Photons from relativistic nuclear collisions

TL;DR

This paper discusses how photons produced in relativistic nuclear collisions provide unique insights into the early stages of these high-energy events, complementing hadron measurements.

Contribution

It explains the physics of photon production in nuclear collisions and highlights their importance for understanding early collision dynamics.

Findings

Photons escape the collision zone with minimal interaction.

Photon measurements reveal information about the initial temperature and conditions.

Photons serve as probes of the early, hot stages of nuclear collisions.

Abstract

Collisions of atomic nuclei at relativistic velocities allow to recreate the conditions encountered in neutron stars or in the early universe micro-seconds after the Big Bang. These reactions are performed in today's largest accelerator facilities, e.g. at CERN in Geneva, at the Relativistic Heavy Ion Collider at Brookhaven, NY or in the planned FAIR facility in Darmstadt Germany. During such a collision the matter is heated up to hundreds of MeV (billions of degrees) and compressed to densities of $3-10$ times the density inside ordinary atomic nuclei (i.e. $10^{17}-10^{18}$ kg/m$^3$). Usually these collisions are studied via the measurement of a multitude of strongly interacting particles, called hadrons, that are emitted at the end of the collision. However, also some photons are created. These photons are of special interest as they allow to look into the early stage of the collisions, because they are only very weakly (namely only electro-magnetically) interacting with the hadrons of the created fireball. This paper elucidates the physics of the photons and what can be learned from them.