Advances in precision measurements of the lifetimes and masses of the charmed baryons at LHCb
The charmed baryon is a class of baryons,that consists of at least one charm quark within.In the spectroscopy of the singly charmed baryons,all these baryons eventually decay into one of the four charmed baryons:Λ+c、Ξ+c、Ξ0 and Ω0c.Therefore,these four ground-state charmed baryons become the cornerstone of the whole charmed baryon spectroscopy.As they decay only via the weak interaction,in which numerous decay modes and different complex interactions are involved,studies of their properties become an important laboratory for exploring the mechanism of the fundamental interactions in particle physics.Precise measurement of their masses and lifetimes enhances our understanding of the non-perturbative property in strong interactions and stringent test of the Standard Model of particle physics.The LHCb experiment,located at the Large Hadron Collider at CERN,is equipped with the position and momentum measurement system and particle identification system of the state-of-art performance.LHCb has accumulated high-statistics proton-proton collision data during the years 2011-2012 and 2015-2018 with an integrated luminosity of 8.6 fb-1,where huge numbers of charmed baryons have been produced.The lifetime order was long believed to be rΞ+c＞τΛ+c＞τΞ0c＞τΩ0c,according to the measurements from fixed target experiments more than 20 years ago,where the Ω0c lifetime was the shortest.However,the situation changed dramatically when LHCb embarked on a series of new measurements of their lifetimes,which have established a new lifetime hierarchy to be τΞ+c＞τΩ0c＞τ Λ+c＞rΞ0c This means the Ω0c lifetime is the second longest and the Ξ0c lifetime becomes the shortest.In particular,the LHCb studies show that the Ω0c lifetime should be nearly four times of the previous measurement.The newly established lifetime hierarchy will help improve our knowledge of quantum chromodynamics in charm hadrons and provide crucial inputs for calibrating theoretical calculations based on the Standard Model.As the majority of excited charm baryons and bottom baryons decays eventually into ground-state charm baryons:Λ+c、Ξ+c、Ξ0c or Ω0c,the masses of the four charmed baryons are crucial base stones for studying the heavy baryons.Therefore,experimental precise determinations of their masses hold significant physical importance for the study of hadron spectroscopy and bottom quark physics.Mass,being a fundamental property of elementary particles,can reflect the dynamics of the strong interactions inside the hadron and the information about the interior structure.It serves as a crucial tool to understand the non-perturbative effects in quantum chromodynamics and the quark confinement.LHCb utilized the data taken during the years 2011-2012 and 2015-2018 and performed the most precise measurements of the masses of theΞ+c and Ω0c baryons.Especially,LHCb improves the mass precision of the Ω0c by four times compared to the previous world-average result,which makes the precisions of all the four charmed baryons less than 0.5 MeV.These accurate results provide stringent tests on various calculations from the theoretical models.In particular,most of the results of the Ω0c mass from the on-market lattice quantum chromodynamics are overall lower than the experimental result,which suggests some non-trivial modifications are needed in the calculations.In this paper,the lifetimes and masses of the ground-state charmed baryons at LHCb are presented.In addition,future prospects at LHCb are discussed.
Large Hadron ColliderLHCb experimentcharmed baryonmasslifetime
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