Exotic Charmonium-like States Research Articles

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.

Exotic Charmonium-Like States at BESIII

Exotic Charmonium-Like States at BESIII

Search for muoproduction of the X(3872) at COMPASS

Exotic charmonium-like states have been observed by various experiments over the last 15 years, but their nature is still under discussion. Photo-(muo)production is a new promising instrument to study them. COMPASS, a fixed target experiment at CERN, analyzed the full set of the data collected with a muon beam between 2002 and 2011, covering the range from 7 GeV to 19 GeV in the centre-of-mass energy of the virtual photon-nucleon system. A signal in the mass spectrum of J/ψπ+π− with the statistical significance of 4.1 σ was observed in the reaction µ+N → µ+(J/ψπ+π−)π±N′. Its mass and width are consistent with those of the X(3872). The shape of the π+π− mass distribution from the observed decay into J/ψπ+π−is different from previous observations for X(3872). The observed signal may be interpreted as possible evidence of a new charmonium state (3872). It could be associated with a neutral partner of X(3872) with C = −1 predicted by a tetraquark model.

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.

Recent Results on Charmonium-like (Exotic) $XYZ$ States at the BESIII/BEPCII Experiment in Beijing/China

With about 12 fb$^{-1}$ collected XYZ data sets, BESIII continues the exploration of the exotic charmonium-like states. In this talk, recent results on the measurements of the spin-parity determination of $Z_{\rm c}(3900)$, as well as on line-shapes of $e^+e^- \rightarrow J/\psi\,\pi\pi, h_{\rm c}\pi\pi, \psi(2S)\,\pi^0\pi^0/\pi^+\pi^-$, and $\pi^+ D^0 D^{*-}$ from open charm are discussed. Also, the recent observation of $e^+ e^- \rightarrow \phi \chi_{c1/2}$ at $\sqrt(s)=4.6$ GeV is reported.

Hidden charm octet tetraquarks from a diquark-antidiquark model

Four exotic charmonium-like states, i.e. $X(4140)$, $X(4274)$, $X(4500)$, and $X(4700)$, have been observed very recently by LHCb Collaboration in the decay process $B^+\to J/\psi \phi K^+$ using the 3${\rm fb}^{-1}$ data of $p\bar p$ collision at $\sqrt s= 7$ and $8$ TeV. In this paper, we investigate systematically the properties of hidden charm tetraquark states. The hidden charm tetraquarks form an octet configuration and a singlet configuration according to flavor $SU(3)$ symmetry. Based on a diquark-antidiquark model, the hidden charm tetraquarks spectra are given. The previous XYZ exotic states altogether with the newly ones $X(4140)$, $X(4274)$, $X(4500)$, and $X(4700)$, can be well classified into certain representations. The spin-parities and masses of the XYZ are predicted, most of which are in agreement with the data. We particularly find that $Z_c(4430)$ may be treated as the first radial excitation of $Z_c(3900)$, while the $Y(1^{--})$ states can be obtained by the first orbital excitation of the $X/Z$ states. Besides, we calculate the decay widths of the hidden charm tetraquarks into two charmed mesons. This work gives a general tetraquark description for the XYZ states, which is helpful to uncover their inner structures.

Study of exotic states in charmonium family at Belle

We report results of a study of exotic charmonium-like states using the data sample accumulated with the Belle detector at the KEKB asymmetric-energy e+e− collider. A search for X(3872) decays to final states with the ηc is presented. We report evidence for a χc1γ mode in the decay B→χc1γK. A study of e+e−→π+π−J/ψ and an observation of a new charmonium-like resonance are described. Also observed are ψ(4040) and ψ(4160) decays to ηJ/ψ.

Hadron spectroscopy from B-factories

We review the recent experimental results on hadron spectroscopy from B-factories focusing on the exotic charmonium-like states. Among them we discuss the X(3872), Y(3940), Z(4430)+, Z(4050)+, Z(4250)+ and Y(4140) states found in B-meson decays, the X(3940) and X(4160) states produced in double charmonium production, the Y(4260), Y(4325), Y(4660) and X(4630) states produced with initial-state radiation in e+e− annihilation and the X(3915), Y(4350) states observed in two-photon collisions.

Exotic Charmoniumlike States

A review of recent experimental results concerning the XYZ charmoniumlike mesons is presented.