Ultrarelativistic nuclear collisions create a strongly interacting state of hot and dense quark–gluon matter that exhibits a remarkable collective flow behavior with minimal viscous dissipation. To gain deeper insights into its intrinsic nature and fundamental degrees of freedom, we determine the speed of sound in an extended volume of quark–gluon plasma using lead–lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The data were recorded by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 0.607 nb−1. The measurement is performed by studying the multiplicity dependence of the average transverse momentum of charged particles emitted in head-on PbPb collisions. Our findings reveal that the speed of sound in this matter is nearly half the speed of light, with a squared value of in natural units. The effective medium temperature, estimated using the mean transverse momentum, is . The measured squared speed of sound at this temperature aligns precisely with predictions from lattice quantum chromodynamic (QCD) calculations. This result provides a stringent constraint on the equation of state of the created medium and direct evidence for a deconfined QCD phase being attained in relativistic nuclear collisions.

中文翻译：

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利用大型强子对撞机上的超相对论铅-铅碰撞提取夸克-胶子等离子体中的声速


超相对论核碰撞产生了热而致密的夸克-胶子物质的强烈相互作用状态，表现出显着的集体流动行为和最小的粘性耗散。为了更深入地了解其内在本质和基本自由度，我们使用铅-铅 (PbPb) 碰撞来确定扩大体积的夸克-胶子等离子体中的声速，每个核子对的质心能量为 5.02 TeV。数据由 CERN LHC 的 CMS 实验记录，对应于 0.607 nb−1 的积分光度。该测量是通过研究正面 PbPb 碰撞中发射的带电粒子的平均横向动量的多重依赖性来进行的。我们的研究结果表明，在这种情况下，声速几乎是光速的一半，其平方值为自然单位。使用平均横向动量估算的有效介质温度为 。在此温度下测得的声速平方与晶格量子色动力学 (QCD) 计算的预测精确一致。这一结果为所创建介质的状态方程提供了严格的约束，并为相对论核碰撞中获得解禁 QCD 相提供了直接证据。