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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$0.241\pm 0.002(\text{stat})\pm 0.016(\text{syst})$in natural units. The effective medium temperature, estimated using the mean transverse momentum, is$219\pm 8(\text{syst})\text{MeV}$. 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.

We report a new measurement of the production of electrons from open heavy-flavor hadron decays (HFEs) at mid-rapidity (|y|<0.7) in Au+Au collisions at$$ \sqrt{s_{\textrm{NN}}} $$$\sqrt{s_{\mathrm{NN}}}$= 200 GeV. Invariant yields of HFEs are measured for the transverse momentum range of 3.5< p_T<9 GeV/cin various configurations of the collision geometry. The HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that inp+pcollisions scaled by the average number of binary collisions, indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the quark-gluon plasma.

Measurements of the charge-dependent two-particle angular correlation function in proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy of$$ \sqrt{s_{\textrm{NN}}} $$$\sqrt{s_{\mathrm{NN}}}$= 8.16 TeV and lead-lead (PbPb) collisions at$$ \sqrt{s_{\textrm{NN}}} $$$\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV are reported. The pPb and PbPb data sets correspond to integrated luminosities of 186 nb⁻¹and 0.607 nb⁻¹, respectively, and were collected using the CMS detector at the CERN LHC. The charge-dependent correlations are characterized by balance functions of same- and opposite-sign particle pairs. The balance functions, which contain information about the creation time of charged particle pairs and the development of collectivity, are studied as functions of relative pseudorapidity (∆η) and relative azimuthal angle (∆ϕ), for various multiplicity and transverse momentum (p_T) intervals. A multiplicity dependence of the balance function is observed in ∆ηand ∆ϕfor both systems. The width of the balance functions decreases towards high-multiplicity collisions in the momentum region<2 GeV, for pPb and PbPb results. Integrals of the balance functions are presented in both systems, and a mild dependence of the charge-balancing fractions on multiplicity is observed. No multiplicity dependence is observed at higher transverse momentum. The data are compared withhydjet,hijing, andamptgenerator predictions, none of which capture completely the multiplicity dependence seen in the data. The comparison of results with different center-of-mass energies suggests that the balance functions become narrower at higher energies, which is consistent with the idea of delayed hadronization and the effect of radial flow.

The hydrodynamic flow-like behavior of charged hadrons in high-energy lead-lead collisions is studied through multiparticle correlations. The elliptic anisotropy values based on different orders of multiparticle cumulants,v₂{2k}, are measured up to the tenth order (k= 5) as functions of the collision centrality at a nucleon-nucleon center-of-mass energy of$$ \sqrt{s_{\textrm{NN}}} $$$\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV. The data were recorded by the CMS experiment at the LHC and correspond to an integrated luminosity of 0.607 nb⁻¹. A hierarchy is observed between the coefficients, withv₂{2}> v₂{4} ≳v₂{6} ≳v₂{8} ≳v₂{10}. Based on these results, centrality-dependent moments for the fluctuation-driven event-by-eventv₂distribution are determined, including the skewness, kurtosis and, for the first time, superskewness. Assuming a hydrodynamic expansion of the produced medium, these moments directly probe the initial-state geometry in high-energy nucleus-nucleus collisions.

The production of prompt$$ {\Lambda}_{\textrm{c}}^{+} $$${\Lambda }_c^{+}$baryons is measured via the exclusive decay channel$$ {\Lambda}_{\textrm{c}}^{+}\to p{\textrm{K}}^{-}{\pi}^{+} $$${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$at a center-of-mass energy per nucleon pair of 5.02 TeV, using proton-proton (pp) and lead-lead (PbPb) collision data collected by the CMS experiment at the CERN LHC. The pp and PbPb data were obtained in 2017 and 2018 with integrated luminosities of 252 and 0.607 nb⁻¹, respectively. The measurements are performed within the$$ {\Lambda}_{\textrm{c}}^{+} $$${\Lambda }_c^{+}$rapidity interval |y|<1 with transverse momentum (p_T) ranges of 3–30 and 6–40 GeV/cfor pp and PbPb collisions, respectively. Compared to the yields in pp collisions scaled by the expected number of nucleon-nucleon interactions, the observed yields of$$ {\Lambda}_{\textrm{c}}^{+} $$${\Lambda }_c^{+}$withp_T>10 GeV/care strongly suppressed in PbPb collisions. The level of suppression depends significantly on the collision centrality. The$$ {\Lambda}_{\textrm{c}}^{+} $$${\Lambda }_c^{+}$/D⁰production ratio is similar in PbPb and pp collisions atp_T>10 GeV/c, suggesting that the coalescence process does not play a dominant role in prompt$$ {\Lambda}_{\textrm{c}}^{+} $$${\Lambda }_c^{+}$baryon production at higherp_T.

The second-order (v₂) and third-order (v₃) Fourier coefficients describing the azimuthal anisotropy of prompt and nonprompt (from b-hadron decays) J/ψ, as well as prompt ψ(2S) mesons are measured in lead-lead collisions at a center-of-mass energy per nucleon pair of$$ \sqrt{s_{\textrm{NN}}} $$$\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV. The analysis uses a data set corresponding to an integrated luminosity of 1.61 nb⁻¹recorded with the CMS detector. The J/ψ and ψ(2S) mesons are reconstructed using their dimuon decay channel. Thev₂andv₃coefficients are extracted using the scalar product method and studied as functions of meson transverse momentum and collision centrality. The measuredv₂values for prompt J/ψ mesons are found to be larger than those for nonprompt J/ψ mesons. The prompt J/ψv₂values at highp_Tare found to be underpredicted by a model incorporating only parton energy loss effects in a quark-gluon plasma medium. Prompt and nonprompt J/ψ mesonv₃and prompt ψ(2S)v₂andv₃values are also reported for the first time, providing new information about heavy quark interactions in the hot and dense medium created in heavy ion collisions.