Influence of the nucleon-nucleon collision geometry on the determination of the nuclear modification factor for nucleon-nucleus and nucleus-nucleus collisions

The influence of the underlying nucleon-nucleon collision geometry on evaluations of the nuclear overlap function (TAB) and number of binary collisions (Ncoll) is studied. A narrowing of the spatial distribution of the hard-partons with large light-cone fraction x in nucleons leads to a downward correction for Ncoll and TAB, which in turn, results in an upward correction for the nuclear modification factor RAB. The size of this correction is estimated for several experimentally motivated nucleon-nucleon overlap functions for hard-partons. It is found to be significant in peripheral nucleus-nucleus and nucleon-nucleus collisions, and is much larger at the LHC energy of s=5.5 TeV than for the RHIC energy of s=200 GeV. The implications for experimental measurements are also discussed.