LHCb上双重味重子的实验研究进展

Experimental study on the doubly heavy baryons at LHCb:A review

本文介绍了LHCb实验上双重味重子的实验研究进展.LHCb实验组利用2016年采集的质子-质子对撞数据,通过中国粒子物理理论家建议的Ξ_(cc)^(++)→Λ_(c)^(+)K^(-)π^(+)π^(+)衰变道观测到由两个较重的粲夸克和一个较轻的上夸克组成的双粲重子Ξ_(cc)^(++),并测得其质量为3621.40±0.72(stat)±0.27(syst)±0.14(Λ_(c)^(+))Me V/c^(2).在此基础上,测得Ξ_(cc)^(++)的寿命为0.2560.022+0.024(stat)±0.014(syst)ps,确认了该粒子主要通过弱相互作用进行衰变的特性;测量了在LHCb探测器接收度范围内Ξ_(cc)^(++)与Λ_(c)^(+)重子产生截面的比值和Ξ_(cc)^(++)→Λ_(c)^(+)K^(-)π^(+)π^(+)衰变分支比的乘积;观测到了Ξ_(cc)^(++)→Ξcπ^(+)和Ξ_(cc)^(++)→Ξc'+π^(+)等新衰变道;联合利用Ξ_(cc)^(++)→Λ_(c)^(+)K^(-)π^(+)π^(+)和Ξ_(cc)^(++)→Ξ_(c)^(+)π^(+)衰变道以及更多实验数据,进一步提升了Ξ_(cc)^(++)重子的质量测量精度.LHCb实验组也利用2011~2018年间采集的所有质子-质子对撞数据,通过Ξ_(cc)^(+)→Λ_(c)^(+)k-π^(+)和Ξ_(cc)^(+)→Ξ_(c)^(+)π^(+)π^(-)过程搜寻了单电荷双粲重子Ξ_(cc)^(+),使得信号灵敏度相比以往结果提高了一个数量级,观测到了Ξ_(cc)^(+)重子存在的迹象;寻找了奇异双粲重子Ω_(cc)^(+),以及含底夸克和粲夸克的3种双重味重子Ξ_(bc)^(+)、Ξ_(cc)^(0)、Ω_(bc)^(0),观察到奇异双粲重子Ω_(cc)^(+)和带电双重味重子Ξ_(bc)^(+)的迹象.这些研究成果开辟了探索强子结构与性质的一个新窗口,将有助于加深对强相互作用本质和强子结构理论的理解.

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 heaw 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 heaw barvons.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 modeΞ_(cc)^(++)→Λ_(c)^(+)K^(-)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/c^(2).Its lifetime has been measured to be 0.256_(-0.022)~(+0.024)(stat)±0.014(syst) picosecond.Its production crosssection times the branching fraction of the Ξ_(cc)^(++)→Λ_(c)^(+)K^(-)π^(+)π^(+) decay relative to that of the Λ_(c)^(+) baryon is measured to be(2.22±0.27±0.29) × 10-4 in the LHCb acceptance.Such discovery is also confirmed with another decay mode of the doubly charmed baryon Ξ_(cc)^(++)→Ξ_(c)^(+)π^(+).The third decay mode Ξ_(cc)^(++)→Ξ_c~('+)π^(+) has been observed and its branching fraction is measured relative to the Ξ_(cc)^(++)→Ξ_(c)^(+)π^(+) decay.The Ξ_(cc)^(++) mass measurement has been updated with both Ξ_(cc)^(++)→Λ_(c)^(+)K^(-)π^(+)π^(+)and Ξ_(cc)^(++)→Ξ_(c)^(+)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)~+→Λ_(c)^(+)K~-π^(+) decay and a new one Ξ_(cc)~+→Ξ_(c)^(+)π^(+)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)^(+),Ξ_(cc)^(0),Ω_(bc)^(0) baryons,which consist of a beauty quark,a charm quark and a light quark.Hints of the strange doubly charmed baryon Ω_(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 E_(cc)~+,the strange doubly charmed baryon Ω_(cc)^(+),and the doubly heavy baryons Ξ_(bc)^(+),Ξ_(bc)^(+) and Ω_(bc)^(0).Their properties,such as the mass,lifetime,production crosssection,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.