在Belle上寻找双粲夸克奇特强子态Tcc

The exotic states with multiple quarks has attracted a lot of interests in recent years, both experimentally and theoretically. The existence of an exotic state with two charm (or anticharm) quarks will unambiguously prove to be a multiquark state. We propose to search for such an isospin-0 doubly charmed exotic state Tcc, composed of two charm quarks and two light antiquarks (including the charge conjugate) in electron-positron collisions, using data collected by Belle and updated Belle II detector. In case of observation, we will measure its mass and production cross-section; and this would be a completely new way how quarks are combined to form resonance states, which largely improves our understanding of quantum chromodynamics. If there were no evidence of such a state, an upper limit on its production will nonetheless be useful input for theory and provides references for future experimental searches. We will start building the analysis framework from the current Belle data which has the highest integrated luminosity of electron-positron collisions in the world, and later add the first data from Belle II operation. Given the different nature of dominating background, the data with centre-of-mass energy at or lower than Upsilon(4S) resonances will be treated separately.

多个夸克的奇特强子态是目前强子物理理论和实验研究的热点，含有两个正（或反）粲夸克的奇特强子态的发现将无可辩驳地证明多夸克态存在。本项目拟利用B工厂上Belle和升级后的Belle II探测器收集的数据，在正负电子对撞中通过直接重建衰变末态和研究反冲质量谱两种方法，搜寻从低于DD阈值到高于D*D阈值质量范围内的双粲夸克奇特强子态Tcc（c c ubar dbar）。如果观测到Tcc，测定其质量和产生截面，将意味着发现夸克通过强相互作用组成共振态的一种全新模式，大大拓展我们对量子色动力学的理解；如现有数据中没有Tcc迹象，对其产生截面的上限也是对理论研究的和下一步实验研究的有用参考。本项目将从Belle现有的世界上亮度最高的正负电子对撞数据开始搭建分析框架，并加入Belle II开始运行后的新数据。针对不同的主要本底类型，分别处理质心能量等于和低于Upsilon(4S)质量的数据。

多个夸克的奇特强子态是目前强子物理理论和实验研究的热点，含两个同味粲夸克的奇特强子态的发现将无可辩驳地证明多夸克态存在。本项目利用Belle实验升级前收集的正负电子对撞数据，在多个DD和DDstar末态中寻找双粲味奇特态Tcc (c c qbar qbar)，并未发现显著的Tcc存在迹象。这意味着该奇特态即使存在，在正负电子对撞中的产生截面也较低，Belle现有数据难以观测到。Belle II升级后取得的对撞数据量较少，现阶段对Tcc寻找的灵敏度提升并无帮助，但Belle II运行结束后积分亮度将增至50倍，届时应重新开展Tcc寻找。