Measurement of transverse single-spin asymmetries for dijet production in polarized proton-proton collisions at s = 200

We report a new measurement of transverse single-spin asymmetries for dijet production in collisions of polarized protons at s = 200     GeV . Correlations between the proton spin and the transverse momenta of its partons, each perpendicular to the proton momentum direction, are probed at high Q 2 ≈ 160     GeV 2 . Evidence for nonzero Sivers effects is measured for the first time in dijets from proton-proton collisions, but only when the jets are sorted by their net charge, which enhances the otherwise canceling opposite-sign u - or d -quark contributions to separate data samples. The resulting asymmetries are compared to recent theoretical calculations. Separately, the associated Sivers observable ⟨ k T ⟩ , the average parton transverse momentum, is extracted using a simple kinematics approach which further enables a determination of the individual partonic contributions to the observed asymmetries.