Physics of ‘the first three fermis’ in high energy heavy ion collisions

Abstract

Using the (lowest-order) perturbative QCD, we argue that high energy heavy ion collisions proceed as follows: ( i) 0 < ρ < 0.3 fm/c entropy production, momenta equilibration due to gg → gg, qg → qg; ( ii) 0.3 < ρ < 2. fm/c: gluon chemical equilibration due to gg → ng; ( ii) 3 < ρ < ρ c = 5–7 fm/c: expansion of gluonic plasma till formation of the ‘mixed phase’, or hadronization. Much higher initial temperatures are suggested, namely T i = 400−500 MeV mat RHIC and T i = 600–900 MeV at LHC. This leads to enhanced charm production and significant modifications of photon and dilepton production.