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 KH: Mini-Symposium: The (Un)Reasonable Effectiveness of Fluid Dynamics in Heavy Ion Collisions II |
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Chair: Gokce Basar, University of North Carolina Room: Whittier |
Wednesday, October 13, 2021 11:30AM - 11:42AM |
KH.00001: Exploring the limits of hydrodynamic simulations of relativistic heavy-ion collisions Mayank Singh, Scott McDonald, Sangyong Jeon, Charles Gale Hydrodynamic simulations of ultrarelativistic heavy-ion collisions have been remarkably successful at explaining experimental data, usually invoking non-zero values of both shear and bulk viscosities to do so. Bulk viscosity is a negative contribution to the kinematic pressure, meaning that large bulk viscosities can possibly lead to negative net kinematic pressure. Unlike cavitation in the standard sense, in relativistic heavy-ion collisions this is not observed to lead to nucleation or bubble formation: a process that is not possible when phase change happens with a smooth crossover. We also demonstrate that net negative kinematic pressure does not necessarily lead to inward flow of fluid. Finally, we explore the phenomenological consequences of imposing the condition that kinematic pressure always stay positive, and we consider observables like mean-pT and vn in AA collisions. |
Wednesday, October 13, 2021 11:42AM - 11:54AM |
KH.00002: Dissipative Magnetohydrodynamics for Non-Resistive Relativistic Plasmas: A flux-conservative formulation with stiff relaxation Elias R Most, Jorge Noronha Based on a 14-moment closure for non-resistive viscous plasmas, we |
Wednesday, October 13, 2021 11:54AM - 12:06PM |
KH.00003: Transfer learning for emulation of hydrodynamic simulations Dan P Liyanage, Derek S Everett, Matthew R Heffernan, Ulrich W Heinz, Irene Ji, Simon Mak, J-F Paquet Extracting the properties of quark-gluon plasma from data taken at the Large Hadron and Relativistic Heavy Ion Colliders requires computationally expensive simulations and Bayesian statistical methods. Bayesian inference requires millions of model simulations which is computationally prohibitive. One, therefore, constructs surrogates that can be trained on data from a sparse set of full model simulations. Obtaining the training data from the full simulations then dominates the computational cost of the analysis. When the model is improved, recalibrating its parameters in principle entails the same cost again. We introduce a computationally much less expensive alternative to build accurate emulators for the improved model with a much smaller number of additional training data, by transferring knowledge about the model parameters from emulators trained on the large set of existing training data for the original model. We validate the new method by building emulators for simulations for different collision systems and collision energies and for different viscous corrections at particlization. We show that the method is surprisingly efficient. Limitations and further improvements of the technique will be discussed. |
Wednesday, October 13, 2021 12:06PM - 12:18PM |
KH.00004: New results from the Trajectum Heavy Ion Code Wilke van der Schee, Govert Nijs Trajectum is a new public state-of-the-art heavy ion code that includes an initial stage, a relativistic hydrodynamic stage and finally an hadronic gas cascade. All its 20 parameters have been obtained from a global analysis that includes among else particle identified flow as a function of transverse momentum. In this talk I will present new results that include observables that are particularly sensitive to the initial stage such as the correlation between the hydrodynamic flow and the mean transverse momentum at fixed multiplicity. This will furthermore include multiple collision systems, including gold collisions at RHIC at several energies and results for oxygen, xenon and lead collisions at the LHC. |
Wednesday, October 13, 2021 12:18PM - 12:30PM |
KH.00005: Investigating the Most Vortical Fluid in Nuclear Collisions with Beam Energy Scan Jinfeng Liao In a non-central nucleus-nucleus collision, the colliding system carries large orbital angular momentum, part of which remains within the hot dense matter created by the collision. This angular momentum turns into complex fluid vorticity structures in the bulk fluid, and eventually manifests itself through the global spin polarization of produced particles (e.g. hyperons). The STAR Collaboration reported the experimental discovery of this novel phenomenon in 2017. A crucial feature in establishing the intepretation is the predicted beam energy dependence, specifically a strong increase of fluid vorticity (and thus the polarization) when the collision beam energy is decresed from O(100) GeV to O(10) GeV range. There remains the interesting question: at which beam energy the truly most vortical fluid will be located. Here we perform a systematic study on the beam energy dependence of hyperon global polarization phenomenon, especially in the interesting Ô(1∼10) GeV region. We find a non-monotonic trend, with the global polarization to first increase and then decrease when beam energy is lowered from 27 GeV down to 3 GeV. The maximum polarization signal has been identified around 7.7 GeV beam energy, where the heavy ion collisions presumably create the most vortical fluid. On the theoretical side, one needs to develop a hydrodynamic theory including the angular momentum conservation in addition to the usual hydrodynamics based on energy-momentum and charge conservation. Recently there has been significant interest in constructing such a new hydrodynamic theory. In this talk, we examine the key conceptual issues for such a theory in the relativistic regime where the orbital and spin components get entangled. We derive the equations for relativistic viscous hydrodynamics with angular momentum through Navier-Stokes type of gradient expansion analysis. (arXiv:2105.13481 & 2105.04060.) |
