Transverse single-spin asymmetry for very forward neutral pion production in polarized collisions at = 510 GeV

Kim, M. H.*; Tanida, Kiyoshi   ; RHICf Collaboration*; 23 of others*

Transverse single-spin asymmetries of very forward neutral pions generated in polarized collisions allow us to understand the production mechanism in terms of perturbative and non-perturbative strong interactions. During 2017 the RHICf Collaboration installed an electromagnetic calorimeter in the zero-degree region of the STAR detector at the Relativistic Heavy Ion Collider (RHIC) and measured neutral pions produced at pseudorapidity larger than 6 in polarized + collisions at = 510 GeV. The large non-zero asymmetries increasing both in longitudinal momentum fraction and transverse momentum have been observed at low transverse momentum GeV/ for the first time at this collision energy. The asymmetries show an approximate scaling in the region where non-perturbative processes are expected to dominate. A non-negligible contribution from soft processes may be necessary to explain the nonzero neutral pion asymmetries.