Nuclear dependence of the transverse-single-spin asymmetry for forward neutron production in polarized collisions at GeV

Aidala, C.*; Hasegawa, Shoichi   ; Imai, Kenichi; Sako, Hiroyuki   ; Sato, Susumu  ; Tanida, Kiyoshi   ; PHENIX Collaboration*; 309 of others*

During 2015 the Relativistic Heavy Ion Collider (RHIC) provided collisions of transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of transverse-single-spin asymmetries with heavy nuclei. Large single-spin asymmetries in very forward neutron production have been previously observed in transversely polarized collisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in collisions predicts only a moderate atomic-mass-number () dependence. In contrast, the asymmetries observed at RHIC in collisions showed a surprisingly strong dependence in inclusive forward neutron production. The observed asymmetry in +Al collisions is much smaller, while the asymmetry in + Au collisions is a factor of three larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed dependence.