The CERES physics programme

The CERES physics programme

CERES is one of the second generation heavy ion experiments at CERN SPS. It is dedicated to the study of e+e- pairs in relativistic nuclear collisions.
In a central collision between two heavy nuclei at relativistic energies one can distinguish three main stages: First, as the nuclei start to overlap, hard scattering processes between the partons inside the nucleons take place redistributing the original beam energy in internal degrees of freedom. The time scale for this stage is ~ 1 fm. This leads to a system consisting of heated and compressed excited matter occupying a volume which is assumed to be the Lorentz contracted volume of the colliding nuclei (~100 fm3). This is the second stage where, if the temperature and density reach high enough values, a deconfined system of quarks and gluons might be formed, the quark-gluon plasma (QGP). Present calculations of lattice QCD set the critical temperature for deconfinment to Tc ~ 200 MeV. But even if Tc is not reached a gas of hadrons (mainly pions) at unusually high temperature and density is formed, usually referred to as the fireball. Typically it will live for a period of a few fm before undergoing a fast expansion and cooling, giving rise to the last stage of the interaction: the QGP hadronizes if it was formed, or the hadron gas becomes decoupled, its constituents not interacting anymore among themselves and making their way to be detected by the experiment.
Dielectrons are produced during the whole space-time evolution of the interaction by different processes. Remembering the Stephan-Boltzmann law, one may note that the major fraction of dielectrons is emitted during the hottest, {\it i. e.} the first part of the collision. Due to their electromagnetic character, they can leave the interaction region without further strong interactions and, therefore, the dielectron spectrum measured by the experiment is the folding of the production rates from early stage of the collision. A careful analysis of the resulting dielectron invariant mass spectrum should, in principle, allow to unfold the whole spacetime history of the nuclear collision.
As a spinoff CERES is also able to study direct photons and high PT pions produced in the nuclear interactions. Photons are converted in the target to a detectable e+e- pair, while pions are identified by their characteristic non asymptotic ring radius in the RICHES. The PT spectrum of pions can be used to infer the degree of thermalization reached in the collision. The high PT part can reflect hard parton processes which took place in the early stages of the collisions.
In 1992 we also run with a special set-up to study QED pairs produced in distant collisions of heavy nuclei. These pairs are produced out of the strong transient Coulomb fields generated when to heavy nuclei pass by each other at an impact parameter larger than their geometrical size ie, not touching each other. One expects high order QED effects to play an important role as the energy and atomic number of the participant nuclei increase. It is also an important process limiting the stability and lifetime of nuclear beams.
We refer to the CERES publications list for further information.


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Alexander Cherlin
Carlos P. de los Heros
© 1997