Studies of the initial and final states of AuAu collisions with BRAHMS

Abstract

When heavy ions collide at ultra-relativistic energy, thousands of particles are emitted and it is reasonable to attempt to use hydrodynamic descriptions, with suitable initial conditions, to describe the time evolution of the collisons. In the longitudinal direction pions seem to exhibit Landau flow. This simple model assumes that all the entropy in the collisions is created the instant the two Lorentz contracted nuclei overlap and that the system then expands adiabatically. The system also displays radial and elliptic flow. Radial flow is manifested as a broadening of the pT distributions with respect to pp collisions. It is typically thought to result from multiple scattering of partons or hadrons before dynamic freeze-out. Elliptic flow occurs when heavy ions do not collide exactly head on. The initial geometrical asymetry is translated into a momentum asymetry via pressure gradiants. Since these gradients are self quenching, strong elliptic flow is thought to be linked to early thermalization and a large initial pressure. Using the concept of limiting fragmentation we attempt to sketch a link between the initial and final states of relativistic heavy ion collisions using new preliminary data from the BRAHMS collaboration on elliptic and radial flow.