Large accelerators are used to propel heavy ions to very high energies. When colliding the ions, nuclear matter can be studied under extreme conditions. If the energy density in the region of overlap between the colliding nuclei is high enough, the highly compressed matter may undergo a phase transition to a QGP.
Table 1.1 summarizes the development of heavy-ion accelerators. The first generation of accelerators were fixed-target machines, where a beam of ions is accelerated to the operating energy and then extracted and steered on to a stationary target. In collider machines two beams traveling in opposite directions are made to collide with each other, making all the kinetic beam energy available in the reactions.
The only operational heavy-ion collider is the Relativistic Heavy-Ion Collider, RHIC [5], at Brookhaven National Laboratory located in Upton, New York. RHIC is capable of colliding a wide variety of particle species from gold nuclei to polarized protons. With a circumference of about 4 km the RHIC ring accelerates gold ions to 99.995% the speed of light, corresponding to an energy of 100 GeV per nucleon.
There are four experiments located at different interaction points along the RHIC ring: PHOBOS, BRAHMS, STAR and PHENIX. The Au+Au collisions at these points produce thousands of particles, requiring high granularity detectors. The different detectors used in the experiments are designed to perform certain types of measurements, each important in the understanding of the physics involved in the collisions. The PHENIX experiment [6] will be described in detail in chapter 2.
The Large Hadron Collider, LHC, is now under construction at CERN. There will be one dedicated heavy-ion experiment called ALICE. The first heavy-ion run at LHC is scheduled for 2007.
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