NLO corrections and factorization for transverse single-spin asymmetries

We present next-to-leading order QCD corrections for the cross sections for $\mathrm{\ell }{p}^{\uparrow }\to hX$, $\mathrm{\ell }{p}^{\uparrow }\to \mathrm{j}\mathrm{e}\mathrm{t}X$ with transversely polarized initial protons. These cross sections are known to be power-suppressed in QCD and probe twist-3 parton correlation functions in the proton. Our calculation exhibits the full complexity of a perturbative QCD analysis beyond leading power, involving in particular various derivatives of the parton correlation functions. We demonstrate that collinear factorization for these single-spin observables holds at one loop. We also present exploratory phenomenological results for the NLO single-spin asymmetry in $e{p}^{\uparrow }\to hX$ and compare to data from the HERMES experiment.