The relativistic heavy-ion collision experiments conducted at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) create fireballs with temperatures that can reach several trillion degrees Kelvin (millions times hotter than the center of the sun) once they reach a state of approximate local thermal equilibrium a short period (∼ 3 × 10−24s) after the collisions. These experiments open a privileged window for studying strongly interacting nuclear matter under extremely hot and dense conditions. These “little bangs” are almost point-like in size (V ∼ 10−42m3) and disappears almost instantaneously (∼ 5 × 10−23s).
The evolution of a relativistic heavy-ion collisions contain multiple stages which are governed by different underlying physics. Right after the collisions, the system is dominated by gluons characterized by an over populated phase-space distribution. The number of gluons is of order ∼ 1/g2
with g < 1 and these gluons carry each a very small fraction of the longitudinal momentum of the incoming nucleus (small-x gluons). During the first 1 fm/c, due to the large occupation number of gluon at leading order in strong coupling g, these saturated small-x gluons will evolve according to the classical Yang-Mills equation of motion. It is believed that the next-to-leading order quantum corrections to the classical field evolution drive the system rapidly towards local isotropy in momentum space and somewhat later to local thermal equilibrium. After 0.3−0.5 fm/c, the system achieves approximately local momentum isotropy; local thermal equilibrium is reached after a few fm/c. The quarks and gluons that are produced after the collision form a strongly coupled plasma (QGP). The dynamics of the QGP can be described by macroscopic viscous hydrodynamics where the viscous corrections account for the remaining deviation from local isotropy and thermal equilibrium. As the system expands and cools, it will smoothly crossover from the QGP phase to a hadron gas phase according to the equation of state (EOS) determined from Lattice QCD calculations. At hadronization, the quark-gluon fluid will convert into hadrons due to confinement. In the hadronic phase, the hadron cascade model can provide us a detailed microscopic description of the evolution. As the fireball continues to expand and cool, the collision rates between the hadronic resonances decrease. First, the inelastic collisions between particles cease and the system reaches chemical freeze-out almost directly after hadronization. After this point, only resonance decays and baryon- antibaryon annihilation can change the particle yields. Regeneration of baryon-antibaryon pairs is a rare process that can be neglected. As the system evolves further, the density of the fireball becomes so low that the mean free time of the particles becomes much larger than the Hubble time (i.e. the time over which the inter particle spacing doubles.). The particles reach kinetic freeze out and subsequently free-stream to the detectors. The figure on the right schematically summarizes the theoretical models and the corresponding codes that we will use to simulate the different stages of heavy-ion collisions.
In relativistic heavy-ion collisions, rare electromagnetic observables like photons and dileptons only interact with the medium through the electromagnetic interaction, which is much weaker than the strong interaction. For this reason, their mean free path is much longer than the system size, and hence they suffer negligible final state interactions after they are produced during the fireball evolution. This advantage over strongly interacting probes makes them the cleanest penetrating probe for the heavy-ion collisions. In Fig. 1.4, we illustrate the qualitative difference between the charged hadron and thermal photon production in one typical heavy-ion collision. Hadrons can only break free at the final kinetic freeze-out surface. Their measured momentum distribution carries indirect time integrated evolution information about the fireball. On the other hand, a large fraction of the thermal photons are produced early inside the fireball. Their momentum distribution preserves the dynamical information of the medium directly at their birth points. Electromagnetic probes can thus provide us with constraints on the early dynamics of the fireball that are complementary to those obtained from the much more abundant hadronic observables.