We discuss the transverse single spin asymmetries AN for very forward
neutrons measured by the PHENIX zero-degree calorimeters (ZDCs) in high-energy
polarized proton-nucleus (p-A) collisions at Relativistic Heavy Ion Collider
(RHIC). The ZDCs cover the psudorapidity range of 6.8<η<8.8 where the neutron
emission angle with respect to the incident proton direction is limited to less
than 2.2mrad. First-ever p-A data taken in the RHIC-2015 run exhibit positive and remarkably
large AN∼0.18 only in p-Au collisions whereas nearly zero AN in
p-Al collisions. In this proceeding, to explain such AN in p-A collisions we present an
important but rather unknown mechanism: ultra-peripheral p-A collisions (UPCs,
also known as Primakoff effects). UPCs lead to very large AN of about 0.35
and have cross sections proportional to Z2 of the nuclei.
UPCs contribute to inclusively measured AN modestly in p-Al collisions and
significantly in p-Au collisions. We show that the Monte Carlo simulations
incorporating both a one-pion exchange (OPE) and UPCs, where virtual
photon-polarized proton interactions follow the MAID 2007 isobar model,
successfully describe the PHENIX data in p-Al and p-Au collisions
simultaneously.