  We report a new measurement of transverse single-spin asymmetries for dijet
production in collisions of polarized protons at $\sqrt{s}$ = 200
$\mathrm{GeV}$. Correlations between the proton spin and the transverse momenta
of its partons, each perpendicular to the proton momentum direction, are probed
at high $Q^2$ $\approx $160 $\mathrm{GeV}^2$. The associated Sivers observable
$\langle k_T \rangle$, the average parton transverse momentum, is extracted
using simple kinematics. Nonzero Sivers effects are observed for the first time
in dijets from proton-proton collisions, but only when the jets are sorted by
their net charge, which enhances the $u$- or $d$-quark contributions to
separate data samples. This also enables a simple kinematic approach for
determination of the individual partonic contributions to the observed
asymmetries.
