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 (T_{A B}) and number of binary collisions (N_coll) 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 N_coll and T_{A B}, which in turn, results in an upward correction for the nuclear modification factor R_{A B}. 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 sqrt(s) = 5.5 TeV than for the RHIC energy of sqrt(s) = 200 GeV. The implications for experimental measurements are also discussed.