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虽然BaBar、Belle、LHCb这些实验给出了X(3872)精确的质量、宽度的测量结果,并得到了其量子数,同时,也给出X(3872)的某些衰变过程的测量结果。但是理解其内部构成,依然需要更多的实验信息。通过对众多实验给出的不同过程的分支比同时分析,发现目前有
$ 32^{+18}_{-32} $ %的分支比对应的衰变末态依然是未知的[70],如表1所示。最新的X(3872)$\rightarrow {\rm{\pi}}^{+}{\rm{\pi}}^{-}{{{\rm{J}}}}/{\rm{\psi}}$ 衰变的绝对分支比实验测量结果,信号显著性仍小于$ 5{\rm{\sigma}} $ [23]。因此,对X(3872)的研究仍然任重而道远。衰变模式 分支比/% ${{\rm{\pi }}^ + }{{\rm{\pi }}^ - }{\rm{J}}/{\rm{\psi }}$ $4.1^{+1.9}_{-1.1}$ ${\rm{\omega}} {\rm J}/{\rm{\psi}}$ $4.4^{+2.3}_{-1.3}$ ${\rm{\gamma}} {\rm J}/{\rm{\psi}}$ $1.1^{+0.6}_{-0.3}$ ${{\rm{D}}^0}{{{\bar {\rm D}}}^{*0}}+{\rm c.c.}$ $52^{+25}_{-14}$ ${\rm{\pi }}^{0}{\rm{\chi}}_{\rm c1}$ $3.6^{+2.2}_{-1.6}$ unknown $32^{+18}_{-32}$ 对X(3872)内部结构的探索,在实验上需要去寻找它的某些敏感衰变过程或者测量某些特定衰变过程的分支比,来为理论提供输入。理想情况下,我们也可以通过Y(4260)
$ \rightarrow {\rm{\gamma}} $ X(3872)的单举过程在BESIII上直接去测量它的衰变绝对分支比。然而由于很高的组合本底,实验上该方法非常困难。但是我们仍可以通过对X(3872)的不同衰变过程的寻找给出每个过程的分支比,并最终得到总的衰变分支比。从BESIII对${\rm X(3872) \rightarrow \rm{\pi}}^{+}{\rm{\pi}}^{-}{{{\rm{J}}}}/{\rm{\psi}}$ [14-15]和X(3872)$\rightarrow {\rm{\omega}} {\rm J}/{\rm{\psi}}$ [18] 的研究来看,它们的本底水平非常低,这将有利于我们对X(3872)未知衰变末态的寻找。BESIII实验上${{{\rm{e}}}}^{+}{{{\rm{e}}}}^{-} \rightarrow {\rm{\gamma}}$ X(3872)的产生截面峰值约为6 pb[14-15,18],X(3872)的样本大约有$6 \times 10^{4}$ 个,这使得在BESIII上能够探索衰变分支比约为0.1%水平的过程。因此,BESIII实验对X(3872)的研究具有优势。在BESIII上,目前在进行的研究包括对X(3872)未知衰变道的寻找,例如,X(3872)
$\to {\rm{\pi}}^{+}{\rm{\pi}}^{-}$ η、${\rm{\pi}}^{+}{\rm{\pi}}^{-}{\rm{\chi}}_{cJ}$ 、${{\pi}}^{0}{\rm{\pi}}^{0}{{\chi}}_{cJ}$ 等末态;研究X(3872)和Y、Z态的关系,例如对Y(4260)$\rightarrow {{\gamma}}$ X(3872),${\rm Z}_{\rm c}(4020) \rightarrow {\rm{\gamma}}$ X(3872)等过程的研究;在更高的质心系能量寻找X(3872)新的产生方式,例如${{{\rm{e}}}}^{+}{{{\rm{e}}}}^{-} \rightarrow {\rm{\omega}}$ X(3872)过程,并测量它与${{\gamma}}$ X(3872),${{\omega}}{{\chi}}_{cJ}$ [71]等过程产生截面的比例。随着BESIII上积累的数据量的增多,我们期望能够发现X(3872)更多的衰变过程,同时在BESIII上的研究能够对X、Y、Z态之间的内在联系进一步地理解。
Study of the Exotic State X(3872) at BESIII
doi: 10.11804/NuclPhysRev.38.38.2021079
- Received Date: 2021-10-23
- Rev Recd Date: 2021-11-29
- Publish Date: 2021-12-20
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Key words:
- X(3872) /
- exotic states /
- BESIII /
- BEPCII
Abstract: Quantum chromodynamics(QCD) allows the existence of exotic states, which beyond baryons and mesons in the quark model. The study of exotic states is one of the hot topics in high-energy experiments. X(3872) was the first candidate for an exotic particle observed experimentally. Experimental and theoretical study on X(3872) has never been stopped, which makes X(3872) also the best exotic state being understood. The Beijing Spectrometer(BESIII) Experiment at the Beijing Electron Positron Collider(BEPCII) collected large data samples for electron-positron collisions with center-of-mass energy from 3.87 to 4.70 GeV. The analysis of these data samples has resulted in a number of achievements on X(3872). In this paper, we will review the research on X(3872) at BESIII and a series of achievements and the latest progress made at BESIII. The study of X(3872) and other exotic states will help us understand the basic interactions of the nature, and it will also provide a strong support for the standard model theory.
Citation: | Longzhou LIAO, Yan NIU, Zhiqing LIU. Study of the Exotic State X(3872) at BESIII[J]. Nuclear Physics Review, 2021, 38(4): 380-388. doi: 10.11804/NuclPhysRev.38.38.2021079 |