重味强子的产生与QGP性质的研究

Recent measurements on heavy flavor hadrons are not precise enough. In particular, open bottom hadrons have never been measured directly in Heavy-ion collisions. The published experimental results are under many assumptions from theoretical models, which leads to both large statistical and systematical uncertainties. Due to their large mass, heavy quarks behave differently from light quarks when interacting with the medium. The properties of heavy flavor hadron, like its energy loss and flow parameters, are sensitive to the dynamics of the hot and dense medium created in the collisions. Therefore, the quantitative measurement of heavy flavor hadrons is a crucial probe to study the QGP matter properties. Heavy flavor physics is one of the most important direction at RHIC in future decade, which is expected to measure degree of the QGP thermalization and test the QCD theory. The measurement of open bottom hadron must rely on the measurement of open charm hadron in sufficient precision. The STAR Heavy Flavor Tracker expected to be in operation and to take data in year 2014, together with continuously increasing RHIC luminosity, will provide us such a never previously existed condition for accomplish this physics goal. In this project, we will take this chance, precisely measure open charm hadrons based on the most advanced silicon pixel technology to reconstruct tracks and heavy flavor decay vertcies. The open bottom hadrons are expected to be measured via leptonic decays with the difference of the decay impact parameter from background, once we know open charm in sufficient precision.

重离子对撞中重味强子的测量精确度并不高，而且迄今为止从未直接测到过底夸克强子，目前发表的实验数据都依赖理论模型来估计，置信度不高且统计和系统误差都很大。由于重味夸克质量较大，其与介质的相互作用与轻夸克预期有显著不同，且重味夸克强子的性质，如能量损失，流体运动参数等都跟碰撞产生的热密系统性质紧密相关，因此重味夸克强子的定量测量是研究QGP物质特性的重要探针。重味物理是未来十年RHIC实验的最重要的课题之一，是测量QGP物质热化程度和验证QCD理论的重要手段。底夸克强子测量的前提要求要对粲强子的产生有足够精确的测量，将在2014年运行取数的STAR重味径迹探测器HFT和不断升级的RHIC对撞亮度，为实现这一物理目标提供了前所未有的条件。在本项目中，我们将抓住这一机遇，利用最先进的基于硅像素精细寻迹和顶点重建方法来精确定量测量开粲强子，并在此基础上利用轻子道衰变长度区分本底的方法测量底夸克强子。

宇宙大爆炸理论认为，在宇宙诞生初期会产生一种夸克、胶子解禁闭的新物质形态——夸克胶子等离子体（QGP）。高能重离子对撞实验可以模拟宇宙爆炸初期的高温高密极端条件。量子色动力学预言重味夸克主要产生在碰撞早期，对研究重味夸克与这种QGP核物质相互作用，研究QGP物质性质有非常重要的意义，同时实验观测也是检验QCD理论的重要途径。.围绕这一科学意义，项目组利用RHIC-STAR实验数据及重味径迹探测器的高位置分辨能力，精确测得粲强子谱的核修正因子和椭圆流，首次定量研究了开粲强子在核物质中的能损，为理论研究重味能损机制提供了重要的实验依据。同时观测到重味强子在高能下仍然参与集体运动，是粲夸克可能达到热化的重要证据，并成功提取了高温高密核物质的输运系数等参数，为定量研究QGP物质性质做出了重要的工作。此外我们还首次提出了重味强子衰变轻子道粲底分离的分析方法，并从实验上成功实现了电子道的粲底分离。.项目共发表文章7篇，包括2篇PRL。其中Phys. Rev. Lett. 113, 142301, 2014的工作被录入本领域的白皮书，被引用140余次。另一篇Phys. Rev. Lett. 118, 212301, 2017发表以来受到科学界广泛关注，在美国Science Daily、Science Newsline、NBC news等媒体上进行了报道。