Single Spin Asymmetry in Forward $pA$ Collisions

Abstract

We compute the transverse single spin asymmetry in light hadron production $p^\uparrow p\to hX$ and $p^\uparrow A\to hX$ including the gluon saturation effect in the unpolarized nucleon/nucleus. In the forward (large-$x_F$) region, the dominant contribution comes from the so-called derivative term associated with the soft gluonic pole. This leads to the cancellation of nuclear effects in $A_N$ which can be tested at RHIC. We also show that the soft fermionic pole disappears in the saturation environment.