Recent developments in the theory of electromagnetic probes in relativistic heavy-ion collisions

The theoretical developments in the study of electromagnetic radiation in relativistic heavy-ion collisions are reviewed. The recent progress in the rates for photon and lepton pair production is discussed. Together with the improvements in the hydrodynamic descriptions of the bulk medium, the combined effort is discussed to resolve the "direct photon flow puzzle" in the RHIC and the LHC experiments. Further prediction of the direct photon production in high multiplicity proton-nucleus collisions at the LHC energy can serve as a signature of the quark gluon plasma formation in these small systems. Phenomenological study of dilepton production at finite net baryon density is highlighted at the collision energies available for the RHIC beam energy scan program.Comment: 6 pages, 3 figures; A plenary talk given at the 7th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions (Hard Probes 2015), June 29-July 3 2015, Montreal, QC, Canad

Electromagnetic Radiation from QCD Matter: Theory Overview

Recent theory developments in electromagnetic radiation from relativistic heavy-ion collisions are reviewed. Electromagnetic observables can serve as a thermometer, a viscometer, and tomographic probes to the collision system. The current status of the "direct photon flow puzzle" is highlighted.Comment: 8 pages, 4 figures; to appear in the proceedings for the Quark Matter 2015 conference, Kobe, Japa

Algebraic methods in sum-product phenomena

We classify the polynomials $f(x,y) \in \mathbb R[x,y]$ such that given any finite set $A \subset \mathbb R$ if $|A+A|$ is small, then $|f(A,A)|$ is large. In particular, the following bound holds : $|A+A||f(A,A)| \gtrsim |A|^{5/2}.$ The Bezout's theorem and a theorem by Y. Stein play important roles in our proof.Comment: introduction revise

Initial state and hydrodynamic modeling of heavy-ion collisions at RHIC BES energies

We present a fully three-dimensional initial state model for relativistic heavy-ion collisions at RHIC Beam Energy Scan (BES) collision energies. The initial energy and net baryon density profiles are produced based on a classical string deceleration model. The baryon stopping and fluctuations during this early stage of the collision are investigated by studying the net baryon rapidity distribution and longitudinal decorrelation of the transverse geometry.Comment: 7 pages, 5 figures, conference proceeding for the Critical Point and Onset of Deconfinement - CPOD201

Viscous Flow in Heavy-Ion Collisions from RHIC to LHC

We present a systematic hydrodynamic study of the evolution of hadron spectra and their azimuthal anisotropy from the lowest collision energy studied at the Relativistic Heavy Ion Collider (RHIC), sqrt(s) = 7.7 A GeV, to the highest energy reachable at the Large Hadron Collider (LHC), sqrt(s) = 5500 A GeV. The energy dependence of the flow observables are quantitatively studied for both the Monte-Carlo Glauber and Monte-Carlo Kharzeev-Levin-Nardi (MC-KLN) models. For MC-Glauber model initial conditions with {\eta}/s = 0.08, the differential charged hadron elliptic flow v_2^{ch}(p_T, sqrt(s)) is found to exhibit a very broad maximum in the region 39 < sqrt(s) < 2760 A GeV. For MC-KLN initial conditions with {\eta}/s = 0.2, a similar "saturation" is not observed up to LHC energies. We emphasize that this "saturation" of elliptic flow arises from the interplay between radial flow and elliptic flow which shifts with sqrt(s) depending on the fluid's viscosity. By generalizing the definition of spatial eccentricity to isothermal hyper-surface, we also calculate {\epsilon}_x on the kinetic freeze-out surface at different collision energies.Comment: 4 pages, 4 figures, proceedings of talk at the 23rd International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, QM2012, August 13-18, 2012, Washington D

Collision Energy Dependence of Viscous Hydrodynamic Flow in Relativistic Heavy-Ion Collisions

