High Baryon Densities Achievable in the Fragmentation Regions of High Energy Heavy-Ion Collisions

Abstract

In high energy heavy-ion collisions, the two colliding nuclei pass through each other leaving behind an almost baryon-free central region. Most of the baryon charges are carried away by the receding nuclear remnants. During the collisions, a large amount of kinetic energy is deposited in the central region where a new form of matter called quark-gluon plasma is formed. At the same time, the col- liding nuclei are highly compressed, resulting in very high baryon densities in the nuclear remnants. In this thesis, we will explore the high baryon density achievable in the nuclear remnants and study the hydrodynamic evolution of the high baryon density matter. To reach this goal, the central region is modeled as classical gluon fields at the initial stage of the collision within the framework of color glass con- densate. We first compute the energy-momentum tensor of classical gluon fields by solving classical Yang-Mills equations semi-analytically using the method of power series expansion in proper time. With the help of the energy-momentum tensor, we further obtain the momentum space rapidity losses and the thermal excitation energies of the receding nuclei by imposing energy momentum conser- vation on the system of gluon fields and receding nuclei. Nuclear compression in high energy heavy-ion collisions is then related to the changes of momentum space rapidity. The baryon densities in the nuclear remnants are found to be more than ten times larger than the normal nuclear density for collision energies attainable at BNL Relativistic Heavy-Ion Collider and CERN Large Hadron Collider. Given the high baryon density and also the large energy density, we further assume par- tonic systems in the nuclear remnants are thermalized baryon-rich quark-gluon plasma and their space-time evolution follows hydrodynamic principles, just like what happens in the central region. Using a realistic equation of state, we solve the 1+1D relativistic hydrodynamic equations with baryon diffusion. We found that baryon charges diffuse from the fragmentation regions to the central region due to fugacity gradients. Possible temperatures and chemical potentials of the high baryon density matter are found to be in regions of the phase diagram of Quantum Chromodynamics (QCD) where a phase transition might happen. This may provide an alternative approach to the ongoing Beam Energy Scan program at RHIC in studying the phase transition and searching for a critical point of the strongly interacting matter.