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
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,
(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
the velocity of light and has reached
a size which is twice as large as that of the incoming
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.