Ultrarelativistic heavy ion collisions recreate in the laboratory the thermodynamical conditions prevailing in the early universe up to ${10}^{-6}\sec$, thereby allowing the study of the quark-gluon plasma (QGP), a state of quantum chromodynamics (QCD) matter with deconfined partons. The top quark, the heaviest elementary particle known, is accessible in nucleus-nucleus collisions at the CERN LHC, and constitutes a novel probe of the QGP. Here, we report the first evidence for the production of top quarks in nucleus-nucleus collisions, using lead-lead collision data at a nucleon-nucleon center-of-mass energy of 5.02 TeV recorded by the CMS experiment. Two methods are used to measure the cross section for top quark pair production (${\sigma }_{t\overline{t}}$) via the selection of charged leptons (electrons or muons) and bottom quarks. One method relies on the leptonic information alone, and the second one exploits, in addition, the presence of bottom quarks. The measured cross sections, ${\sigma }_{t\overline{t}}=2.54_{-0.74}^{+0.84}$ and ${2.03}_{-0.64}^{+0.71}\mu \mathrm{b}$, respectively, are compatible with expectations from scaled proton-proton data and QCD predictions.

中文翻译：

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核-核碰撞中顶级夸克生产的证据

超相对论性重离子碰撞在实验室中重现了早期宇宙中普遍存在的热力学条件，直至 ${10}^{-6}秒$从而可以研究夸克-胶子等离子体（QGP），这是一种具有有限束缚态的量子色动力学（QCD）物质。顶夸克是已知的最重的基本粒子，可在CERN LHC的核-核碰撞中接近，构成了QGP的新型探针。在这里，我们使用CMS实验记录的在5.02 TeV的核子-核子质心中心的铅-铅碰撞数据，报告了在核-核碰撞中产生顶夸克的第一个证据。两种方法用于测量顶级夸克对的横截面（${\sigma }_{Ť\overline{Ť}}$），通过选择带电的轻子（电子或介子）和底夸克。一种方法仅依赖于瘦子信息，第二种方法还利用了底部夸克的存在。测量的横截面${\sigma }_{Ť\overline{Ť}}=2.54_{-0.74}^{+0.84}$ 和 ${2.03}_{-0.64}^{+0.71}\mu \mathrm{b}$分别与按比例绘制的质子-质子数据和QCD预测的期望兼容。