Speaker
Description
The first high energy polarized proton-nuclei collisions at RHIC in 2015 give us opportunities to study unexplored reaction mechanisms of hadron production in the forward region. In PHENIX, single transverse spin asymmetries (AN s) of forward (6.8 < η < 8.8) neutron production in √sNN = 200 GeV p+Al, and p+Au collisions are measured, and an unexpected strong A dependence in AN is found. The cross section and AN from the p + p data, which covers the nonperturbative region (pT < 0.2 GeV/c), have been well described by a one pion exchange (OPE) model in Regge theory. In the OPE model, AN arises mainly from the interference of the helicity flip amplitude via pion exchange and the helicity nonflip amplitude via a1-Reggeon exchange. However, this model cannot describe the observed A dependence.
Since our data covers small −t range (< 0.5 (GeV/c)2), electromagnetic interaction may not be ignorable for the large Z nucleus, and ultra peripheral collisions (UPC) and Coulomb nuclear interference (CNI) can also play important role in asymmetry. In order to study competing effects, a correlation study using beam beam counters (which detect charged particles at 3.1 < η < 3.9) was done which can reduce or enhance their relative contributions. The resulting asymmetries demonstrated drastic dependence depending on hit requirements in the beam beam counters.
In this talk, the AN results and current progress in the interpretation of the data will be presented.
