Our current understanding on the evolution of heavy ion collisions

High energy heavy ion collisions Theory predicts that heating or compressing normal nuclear matter results in deconfined quark-gluon matter when the temperature and density are above a critical value. The constituents of normal nuclear matter, a system at low temperature and density, are hadrons (p, n, pions, Deltas...) while the constituents of quark-gluon matter are quarks (u, d, s... in 3 colours) and gluons in a deconfined state. In the laboratory sufficient heating and compression is reached in collisions of heavy ions at high energies to produce quark-gluon matter.

Space time diagram for the time evolution of the colliding system

The two nuclei start colliding at time t=0. Almost immediately afterwards, at t ~1 fm/c ~3x10^-24 sec, a super-dense and hot state of quark-gluon matter is created of energy density epsilon ~3 GeV per fm³, approximately 20 times the normal matter density, and a temperature estimated to be T ~235 MeV, approximately 3x10¹² (this is more than 5 orders of magnitude hotter than the interior of the sun!). The extreme pressure gradients in this new state of matter drive an explosive type of expansion. After a few fm/c (about 10^-23 sec), the density has already decreased by a factor three when the ``hadronization'' of the quark-gluon matter starts and the first hadronic constituents of normal matter emerge from the highly exited state (``chemical freeze-out''). Their temperature is still very high (T ~175 MeV), but the system continues to cool quickly and after a few times 10^-23 sec its density is small enough that the thermally produced particles stop having strong interactions and they can fly freely to the detectors (``thermal freeze-out'').

On the right hand side, the time evolution of the created ``fireball'' is shown in the plane transverse to the direction of the collision. In this direction where, all the expansion is due to the pressure gradients in the ``fireball'', the system expands at the moment that the system is in the state of decoupling from strong interactions with half the velocity of light and has reached a size which is twice as large as that of the incoming projectile.

The phase diagram The temperature and density attained in heavy-ion collisions at the SPS allow to place our measurements in the general phase diagram of nuclear matter. As water comes in different phases (solid, liquid, gas), so nuclear matter can come in its normal hadronic form and at sufficiently high temperature and density, in the form of a deconfined state of quarks and gluons. While previous experiments (at SIS and AGS) approached the transition region between the two phases (indicated by the green hatched region), the SPS experiments got the first real glance of it. Also shown is a likely expansion trajectory of the ``fireball'' formed in Lead on Lead collisions at the SPS.

See Image Library for ps file format of the figures.