标题： 在LHC上通过ALICE研究铅-铅碰撞中核重叠的相干J/$ψ$光生过程 显示英文标题

标题： Coherent J/$ψ$ photoproduction in Pb-Pb collisions with nuclear overlap studied with ALICE at the LHC

摘要： 重夸克矢量介子在超相对论核的电磁相互作用中的光致产生对核内的胶子分布敏感，因此对冷核物质效应如阴影效应或部分子饱和敏感。 除了在超外围碰撞中已知的矢量介子产生现象外，在LHC和RHIC能量下，外围和半中央核-核碰撞中，非常低的横向动量（$p_T < $~0.3 GeV/$c$）下观测到超过预期强子J/$\psi$产生的过量现象，被解释为在铅-铅碰撞中发生相干J/$\psi$光致产生的第一个迹象。 ALICE合作组发表了在 $\sqrt{s_{\rm NN}}$ = 2.76~TeV和前向快速度（$2.5<y<4.0$）下外围和半中央铅-铅碰撞中的J/$\psi$ 相干光致产生截面。 使用LHC运行-2数据，ALICE在中 rapidity ($|y|<0.9$) 和前向 rapidity 的$\sqrt{s_{\rm NN}}$= 5.02 TeV 的外围Pb-Pb碰撞中展示了初步结果。 由于中央圆柱体的跟踪分辨率非常好，也能够提取$p_T$-微分截面，这加强了观察到的J/$\psi$过量的光生起源。 对这种低-$p_T$过量的定量理解提出了重大的理论挑战，因为J/$\psi$光生取决于碰撞动力学以及光子通量和光核截面。 在本次会议中，我们介绍了ALICE对边缘Pb-Pb碰撞中J/$\psi$光致产生截面的最新测量结果，重点在于在$\sqrt{s_{\rm NN}}$= 5.02~TeV时的二μ子衰变通道的新前向测量结果。这些结果将被讨论并与几种涉及核重叠的Pb-Pb碰撞中J/$\psi$光致产生的模型计算进行比较。 显示英文摘要

摘要： The photoproduction of heavy vector mesons in the electromagnetic interactions of ultra-relativistic nuclei is sensitive to the gluon distribution in the nucleus and thus to cold nuclear matter effects like shadowing or parton saturation. Besides the well known observations of vector meson production in ultra-peripheral collisions, first observations of an excess over the expected hadronic J/$\psi$ production at very low transverse momentum ($p_T < $~0.3 GeV/$c$) in peripheral and semi-central nucleus-nucleus collisions both at LHC and RHIC energies were interpreted as the first sign of coherent J/$\psi$ photoproduction occurring in Pb-Pb collisions with nuclear overlap. The ALICE Collaboration published the J/$\psi$ coherent photoproduction cross sections in peripheral and semi-central Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76~TeV and forward rapidity ($2.5<y<4.0$). Using the LHC Run-2 data, ALICE presents preliminary results in peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV at mid-rapidity ($|y|<0.9$) and forward rapidity. Thanks to the very good tracking resolution of the central barrel, the extraction of the $p_T$-differential cross section was also possible, strengthening the photoproduction origin of the observed J/$\psi$ excess. The quantitative understanding of this low-$p_T$ excess poses significant theoretical challenges since the J/$\psi$ photoproduction depends on the collision dynamics as well as on the photon-flux and the photonuclear cross section. In this proceeding, we present the latest ALICE measurements on J/$\psi$ photoproduction cross section in peripheral Pb-Pb collisions, with emphasis on the new forward measurement in the dimuon decay channel at $\sqrt{s_{\rm NN}}$ = 5.02~TeV. These results will be discussed and compared to several model calculations of J/$\psi$ photoproduction in Pb-Pb collisions with nuclear overlap.