Session S12: Mini-Symposium on Initial State and Early-time Dynamics of Heavy Ion Collisions I

The initial state and early time dynamics in heavy-ion collisions

The standard picture of heavy ion collisions is that large systems (collisions of large nuclei like Au+Au and Pb+Pb) create a quark-gluon plasma that exhibits collective behavior indicative of nearly inviscid hydrodynamical evolution. Recently, data from small systems (collisions of a small projectile and a large target like d+Au and p+Pb) have been found to exhibit strikingly similar evidence for collective behavior. Contrariwise, while there is a wealth of evidence for particle suppression and jet attenuation in large systems, such effects are notably absent in small systems. In this talk we give an introduction to the initial state and early time dynamics of heavy ion collisions, giving an overview of results in large and small systems.   

(3+1)D Viscous Anisotropic Hydrodynamics for Nonconformal Fluids

Anisotropic hydrodynamics improves upon standard viscous hydrodynamics by treating certain large dissipative corrections non-perturbatively. Relativistic heavy-ion collisions feature two such large viscous effects: (i) Strongly anisotropic expansion along the beam direction generates a large shear stress, leading to very different longitudinal and transverse pressures at early times. (ii) Critical fluctuations near the quark-hadron phase transition lead to a large bulk pressure on the conversion surface between hydrodynamics and a hadronic cascade description of the final collision stage. We present a new dissipative hydrodynamic formulation for non-conformal fluids where both of these effects are treated non-perturbatively using an anisotropic leading-order distribution function with two momentum-space deformation parameters. Generalized Landau matching conditions are required to fix these parameters, accounting for the longitudinal and transverse pressures. Residual shear stresses are treated perturbatively, as in standard viscous hydrodynamics. The resulting hydrodynamic evolution equations are derived from the Boltzmann equation in 3+1 dimensions and tested in a (0+1)-dimensional Bjorken expansion. Comparisons with viscous hydrodynamical frameworks are presented.   

Anisotropic Hydrodynamics with a Realistic Scalar Scattering Kernel

Prior studies of non-equilibrium dynamics using anisotropic hydrodynamics have all relied on the use of the relativistic Anderson-Witting scattering kernel. In this paper, we make the first study of the impact of using a more realistic scalar scattering kernel in anisotropic hydrodynamics. For this purpose, we consider a conformal transversally homogenous system undergoing boost-invariant Bjorken expansion and take the collisional kernel to be given by the leading order $2 \leftrightarrow 2$ scattering kernel in scalar $\lambda \phi^4$. We consider both classical and quantum statistics in order to assess the impact of Bose enhancement on the dynamics. We also determine the anisotropic non-equilibrium attractor of a system subject to this collisional kernel in the context of anisotropic hydrodynamics. We find that, when the shear viscosity to entropy density ratio ($\eta/s$) obtained using the Anderson-Witting and scalar collisional kernels is matched, the system develops a higher degree of early-time momentum-space anisotropy given the same value of $\eta/s$. Additionally, we find that taking into account Bose enhancement further increases early-time momentum-space anisotropy.   

Quark and Gluon Collective Modes with Momentum Anisotropy

The Self-energies and dispersion relations of the quark and gluon collective excitations are modified by medium effects in a hot and momentum anisotropic quark-gluon plasma. This can affect various observables of the heavy-ion collision experiments and provide information about earlier stages of the quark-gluon plasma evolution. We calculate the hard loop self-energies and dispersion relations of the collective excitations for both of the quark and gluon modes with various types and strengths of momentum anisotropy. We show the full direction dependence of the effective masses and unstable modes of the quark and gluon quasiparticles.   

Free-streaming as a pre-equilibrium model for use with (3+1)-dimensional hydrodynamical evolution models

A (3+1)-dimensional free-streaming module has been developed that converts an initial energy momentum tensor for a system of massless partons with arbitrary initial energy density but zero collective flow at time zero into an evolved energy momentum tensor at any later time. By writing the evolved energy momentum tensor at that later time in hydrodynamic form one obtains non-trivial initial conditions, including initial collective flow profiles and dissipative flows, for subsequent hydrodynamic evolution.   

Measurement of charged particle mixed higher order flow harmonics and nonlinear response coefficients in PbPb collisions

Higher-order flow harmonics can be measured either with respect to the event plane of the same order, a lower order event plane, or a mixture of lower order planes. Studies of flow harmonics using the same order event plane have been used to extract the transport properties of the hot and dense medium produced in the collisions and to explore initial state effects. The mixed higher-order harmonics have been proposed to have sensitivity to initial conditions and shear viscosity over entropy density ratio of the medium during the hydrodynamic evolution and at freeze-out. In this talk, the mixed higher order flow harmonics and nonlinear response coefficients of charged particles are measured as a function of $p_{T}$ and centrality in PbPb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV and 5.02 TeV with the CMS detector. The results are obtained using the scalar-product method, and cover a $p_{T}$ range from 0.3 to 8.0 GeV/c, pseudorapidity $|\eta|<0.8$, and a centrality range of 0 - 60\%. It is observed that the nonlinear response coefficients of the odd harmonics are larger than the even harmonic ones for $p_{T}$ less than 3 GeV/c. The results are compared with hydrodynamic predictions with different shear viscosity to entropy density ratios and different initial conditions.