3±0.7)  MeV/c2, where the first error is statistical and the second systematic, and the width is less than 16 MeV at the 90% confidence level. The products of the Born cross sections for e+e-→π+π-X(3823) and the branching ratio B[X(3823)→γχc1,c2] are also measured. These measurements are in good agreement with the assignment of the X(3823) state as the ψ(1 3D2) charmonium state.

Study of e(+)e(-)→ωχ(cJ) at center of mass energies from 4.21 to 4.42 GeV

Based on data samples collected with the BESIII detector at the BEPCII collider at nine center of mass energies from 4.21 to 4.42 GeV, we search for the production of e^{+}e^{-}→ωχ_{cJ} (J=0, 1, 2). The process e^{+}e^{-}→ωχ_{c0} is observed for the first time, and the Born cross sections at sqrt[s]=4.23 and 4.26 GeV are measured to be (55.4±6.0±5.9) and (23.7±5.3±3.5)  pb, respectively, where the first uncertainties are statistical and the second are systematic. The ωχ_{c0} signals at the other seven energies and the e^{+}e^{-}→ωχ_{c1} and ωχ_{c2} signals are not significant, and the upper limits on the cross sections are determined. By examining the ωχ_{c0} cross section as a function of center of mass energy, we find that it is inconsistent with the line shape of the Y(4260) observed in e^{+}e^{-}→π^{+}π^{-}J/ψ. Assuming the ωχ_{c0} signals come from a single resonance, we extract the mass and width of the resonance to be (4230±8±6)  MeV/c^{2} and (38±12±2)  MeV, respectively, and the statistical significance is more than 9σ.

Selection rules for hadronic transitions of XYZ mesons

Many of the XYZ mesons discovered in the last decade can be identified as bound states of a heavy quark and antiquark in Born-Oppenheimer (BO) potentials defined by the energy of gluon and light-quark fields in the presence of static color sources. The mesons include quarkonium hybrids, which are bound states in excited flavor-singlet BO potentials, and quarkonium tetraquarks, which are bound states in BO potentials with light-quark+antiquark flavor. The deepest hybrid potentials are known from lattice QCD calculations. The deepest tetraquark potentials can be inferred from lattice QCD calculations of static adjoint mesons. Selection rules for hadronic transitions are derived and used to identify XYZ mesons that are candidates for ground-state energy levels in the BO potentials for charmonium hybrids and tetraquarks.