Observation of a Near-Threshold Structure in the K^{+} Recoil-Mass Spectra in e^{+}e^{-}→K^{+}(D_{s}^{-}D^{*0}+D_{s}^{*-}D^{0})

Evidence of a J/ψK_{S}^{0} Structure in B^{0}→J/ψϕK_{S}^{0} Decays

An amplitude analysis of B^{0}→J/ψϕK_{S}^{0} decays is performed using proton-proton collision data, corresponding to an integrated luminosity of 9  fb^{-1}, collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV. Evidence with a significance of 4.0 standard deviations of a structure in the J/ψK_{S}^{0} system, named T_{ψs1}^{θ}(4000)^{0}, is seen, with its mass and width measured to be 3991_{-10}^{+12} _{-17}^{+9}  MeV/c^{2} and 105_{-25}^{+29} _{-23}^{+17}  MeV, respectively, where the first uncertainty is statistical and the second systematic. The T_{ψs1}^{θ}(4000)^{0} state is likely to be the isospin partner of the T_{ψs1}^{θ}(4000)^{+} state, previously observed in the J/ψK^{+} system of the B^{+}→J/ψϕK^{+} decay. When isospin symmetry for the charged and neutral T_{ψs1}^{θ}(4000) states is assumed, the signal significance increases to 5.4 standard deviations.

Observation of a Resonant Structure near the D_{s}^{+}D_{s}^{-} Threshold in the B^{+}→D_{s}^{+}D_{s}^{-}K^{+} Decay

An amplitude analysis of the B^{+}→D_{s}^{+}D_{s}^{-}K^{+} decay is carried out to study for the first time its intermediate resonant contributions, using proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV. A near-threshold peaking structure, referred to as X(3960), is observed in the D_{s}^{+}D_{s}^{-} invariant-mass spectrum with significance greater than 12 standard deviations. The mass, width, and the quantum numbers of the structure are measured to be 3956±5±10  MeV, 43±13±8  MeV, and J^{PC}=0^{++}, respectively, where the first uncertainties are statistical and the second systematic. The properties of the new structure are consistent with recent theoretical predictions for a state composed of cc[over ¯]ss[over ¯] quarks. Evidence for an additional structure is found around 4140 MeV in the D_{s}^{+}D_{s}^{-} invariant mass, which might be caused either by a new resonance with the 0^{++} assignment or by a J/ψϕ↔D_{s}^{+}D_{s}^{-} coupled-channel effect.

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.

An updated review of the new hadron states

The past decades witnessed the golden era of hadron physics. Many excited open heavy flavor mesons and baryons have been observed since 2017. We shall provide an updated review of the recent experimental and theoretical progresses in this active field. Besides the conventional heavy hadrons, we shall also review the recently observed open heavy flavor tetraquark statesX(2900) andTcc+(3875)as well as the hidden heavy flavor multiquark statesX(6900),Pcs(4459)0,Zcs(3985)-,Zcs(4000)+, andZcs(4220)+. We will also cover the recent progresses on the glueballs and light hybrid mesons, which are the direct manifestations of the non-AbelianSU(3) gauge interaction of the Quantum Chromodynamics in the low-energy region.

Evidence for a Neutral Near-Threshold Structure in the K_{S}^{0} Recoil-Mass Spectra in e^{+}e^{-}→K_{S}^{0}D_{s}^{+}D^{*-} and e^{+}e^{-}→K_{S}^{0}D_{s}^{*+}D^{-}

We study the processes e^{+}e^{-}→K_{S}^{0}D_{s}^{+}D^{*-} and e^{+}e^{-}→K_{S}^{0}D_{s}^{*+}D^{-}, as well as their charge conjugated processes, at five center-of-mass energies between 4.628 and 4.699 GeV, using data samples corresponding to an integrated luminosity of 3.8  fb^{-1} collected by the BESIII detector at the BEPCII storage ring. Based on a partial reconstruction technique, we find evidence of a structure near the thresholds for D_{s}^{+}D^{*-} and D_{s}^{*+}D^{-} production in the K_{S}^{0} recoil-mass spectrum, which we refer to as the Z_{cs}(3985)^{0}. Fitting with a Breit-Wigner line shape, we find the mass of the structure to be (3992.2±1.7±1.6)  MeV/c^{2} and the width to be (7.7_{-3.8}^{+4.1}±4.3)  MeV, where the first uncertainties are statistical and the second are systematic. The significance of the Z_{cs}(3985)^{0} signal is found to be 4.6σ including both the statistical and systematic uncertainty. We report the Born cross section multiplied by the branching fraction at different energy points. The mass of the Z_{cs}(3985)^{0} is close to that of the Z_{cs}(3985)^{+}. Assuming SU(3) symmetry, the cross section of the neutral channel is consistent with that of the charged one. Hence, we conclude that the Z_{cs}(3985)^{0} is the isospin partner of the Z_{cs}(3985)^{+}.

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.

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.

