# Nuclear-modification factor of charged hadrons at forward and backward rapidity in $p$$+$Al and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

## TL;DR

This study measures how charged hadron production in proton-nucleus collisions at 200 GeV varies with rapidity and collision centrality, revealing suppression at forward rapidity and enhancement at backward rapidity, with results compared to nuclear parton distribution models.

## Contribution

It provides new measurements of nuclear modification factors at forward and backward rapidities in $p$+Al and $p$+Au collisions, highlighting differences in nuclear effects and testing theoretical models.

## Key findings

- Suppression of charged hadrons at forward rapidity in central collisions.
- Enhancement of charged hadrons at backward rapidity, larger in $p$+Au than $p$+Al.
- Nuclear parton distribution functions agree at forward rapidity but not at backward rapidity.

## Abstract

The PHENIX experiment has studied nuclear effects in $p$$+$Al and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV on charged hadron production at forward rapidity ($1.4<\eta<2.4$, $p$-going direction) and backward rapidity ($-2.2<\eta<-1.2$, $A$-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.