在LHCb实验上寻找Lambda_b->Lambda+X(3872)

Both Λb and X(3872) hadrons are not extensively studied yet, and they are hot topics of the heavy flavor physics. As the first one of the so called “XYZ” particles, X(3872) was first observed in B+->X(3872)K+ decay. Based on the symmetry inside heavy flavor hadrons, Λb->Λ+X(3872) is expected to have similar branching fraction. So far, X(3872) has not been directly observed in b-baryon decay. Here we propose a search for Λb->Λ+X(3872) decay events at LHCb experiment. Due to the smallness of the branching fraction (10^{-6}) estimated based on B meson decays, the expected number of signal events is small, while the combinatorial background would be huge. We plan to use multi-variable analysis method to suppress the background while keeping a high efficiency for signal. If we observe significant signal yield, a measurement of the branching fraction relative to Λb->Λ+ψ(2S) will be made, otherwise, an upper limit on the relative branching fraction will be reported. It will be the first measurement of this decay channel, and the results will shed some light on the production mechanism of chromium in b-hadron decay by testing the heavy quark effective theory and the QCD behind it. Furthermore, by scrutinizing the invariant mass spectra of J/ψπ and J/ψΛ of Λb events without X(3872), some new hadron states, like tetra-quark or penta-quark, might be unveiled.

Λb和X(3872)都是尚未得到充分研究的粒子，对其性质的研究是当前重味物理研究的前沿课题。X(3872)是在b介子的衰变中发现的，目前尚未在b重子衰变中观察到。本项目提出在LHCb数据中寻找Λb->Λ+X(3872)衰变。LHC上较高的Λb产生截面和LHCb探测器在重建重味粒子方面的优良性能使得在LHCb上可以收集到大量的Λb事例。理论预期该衰变的分支比为~10^{-6}，我们计划通过多变量分析的方法来压低本底，提取信号。如果观察到显著信号，将用Λb->Λ+ψ(2S)衰变作为参考道对其分支比进行测量；若没有观察到显著信号，将对其相对分支比设置上限。这将是对该衰变道的首次搜寻，有助于理解b强子衰变中粲偶素的产生机制，检验重味夸克有效理论。此外，在非X(3872)共振态下的J/ψπ和J/ψΛ的不变质量谱可分别用来寻找四夸克和五夸克态。

本项目使用LHCb实验在一期（2011-2012）和二期（2015-2018）运行期间取得的质子质子对撞数据来寻找Λ_b->Λ+X(3872)事例并测量该衰变的分支比。该寻找的结果可以为理解X(3872)粒子的结构提供重要信息。我们采取了盲分析的策略来避免引入人为相关的误差，并通过参考道来优化事例筛选和估计信号的预期显著度。经过事例筛选优化，我们得到了约2300个参考衰变道Λ_b->Λ+ψ(2S)过程的事例，对两个衰变道衰变分支比R在0.02到0.06的情况下可以观察到约50到160个信号，信号显著度在5-20σ。我们对效率也做了细致的估计。目前分析工作初步完成，需要经过LHCb工作组审查和合作组审查等阶段才能打开盲区查看结果并投稿发表。预计该过程尚需数月时间。此外，项目计划中的J/ψπ和J/ψΛ的共振态寻找也有了初步结果，在实验数据中并没有发现明显的信号。但是，我们数据中发现Λ_b->Λ+J/ψππ的贡献很大。这也是一个此前没有观察到的过程，而且我们在该过程中看到了一些共振态的存在。我们将在下一步对该过程的分支比也进行测量。