Using a (2+1)-d viscous hydrodynamical model, we study the dependence of flow observables on the collision energy ranging from sqrt(s)=7.7 A GeV at the Relativistic Heavy Ion Collider (RHIC) to sqrt(s)=2760 A GeV at the Large Hadron Collider (LHC). With a realistic equation of state, Glauber model initial conditions and a small specific shear viscosity eta/s = 0.08, the differential charged hadron elliptic flow v_2^{ch}(p_T,sqrt(s)) is found to exhibit a very broad maximum as a function of sqrt(s) around top RHIC energy, rendering it almost independent of collision energy for 39 < sqrt(s) < 2760 A GeV. Compared to ideal fluid dynamical simulations, this "saturation" of elliptic flow is shifted to higher collision energies by shear viscous effects. For color-glass motivated MC-KLN initial conditions, which require a larger shear viscosity eta/s = 0.2 to reproduce the measured elliptic flow, a similar "saturation" is not observed up to LHC energies, except for very low p_T. We emphasize that this "saturation" of the elliptic flow is not associated with the QCD phase transition, but arises from the interplay between radial and elliptic flow which shifts with sqrt(s) depending on the fluid's viscosity and leads to a subtle cancellation between increasing contributions from light and decreasing contributions from heavy particles to v_2 in the sqrt(s) range where v_2^{ch}(p_T,sqrt(s)) at fixed p_T is maximal. By generalizing the definition of spatial eccentricity epsilon_x to isothermal hyper-surfaces, we calculate epsilon_x on the kinetic freeze-out surface at different collision energies. Up to top RHIC energy, sqrt(s)=200 A GeV, the fireball is still out-of-plane deformed at freeze out, while at LHC energy the final spatial eccentricity is predicted to approach zero.Comment: 12 pages, 9 figure

Dynamical initial state model for relativistic heavy-ion collisions

We present a fully three-dimensional model providing initial conditions for energy and net-baryon density distributions in heavy ion collisions at arbitrary collision energy. The model includes the dynamical deceleration of participating nucleons or valence quarks, depending on the implementation. The duration of the deceleration continues until the string spanned between colliding participants is assumed to thermalize, which is either after a fixed proper time, or a fluctuating time depending on sampled final rapidities. Energy is deposited in space-time along the string, which in general will span a range of space-time rapidities and proper times. We study various observables obtained directly from the initial state model, including net-baryon rapidity distributions, 2-particle rapidity correlations, as well as the rapidity decorrelation of the transverse geometry. Their dependence on the model implementation and parameter values is investigated. We also present the implementation of the model with 3+1 dimensional hydrodynamics, which involves the addition of source terms that deposit energy and net-baryon densities produced by the initial state model at proper times greater than the initial time for the hydrodynamic simulation.Comment: 16 pages, 15 figures; Accepted by Phys. Rev.

Dynamical initialization and hydrodynamic modeling of relativistic heavy-ion collisions

We present a fully three-dimensional model providing initial conditions for energy and conserved charge density distributions in heavy ion collisions at RHIC Beam Energy Scan (BES) collision energies. The model includes the dynamical deceleration of participating nucleons or valence quarks. It provides a realistic estimation of the initial baryon stopping during the early stage of collisions. We also present the implementation of the model with 3+1 dimensional hydrodynamics, which involves the addition of source terms that deposit energy and net-baryon densities produced by the initial state model at proper times greater than the initial time for the hydrodynamic simulation. The importance of this dynamical initialization stage on hadronic flow observables at the RHIC BES is quantified.Comment: 4 pages, 3 figures, conference proceeding for Quark Matter 201

The road to precision: Extraction of the specific shear viscosity of the quark-gluon plasma

Recent progress in extracting the specific shear viscosity of the Quark-Gluon Plasma from phenomenological studies is highlighted.Comment: 6 pages; 6 figures; invited feature article on Nuclear Physics News Vol. 25, issue 2, 201

Longitudinal Dynamics of High Baryon Density Matter in High Energy Heavy-Ion Collisions

In high energy heavy-ion collisions, the two colliding nuclei pass through each other leaving behind an almost baryon free central rapidity region. Most of the baryons are carried away by the nuclear remnants and are located in the so-called fragmentation regions. In previous papers \cite{Li:2016wzh,Li:2018ini}, it has been argued that very high baryon densities, more than ten times larger than the normal nuclear density, can be achieved in these fragmentation regions. In this paper, we assume the high baryon density matter is thermalized at the same time as the baryon-free quark-gluon plasma in the central rapidity region. We perform a 1+1D (temporal + longitudinal) hydrodynamic simulation covering both the fragmentation regions and the central rapidity region with the baryon diffusion equation included. Baryons are found to diffuse from the fragmentation regions to the central rapidity region driven by fugacity gradients. The baryon chemical potential at freezeout monotonically increases from the central rapidity region to the fragmentation regions, suggesting a rapidity scan in high energy heavy-ion collisions might be helpful in searching for the critical point of the QCD phase diagram.Comment: 14 pages, 17 figure