Expansion Of Quark-gluon Plasma Research Articles

Anisotropic flow and flow fluctuations of identified hadrons in Pb–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

The first measurements of elliptic flow of π±, K±, textrm{p}+overline{textrm{p}} , {textrm{K}}_{textrm{S}}^0 , Lambda +overline{Lambda} , ϕ, {Xi}^{-}+{overline{Xi}}^{+} , and {varOmega}^{-}+{overline{varOmega}}^{+} using multiparticle cumulants in Pb–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV are resented. Results obtained with two- (v2{2}) and four-particle cumulants (v2{4}) are shown as a function of transverse momentum, pT, for various collision centrality intervals. Combining the data for both v2{2} and v2{4} also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous pT-differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low pT and the approximate scaling with the number of constituent quarks at intermediate pT, are similarly present in the four-particle correlations and the combinations of v2{2} and v2{4}. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide pT range.

Second order hydrodynamics based on effective kinetic theory and electromagnetic signals from QGP

We study the thermal dilepton and photon production from relativistic heavy ion collisions in presence of viscosities by employing the recently developed second order dissipative hydrodynamic formulation estimated within a quasiparticle description of thermal quantum chromo-dynamics (QCD) medium. The sensitivity of shear and bulk viscous pressures to the temperature dependence of relaxation time is analyzed within one- dimensional boost invariant expansion of quark gluon plasma (QGP). The dissipative corrections to the phase-space distribution functions up to first order in gradients are obtained from the Chapman–Enskog like iterative solution of effective Boltzmann equation in the relaxation time approximation. Thermal dilepton and photon production rates for QGP are calculated by employing this viscous modified distribution function. Yields of these particles are quantified for the longitudinal expansion of QGP with different temperature dependent relaxation times. Our analysis employing this second order hydrodynamic model indicates that the spectra of dileptons and photons get enhanced by both bulk and shear viscosities and are well behaved. Also, these particle yields are found to be sensitive to relaxation time. Further, we do a comparison of these particle spectra with a standard hydrodynamic formulation.

Electromagnetic Response in an Expanding Quark–Gluon Plasma

The validity of conventional Ohm’s law is tested in the context of a rapidly evolving quark–gluon plasma produced in heavy-ion collisions. Here, we discuss the electromagnetic response using an analytical solution in kinetic theory. As conjectured previously, after switching on an electric field in a nonexpanding plasma, the time-dependent current is given by J(t)=(1−e−t/τ0)σ0E, where τ0 is the transport relaxation time and σ0 is the steady-state electrical conductivity. Such an incomplete electromagnetic response reduces the efficiency of the magnetic flux trapping in the quark–gluon plasma, and may prevent the observation of the chiral magnetic effect. Here, we extend the study to the case of a rapidly expanding plasma. We find that the decreasing temperature and the increasing transport relaxation time have opposite effects on the electromagnetic response. While the former suppresses the time-dependent conductivity, the latter enhances it.

Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb–Pb and Xe–Xe collisions

Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons v2{ΨSP} in collisions of Pb ions at center-of-mass energy per nucleon–nucleon pair sNN=2.76 TeV and Xe ions at sNN=5.44 TeV are reported. The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum for the 5–70% and 0.2–6 GeV/c ranges, respectively. The ratio between v2{ΨSP} and the elliptic flow coefficient relative to the participant plane v2{4}, estimated using four-particle correlations, deviates by up to 20% from unity depending on centrality. This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies. The differences can be interpreted as a decorrelation of the neutron spectator plane and the reaction plane because of fragmentation of the remnants from the colliding nuclei, which points to an incompleteness of current models describing the initial state fluctuations. A significant transverse momentum dependence of the ratio v2{ΨSP}/v2{4} is observed in all but the most central collisions, which may help to understand whether momentum anisotropies at low and intermediate transverse momentum have a common origin in initial state fluctuations. The ratios of v2{ΨSP} and v2{4} to the corresponding initial state eccentricities for Xe–Xe and Pb–Pb collisions at similar initial entropy density show a difference of (7.0±0.9)% with an additional variation of +1.8% when including RHIC data in the TRENTo parameter extraction. These observations provide new experimental constraints for viscous effects in the hydrodynamic modeling of the expanding quark–gluon plasma produced in heavy-ion collisions at the LHC.

