Session K10: Minisymposium on Probing High Baryon Density Physics I

The Many Faces of Dense Baryonic Matter inside of Neutron Stars

Gravity compresses the matter in the core regions of neutron stars to densities that are several times greater than the density of ordinary atomic nuclei. This provides a high-pressure environment in the core regions of neutron stars where several different subatomic particle processes are expected to compete with each other and even novel states of matter exist. The most spectacular possibilities involve the generation of hyperons and baryon resonances, boson condensates, and/or the formation of color superconducting quark matter. Combined with the unprecedented progress in observational astrophysics, this makes neutron stars superb astrophysical laboratories for a range of physical studies which shed light on the structure and equation of state of dense baryonic matter. In this talk I will begin with providing an overview of our current understanding of the core-composition of neutron stars. Models for the equation of state of dense neutron star matter will then be presented, which are constrained by the latest nuclear and astrophysical data. Particular emphasis will be put on the quark-hadron phase transition in the core regions of neutron stars, which could be driven by changes in the rotation periods of neutron stars. Finally, the phase diagram of hot and dense proto neutron star matter will be discussed and possible instabilities in such matter will be pointed out. The information gained from these investigations provides information about the nuclear equation of state that is complementary to what is expected from the study of gravitational waves from neutron star-black holes and binary neutron star mergers, high baryon density QCD from lattice calculations, and high baryon density physics from low energy heavy-ion collisions at RHIC and GSI.   

Constraining Superfluidity in Dense Matter from the Cooling of Isolated Neutron Stars

A quantitative analysis of luminosity and temperature observations from isolated neutron stars constrains two nucleon superfluid critical temperatures: the neutron triplet superfluid critical temperature and the proton singlet superconducting critical temperature. The neutron star observations imply that the most likely value for the neutron triplet critical temperature is $2.09^{+4.37}_{-1.41} \times 10^{8}$ K, while the most likely value for the proton singlet critical temperature is $7.59^{+2.48}_{-5.81} \times 10^{9}$ K. Our results assumed minimal cooling, and these results are only valid when Vela is removed from our data set. The inclusion of Vela increased the gaps significantly, as Vela has a very low temperature for its young age. We present preliminary results which include enhanced cooling and variations of the neutron star masses and equation of state parameters. These preliminary results show that Vela is likely more massive and has a direct Urca cooling process which explains its low temperature.   

Locating the QCD critical point using holographic black holes

We use the gauge/gravity duality to map thermodynamic fluctuations of black holes onto fluctuations of baryon charge in a hot and baryon dense Quark-Gluon Plasma (QGP). Our approach gives results that are in quantitative agreement with state-of-the-art lattice simulations for the QCD equation of state at finite baryon density and the moments of fluctuations of baryon charge, while simultaneously encompassing nearly-perfect fluidity. This framework provides a definite prediction for the QCD critical point, which is found to be within the reach of low collision energy experiments at RHIC and also the CBM experiment at FAIR. We also determine the temperature and baryon chemical potential dependence of the bulk viscosity and the coefficients that characterize the transport of baryon charge, electric charge, and strangeness in the QGP.   

Baryon clustering in high energy heavy ion collisions, with and without a critical point

Clustering in systems with attractive forces are known in many physical settings: for example in formation of globular clusters in Galaxies. Formation of nuclear fragments is a well known phenomenon in low energy nuclear collisions, especially at temperatures $T\sim 10\, MeV$ near gas-liquid critical point. This work is however about a freezeout stage of high energy collisions, with $T=100-150\, MeV$, where clustering is reflected by subtle correlations, such as large kurtosis of the baryon distribution. We use classical molecular dynamics and Langevin equation to model the system. In the vicinity of the hypothetical QCD critical point one expects certain modification of the nuclear forces, with a critical mode adding weak long-range attraction, and we study how it may influence the baryon clustering.   

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 study various observables obtained directly from the initial state model, including net-baryon rapidity distributions, 2-particle rapidity correlations, and 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. The importance of this pre-equilibrium stage on hadronic flow observables at the RHIC BES will be quantified.   

Hydrodynamic Evolution of the High Baryon Density Matter in High Energy Heavy-Ion Collisions

In high energy heavy-ion collisions, the colliding nuclei pass through each other, leaving behind an almost baryon-free central region. This property of transparency in high energy collisions is different from the nuclear stopping in the low energy collisions. I will argue that very high baryon density (more than ten times larger than the normal nuclear density) can be achieved in the fragmentation regions of high energy heavy-ion collisions. This very high baryon density matter is further assumed to follow hydrodynamic equations in the subsequent space-time evolution. Baryons are found to diffuse from the forward/backward rapidity regions to the mid-rapidity region. I will also talk about the potential relevance to exploring the QCD phase diagram in high energy heavy-ion collisions, which may be an alternative to the low energy Beam Energy Scan program at RHIC.   

Search for the QCD critical point through the rapidity dependence of cumulants

In the coming Beam Energy Scan, RHIC will have much higher luminosity at $\sqrt{s}=20$ GeV than it will at $\sqrt{s}=10$ GeV and below. With the STAR iTPC upgrade in place they will be able to reach proton rapidities up to $|y|\sim 0.8$ where the baryon chemical potential $\mu_B$ in $\sqrt{s}=20$ GeV collisions is somewhat higher than at mid-rapidity.  They may therefore be able to use the high statistics at this and nearby collision energies to vary $\mu_B$ somewhat by varying $y$, as well as by scanning down to the lowest possible collision energies. By employing Ising universality together with a phenomenologically motivated freeze-out prescription, we demonstrate that the rapidity dependence of Gaussian and non-Gaussian cumulants is sensitive to the presence of the critical point and exhibits a characteristic pattern as indicated by critical universality. If there is a critical point to be found in the regime that RHIC will explore, we propose the rapidity dependence of cumulants as a complementary route to finding signs of its presence. In particular, it is quite plausible that the rapidity dependence of cumulants will change qualitatively if one passes the critical point during the RHIC beam energy scan.