Abstract for DIS 2020

Measurement of transverse single spin asymmetries at forward rapidities by the STAR experiment  at √s = 200 and 500 GeV

We present recent results of transverse single spin asymmetries (TSSA) for neutral pions using the Forward Meson Spectrometer at STAR at center of mass energies of 200 and 500 GeV in proton-proton collision.  Combining the results from the two energies shows that the pion TSSA increases with Feynman-x continuously. Comparisons with previous measurements show that the pion TSSA is mostly independent of energy scale from 20 GeV to 500GeV. It is found that pions with no nearby particles tend to have higher TSSA which may suggest different mechanisms for the TSSAs for different sub-groups of pions.  In order to separate the contribution of initial and final state effects at both energies, we also measured TSSAs for the electromagnetic jets and the Collins asymmetry through the TSSA of neutral pions inside the electromagnetic jet. The jet TSSA follows the Feynman-x behavior of the pion TSSA, but with a significantly smaller amplitude of the asymmetry. The Collins asymmetry is consistent with zero. These results provide rich information to understand the neutral pion TSSA.