Nuclear-modification factor of charged hadrons at forward and backward rapidity in p + Al and p + Au collisions at √sNN = 200 GeV

The PHENIX experiment has studied nuclear effects in p + Al and p + Au collisions at √sNN = 200 GeV on charged hadron production at forward rapidity (1.4 <η< 2.4, p-going direction) and backward rapidity (−2.2 <η< −1.2, A-going direction). Such effects are quantified by measuring nuclear modification factors as a function of transverse momentum and pseudorapidity in various collision multiplicity selections. In central p + Al and p + Au collisions, a suppression (enhancement) is observed at forward (backward) rapidity compared to the binary scaled yields in p + p collisions. The magnitude of enhancement at backward rapidity is larger in p + Au collisions than in p + Al collisions, which have a smaller number of participating nucleons. However, the results at forward rapidity show a similar suppression within uncertainties. The results in the integrated centrality are compared with calculations using nuclear parton distribution functions, which show a reasonable agreement at the forward rapidity but fail to describe the backward rapidity enhancement