Experimental study on the doubly heavy baryons at LHCb:A review
The doubly heavy baryons are formed by two heavy quarks and one light quark,as predicted by the quark model.They are expected to have different internal structures than those of the normal baryons such as the proton and the neutron,therefore have unique properties.Study and search for the doubly heavy baryons is one of the key research topics for both the theoretical and experimental particle physics.The LHCb experiment is one of the four large ones on the Large Hadron Collider at CERN.It has excellent vertexing,tracking and particle-identification performances,and a flexible and efficient trigger system,which make it an ideal place to perform experimental studies of the doubly heavy baryons.Using the proton-proton collision data collected by the LHCb experiment in 2016,the doubly charmed baryon consisting of two charm quarks and one up quark,the Ξ++cc baryon,has been discovered for the first time with the decay mode E++cc → Λ+cK-π+π+as suggested by the Chinese theorists.Its mass has been measured to be 3621.40±0.72(stat)±0.27(syst)±0.14(Λ+c)MeV/c2.Its lifetime has been measured to be 0.256+0.024-0.022(stat)±0.014(syst)picosecond.Its production cross-section times the branching fraction of the Ξ++cc → Λ+cK-π+π+decay relative to that of the Λ+c baryon is measured to be(2.22±0.27±0.29)x 10-4 in the LHCb acceptance.Such discovery is also confirmed with another decay mode of the doubly charmed baryon Ξ++cc→ Ξ+cπ+.The third decay mode Ξ++cc→Ξ'c+π+has been observed and its branching fraction is measured relative to the Ξ++cc → Ξ+cπ+decay.The Ξ++cc mass measurement has been updated with both Ξ++cc → Λ+cK-π+π+and Ξ++cc → Ξc+π+decays and using the proton-proton collision data taken by the LHCb experiment from 2016 to 2018,and the precision has been improved by a factor of two.Using all the proton-proton collision data taken by the LHCb experiment during its first and second run periods,and the Ξ+cc → Λc+K-π+decay and a new one Ξ+cc → Ξ+cπ π-,the search for the doubly charmed baryon with a single charge Ξ+cc has been updated.The sensitivity is increased by one order of magnitude,and a hint is seen in data.First searches have been performed for the strange doubly charmed baryon Ω+cc,also for the Ξ+bc,Ξ0bc,Ω0bc baryons,which consist of a beauty quark,a charm quark and a light quark.Hints of the strange doubly charmed baryonΩ+cc and the charged doubly heavy baryon Ξ+bc are seen in data.These achievements open a new window on studying hadron structures and properties,which will deepen our understanding of the strong interaction and the quark model.The LHCb detector has been upgraded in 2019-2021,and is scheduled to be upgraded again in 2033-2034.The LHCb experiment is expected to collect 300 fb-1 of proton-proton collision data by the end of the LHC operation.Based on such a huge dataset,the LHCb experiment will be able to observe the doubly charmed baryon Ξ+cc,the strange doubly charmed baryon Ω+cc,and the doubly heavy baryons Ξ+bc,Ξ0bc and Ωbc.Their properties,such as the mass,lifetime,production cross-section,will be measured.In the meanwhile,the excited states of the doubly heavy baryons will also be searched.Such experimental studies will further deepen our understanding of the strong interaction and the quark model.
quark modeldoubly heavy baryonsLHCb experimentmass measurementlifetime measurementproduction measurement
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