First Evidence for the First Neutral Open-strange Hidden-charm Tetraquark State


The BESIII Collaboration recently reported evidence of a new exotic multiquark 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 neutral hidden-charm tetraquark with non-zero strangeness candidate, thus marking a new milestone in exploration of the genuine properties of the family of exotic multiquark hadrons. The paper has been published in Physical Review Letters.

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 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) is important to confirm and establish the properties of Zcs(3985).

This study is based on data sets taken at energy points ranging from 4.628 GeV to 4.699 GeV. Here the reactions of e^+e^-→Ks(Ds^+D^*-+Ds^*+D^-) are filtered out based on the partial reconstruction method, where the promptly produced KS and one of the ground-state charmed mesons Ds^+ or D^- are detected, and signal candidates were identified by a distinct peak at the excited D^*- or Ds^*+ mass in the Ks^0Ds^+ or Ks^0D^- 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 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's first experimental report on the existence of the neutral open-strange hidden-charm tetraquark state with 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 of its existence and crucial information on its properties. In the future, more data sets in this energy region will be necessary for further systematic studies of hidden-charm tetraquark states.

Fig. Signal of Zcs(3985)^0 in the KS recoil-mass spectra (Image by IHEP)

Contact Information

Mr. GUO Lijun