Bulletin of the American Physical Society
2021 Fall Meeting of the APS Division of Nuclear Physics
Volume 66, Number 8
Monday–Thursday, October 11–14, 2021; Virtual; Eastern Daylight Time
Session JH: Mini-Symposium: The (Un)Reasonable Effectiveness of Fluid Dynamics in Heavy Ion Collisions I |
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Chair: Jorge Noronha, University of Illinois at Urbana-Champaign Room: Whittier |
Wednesday, October 13, 2021 9:30AM - 10:06AM |
JH.00001: Hydrodynamics in heavy ion collisions Invited Speaker: J-F Paquet The quark-gluon plasma is a new phase of matter that can be produced by colliding large nuclei at velocities close to the speed of light. When produced in collider experiments, the plasma expands and cools down rapidly, with quarks and gluons recombining into hadrons within $10^{-22}$ seconds. There is evidence that the strength of the interactions within the quark-gluon plasma, as well as its high density, allows it to maintain a fluid-like collective behaviour despite its explosive expansion. Indeed, numerical simulations based on relativistic viscous hydrodynamics have been shown to describe a broad range of measurements from the Relativistic Heavy-Ion Collider and the Large Hadron Collider. Yet this represents an atypical application of hydrodynamics: the quark-gluon plasma is extremely small and short-lived, with considerable spatial gradients and seemingly few constituents. The apparent success of hydrodynamics under these extreme conditions raises questions about our understanding of its domain of applicability, and provides a valuable opportunity to extend our knowledge of the topic. In this talk, I will present an introductory overview of the situation, highlighting recent progress and remaining challenges. |
Wednesday, October 13, 2021 10:06AM - 10:18AM |
JH.00002: Causality violations in realistic simulations of nuclear collisions Christopher Plumberg, Dekrayat K Almaalol, Travis Dore, Jorge Noronha, Jacquelyn Noronha-Hostler Causality is violated up to 75% in the early stages of state-of-the-art heavy-ion hydrodynamic simulations. Only after 2-3 fm/c of evolution, do we find that ~50% of the fluid cells are definitely causal. Inclusion of pre-equilibrium evolution significantly reduces the number of acausal cells, but does not eliminate them. Our findings imply that relativistic causality imposes constraints on the available parameter space of heavy-ion collision simulations. |
Wednesday, October 13, 2021 10:18AM - 10:30AM |
JH.00003: Dynamics of Non-Gaussian Hydrodynamic Fluctuations Gokce K Basar, Xin An, Mikhail Stephanov, Ho-Ung Yee In the context of the search for the QCD critical point we present dynamical evolution equations for Non-Gaussian fluctuations in hydrodynamics. We introduce a novel generalization of the Wigner transform to multi-point correlators and derive the evolution equations for three- and four-point Wigner functions for the problem of nonlinear stochastic diffusion with multiplicative noise. The formalism and the results we present are very general and would pertain to problems where non- linearity and non-Gaussian fluctuations are of interest. |
Wednesday, October 13, 2021 10:30AM - 10:42AM |
JH.00004: Universal dynamics near the O(4) critical point, and soft pion yields in heavy ion collisions. Derek Teaney, Alexander Soloviev, Eduardo Grossi, Adrien Florio, Fanglida Yan I present a stochastic hydrodynamic description of the O(4) phase transition, and present numerical work showing how the axial current-current correlator evolves near the psuedo-critical point. At high temperature the theory matches smoothly with ordinary hydrodynamics, while at low temperatures the theory matches smoothly with a non-abelian superfluid theory capturing broken chiral symmetry. This theory describes the dynamics of soft pions coupled to hydrodynamic modes. We use this theory to present estimates for the yields of soft pions in heavy ion collisions, which are chronically underpredicted by traditional hydrodynamic simulations of heavy ion events. |
Wednesday, October 13, 2021 10:42AM - 10:54AM |
JH.00005: On the convergence of the hydrodynamics of rotating plasmas Casey C Cartwright, Matthias Kaminski, Markus Garbiso Amano, Jorge Noronha, Enrico Speranza In this presentation I will report on novel theoretical methods demonstrating the effectiveness of hydrodynamics in rotating systems. Recently developed techniques making use of complex analysis and the mathematical theory of plane curves allow one to derive the radius of convergence of the linearized hydrodynamic series. Utilizing these techniques I will show how the convergence of the hydrodynamic series of a particular, strongly coupled, rotating plasma can be expressed in terms of non-rotating quantities. |
Wednesday, October 13, 2021 10:54AM - 11:06AM |
JH.00006: Non-conformal attractor in boost-invariant plasmas Sunil Jaiswal, Chandrodoy Chattopadhyay, Lipei Du, Ulrich W Heinz, Subrata Pal We explore the far-off-equilibrium dynamics of a (0+1)-dimensionally expanding non-conformal system with Bjorken symmetry using kinetic theory and hydrodynamics. It is shown that the breaking of conformal invariance by the introduction of even a small mass (in units of the local temperature) can drastically modify the well-known attractor for the shear Reynolds number previously observed in massless systems. For generic nonzero particle mass, neither shear nor bulk viscous pressure relax quickly to a non-equilibrium attractor; universal hydrodynamic behavior is manifest only at small values of the inverse Reynolds numbers. In kinetic theory, the scaled longitudinal pressure, which is a combination of the scaled shear and bulk viscous pressures, exhibits fast decay to an early-time attractor, driven by the rapid longitudinal expansion of the medium. We demonstrate that second-order dissipative hydrodynamics, based on a gradient expansion around local thermal equilibrium, fails to accurately describe the attractor. These results will be discussed in the light of fixed lines and fixed points of kinetic theory and hydrodynamics. A modified anisotropic hydrodynamic prescription that reproduces the attractor and provides excellent agreement with kinetic theory will be presented. |
Wednesday, October 13, 2021 11:06AM - 11:18AM |
JH.00007: Transient Relativistic Fluid Dynamics in a General Hydrodynamic Frame Enrico Speranza, Jorge Noronha, Michal Spalinski We propose a new theory of second order viscous relativistic hydrodynamics which does not impose any frame conditions on the choice of the hydrodynamic variables. It differs from Mueller-Israel-Stewart theory by including additional degrees of freedom, and its first-order truncation reduces to Bemfica-Disconzi-Noronha-Kovtun theory. Causality and stability hold at the linearized level if suitable conditions on transport coefficients are met. Apart from its conceptual implications, this new theory has potential applications to modeling the physics of quark-gluon plasma as well as viscous simulations of neutron star mergers. As an illustrative example we consider Bjorken flow solutions to our equations and identify variables which make an early-time attractor manifest. |
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