Two-particle Transverse Momentum Correlations Research Articles

Extraction of the specific shear viscosity of quark-gluon plasma from two-particle transverse momentum correlations

The specific shear viscosity, eta /s, of the quark-gluon plasma formed in ultrarelativistic heavy-ion collisions at RHIC and LHC is estimated based on the progressive longitudinal broadening of transverse momentum two-particle correlators, G_2, reported as a function of collision centrality by the STAR and ALICE experiments. Estimates are computed as a function of collision centrality using the Gavin ansatz which relates the G_2 longitudinal broadening to the specific shear viscosity. Freeze out times required for the use of the ansatz are computed using a linear fit of freeze out times reported as a function of the cubic root of the charged particle pseudorapidity density ({mathrm{d}}N_{mathrm{ch}}/deta )^{1/3}. Estimates of eta /s based on ALICE data exhibit little to no dependence on collision centrality at LHC energy, while estimates obtained from STAR data hint that eta /s might be a function of collision centrality at top RHIC energy.

Characterizing the medium created in Pb–Pb collisions at by means of the evolution of two-particle transverse momentum correlations

Two-particle correlations are powerful tools for studying the medium produced in heavy-ion collisions. In particular, two-particle transverse momentum correlations enable measurements of the collision dynamics sensitive to momentum currents. Their evolution with collision centrality, which is related to the system lifetime, provides information about the shear viscosity, ɳ/S, and the system relaxation time, . We report on measurements of two-particle transverse momentum correlations as a function of centrality in Pb-Pb collisions using the ALICE detector at the LHC. The centrality dependence of the near-side peak of the correlation function, particularly in the longitudinal dimension, provides information about the shear viscosity and the system relaxation time of the produced medium. The data are compared to predictions from selected Monte Carlo models. The charge independent momentum correlator exhibits a longitudinal broadening from peripheral to central collisions that is qualitatively consistent with expectations from a model with viscous effects. We will discuss an interpretation of the observed broadening in the context of this model.

Two-Particle Transverse Momentum Correlations in Pb–Pb Collisions at $$\sqrt {{{{\text{s}}}_{{{\text{NN}}}}}} = 2.76$$ TeV

Two-particle correlations are powerful tools for studying the medium produced in heavy-ion collisions. In particular, two-particle transverse momentum correlations enable measurements of the collision dynamics sensitive to momentum currents. Their evolution with collision centrality, which is related to the system lifetime, provides information about the shear viscosity, $$\eta {\text{/}}s$$, and the system relaxation time, $${{\tau }_{\pi }}$$. We report on measurements of two-particle transverse momentum correlations as a function of centrality in Pb‒Pb collisions using the ALICE detector at the LHC. The centrality dependence of the near side peak of the correlation function, particularly its longitudinal width, provides information about the shear viscosity of the produced medium. The data are compared to predictions from selected Monte Carlo models. The charge independent momentum correlator exhibits a longitudinal broadening from peripheral to central collisions that is qualitatively consistent with expectations from a model with viscous effects. An interpretation of the observed broadening in the context of this model will be discussed.

Recent results on event-by-event fluctuations from the RHIC Beam Energy Scan program in the STAR experiment

Event-by-event fluctuations of global observables in relativistic heavy-ion collisions are studied as probes for the QCD phase transition and as tools to search for critical phenomena near the phase boundary. Dynamical fluctuations in mean transverse momentum, identified particle ratios and conserved quantities (such as net-charge, net-baryon) are expected to provide signatures of a de-confined state of matter. Non-monotonic behavior in the higher-moments of conserved quantities as a function of beam energy and collision centrality are proposed as signatures of the QCD critical point. To study the QCD phase transition and locate the critical point, the STAR experiment at RHIC has collected a large amount of data for Au+Au collisions from = 7.7 - 200 GeV in the RHIC Beam Energy Scan (BES) program. We present the recent beam energy scan results on dynamical fluctuations of particle ratios and two-particle transverse momentum correlations at mid-rapidity. Higher-moments of the net-charge and net- proton multiplicity distributions as a function of beam energy will be presented. We give a summary of what has been learnt so far and future prospectives for the BES-II program.

Rapidity Dependence of Transverse Momentum Correlations from Fluctuating Hydrodynamics

Interest in the development of the theory of fluctuating hydrodynamics is growing [1]. Early efforts suggested that viscous diffusion broadens the rapidity dependence of two-particle transverse momentum correlations [2]. That work stimulated an experimental analysis by STAR [3]. In this work we present results of correlation observables computed using the second order Israel-Stewart hydrodynamics with stochastic noise and latest theoretical equations of state and transport coefficients. We also compute these observables using the first Navier-Stokes theory. We find that the second order theory better describes the data and also provides some novel features forced by causal constraints of the second order theory.

Energy dependence of transverse momentum fluctuations inPb+Pbcollisions at the CERN Super Proton Synchrotron (SPS) at20Ato158AGeV

Results are presented on event-by-event fluctuations of transverse momenta pT in central Pb+Pb interactions at 20A, 30A, 40A, 80A, and 158A GeV. The analysis was performed for charged particles at forward center-of-mass rapidity (1.1<yπ*<2.6). Three fluctuation measures were studied: the distribution of average transverse momentum M(pT) in the event, the ΦpT fluctuation measure, and two-particle transverse momentum correlations. Fluctuations of pT are small and show no significant energy dependence in the energy range of the CERN Super Proton Synchrotron. Results are compared with QCD-inspired predictions for the critical point, and with the UrQMD model. Transverse momentum fluctuations, similar to multiplicity fluctuations, do not show the increase expected for freeze-out near the critical point of QCD.18 MoreReceived 10 November 2008DOI:https://doi.org/10.1103/PhysRevC.79.044904©2009 American Physical Society

Incident energy dependence ofptcorrelations at relativistic energies

We present results for two-particle transverse momentum correlations, <dpt,i dpt,j>, as a function of event centrality for Au+Au collisions at sqrt(sNN) = 20, 62, 130, and 200 GeV at the Relativistic Heavy Ion Collider. We observe correlations decreasing with centrality that are similar at all four incident energies. The correlations multiplied by the multiplicity density increase with incident energy and the centrality dependence may show evidence of processes such as thermalization, minijet production, or the saturation of transverse flow. The square root of the correlations divided by the event-wise average transverse momentum per event shows little or no beam energy dependence and generally agrees with previous measurements at the Super Proton Synchrotron.

Transverse momentum fluctuations in nuclear collisions at158AGeV

Results are presented on event-by-event fluctuations in transverse momentum of charged particles, produced at forward rapidities in p+p, C+C, Si+Si, and Pb+Pb collisions at 158AGeV. Three different characteristics are discussed: the average transverse momentum of the event, the ΦpT fluctuation measure, and two-particle transverse momentum correlations. In the kinematic region explored, the dynamical fluctuations are found to be small. However, a significant system size dependence of ΦpT is observed, with the largest value measured in peripheral Pb+Pb interactions. The data are compared with predictions of several models.8 MoreReceived 17 November 2004DOI:https://doi.org/10.1103/PhysRevC.70.034902©2004 American Physical Society