Session X12: Ultrarelativistic Heavy-Ion Collisions

Heavy quarkonium dynamics at next-to-leading order in the binding energy over temperature

We present a systematic method for solving the Lindblad equation for heavy-quarkonium dynamics in the quark-gluon plasma which accounts for corrections that are next-to-leading order (NLO) in the ratio of the binding energy of the state and the temperature.  The method used relies on mapping the three-dimensional Lindblad evolution to the solution of the one-dimensional Schrodinger evolution with stochastically sampled quantum jumps.  We demonstrate how to achieve this dimensional reduction by writing the NLO complex Hamiltonian and jump operators in the spherical basis in which the operators act only on the radial part of the wave function.  As result, one can implement the quantum trajectories method to solve the NLO Lindblad equation.  Using the resulting NLO framework we can more reliably extend the calculation of heavy-quarkonium suppression to lower temperatures than is possible with the LO formalism.   

Quantify phenomenological effects of causality constraints in the hydrodynamic description of relativistic heavy-ion collisions

We study theoretical uncertainties in the hydrodynamic description of relativistic heavy-ion collisions by examining the full nonlinear causality conditions [1] and quantifying their effects on flow observables [2]. The causality conditions impose physical constraints on the maximum allowed values of inverse Reynolds numbers during the hydrodynamic evolution. We develop a new numerical scheme to impose the necessary and sufficient causality conditions on individual fluid cells during the evolution. We find that the necessary causality condition can effectively stabilize event-by-event hydrodynamic simulations with large pressure gradients. Performing systematic simulations with and without the necessary and sufficient causality conditions, we quantify their effects on flow observables in p+Au and Au+Au collisions at the top RHIC energy and p+Pb and Pb+Pb collisions at LHC. Their impacts on the global Bayesian extraction of the QGP transport properties will be discussed.

[1] F. S. Bemfica, M. M. Disconzi, V. Hoang, J. Noronha, and M. Radosz, “Nonlinear Constraints on Relativistic Fluids Far from Equilibrium,‘’ Phys. Rev. Lett. 126, 222301 (2021)

[2] C. Chiu and C. Shen, “Exploring theoretical uncertainties in the hydrodynamic description of relativistic heavy-ion collisions,” Phys. Rev. C 103, 064901 (2021)   

Probing early-time longitudinal dynamics with the Λ hyperon's spin polarization in relativistic heavy-ion collisions

We present a systematic study of the hyperon global polarization's sensitivity to the collision systems' initial longitudinal flow velocity using 3+1D hydrodynamic simulations [1]. By explicitly imposing local energy-momentum conservation when mapping the initial collision geometry to macroscopic hydrodynamic fields, the evolution of systems' orbital angular momentum (OAM) and fluid vorticity are studied. We find that a simultaneous description of the Λ 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 constrain the initial longitudinal flow size 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 examine the effects of the new thermal shear gradients on the hyperon's polarization. The gradients of µB/T can change the ordering between Λ's and Λ's polarization. Finally, we extend calculations to event-by-event simulations for the isobar, Cu+Au, Au+Au, and U+U collisions at the top RHIC energies and study the system-size dependence of novel correlations among hyperons polarization and charge hadrons' averaged transverse momentum and anisotropic flow coefficients. Measuring these correlations will verify the OAM-vorticity-polarization paradigm in heavy-ion collisions.   

Production of π, K, and proton(anti-proton) in d+Au collisions at √SNN = 19.6, 39, and 62.4 GeV

Measurements of identified particles in d+Au collisions will provide fundamental information for understanding cold nuclear matter effects on particle production, such as Cronin enhancement, nuclear shadowing, and gluon saturation. It will also be helpful to study particle production mechanisms in the hot QGP droplet.

In this talk, we will present the working progress for measurements of π, K, and proton(anti-proton) production in d+Au collisions at √S_NN = 19.6, 39, and 62.4 GeV from STAR collaboration. The particle yield and ratio will be studied as a function of transverse momentum, multiplicity, and collisions energies. Such measurements will be compared with those in Au+Au collisions with the same collisions energy and participant for further understanding the cold or hot nuclear matter effect.   

