Progress on study of hidden-charm tetraquark states at LHCb

At the low-energy scale, quarks are always confined inside hadrons. Such a color confinement phenomenon is not yet fully understood in quantum chromodynamics (QCD), while hadron spectroscopy is a crucial way to probe it. In particular, the past decades have witnessed the prosperity of exotic hadrons composed of more than three valence quarks, which provide new insights into the color confinement regime. The LHCb experiment at the Large Hadron Collider (LHC) has been making remarkable contributions to the studies on exotic hadrons. This review presents the progress of studies on hidden-charm tetraquark resonances from the LHCb experiment.First is the study of hidden-charm states composed of cc¯qq¯(q=u/d), in particular, the χc1(3872) state. As the first discovered and most widely studied heavy exotic state, the χc1(3872) state has attracted lots of interest, but there is yet no consensus on its nature. LHCb performed a series of analyses targeting a better understanding of its internal structure, including measurements of its production cross-sections in proton-proton collisions, its decay properties, and its lineshape. In particular, LHCb tried for the first time to describe χc1(3872) with a Flatté lineshape to take into account its proximity to the D0D¯*0 threshold, and performed a pole study. LHCb has also been investigating other hidden-charm states through both hidden-charm and open-charm decay modes. Using hidden-charm decay modes, LHCb observed a series of X states in the J/ψϕ system of the B+→J/ψϕK+ decay, as well as in the J/ψϕ spectrum from diffractive processes of proton-proton collisions. LHCb also studied the B0→ψ'π−K+ decay, leading to the measurement of the spin-parity number of Zc(4430)+ and the identification of new exotic contributions. As for open-charm decays, LHCb saw a huge threshold enhancement in the Ds+Ds− system from the B+→Ds+Ds−K+ decay. It can be modelled by a X(3960) resonance with spin-parity 0++, which could be a strong candidate for a cc¯ss¯ tetraquark. Besides, four charmonium states are observed from the amplitude analysis of B+→D*±D∓K+, out of which χc1(4010) is likely to have an exotic contribution. Second is the study on hidden-charm tetraquark states containing a strange quark. In the amplitude analysis of the B+→J/ψϕK+ decay, LHCb observed a significant contribution from Zcs(4000)+→J/ψK+, which is a manifestly exotic tetraquark state containing a strange quark. Its spin-parity is determined to be 1+. It is still to be investigated whether it is the same state as Zcs(3985)− that which was observed earlier by BESIII e+e−→K+(Ds−D*0+Ds*−D0). Later, LHCb extended the study to the B0→J/ψϕKS0 decay, the isospin symmetric mode of B+→J/ψϕK+, and performed a joint amplitude study of the two modes. A neutral Zcs state is found B0→J/ψϕKS0 with a significance larger than three standard deviations. Its mass and width are consistent with those of Zcs(4000)+, so they are likely isospin partners. Finally is the discovery of the four-charm tetraquark state. Theories predict the existence of four-charm tetraquark states as early as right after the observation of the charm quark. The operation of LHC makes it possible to experimentally explore it, for which the J/ψ-pair mode is considered a golden channel. In the invariant-mass spectrum of J/ψ-pair candidates, LHCb observed a peaking structure around 6.9 GeV/c2, which was interpreted as a new resonance named X(6900), and a wide bump above the J/ψ-pair mass threshold. The X(6900) state is a candidate for a four-charm tetraquark, and has been confirmed by the ATLAS and CMS experiments afterwards.In summary, LHCb continues to promote the studies on heavy exotic hadrons by both discovering new resonances and deepening the understanding of existing states. Along with the current third running period, LHCb will accumulate a much larger statistic. We can thus expect more exciting outcomes from LHCb on exotic hadrons in the future.