【作者】 张弛；

【导师】 陈申见； 张景芝；

【作者基本信息】 南京大学 ， 粒子物理与原子核物理， 2015， 博士

【摘要】 粲偶素能谱及其衰变是研究处于微扰和非微扰QCD能区之间的过渡区域的强相互作用动力学的理想场所。在过去的几十年间,粲偶素能谱及其衰变的测量取得了一系列重大的成果,为唯象模型提供了重要的实验约束。ψ(3770)是位于粲介子对打开阈值之上的最轻的粲偶素共振态,被认为是由13D1和23S1态混合而成的面系统。ψ(3770)曾被认为几乎全部衰变到纯净的DD末态,直到BES合作组观测到了首个非DD强子衰变模式寻找其他的非DD衰变已经成为了近些年来的热门课题。中间态介子圈模型给出了这两个非DD衰变分宽度的理论计算：和此理论模型给出的误差范围较大,急需实验测量的结果给予检验。最近,BEPCⅡ上的BESⅢ探测器在3.773 GeV上累积采集到积分亮度为2.92 fb-1的数据,这就为我们观测ψ(3770)和ηc(ηc(2S))之间的辐射跃迁提供了机会。综合考虑到的衰变率和本底水平,在BESⅢ上我们选择利用的衰变模式作为首次尝试来寻找ψ(3770)辐射跃迁到过程。由于没有观测到明显的信号事例,我们给出了90%置信度水平的联合分支比上限：如果结合我们的结果和的世界平均测量值,我们得到90%置信度水平的分支比上限：此测量的分支比上限值在理论预期的范围之内,但是并不能挑战理论预言。在此分析中,我们也利用衰变模式测量了E1跃迁过程的分支比作为分析方法的检查,利用过程测量了强子衰变的分支比,结果分别所测量的的结果在倍标准误差范围内是一致的,然而,测量的值在一倍标准误差范围内不一致。夸克偶素(比如J/ψ、T等)以及其他介子的不可见衰变为寻找标准模型(SM)之外的新物理提供了平台。这是因为在标准模型框架下,唯一允许的夸克偶素不可见衰变模式是通过湮灭成一个虚Zo玻色子衰变到中微子对vv。标准模型同时也预言了分支比：然而,如果实验上观测到信号的增强,将意味着标准模型之外的新物理的存在。比如通过传递一个自旋为1的电中性规范玻色子U衰变到轻暗物质(LDM)粒子,可以极大地提高夸克偶素的不可见衰变率。这就是我们寻找J/ψ不可见衰变的动机。我们利用BESⅢ获取的衰变事例,通过ψ(2S)→π+π-J/ψ跃迁过程来研究J/ψ粒子的不可见衰变。我们从事例中重建出两个低动量的π介子,利用π+π-的反冲不变质量谱标记J/ψ粒子,并要求在π+π-J/ψ候选事例中没有多余可探测的粒子。同时,我们也遍举重建出事例,通过计算相对分支比B(J/ψ→invisible)/B(J/ψ→μ+μ-)来减少系统误差。扣除相应的本底后,我们并没有发现明显的不可见衰变事例,因此给出90%置信度水平的相对分支比上限：由于数据统计量的提高和分析方法的改善,与之前BESⅡ的测量结果相比,我们给出的上限几乎提高了一个数量级,更加接近于标准模型的预言。更多还原

【Abstract】 Charmonium spectrum and decays are an ideal place for studying the strong in-teraction dynamics in the interplay of perturbative and non-perturbative QCD regime. In the past decades there have been significant progresses made on the measuremen-t of charmonium spectrum and their decays, which provide important constraints on phenomenological approaches.The ψ(3770) resonance, as the lightest charmonium state lying above the open charm threshold, is generally assigned to be a dominant 13D1 momentum eigenstate with a small 2351 admixture. It has been thought almost entirely to decay to pure DD final states until BES Collaboration observed the first hadronic non-DD decay mod-Searching for other non-DD decays of the ψ(3770) has been one of the most interesting topics in the decades. The intermediate meson loop (IML) model predicts keV, both of which have large uncertainties and need con-straints from experimental measurements. Recently, totally 2.92 fb-1 data has been accumulated at (?)s= 3.773 GeV with the BESIII detector at BEPCII, which of-fers us great opportunities to observe the radiative transitions between ψ(3770) and ηc(ηc(2S)).The decay mode η is chosen as the first try to search for ηc(η)c(2S)) in ψ(3770) radiative transition at BESIII considering the decay rate of ηc(ηc(2S)) and background level. No significant excess of signal events above back-ground is observed. We set upper limits at a 90% confidence level for the product branching fractions to be < 5.6 x 106. Combining our re-sult with world-average values of we find the branching at a 90% confidence level. The measured upper limits are within the range of the theoretical predictions but cannot chanllenge them. In this analysis, we also mea-sure the branching fraction of E1 transition ψ(3770)-γxc1 using the decay mode and the branching fraction of hadronic decay using the process e+e→γISRJ/ψ, which give the results (2.33±0.65± 0.43)×10-3 and B{ respectively. The measured branching fraction for ψ(3770)→γ7Xc1 is consistent with the CLEO result in one standard deviation (1σ), while the one for at odds with the value of Particle Data Group (PDG) in 1σ.Invisible decays of quarkonium states such as J/1ψ, T, etc., and other mesons provide a window on physics beyond the standard model (SM). This is because the only allowed invisible decay of quarkonium states in the SM is the decay to vv via annihilation into a virtual Z0 boson. The SM predicts ). However, new physics beyond the SM might enhance the branching fraction of J/ψ invisible decays. One possibility is the decay into light dark matter (LDM) particles mediated by a new, electrically neutral spin-1 gauge boson U, which could significantly increase the invisible decay rate. This motivates the search for invisible decays of the J/ψ.At BESⅢ, we do so by looking for evidence of the decay of the J/ψ into unde-tectable final states recoiling against the di-pion system in using a sample of (106.41±0.86) ×106 ψ(2S) mesons. We also exclusively reconstruct the events from the process ψ(2S) which is used to calculate the ratio of invisible J/ψ decays to to cancel some common uncertain-ties. After subtracting the background, we find no evidence for J/ψ→invisible and set an upper limit on the ratio at 3.6 x 10-3 at the 90% C.L.. This limit is almost an order of magnitude closer to the SM prediction than the best previous limit from BESⅡ, due to the improvements of statistics and analysis method.更多还原