Speaker
Description
Sizeable Transverse Single Spin Asymmetries have been observed at large $x_{\mathrm{Feynman}}$ (TSSAs) since the mid-1970s with fixed-target experiments, and were confirmed at the Relativistic Heavy Ion Collider experiments up to higher energy. The TSSAs are an important probe for non-perturbative spin-momentum correlations both in initial- and final-state strong nuclear interactions. They are described within two complementary frameworks: the collinear twist-3 (CT3) and the Transverse-Momentum-Dependent (TMD) schemes. We show the most recent measurements of TSSAs in inclusive production of $\pi^0$ and $\eta$ mesons obtained at the sPHENIX experiment, as a function of the meson transverse momentum $p_{\mathrm{T}}$ and $x_{\mathrm{Feynman}}$. The sPHENIX apparatus includes a large acceptance granular electromagnetic calorimeter (EMCal), as well as a high data rate plus trigger system, thus sampling 107 $\mathrm{pb}^{-1}$ of transversely polarized $p^\uparrow+p^\uparrow$ data during the RHIC run of 2024. This allows us to measure TSSAs in a kinematic region that spans central rapidity up to moderate forward rapidity ($0 < \eta < 2$), extending significantly the kinematic coverage given by previous RHIC measurements. In the central region at small $p_{\mathrm{T}}$, and thus at small $x_{\mathrm{Feynman}}$, TSSAs are particularly sensitive to gluon contributions and allow to constrain the collinear twist-3 trigluon functions as well as the gluon Sivers TMD functions.
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