Sketch of collision of two nuclei
Top: 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. Bottom: In the laboratory sufficient heating and compression is reached in collisions of heavy ions at high energies to produce quark-gluon matter.
Time evolution of collision
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.
Phase diagram of matter
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.
In order to provide Lead beams in the North and West Experimental areas the Lead ion source (Pb ions), the Linac3, the Proton Synchrotron Booster (PBS), the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS) in succession accelerate the beam up to a total energy of 33 TeV.
Photo of the Heavy Ion Linac 3
Picture of the Heavy Ion Linac3 area. Built in 1993 as a special project under the leadership of the CERN PS Division, the Injector consists of an ECR Ion Source built at GANIL (Caen, France), a magnetic filter and a Radio Frequency Quadrupole, RFQ, built at Legnaro (INFN Italy) and a Linac constructed at GSI (Darmstadt, Germany). The project also included participation of India and Sweden.
NA49 layout from geometry in the Geant simulation program
The NA49 experimental setup consists of two large super-conducting magnets of 1~m vertical gap and 2~m radius in which two Vertex Time Projection Chambers (TPC) detect charged particles. The coverage of the experiment for large momentum particles is augmented downstream by an other two larger TPCs, 4~m long, and a set of Time of Flight (TOF) detector arrays.
Photographs of NA49 experiment
Photo 1-7: Views of the target area Photo 7: The two spokesmen of NA49, Reinhard Stock, Frankfurt (1991-1996), sitting and standing, Peter Seyboth, MPI Munich (1997-present), in the target area of NA49 in front of the first super-conducting vertex magnet containing one of the TPCs. Photo 8: Perspective view of NA49 Photos 9-10: The Main TPC suspended from the top and the Time-of-Flight (TOF) wall
View looking into the MTPC field cage
The field cage on one of the MAIN Time Projection Chambers. The aluminized mylar electrodes define a very uniform electric field in which the ionization produced by charged particles crossing the detector drifts to the top where it is amplified and recorded by the readout chambers electronics.
Points and tracks; reconstruction in MAIN TPC
Display of a stage of the track reconstruction in the NA49 Main Time Projection Chambers. The measured ionization produced by charged particles has been reduced to space points by a cluster finder algorithm. Tracks are being searched for and fitted to the measured points. After a laborious and recursive process more than 98\% of all charged particles traversing the detector are found and reconstructed.
Event display STAR
NA49 event display of the reconstructed tracks emanating from the ``little bang'' created in a central collision of Lead projectile with a Lead nucleus.
Reconstructed event in all TPcs
Central collision of lead projectile on a Lead nucleus at 158 GeV/nucleon as measured by the four large Time Projection Chambers (TPC) of the NA49 experiment