Measurement of transverse single-spin asymmetries for dijet production in polarized $p$+$p$ collisions at $\sqrt{s}$ = 200 GeV at STAR

We report a new measurement of transverse single-spin asymmetries for pair-production of jets in collisions of transversely polarized protons at $\sqrt{s}$ = 200 GeV with data taken in 2012 and 2015 at STAR. The correlation between the transverse momentum of a parton ($\vec k_T$) and the transverse spin ($\vec S$) of its proton, moving in the longitudinal ($\vec p$) direction, is probed at a high $Q^2$ scale ($\sim$160 GeV$^2$). The corresponding Sivers $\langle k_T \rangle$ is calculated based on a simple kinematic model. By employing charge-tagging to separately enhance $u$- and $d$-quark contributions, we see non-zero Sivers effects for the first time in dijet production with transversely polarized proton collisions. The individual parton contributions ($u$, $d$, gluon+sea) to the measured $\langle k_T \rangle$ are extracted through bin-by-bin matrix inversion of the charge-sorted $\langle k_T \rangle$ results.