Overview of most recent BESIII results

Since 2009, the BESIII experiment has been colliding e+e− beams in the energy range between 2.0 and 4.9 GeV. This allowed the BESIII Collaboration to collect three of the largest data sets of charmonium resonances (J/ψ, ψ(2S), and ψ(3770)) in the world as well as other data sets at the centre-of-mass energies above 3.8 GeV. These datasets allow BESIII to deepen the knowledge of the properties of charmonia; investigate the light hadrons spectra; probe the decay and production mechanism of charmed mesons; search for the exotic XYZ hadrons and to understand their nature; perform precision Quantum ChromoDynamics measurements (such as R-value measurements and Form Factors estimations). Some of the most recent and noteworthy results from the BESIII collaboration are presented.

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.

Implications of the Zcs(3985) and Zcs(4000) as two different states

Recently, the hidden charm tetraquark states Z_cs(3985) and Z_cs(4000) with strangeness were observed by the BESIII and LHCb collaborations, respectively, which are great breakthroughs for exploring exotic quantum chromodynamics (QCD) structures. The first and foremost question is whether they are the same state. In this work, we explore the implications of the narrower state Z_cs(3985) in BESIII and the wider one Z_cs(4000) in LHCb as two different states. Within a solvable nonrelativistic effective field theory, we include the possible violations of heavy quark spin symmetry and SU(3) flavor symmetry in a comprehensive approach. If Z_cs(3985) and Z_cs(4000) are two different states, our results show that Z_cs(4000)/Z_cs(3985) is the pure (|D‾_s^∗D〉+/-|D‾_sD^∗〉)/2 state, and the SU(3) flavor partner of Z_c(3900) is Z_cs(4000) rather than the Z_cs(3985). Another two important consequences are the existence of a tensor D‾_s^∗D^∗ resonance with mass about 4126 MeV and width 13 MeV, and the suppression of the decay mode Z_cs(3985)→J/ψK. The two consequences can be tested in experiments and distinguish the two-state interpretation from the one-state scheme.

Observation of New Resonances Decaying to J/ψK^{+} and J/ψϕ

The first observation of exotic states with a new quark content cc[over ¯]us[over ¯] decaying to the J/ψK^{+} final state is reported with high significance from an amplitude analysis of the B^{+}→J/ψϕK^{+} decay. The analysis is carried out using proton-proton collision data corresponding to a total integrated luminosity of 9  fb^{-1} collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV. The most significant state, Z_{cs}(4000)^{+}, has a mass of 4003±6_{-14}^{+4}  MeV, a width of 131±15±26  MeV, and spin parity J^{P}=1^{+}, where the quoted uncertainties are statistical and systematic, respectively. A new 1^{+}  X(4685) state decaying to the J/ψϕ final state is also observed with high significance. In addition, the four previously reported J/ψϕ states are confirmed and two more exotic states, Z_{cs}(4220)^{+} and X(4630), are observed with significance exceeding 5 standard deviations.

Clues about the Nature of Multiquark States

We present a comprehensive study of the decay width of multiquark states containing different color singlet components in a coupled-channel approach. We show how the decay width can provide in-depth information about the nature of a coupled-channel resonance. An unexpected behavior of the decay width of a multiquark state could be pointing to a relevant role of the coupled-channel dynamics, aiming at the channel responsible for the formation of the resonance. The symmetrical situation between meson- and baryon-like multiquarks is highlighted. Our study accounts for the existence of narrow resonances with large phase spaces. In the case of resonances far from their detection channel, it is the mass difference with the formation channel that determines their decay width. The larger the binding, the larger the decay width, even though the phase space to the detection channel gets reduced. The trends noticed cast doubts on the molecular assignment of some multiquark candidates. Finally, we wonder about the existence and properties of multiquark partners in other flavor sectors.

Explaining the Many Threshold Structures in the Heavy-Quark Hadron Spectrum

Tremendous progress has been made experimentally in the hadron spectrum containing heavy quarks in the last two decades. It is surprising that many resonant structures are around thresholds of a pair of heavy hadrons. There should be a threshold cusp at any S-wave threshold. By constructing a nonrelativistic effective field theory with open channels, we discuss the generalities of threshold behavior, and offer an explanation of the abundance of near-threshold peaks in the heavy quarkonium regime. We show that the threshold cusp can show up as a peak only for channels with attractive interaction, and the width of the cusp is inversely proportional to the reduced mass relevant for the threshold. We argue that there should be threshold structures at any threshold of a pair of heavy-quark and heavy-antiquark hadrons, which have attractive interaction at threshold, in the invariant mass distribution of a heavy quarkonium and light hadrons that couple to that open-flavor hadron pair. The structure becomes more pronounced if there is a near-threshold pole. Predictions of the possible pairs are also given for the ground state heavy hadrons. Precisely measuring the threshold structures will play an important role in revealing the heavy-hadron interactions, and thus understanding the puzzling hidden-charm and hidden-bottom structures.