BESIII上Ds+介子到包含三个带电pi介子的单举衰变过程研究

Research on lepton universality has become a very hot topic in the field of particle physics. Recent reports from a number of high energy experiments on studying B semileptonic decays of the form Bbar -> D(*) l- nu, with l = e, mu or tau suggest deviations from Standard Model based predictions, and has drawn a wide range of interests from both experiments and theorist. Among these experiments, the LHCb measurement of R(D*) by reconstructing tau lepton from three charged pions (tau- -> pi-pi+pi-X) offers the result with the smallest statistical uncertainty. This type of measurement is rather promising as tremendous amount of LHCb data are readily available. Still, the most important issue of reconstructing tau lepton from the inclusive decay mode of tau- -> pi-pi+pi-X is sufficient knowledge on the backgrounds B -> D*Ds，with Ds decaying into final states including three charged pions. Due to the LHCb experiment itself being hadron collider based, the capability of studying the inclusive decay of Ds+ -> pi+pi-pi+X with LHCb alone is vastly limited. However, electron-positron collider based experiments such as the BESIII experiment have unique advantages in studying inclusive decays with the double-tagging method. Based on ~3.19 fb-1 BESIII data taken at the center-of-mass energy of 4.178 GeV in 2016, this project plans to measure precisely the branching fraction of the inclusive Ds+ decays involving at least three charged pions, as well as the invariant mass spectra of each of the pi+pi- pairs from the inclusive mode. All these measurements will offer important inputs for the LHCb experiment to lower the systematic uncertainty on R(D*), and therefore, lead to more stringent tests on lepton universality.

轻子普适性的相关研究是当前粒子物理领域非常热门的一个方向。在相关实验中LHCb利用tau轻子衰变到包含三个带电pi介子的模式（tau- -> pi-pi+pi-X）重建tau轻子的方式对R(D*)的测量结果具有统计误差最小的优势。但是tau- -> pi-pi+pi-X的重建方法存在的最大问题是需要对B -> D*Ds，Ds 衰变到包含三个带电pi介子的单举模式这种背景有充分的了解，像BESIII这样的电子对撞实验设备在研究单举衰变方面有着独特的优势，可以利用双标方法来详细研究这样的过程。该项目计划利用BESIII实验于2016年在4.178GeV质心能量点上收集到的的3.19fb-1的数据，精确测量Ds+介子到至少三个带电pi介子的单举衰变道的分支比，以及pi介子对的不变质量谱。这些测量工作可以协助LHCb实验很好的降低相关系统误差，从而对轻子普适性做出更严格的检验。

轻子普适性的相关研究是当前粒子物理领域非常热门的一个方向。在相关实验中LHCb利用tau轻子衰变到包含三个带电pi介子的模式(tau- -> pi-pi+pi-X)重建tau轻子的方式对R(D*)的测量结果具有统计误差最小的优势。但是tau- -> pi-pi+pi-X的重建方法存在的最大问题是需要对B -> D*Ds,Ds 衰变到包含三个带电pi介子的单举模式这种背景有充分的了解,像BESIII这样的电子对撞实验设备在研究单举衰变方面有着独特的优势,可以利用双标方法来详细研究这样的过程。该项目利用了BESIII实验于2016年在4.178GeV质心能量点上收集到的3.19fb-1的数据，以及更多的在4.178GeV能量点附近的数据，测量了Ds+介子到末态包含三个带电pi介子的相关衰变道的分支比和不变质量谱分布。该项目相关已公开的研究结果包括了Ds+ -> K+ K- pi+ pi- pi+，Ds+ -> pi+ pi- pi+，和Ds+ -> pi+ pi- pi+ X衰变过程。这些测量可以用来完善Ds+衰变模型，协助LHCb实验很好的降低相关系统误差，从而对轻子普适性做出更严格的检验。