高能重离子对撞是核与粒子物理的前沿热点之一，其最主要的目标是寻找夸克胶子等离子体（QGP）存在的证据并研究QGP的性质。在研究QGP的诸多信号中，J/ψ是其中最重要的探针之一。经过几十年实验和理论的发展，J/ψ在重离子对撞中产额的改变可以被冷核物质效应加上热介质效应很好的描述。最近，ALICE合作组观测到J/ψ在极低横动量（< 0.3 GeV/c）有显著的异常增强。该增强不能被以上提到的各种效应解释，极有可能源自J/ψ的相干光致产生。目前，对极低横动量J/ψ的研究，十分稀少，亟需大量的实验测量与理论探索。本项目利用RHIC STAR的实验数据，精确测量在不同对撞系统、不同中心度下极低横动量J/ψ的产额，辅以理论计算，为人们理解相干光致产生、重核中的部分子分布、冷核物质效应和QGP的特性提供实验和理论依据。

Relativistic heavy ion collision is one of the major frontiers in nuclear and particle physics. The main goal of high energy heavy ion collision is to find the existence of QGP (Quark-Gluon Plasma) and to study its properties. J/ψ is one of most important probes of QGP. After decades of experimental and theoretical efforts, the modification of J/ψ yield in heavy ion collisions can be described by the cold nuclear matter and hot medium effects. Recently, an abnormal excess of J/ψ yield at very low transverse momentum (< 0.3 GeV/c) has been observed by the ALICE collaboration, which cannot be explained by the hadronic J/ψ production modified by the cold or hot medium effects. The observed excess may originate from the coherent photon-nucleus interactions. At present, the studies of J/ψ production at very low transverse momentum are very rare, which need more experimental and theoretical explorations. In this project, we propose to do precise experimental measurements and theoretical calculations of J/ψ production at very low transverse momentum in different collision systems and centralities, which can shed new light on the understanding of coherent photon-production, parton distributions in nuclei, cold nuclear matter effects, and the properties of QGP.