Nuclear Dependence of the Transverse-Single-Spin Asymmetry for Forward Neutron Production in Polarized p+A Collisions at √sNN = 200 GeV

During 2015, the Relativistic Heavy Ion Collider(RHIC) provided collisionsof transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of transverse-single-spin asymmetrieswith heavy nuclei. Large single-spin asymmetriesin very forward neutron production have been previously observed in transversely polarized p+p collisionsat RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetryin p+p collisionspredicts only a moderate atomic- mass-number(A) dependence. In contrast, the asymmetriesobserved at RHIC in p+A collisionsshowed a surprisingly strong A dependence in inclusive forward neutron production. The observed asymmetryin p+Al collisionsis much smaller, while the asymmetryin p+Au collisionsis a factor of 3 larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed A dependence. © 2018 authors. Published by the American Physical Society. (Less)