Rep Prog Phys. 2024 Jun 20;87(7). doi: 10.1088/1361-6633/ad4b9b.
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. 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.
超相对论性核碰撞产生一种热且密的夸克-胶子物质的强相互作用状态，这种状态展现出显著的集体流行为，粘性耗散极小。为了更深入了解其内在本质和基本自由度，我们利用质心能量为每核子对5.02 TeV的铅-铅（PbPb）碰撞，在扩展体积的夸克-胶子等离子体中确定声速。数据由欧洲核子研究组织大型强子对撞机上的CMS实验记录，对应积分亮度为0.607纳靶。该测量通过研究对头PbPb碰撞中发射的带电粒子平均横向动量的多重性依赖关系来进行。我们的研究结果表明，这种物质中的声速接近光速的一半，在自然单位下其平方值为0.241±0.002（统计）±0.016（系统）。使用平均横向动量估计的有效介质温度为219±8（系统）MeV。在此温度下测得的声速平方与格点量子色动力学（QCD）计算的预测精确吻合。这一结果对所产生介质的状态方程提供了严格约束，并为在相对论性核碰撞中达到解禁闭的QCD相提供了直接证据。
