Speaker
Description
We calculate the single transverse spin asymmetry in polarized proton-proton and polarized proton-nucleus collisions ($A_N$) generated by a partonic lensing mechanism. The polarized proton is considered in the quark-diquark model while its interaction with the unpolarized target is calculated using the small-$x$/saturation approach. The phase required for the asymmetry is caused by a final-state gluon exchange between the quark and diquark, as is standard in the lensing mechanism of Brodsky, Hwang and Schmidt. The expression we obtain for the asymmetry $A_N$ of the produced quarks has the following properties:(i) The asymmetry is generated by the dominant elastic scattering contribution and $1/N^2_c$ suppressed inelastic contribution;(ii) The asymmetry grows or oscillates with the produced quark's transverse momentum $p_T$ until the momentum reaches the saturation scale $Q_s$, and then only falls off as $1/p_T$ for larger momenta;(iii) The asymmetry decreases with increasing atomic number $A$ of the target for $p_T$ below or near $Q_s$, but is independent of $A$ for $p_T$ significantly above $Q_s$. We discuss how these properties may be qualitatively consistent with data published by the PHENIX collaboration and with preliminary data reported reported by the STAR collaboration.