Measurement of prompt [formula omitted]-meson production and azimuthal anisotropy in Pb–Pb collisions at [formula omitted

The production yield and angular anisotropy of prompt Ds+ mesons were measured as a function of transverse momentum (pT) in Pb–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02TeV collected with the ALICE detector at the LHC. Ds+ mesons and their charge conjugates were reconstructed at midrapidity (|y|<0.5) from their hadronic decay channel Ds+→ϕπ+, with ϕ→K−K+, in the pT intervals 2<pT<50GeV/c and 2<pT<36GeV/c for the 0–10% and 30–50% centrality intervals. For pT>10GeV/c, the measured Ds+-meson nuclear modification factor RAA is consistent with the one of non-strange D mesons within uncertainties, while at lower pT a hint for a Ds+-meson RAA larger than that of non-strange D mesons is seen. The enhanced production of Ds+ relative to non-strange D mesons is also studied by comparing the pT-dependent Ds+/D0 production yield ratios in Pb–Pb and in pp collisions. The ratio measured in Pb–Pb collisions is found to be on average higher than that in pp collisions in the interval 2<pT<8GeV/c with a significance of 2.3σ and 2.4σ for the 0–10% and 30–50% centrality intervals. The azimuthal anisotropy coefficient v2 of prompt Ds+ mesons was measured in Pb–Pb collisions in the 30–50% centrality interval and is found to be compatible with that of non-strange D mesons. The main features of the measured RAA, Ds+/D0 ratio, and v2 as a function of pT are described by theoretical calculations of charm-quark transport in a hydrodynamically expanding quark–gluon plasma including hadronisation via charm-quark recombination with light quarks from the medium. The pT-integrated production yield of Ds+ mesons is compatible with the prediction of the statistical hadronisation model.

Radiative p_{perp }-broadening of fast partons in an expanding quark\u2013gluon plasma

We study contribution of radiative processes to p_{perp }-broadening of fast partons in an expanding quark–gluon plasma. It is shown that the radiative correction to langle p_{perp }^2rangle for the QGP produced in AA-collisions at RHIC and LHC may be negative, and comparable in absolute value with the non-radiative contribution. We have found that the QGP expansion enhances the radiative suppression of p_perp -broadening as compared to the static medium.

Recent development of hydrodynamic modeling in heavy-ion collisions

We present a concise review of the recent development of relativistic hydrodynamics and its applications to heavy-ion collisions. Theoretical progress on the extended formulation of hydrodynamics toward out-of-equilibrium systems is addressed, with emphasis on the so-called attractor solution. Moreover, recent phenomenological improvements in the hydrodynamic modeling of heavy-ion collisions with respect to the ongoing beam energy scan program, the quantitative characterization of transport coefficients in three-dimensionally expanding quark–gluon plasma, the fluid description of small colliding systems, and certain other interdisciplinary connections are discussed.

Calculation of power spectrum of the little bangs

The power spectrum of the momentum distributions of particles have been estimated with Optical Glauber and Monte Carlo Glauber initial conditions for relativistic heavy ion collisions. The evaluation procedure adopted in this work is analogous to the one used in the calculation of power spectrum in Cosmic Microwave Background Radiation (CMBR). The power spectrum due to perturbations in the phase space distribution of the particles has also been evaluated. The perturbation in phase space has been evolved through the Boltzmann transport equation in an expanding quark–gluon plasma (QGP) background. The expansion of the QGP has been treated within the purview of (3[Formula: see text]+[Formula: see text]1)-dimensional relativistic hydrodynamics. We observe that the nonequilibrium effects introduced as perturbations in the phase space distributions can be traced through its variation with temperature which is distinctly different from the case of vanishing perturbation. A relation has been derived between the power spectrum and the flow harmonics.

