The BESIII Collaboration recently reported the evidence of a new exotic multi-quark state, namely Zcs(3985)0. It is the neutral partner of the charged tetraquark Zcs(3985)+ observed in 2021 at BESIII. This is the first candidate of the neutral hidden-charm tetraquark with non-zero strangeness, which marks a new milestone in exploration of the genuine properties of the family of the exotic multi-quark hadrons. The paper has been published in Physical Review Letters [Phy. Rev. Lett. 129, 112003 (2022)].
Studies of exotic hadrons, i.e., those that contain more valence-quark constituents than the conventional three-quark baryons and two-quark mesons, provide unique access to the exploration of the non-perturbative effects in QCD theory and color confinement mechanisms. In 2021, BESIII discovered the first open-strange hidden charm tetraquark Zcs(3985)+, which has unit charge. In theory, the existence of its neutral charge partner, Zcs(3985)0, is expected. The experimental exploration of the neutral Zcs(3985)0 is important to confirm and establish the property of the Zcs(3985)+.
This study is implemented based on the data sets taken at the energy points ranging from 4.628 GeV to 4.699 GeV, where the charged Zcs(3985)+ is observed. Here the reactions of e+e-→KS(Ds+D*-+Ds*+D-) are filtered out based on partial reconstruction method, where the promptly produced KS and one of the ground-state charmed mesons Ds+ or D- are detected, and signal candidate are identified by a distinct peak at the excited D*- or Ds*+ mass in the KS0Ds+ or KS0D- recoil-mass spectrum. Then a clear enhancement over known backgrounds and conventional meson contributions in the KS recoil-mass spectra is seen near the Ds+D*- and Ds*+D- mass thresholds. A resonance fit of a hypothesis of the neutral Zcs(3985) decaying into the Ds+D*- and Ds*+D- final states is carried out and the observed excess is very well described. The significance of the Zcs(3985)0 is found to be 4.6 standard deviations. The mass of the neutral Zcs(3985)0 is close to that of the charged Zcs(3985)+. Under the SU(3) symmetry, the cross sections of the neutral channel are consistent with those of the charged one. Hence, we conclude that neutral Zcs(3985)0 is the isospin partner of the charged Zcs(3985)+. Hence, their isospin quantum numbers are established to be half.
This is the world-first experimental report on the existence of the neutral open-strange hidden-charm tetraquark state. which has a new-type quark structure (c c-bar s d-bar). Following the discovery of the charged Zcs(3985) at BESIII, the evidence of the neutral partner at BESIII provides critical confirmation on its existence and crucial information on its properties. In the future, more data sets in this energy region become necessary for further systematic studies of the hidden-charm tetraquark states.
The PRL publication: https://link.aps.org/doi/10.1103/PhysRevLett.129.112003
The INSPIRE entry: https://inspirehep.net/literature/2074678