Photoproduction of J/$ψ$ and dileptons in Pb-Pb collisions with nuclear overlap

arXiv - PHYS - Nuclear Experiment Pub Date : 2024-06-18 DOI:arxiv-2406.12630

Nicolas Bizéfor the ALICE collaboration

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Abstract

Photon-photon reactions and the production of J/$\psi$ meson through photonuclear reactions have been extensively studied in ultra-peripheral heavy-ion collisions, in which the impact parameter is larger than twice the nuclear radius. In recent years, coherently photoproduced J/$\psi$ and dilepton production via photon-photon interactions have also been observed in nucleus-nucleus (A-A) collisions with nuclear overlap. The former can help to constrain the nuclear gluon distributions at low Bjorken-$x$ and high energy, while the latter could be used to further map the electromagnetic fields produced in heavy-ion collisions. In addition, these measurements can shed light on the theory behind photon-induced reactions in A-A collisions with nuclear overlap, including possible interactions of the measured probes with the formed and fast expanding quark-gluon plasma. Since the produced quarkonium is expected to keep the polarization of the incoming photon due to $s$-channel helicity conservation, the photoproduction origin of the J/$\psi$ yield excess at very low transverse momentum, $p_{\rm T}$, can be confirmed by the measurement of the J/$\psi$ polarization. The ALICE detector can perform quarkonium production measurements at both mid ($|y|<0.9$) and forward ($2.5

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核重叠 Pb-Pb 碰撞中 J/$ψ$ 和二重子的光生成

光子-光子反应和通过光子核反应产生J/$\psi$介子已经在超外围重离子碰撞中得到了广泛的研究，在这种碰撞中，碰撞参数大于核半径的两倍。近年来，在核重叠的核-核（A-A）对撞中也观测到了通过光子相互作用相干产生的J/$\psi$和二轻子。前者有助于在低Bjorken-$x$和高能量下约束核胶子分布，而后者可用于进一步绘制重离子碰撞中产生的电磁场。此外，这些测量还可以揭示核重叠的A-A对撞中光子诱导反应背后的理论，包括测量探针与已形成并快速膨胀的夸克-胶子等离子体之间可能存在的相互作用。由于s$沟道褶皱守恒，产生的夸克鎓预计会保持进入光子的偏振，因此可以通过测量J/$\psi$的偏振来证实J/$\psi$在很低的横动量（$p_{\rm T}$）下产生过量的光生源。ALICE探测器可以在中速（$|y|<0.9$）和前速（$2.5

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