Strong diffusion effect of charm quarks on J/ψ production in Pb–Pb collisions at the LHC

We study the J/ψ production based on coalescence model at sNN=2.76 and 5.02 TeV Pb–Pb collisions. With the colliding energy increasing from 2.76 TeV to 5.02 TeV, the number of charm pairs is enhanced by more than 50%. However, the ratio of J/ψ inclusive nuclear modification factors RAA5.02 TeV/RAA2.76 TeV is only about 1.1∼1.2. We find that the regeneration of J/ψ is proportional to the densities of charm and anti-charm quarks, instead of their total numbers. The charm quark density is diluted by the strong expansion of quark gluon plasma, which suppresses the combination probability of heavy quarks and J/ψ regeneration. This effect is more important in higher colliding energies where QGP expansion is strong. We also propose the ratio NJ/ψ/(Nc)2 as a measurement of c and c¯ coalescence probability, which is only affected by the heavy quark diffusions in QGP, and does not depend on the inputs such as cold nuclear matter effects and cross sections of charm quark production. Further more, we give the predictions at the energy of Future Circular Collider (sNN=39 TeV).

Bottomonium continuous production from unequilibrium bottom quarks in ultrarelativistic heavy ion collisions

We employ the Langevin equation and Wigner function to describe the bottom quark dynamical evolutions and their formation into a bound state in the expanding Quark Gluon Plasma (QGP). The additional suppressions from parton inelastic scatterings are supplemented in the regenerated bottomonium. Hot medium modifications on ϒ(1S) properties are studied consistently by taking the bottomonium potential to be the color-screened potential from Lattice results, which affects both ϒ(1S) regeneration and dissociation rates. Finally, we calculated the ϒ(1S) nuclear modification factor RAArege from bottom quark combination with different diffusion coefficients in Langevin equation, representing different thermalization of bottom quarks. In the central Pb–Pb collisions (b=0) at sNN=5.02 TeV, we find a non-negligible ϒ(1S) regeneration, and it is small in the minimum bias centrality. The connections between bottomonium regeneration and bottom quark energy loss in the heavy ion collisions are also discussed.

Thermal production of charm quarks in heavy ion collisions at the Future Circular Collider

By solving the rate equation in an expanding quark–gluon plasma (QGP), we study thermal production of charm quarks in central Pb + Pb collisions at the Future Circular Collider. With the charm quark production cross section taken from the perturbative QCD at the next-to-leading order, we find that charm quark production from the QGP can be appreciable compared to that due to initial hard scattering between colliding nucleons.

Hot medium effects on [formula omitted] production in [formula omitted] collisions at [formula omitted

Based on a kinetic description of J/ψ dissociation and production in an expanding quark–gluon plasma that is described by a 2+1 dimensional ideal hydrodynamics, we have studied the hot medium effects on J/ψ production in p+Pb collisions at sNN=5.02 TeV. Including also the cold nuclear matter effects, we are able to reproduce recent experimental results on the nuclear modification factor RpPb(J/ψ) measured by the ALICE Collaboration. We have also made predictions for the RpPb of J/ψ and the double ratio RpPbpro(ψ′)/RpPbpro(J/ψ) of prompt quarkonia produced in the most central 10% p+Pb collisions. We find that different from the cold nuclear matter effects, the RpPb(J/ψ) is slightly smaller than that in the minimum bias collisions, and the double ratio is significantly less than one at backward rapidity.

Anomalous transport processes in turbulent non-Abelian plasmas

Turbulent color fields, which can arise in the early and late stages of relativistic heavy ion collisions, may contribute significantly to the transport processes in the matter created in these collisions. We review the theory of these anomalous transport processes and discuss their possible phenomenology in the glasma and quasistationary expanding quark–gluon plasma.

Correlation as a signature of strongly coupled quark matter

Massive heavy quarks, which are pair produced back to back initially in high-energy nuclear collisions, are sensitive to early dynamical conditions. The strong collective partonic ‘wind’ from the fast expanding quark–gluon plasma created in high-energy nuclear collisions modifies the correlation pattern significantly. While the hot and dense medium in collisions at the RHIC ( GeV) can only smear the initial back-to-back correlation, a clear and strong near-side correlation is expected at the LHC ( GeV). This is considered as a signature for early thermalization and the strongly coupled quark–gluon plasma.

