Probing Thermalizing Nuclear Matter with Hard Photons

Abstract

To study experimentally nuclear matter in its various phases, heavy ions are collided to convert the projectile kinetic energy into compression and heat. By selecting adequately the projectile energy and the violence of the collision, highly excited nuclear matter can thus be formed and studied in its various phases : liquid, gas, hadron gas, or quark-gluon plasma. Such a dynamical exploration of the nuclear phase diagram leaves however nuclear matter in well defined excited states only for fleetingly short instants, challenging their experimental study. Within this context the ideal probe must convey the information of interest to the detector without being affected neither by the collision dynamics nor by final state interactions. It must escape from the nuclear medium without strong interaction, as do electromagnetic particles, and its formation time must decouple from the typical collision time, or, in other words, the probe must be created quasi-instantaneously in a frozen source.