The transversely polarized proton structure has been a fascinating topic over the decades. It was observed in many experiments that the hadrons produced in a polarized e-p or p-p collision had significant transverse single spin asymmetry(TSSA). Complex models like transverse momentum dependent parton distribution function(TMD) and collinear twist-3 factorization were proposed to explain the mechanism which still needs more data to understand. The Relative Heavy Ion Collider in Brookhaven National Laboratory provides a unique opportunity to probe the polarized proton structure in proton-proton collision.
We measured the transverse single spin asymmetry of inclusive neutral pion at 200 and 500 GeV using the Forward Meson Spectrometer at RHIC-STAR. The result shows that the pion TSSA increase as the Feynman-x and it is independent of energy scale. We investigated the event topology of the neutral pion, and found that the pions with no nearby particles tended to have higher TSSA. We also measured the Collins asymmetry using the electromagnetic jet to study the final state effect, and the result is consistent with zero for both collision energies.