Wednesday, October 13, 2021 12:30PM - 12:42PM |
KH.00006: Probing early-time longitudinal dynamics with the $\Lambda$ hyperon's spin polarization in relativistic heavy-ion collisions Sangwook Ryu, Vahidin Jupic, Chun Shen We systematically study the hyperon global polarization's sensitivity to the collision systems' initial longitudinal flow velocity in hydrodynamic simulations. By explicitly imposing local energy-momentum conservation when mapping the initial collision geometry to macroscopic hydrodynamic fields, we study the evolution of systems' orbital angular momentum (OAM) and fluid vorticity. We find that a simultaneous description of the $\Lambda$ hyperons' global polarization and the slope of pion's directed flow can strongly constrain the size of longitudinal flow at the beginning of hydrodynamic evolution. We extract the size of the initial longitudinal flow and the fraction of orbital angular momentum in the produced QGP fluid as a function of collision energy with the STAR measurements in the RHIC Beam Energy Scan program. We find that there is about 100-200 $\hbar$ OAM that remains in the mid-rapidity fluid at the beginning of hydrodynamic evolution. We further exam the effects of different hydrodynamic gradients on the spin polarization of $\Lambda$ and $\bar{\Lambda}$. The gradients of $\mu_B/T$ can change the ordering between $\Lambda$'s and $\bar{\Lambda}$'s polarization. |
Wednesday, October 13, 2021 12:42PM - 12:54PM |
KH.00007: Normal and anomalous Transport in large and small collision-systems ROY LACEY A primary aim of current nuclear science research at RHIC and the LHC is to delineate the normal and anomalous transport properties of the QGP produced in ion-ion collisions. I will show that the comprehensive RHIC and LHC data sets for different beam energies and collision systems in tandem with novel correlators and scaling functions provide unique insight into the transport properties of the QCD-matter created in small and large systems. I will also present specific testable predictions for future anisotropy measurements in small systems such as O+O, p+Au, and d+Au at RHIC (0.20 TeV) and O+O and p+Pb at the LHC (7.0 TeV), derived from the scaling functions. |
Wednesday, October 13, 2021 12:54PM - 1:06PM |
KH.00008: Correlations Measure Partial Thermalization in High Energy Collisions George S Moschelli, Zoulfekar Mazloum, Sean Gavin Is thermalization necessary for hydrodynamic flow in nuclear collisions? The discovery of flow-like azimuthal correlations in pA and high-multiplicity pp collisions raises profound questions about the onset of collective flow and its relation to hydrodynamics. We seek independent experimental information on the degree of thermalization in order to identify those hydrodynamic collision systems in which flow is sensitive to equilibrium QCD properties. We aim to develop a protocol for identifying the degree of thermalization using a combination of momentum and multiplicity correlation. To study the effect of thermalization on these correlations, we use Boltzmann equation in the relaxation time approximation with Langevin noise. We derive a new non-equilibrium transport equation for the two-body distribution function that is consistent with the conservation laws obeyed by microscopic scattering processes. We find that transverse momentum fluctuations in peripheral PbPb collisions at LHC markedly deviate from equilibrium behavior. We propose new measurements that can provide more refined information. |
Wednesday, October 13, 2021 1:06PM - 1:18PM |
KH.00009: Collective evolution of a parton in the vacuum: the ultimate partonic "droplet", non-perturbative QCD, and quantum entanglement. Parker Gardner, Austin Baty, Wei Li In this talk we will discuss the non-perturbative QCD evolution of a single parton in the vacuum. We postulate that such a parton will develop the long-range collective effects of a multi-parton system, reminiscent of those observed in high-energy hadronic or nuclear interactions with large final-state particle multiplicity. Proton-Proton collisions at the Large Hadron Collider showed surprising signatures of a strongly interacting, thermalized quark-gluon plasma, which was thought only to form in collisions of large nuclear systems. Another puzzle observed earlier in e+e- collisions is that production yields of various hadron species appear to follow a thermal-like distribution with a common temperature. We will begin this talk by proposing searches for thermal and collective properties of a single parton propagating in the vacuum using high multiplicity jets in high-energy elementary collisions. We will then present studies on several observables using the PYTHIA 8 Monte Carlo event generator. Experimental observation of such long-range collectivity will offer a new view of non-perturbative QCD dynamics of multi-parton systems at the smallest scales. Absence of any collective effect may offer new insights into the role of quantum entanglement in the observed thermal behavior of particle production in high energy collisions. Please see our paper at https://arxiv.org/abs/2104.11735. |
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