Advances in the experimental study of pentaquark hadrons at LHCb
In the Standard Model of particle physics,the strong interaction is described by quantum chromodynamics(QCD).Although the asymptotic freedom of QCD at high energies has been extensively validated through experiments,our understanding of its non-perturbative characteristics at low energies remains limited,with the phenomenon of quark confinement in strong interactions being a longstanding global challenge.Hadron spectroscopy serves as a crucial means for understanding strong interactions.Pentaquark states,as one kind of exotic hadronic states with rich internal structures,provide a unique platform for studying strong interactions.Since the first discovery of pentaquark states by the LHCb experiment in 2015,both theoretical and experimental research on pentaquark states has developed rapidly,promoting each other.In this paper,we will introduce the progress made by the LHCb experiment in the study of pentaquark states,and look forward to the opportunities for pentaquark research brought by the new data taken and the future upgrade program from the LHCb experiment.The experimental search for pentaquark states has a long and controversial history,especially in early 2000s.However,the situation changed in 2015 when the LHCb experiment discovered several pentaquark candidates decaying into J/ψp inΛ0 → J/ψpK-decays.This was a major turning point in exotic baryon spectroscopy.In 2015,the LHCb experiment reported observations of J/ψp resonance structures in the decay of Λ0b → J/ψpK-,consistent with the hidden-charm pentaquark state with quark component ccuud.Initial observations showed a broader Pc(4380)+and a narrower Pc(4450)+resonance state.More recent results in 2019 have shown finer pentaquark structures.The peak at 4450 MeV was resolved into two states of Pc(4440)+and Pc(4457)+.In addition,a new narrower peak Pc(4312)+was observed.But the wider Pc(4380)+state awaits further confirmation.Due to the SU(3)flavour symmetry,one would expect the existence of strange pentaquark states with hidden-charm,which would decay into J/ψΛ.Within the Ξ-b → J/ψΛK-decay,the first evidence for strange pentaquark candidate Pcs(4459)0 was observed.With tireless efforts,the first discovery was finally made.A new pentaquark candidate PΛψs(4338)0 with quark content c(c)uds was uncovered in the B-→ J/ψΛ(p)decays.To date,we have observed a series of hidden-charm pentaquark candidates containing cc quark pairs,including Pc(4312)+,Pc(4440)+,Pc(4457)+and PΛψs(4338)0.This initial family of hidden-charm pentaquarks exhibits striking features.These candidates have very narrow widths,lie below or are very close to the mass thresholds of the charmed baryons and anticharm mesons,and clearly exhibit the characteristics of the"molecular"type of pentaquark states.The discovery of this series of pentaquark states provides important clues for us to study pentaquark states in depth.In order to better understand the whole picture of pentaquarks,it is necessary to search for pentaquark states of different quark types,such as with open charm,double charm,etc.More experimental evidence is needed as to whether the internal structures of the observed states are"molecular"or compact,and we can measure their spin-parity quantum numbers and compare them with theoretical predictions.Investigation of their more decay channels would also be helpful.Most of these studies require more data.Thankfully,the LHCb experiment will accumulate data samples of 50 fb-1 integral luminosity over the next 10 years,and even 300 fb-1 is planned after the Phase Ⅱ upgrade in 2033.These will provide a powerful big-science platform to support new discoveries as well as deeper exploration of pentaquark states.
Large Hadron ColliderLHCb experimentexotic hadronspentaquark states
万方数据
维普
