Nuclear-modification factor of charged hadrons at forward and backward rapidity in Al and Au collisions at GeV

Aidala, C.*; Hasegawa, Shoichi   ; Imai, Kenichi*; Sako, Hiroyuki   ; Sato, Susumu  ; Tanida, Kiyoshi   ; PHENIX Collaboration*; 297 of others*

The PHENIX experiment has studied nuclear effects in Al and Au collisions at GeV on charged hadron production at forward rapidity (, -going direction) and backward rapidity (, -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 Al and Au collisions, a suppression (enhancement) is observed at forward (backward) rapidity compared to the binary scaled yields in + collisions. The magnitude of enhancement at backward rapidity is larger in Au collisions than in 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.