In the March 12, 2021 issue ofPhysical Review Letters, the BESIII collaboration reports the discovery of an exotic multiquark structure, dubbedZcs(3985), that is produced in the process ofe+e-→K+(Ds¯D＊0+Ds＊¯D0) at ane+e-center-of-mass energy of 4.68 GeV. The structureZcs(3985) is observed to decay to a charged strange-charmed meson plus a neutral charmed meson,i.e.,Ds¯D＊0+Ds＊¯D0, and has a mass of 3.98 GeV/c2. This is the first candidate for a tetraquark meson containing hidden-charm with nonzero strangeness. This paper was selected as an “Editors’Suggestion” for that issue of the journal and was prominently featured on the APS synopsis website.

Studies of exotic hadron states,i.e., those that contain more valence-quark constituents than the conventional three-quark baryons and two-quark mesons, provide unique access to details of the nonperturbative effects and color confinement mechanisms in quantum chromodynamics (QCD) theory. The existence of hidden-charm tetraquarks with nonzero strangeness, noted asZcs, was predicted by many theoretical models, but until now, no definitive experimental observations have been reported.

Illustration for the tetraquark constituent

Schematic diagram of the enhancement structure

The BESIII discovery emerged from analyses of data sets taken at five center-of-mass energy points ranging from 4.628 GeV to 4.698 GeV in 2020 with a total integrated luminosity of 3.7 fb-1. Using a novel partial reconstruction method that relies on the detection of the charged kaon and oppositely charged Ds meson,events corresponding toe+e-→K+(Ds¯D＊0+Ds＊¯D0)were identified by a distinct peak atD＊0mass in theK+Ds¯recoil mass spectrum. After removing randomcombination backgrounds, a clear enhancement was seen near theDs¯D＊0andDs＊¯D0mass thresholds that could not be attributed to any known conventional excited(strange-) charmed mesons. Therefore, the hypothesis of the presence of an exotic stateZcs(3985) in theDs¯D＊0andDs＊¯D0mass spectrum was introduced and found to fit the observed excess very well. A total of 127Zcs(3985)decays toDs¯D＊0orDs＊¯D0were observed with a global significance of 5.3 standard deviations. TheZcs(3985)has one unit of charge, one unit of strangeness, and couples strongly toDs¯D＊0orDs＊¯D0, a set of properties that can only be satisfied by a minimalccsūfour-quark substructure. Hence, it is a strong candidate for the predicted open-strange charmonium-like tetraquark.

A search forZcswas among the prominent physics motivations, proposed in the BESIII white paper published in 2020, for increasing the centerof-mass energy reach as part of the BEPCII upgrade.TheZcsdiscovery is the first major new BESIII result made possible by the success of the collider upgrade program, and represents an important milestone in the collaboration’s program to investigate and understand the underlying physics associated with the spectrum of the hidden-charm tetraquark states. A major derivative from this quest would be an improved understanding of non-perturbative effects in QCD theory.

TheZcs(3985) observation represents a significant breakthrough that opens up a new dimension of the tetraquark meson spectrum. Curiously,Zcs(3985)production is more pronounced at center-of-mass energies around 4.68 GeV than at nearby higher or lower energies, which indicates that this energy may be of particular interest. More BESIII data spanning the 4.68 GeV energy region is needed to thoroughly explore theZcsproduction mechanism, measure its spinparity quantum numbers, and search for its neutralZcscounterpart as well as heavierZcssiblings.