Sequential melting of charmonium states in an expanding quark–gluon plasma and J/ψ suppression at RHIC and LHC energy collisions

We have developed a hydrodynamic model to study sequential melting of charmonium states in an expanding quark–gluon plasma (QGP) medium. According to the initial fluid temperature profile, J/ψ's are randomly distributed in the transverse plane. As the fluid evolves in time, the free streaming J/ψ's are suppressed if the local fluid temperature exceeds a critical temperature. PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions at mid-rapidity are explained by sequential melting of the charmonium states, χc, ψ′ and J/ψ, in the expanding medium. The critical temperatures TJ/ψ ≈ 2.09Tc and agree with lattice-motivated calculations. The feed-down fraction F depends on whether the cold nuclear matter effect is included or not. It changes from F = 0.3 with cold nuclear matter effect included to F = 0.5 when the effect is neglected. The model fails to reproduce the PHENIX data on the centrality dependence of J/ψ suppression in Cu+Cu collisions at mid-rapidity, indicating that the mechanism of J/ψ suppression is different in Au+Au and Cu+Cu collisions. We also use the model to predict for the centrality dependence of J/ψ suppression in Pb+Pb collisions at LHC energy, = 5500 GeV. In LHC energy, J/ψ's are more suppressed in mid central collisions than in Au+Au collisions at RHIC energy.

Anomalous viscosity of an expanding quark–gluon plasma

We argue that an expanding quark–gluon plasma has an anomalous viscosity, which arises from interactions with dynamically generated colour fields. The anomalous viscosity dominates over the collisional viscosity for large velocity gradients or weak coupling. This effect may provide an explanation for the apparent near-perfect liquidity of the matter produced in nuclear collisions at RHIC without the assumption that it is a strongly coupled state.

The flavours of the quark–gluon plasma

Flavour probes depend on the transport properties of the quark–gluon plasma. An expanding quark–gluon plasma has an anomalous viscosity, which arises from interactions with dynamically generated colour fields. The anomalous viscosity dominates over the collisional viscosity for weak coupling and at not too late times. In addition to possibly providing an explanation for the apparent nearly ‘perfect’ liquidity of the matter produced in nuclear collisions at RHIC, similar anomalous effects can affect the rate of flavour equilibration and the diffusion constant of heavy quarks.

Mach shocks induced by partonic jets in expanding quark–gluon plasma

We study Mach shocks generated by fast partonic jets propagating through a deconfined strongly-interacting matter. Our main goal is to take into account different types of collective motion during the formation and evolution of this matter. We predict a significant deformation of Mach shocks in central Au + Au collisions at RHIC and LHC energies as compared to the case of jet propagation in a static medium. The observed broadening of the near-side two-particle correlations in pseudorapidity space is explained by the Bjorken-like longitudinal expansion. Three-particle correlation measurements are proposed for a more detailed study of the Mach shock waves.

EFFECTS OF CURVATURE AND INTERACTIONS ON THE DYNAMICS OF THE DECONFINEMENT PHASE TRANSITION

We study the dynamics of first-order confinement-deconfinement phase transition through nucleation of hadronic bubbles in an expanding quark–gluon plasma in the context of heavy ion collisions for interacting quark and hadron gas and by incorporating the effects of curvature energy. We find that the interactions reduce the delay in the phase transition whereas the curvature energy has a mixed behavior. In contrast to the case of early Universe phase transition, here lower values of surface tension increase the supercooling and slow down the hadronization process. Higher values of bag pressure tend to speed up the transition. Another interesting feature is the start of the hadronization process as soon as the QGP is created.

Jet quenching at intermediate RHIC energies

The final state energy loss of fast partons penetrating a longitudinally expanding quark–gluon plasma of effective gluon rapidity density dNg/dy=650–800 is evaluated and incorporated together with the multiple initial state Cronin scattering in the lowest order perturbative QCD hadron production formalism. Predictions for the neutral pion attenuation in central Au+Au collisions at the intermediate RHIC energy of sNN=62 GeV relative to the binary collision scaled p+p result are given. The quenching is found to be a factor of 2–3 with a moderate transverse momentum dependence and the attenuation of the away side di-hadron correlation function is estimated to be 3–5-fold.