Di-Leptons at CERN

Abstract

One of the premier challenges of the ultra-relativistic reaction physics program is to gain information on the space-time history of heavy-ion reactions. This is by no means a trivial undertaking, because all that is experimentally attainable is the measurement of the asymptotic momentum states of the final products of the reaction. Measuring two-particle correlations of hadrons emitted during the reaction provides at least an indirect way of obtaining space-time information.¹ Hadronic probes, however, have large final state interactions and thus are not sensitive to the initial high-density and high-temperature phase of a heavy-ion reaction. Consequently, any information embedded in hadronic dynamics is completely masked by multiple scatterings. Dileptons are not disturbed by the hadronic environment even though they are produced at all stages of the collisions as they have long mean free paths. They are dubbed “clean probes” of the collision dynamics. This is what we need, if we want to learn about possible phase transitions (quark-gluon-plasma formation, restoration of chiral symmetry,...) in the early stages of ultra-relativistic heavy-ion collisions.