Surface Emission of Quark Gluon Plasma at RHIC and LHC

Within the framework of a factorization model, we study the behaviour of nuclear modification factor in Au--Au collisions at RHIC and Pb--Pb collisions at LHC. We find that the nuclear modification factor is inversely proportional to the radius of the quark--gluon plasma and is dominated by the surface emission of hard jets. We predict the nuclear modification factor R _AA^LHC~0.15 in central Pb--Pb collisions at LHC. The study shows that the factorization model can be used to describe the centrality dependence of nuclear modification factor of the high transverse momentum particles produced in heavy ion collisions at both RHIC and LHC.