利用4.01GeV正负电子对撞数据对粲偶素、粲介子产生与衰变的研究

Several charmonia and charmonium-like states were discovered with the huge statistics of data accumulated by B factories. The exotic state X(3872) is one of discovered states, but not well understood. It could be a molecule state of D*D, or the P wave triplet state chi_c1(2P), or the D wave singlet eta_c(1D). It is hard to conclude the nature of X(3872) until more extensive experimental and theoretical studies are performed. The decay constant of the strange charmed meson Ds (fDs for short) is also very interesting topic. At present, there is a remarkable discrepancy between the world average of the measurements of fDs and the theoretical calculations. It can be explained that the model used in the calculation is not representative, but also that there could be new physics beyond the standard model. This can only be answered by more precision measurements and theoretical calculations. At tau-charm energy area, BESIII is the e+e- collision experiment with the highest luminosity and the most sable performance. We plan to use the 477pb-1 data sample collected at 4.01GeV by BESIII to study the production, decay of charmonia and charmed mesons. The topics includes the search for X(3872) through radiative transition, the hadronic transition to charmonia states, the open charm cross section measurement at 4.01GeV, measurement of the decay constant of Ds and so on. All these studies will help us to understand the physical phenomenon mentioned above and to improve the developments of particle physics theory.

在B工厂海量正负电子对撞数据中，大量粲偶素和类粲偶素粒子被发现。X(3872)便是其中代表之一，它可能是粲介子对组成的分子态、也有可能是P波三重态、D波单态，但这些猜测远没有定论，仍需更深入的实验研究来回答。奇异粲介子的衰变因子的测量结果和理论计算存在较明显的差异，可能是理论计算中机制考虑不全，也有可能存在标准模型之外的新物理，这需要更多更精确的实验测量和理论计算。BESIII是世界目前在tau粲能区亮度最高、运行最稳定、性能优秀的大型实验装置。我们计划利用BESIII在4.01GeV能量点采集的约477pb-1的数据，对粲偶素、粲介子的产生和衰变做出一系列研究，包括X(3872)的寻找、4.01GeV粲偶素强跃迁的研究、粲介子对产生截面的测量、奇异粲偶素衰变因子的测定等课题。这些研究工作将加深对现有这些物理现象的理解，促进粒子物理理论的发展和完善。

本项目研究基于BESIII实验在4.009GeV采集的正负电子对撞数据，主要内容包括通过psi(4040)辐射衰变寻找X(3872)、psi(4040)强子跃迁观测、粲介子衰变以及粲介子产生截面测量的研究等。其中，我们首次观测到4.009GeV下e+e-到eta J/psi过程，信号显著性达到10倍标准偏差，截面测量值为(32.1+-2.8+-1.3) pb。如果假设该过程中eta J/psi末态全部通过psi(4040)产生，那么psi(4040)到eta J/psi的跃迁率为(5.2+-0.5+-0.2+-0.5)*10^-3。我们精确测量了D^0*到gamma D^0和pi0 D^0的分支比，假设没有其他衰变末态，其测量结果为Br(D0*->gamma D0)=(65.5+/-0.8+/-0.5)%，Br(D0*->pi0 D0)=(34.5+/-0.8+/-0.5)%，该结果与之前的实验结果一致，但精度显著提高，为目前世界上最好的单次测量。我们还完成了Ds的纯轻衰变Ds->mu nu，tau nu衰变常数的测量，以有限的统计量，获得了具有竞争力的实验结果，与最近的unquenched的格点计算一致。这些研究结果能够帮助我们更好的理解该能区粲偶素、类粲偶素以及粲介子的产生与衰变，并促进其理论发展，改进相关实验的精度。