The mean lifetimes of light nuclei and self-consistency of the statistical hadronization model

The statistical hadronization model (SHM) describes the yields of hadrons and nuclei in heavy ion collisions. SHM parametrizes the data in terms of a small number of universal parameters such as the chemical freeze-out temperature and chemical potentials. SHM achieves a remarkable phenomenological success over nine orders of magnitude in the hadron masses and several orders of magnitude in the energy of the colliding ions. In this talk, I will discuss the consistency of the assumptions underlying SHM by focusing on the yield of light nuclei whose binding energies are much lower than the chemical freeze-out temperature predicted by the model. The statistical hadronization model can be used to derive the bounds on the mean lifetimes of the light nuclei in a thermally equilibrated hadronic gas. The analysis shows that these lifetime bounds are badly violated at the chemical freeze-out temperature predicted by SHM.   

QCD equation of state at finite density with a critical point from an alternative expansion scheme.

    In Ref. [1], results for the QCD equation of state from the lattice Taylor expansion were combined with the 3D Ising model critical behavior, to build a family of equations of state which match the first principle results and contain a critical point in the expected universality class for QCD. This family of equations of state was limited to chemical potentials 0 ≤ μ_B ≤ 450 MeV, due to the limitations of the Taylor expansion. In Ref. [2],  an alternative expansion scheme was introduced, for extrapolating the lattice QCD equation of state to finite chemical potential. In this research, we combine these two approaches to obtain a family of equations of state in the range 0 ≤ μ_B ≤ 700 MeV and  30 MeV ≤ T ≤ 800  MeV, that match the lattice QCD results at small density and contain a 3D-Ising model critical point. With these new equations of state, we substantially extend the coverage of the QCD phase diagram.

Our open-source code allows the user to choose the position and strength of the critical point. Our results provide input for hydrodynamical simulations at finite T and unprecedentedly large μ_B and will help constrain the location of the critical point through a comparison with experimental data from the Second Beam Energy Scan at RHIC.   

Equilibrium and Dynamical Properties of Hot and Dense Quark-Gluon matter from Holographic Black Holes

By using gravity/gauge correspondence, we employ an Einstein-Maxwell-Dilaton model to compute the equilibrium and out-of-equilibrium properties of a hot and baryon rich strongly coupled quark-gluon plasma. The family of 5-dimensional holographic black holes, which are constrained to mimic the lattice QCD equation of state at zero density, is used to investigate the temperature and baryon chemical potential dependence of the equation of state [1]. We also obtained the baryon charge transport coefficients, the bulk and shear viscosities as well as the drag force and langevin diffusion coefficients associated with heavy quark jet propagation and the jet quenching parameter of light quarks in the baryon dense plasma, with a particular focus on the behavior of these observables on top of the critical end point and the line of first order phase transition predicted by the model.

[1] Grefa, J., Noronha, J., Noronha-Hostler, J., Portillo, I., Ratti, C., Rougemont, R. 10.1103/PhysRevD.104.034002   

Quantifying the uncertainty on the location of the holographic critical point

In Quantum Chromodynamics (QCD), we study the behavior of strongly interacting matter made up of quarks and gluons. The transition between the confined and low-energy phase called hadron gas and the deconfined and hot quark gluon plasma phase is a smooth crossover at vanishing density. However, it has been conjectured the crossover must evolve into a line of first order phase transition with a critical end point. By using an Einstein-Maxwell-Dilaton (EMD) model, fixed to reproduce the Lattice-QCD equation of state at vanishing chemical potential, we predict the location of a critical end point in the phase diagram. Two free functions in the EMD model are fixed to reproduce the lattice equation of state, a scalar dilation potential V(φ), and another corresponding to the coupling between the Maxwell and dilation fields, f(φ). By modifying these free functions, we study a possible change in the predicted location of the critical point in the phase diagram.