Speaker
Description
Large Transverse Single Spin Asymmetries ($A_N$) for forward rapidity inclusive hadron production have been observed over a wide range of center of mass energies. This large $A_N$ has been described by phenomenological TMD models like Sivers and Collins mechanisms for initial and final state effects respectively, or by higher twist contributions in the initial and final states. It is necessary to go beyond inclusive hadron measurements of $A_N$ to isolate the Sivers contributions, and we need to measure fragments in jets to account for the Collins contributions.
The STAR forward electromagnetic calorimeter, Forward Meson Spectrometer (FMS), measures $\pi^0$ and $\eta$ mesons at high $x_{F}$ (0.2-0.6) where $A_N$ is known to be large. With the addition of pre-shower and post-shower detectors in 2015-2017, FMS is capable of detecting direct photons and Drell-Yan (DY) pairs and, hence, capable of testing the predicted change in sign of $A_N$ in DY and direct photons compared to semi-inclusive DIS. Observation of large $A_N$ for isolated $\pi^0$ is still a puzzle. In this context, we take advantage of the large acceptance of FMS to reconstruct electromagnetic jets. We will show $A_N$ for electromagnetic jets and neutral pions and Collins measurements for $\pi^0$'s in jets from $p\uparrow$ + $p\uparrow$ collisions at $\sqrt{s}$ = 500 GeV.
