Abstract：
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 of0.241±0.002(stat)±0.016(syst)in natural units. The effective medium temperature, estimated using the mean transverse momentum, is219±8(syst)MeV. 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.
摘要：
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，该状态表现出显着的集体流动行为，并且粘性耗散最小。为了更深入地了解其内在性质和基本自由度，我们使用铅-铅（PbPb）碰撞以每核子对5.02TeV的质心能量确定夸克-胶子等离子体的扩展体积中的声速。数据通过CMS实验在CERNLHC上记录，对应于0.607nb-1的积分光度。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性依赖性来进行测量。我们的发现表明，在这种情况下，声速几乎是光速的一半，以自然单位表示的平方值为0.241±0.002(stat)±0.016(syst)。有效介质温度,使用平均横向动量估计，为219±8(syst)MeV。在此温度下测得的平方声速与晶格量子色动力学（QCD）计算的预测精确一致。此结果对所创建介质的状态方程提供了严格的约束，并直接证明了在相对论核碰撞中达到了定义的QCD阶段。
