A report from the LHCb experiment.
Half a century since its inception, quantum chromodynamics (QCD) continues to prove itself as the correct description of the strong interaction between quarks and gluons. At low energies, however, perturbative calculations in QCD are not possible. Therefore, understanding the properties of hadrons usually requires the development of phenomenological models.
The study of exotic hadrons made up of more than three quarks offers a powerful way to gain a deeper understanding of the non-perturbative behaviour of QCD. The LHC has so far discovered no fewer than 23 new exotic hadrons, most of which were first observed by the LHCb experiment. In March 2021 the LHCb collaboration reported the observation of two tetraquarks with the c c u s quark content – named Tθψs1(4000)+ and Tθψs1(4220)+ – in the decay B+ → J/ψφK+. Now, based on a study of the isospin-symmetry-related decay B0 → J/ψφK0S using a sample of about 2000 candidate events, the collaboration has found evidence for a new tetraquark state, Tθψs1(4000)0, with a minimal quark content c c d s . The name of the new state follows a convention introduced by LHCb in 2022 to help simplify the exotic- hadron vista.
The Tθψs1(4000)0 state was found as a resonance in the J/ψK0S mass spectrum through an amplitude analysis, and is characterised as a horizontal band in the Dalitz plot (figure 1). Imposing isospin symmetry for all intermediate states except for the Tθψs1(4000)+/0 in the two B-meson decays, the signal significance is measured to be 4.0σ. The mass and width are found to be equal to those of the Tθψs1(4000)+ within uncertainties, which is consistent with the new state being an isospin partner of the Tθψs1(4000)0. If isospin symmetry between the two states is further applied, the Tθψs1(4000)0 significance increases to 5.4σ.
The Tθψs1(4000)+ and Tθψs1(4000)0 states are not the only pair of isospin partners of hidden-charm tetraquark candidates with strangeness. Recently the BESIII collaboration reported signals of the Tψs(3985)+ and Tψs(3985)0 states with a minimal quark content of c c u s and c c d s , respectively. Although the tetraquark candidates seen by BESIII and LHCb have similar masses, the natural widths measured by each experiment are significantly different, indicating that they are distinct states.
Further studies and theoretical inputs are needed to determine the inner structure of such hidden-charm tetraquark candidates, for example whether they are compact tetraquarks, hadron molecules or produced due to kinematic effects. Despite continuous efforts, the detailed mechanisms responsible for binding multi-quark states have remained mysterious. With the start of LHC Run 3 and a new upgraded detector, the LHCb collaboration can look forward to finding further exotic states that shed light on the low-energy behaviour of QCD relevant to hadronic matter.