RHIC能区J/psi产生及其与带电强子之间关联的实验研究

The primary gole of the Relativistic Heavy Ion Collider (RHIC) is to study the properties of hot and dense QCD matter created in relativistic heavy-ion collisions. One important probe of the hot and dense matter is J/psi, the bound state of charm quark and its anti-quark. On one hand, J/psi yield in relativistic heavy-ion collisions will be suppressed due to the Debye color-screening effect in the deconfined medium. On the other hand, the thermalized charm quark and anti-charm quark from different initially produced pairs may recombine into bound states thus increase the J/psi yield in relativistic heavy-ion collisions. These two competing effects have different dependence on the properties of hot and dense medium and its evolution, as well as the transverse momentum of J/psi. Experiemtal study of J/psi production in a broad transverse momentum range and its dependence on beam-energy can provide important information of the hot and dense matter created at RHIC.Jet is another important probe of the hot and dense matter. Due to its production mechanism, the parton opposite to J/psi is either a heavy quark jet or a gluon jet.The measurements of the identified particle composition and its transverse momentum distribution on the away side with respect to J/psi trigger can provide the information on whether the parton is a heavy quark jet or gluon jet. Once this is determined, J/psi can be used as a gluon jet or heavy quark jet tag to study the jet quenching mechanism in the hot and dense matter created at RHIC. In this project, the newly installed large area Time-Of-Flight (TOF) and Muon Telescope Detector (MTD), contributed by STAR-China collaboratoin, will be used to perform the measurement proposed above, namely the production and correlation of J/psi at RHIC , to study the property of the hot and dense matter in detail.

相对论重离子碰撞的主要物理目标是研究高温高密极端条件下物质的本质和特性。J/psi一直被公认为研究高温高密夸克物质的黄金探针。理论研究表明,J/psi在高温高密夸克物质中一方面会因为色荷德拜屏蔽效应造成产额压低，另一方面会因为热化的粲夸克通过聚合效应重新产生J/psi导致其产额增强。这两种竞争效应有不同的横动量依赖关系，且与高温高密夸克物质的特性及其动力学演化过程密切相关。通过大横动量范围J/psi产额的精确测量和其对碰撞能量的依赖可以深入研究高温高密物质的特性。喷注是高温高密夸克物质特性研究的另一重要探针，通过研究J/psi背向强子的粒子组分及其横动量分布可以研究J/psi背向喷注的类型，并可通过J/psi标记来深入研究高温高密物质中的喷注淬火机制。本项目旨在利用RHIC-STAR中国组最近为合作组贡献的大面积飞行时间探测器TOF和缪子探测器MTD,进行J/psi产生和关联的深入研究。

相对论重离子碰撞的主要目标是寻找夸克胶子等离子体并研究其特性，J/psi在夸克胶子等离子碰撞中会由于色荷德拜屏蔽和粲夸克“聚并”重产生这两个效应造成产额调制，被认为是夸克胶子等离子体的黄金探针之一。本项目围绕J/psi的产生完成了五项实验研究，包括：1）利用近年来中美联合研制的缪子望远镜探测器(MTD)测量了200GeV金核-金核对撞中横动量范围0-14 GeV/c内J/psi核修正因子，及其与横动量、对撞中心度的依赖关系，这不仅是首次在RHIC将J/psi的核修正因子测量扩展到如此大的动量范围，也是目前RHIC最精确的J/psi核修正因子测量结果。这为相对论重离子对撞中J/psi的色荷德拜屏蔽效应和重产生效应的研究，并以此研究夸克胶子等离子体的特性提供了重要的实验数据。2）进一步测量了200GeV金核-金核对撞中J/psi的椭圆流。项目组成员作为主要作者之一在Phys. Rev. Lett.上发表了D0介子的椭圆流结果。实验发现D0的椭圆流和轻味强子相当，表明粲夸克可能接近热化。在此基础上测量J/psi的椭圆流对研究粲夸克聚并重产生J/psi的时空特性具有重要的物理意义。实验观测发现J/psi的椭圆流很小，与热化夸克聚并重产生的模型有较大差别，与输运模型计算结果相近。表明J/psi的重产生是在整个演化过程中产生，此时粲夸克还没有建立足够的流。 3）利用RHIC第一期能量扫描的数据测量了J/psi的核修正因子与对撞能量之间的依赖关系，以获得色荷德拜屏蔽效应和粲夸克聚并重产生效应的竞争与对撞能量的依赖。4）测量了铀核-铀核对撞中J/psi的产生，并与金核-金核对撞进行了对比，研究了J/psi的产额压低与对撞系统的依赖关系。发现两个对撞系统中J/psi的核修正因子非常接近。5）测量了500 GeV质子-质子对撞中J/psi的相对产额与带电强子相对产额之间的关联。发现两者之间具有超线性关联。具体物理根源还不清楚。以上这些实验结果加深了我们对高温高密介质特性的理解，产生了较好的科学影响。通过本项目，也培养了几位很有潜力的青年人才。