Session K14: Hydrodynamics in Heavy Ion Collisions

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We introduce an improved form for the anisotropic hydrodynamics distribution function which explicitly takes into account the free-streaming and equilibrating contributions separately. We demonstrate that with this improvement one can better reproduce exact results available in the literature for the evolution of moments of the distribution function, in particular, for moments which contain no powers of the longitudinal momentum in their definition (m = 0 moments). Using the resulting dynamical equations, we extract the non-equilibrium attractor associated with our improved aHydro ansatz and demonstrate that the improvement also allows one to better reproduce the exact dynamical attractor obtained using kinetic theory in the relaxation time approximation, particularly at early rescaled times and for m = 0 moments.   

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We establish the existence of a far-from-equilibrium attractor in weakly-coupled gauge theory undergoing one-dimensional Bjorken expansion. We demonstrate that the resulting far-from-equilibrium evolution is insensitive to certain features of the initial condition, including both the initial momentum-space anisotropy and initial occupancy. We find that this insensitivity extends beyond the energy-momentum tensor to the detailed form of the one-particle distribution function. Based on our results, we assess different procedures for reconstructing the full one-particle distribution function from the energy-momentum tensor along the attractor and discuss implications for the freeze-out procedure used in the phenomenological analysis of ultra-relativistic nuclear collisions.   

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The dynamics of massless particles with quartic interactions in Friedman-Lemaitre-Robertson-Walker spacetime are studied through solutions to the nonlinear relativistic Boltzmann equation. Using a new generation function method, the Boltzmann equation is expressed as an infinite set of coupled ordinary differential equations for suitably defined moments of the distribution function. Our new formulation allows for numerical solution of the evolution of the moments, which in turn can be used to recover the full distribution function. Furthermore, the linearized Boltzmann operator for this system is diagonalized, and the corresponding eigenfunctions and eigenvalues are given in explicit form. Finally, a comparison between our results and previous calculations for the dynamics of particles with constant cross section interactions is made.   

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The measurement of the correlation between flow harmonics $v_n$ and average transverse momentum $[p_\mathrm{T}]$ for n = 2, 3 and 4 with the ATLAS detector at the LHC is presented, based on 3$\mu \mathrm{b}^{-1}$ Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44$ TeV and 22$\mu \mathrm{b}^{-1}$ Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. The observable is the Pearson's correlation coefficient $\rho(v_n^2,[p_\mathrm{T}])$ which is measured for several ranges in $p_\mathrm{T}$ and pseudorapidity $\eta$. The results show a strong dependence of the $\rho(v_n^2,[p_\mathrm{T}])$ on centrality, $p_\mathrm{T}$ and $\eta$ ranges. Comparison of $\rho(v_n^2,[p_\mathrm{T}])$ measured using two different methods of event class definitions : the number of charged particles at mid-rapidity and transverse energy at the forward pseudorapidity, shows significant differences between these two methods indicating a strong influence of centrality fluctuations. When compared with theory models it is seen that the models can capture some of the qualitative trends of the measurement but they cannot describe all the quantitative trends. This measurement provides insights on the initial-state geometry and final-state dynamics in heavy ion collisions.   

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Estimates of the specific shear viscosity, $\eta/s$, of the quark-gluon plasma formed in ultrarelativistic heavy-ion collisions at RHIC and LHC based on the longitudinal broadening of transverse momentum two-particle correlator, $G_2$, were published by both STAR and ALICE collaborations. In this work using the progressive evolution with collision centrality of the correlator longitudinal widths in both systems, values of $\eta/s$ are computed as a function of charged particle pseudorapidity density 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 (${\rm d}N_{\rm ch}$/d$\eta)^{1/3}$ for different collision systems. Estimated values of $\eta/s$ based on ALICE data exhibit little to no dependence on charged particle pseudorapidity density at LHC energy, while estimates obtained from STAR data hint that $\eta/s$ might be a function of charged particle pseudorapidity density at top RHIC energy.   

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We present hydrodynamic predictions of anisotropic flow coefficients and average transverse momenta for O-O collisions at both $\sqrt{s_{\rm{NN}}} =$ 200 and 6.5 TeV energies. In particular, we investigate the effect of the relevant degrees of freedom in the initial nuclear state using the T$_{\rm R}$ENTo model. Alpha clustering is implemented using Lattice QCD calculations, and the corresponding hydrodynamic predictions are compared to modeling the nucleons independently using a Wood Saxon distribution with and without constituent quarks. We use VISHNN 2+1 hydrodynamics and the UrQMD hadronic afterburner to evolve the system, and the transport properties inputted result from a Bayesian analysis on p-Pb and Pb-Pb collision data at the LHC. We will also access validity of hydrodynamics for O-O collisions by examining the Knudsen and Reynold values during the evolution.   

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Initial state geometry is a key quantitative component of descriptions of collective behavior in heavy ion collisions. Phenomenological models of the initial state are typically used in hybrid models tuned to observables such as charged hadron multiplicity and coefficients of the spatial Fourier decomposition, $v_n$. While this approach has demonstrated success, these models often fail to accurately predict many-body observables such as event plane correlations and nonlinear response coefficients. In this talk, we use state of the art simulations of heavy ion collisions to demonstrate that Color Glass Condensate-inspired models such as IP-Glasma are better able to simultaneously describe and predict many-body observables. We demonstrate the advantage of interpretable microscopic models of the initial stage of heavy ion collisions over parametric models and motivate their use in Bayesian inference. Through the use of an interpretable model of microscopic initial state physics, we can gain further insight into the properties of collective behavior in strongly-interacting matter and reduce systematic uncertainties and unphysical cross-correlations in order to precisely quantify the dynamics observed in heavy ion collisions.   

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Characterization of the Chiral Magnetic Effect (CME) and the Chiral Magnetic Wave (CMW) in the quark-gluon plasma (QGP) produced in heavy-ion collisions can provide critical insights into anomalous transport in the QGP and the connections between chiral symmetry restoration, axial anomaly, and gluonic topology. The CME and the CMW lead to dipole and quadrupole charge separation signals respectively in heavy-ion collisions. Consequently, their detection and characterization are keys to understanding the anomalous transport in the QGP. We will present our recent measurements on dipole and quadrupole charge separation measurements, obtained with $\mathrm{R^{(d)}_{\Psi_m}(\Delta S_{d})}$ correlator~[1,2], for $d$+Au and Au+Au collisions at $\sqrt{s_{NN}}=200$~GeV. Results will be also compared to the different background- and CME (CMW)-driven charge separation models.\\ [1] N. Magdy et al. Phys.Rev.C 97 6, 061901 (2018) \\ [2] N. Magdy et al. Phys.Lett.B 811, 135986 (2020)   

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Transverse momentum distributions of identified hadrons provide important information on the transverse expansion and the freeze-out properties of the hot and dense matter created in relativistic heavy-ion collisions. In 2018, the STAR experiment collected large datasets of isobaric collisions of Ru+Ru and Zr+Zr at $\sqrt{s_{\rm {NN}}}=200$ GeV, which provide a good opportunity to study the charged particle spectra in these collisions with great precision. In this presentation, we will report analysis progresses towards measuring $\pi^\pm$,$K^\pm$, proton and antiproton spectra as a function of transverse momentum at mid-rapidity for different centrality classes. Particle species are identified utilizing the specific energy loss measured in the Time Projector Chamber, and the time of flight information measured by the Time Of Flight detector. Physics implications of these measurements will be discussed.