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Single Spin Asymmetry at Large x_F and k_T

The large single spin asymmetries observed at high momentum fractions x_F and transverse momenta k_T in p^\uparrow p -> π(x_F,k_T)+X as well as in pp -> Λ^\uparrow(x_F,k_T)+X suggest that soft helicity flip processes are coherent with hard scattering. Such coherence can be maintained if x_F -> 1 as k_T -> \infty, while k_T^2(1-x_F) \sim Λ_QCD^2 stays fixed. The entire hadron wave function, rather than a single quark, then contributes to the scattering process. Analogous coherence effects have been seen experimentally in the Drell-Yan process at high x_F. We find that the p^\uparrow p -> π(x_F,k_T)+X production amplitudes have large dynamic phases and that helicity flip contributions are unsuppressed in this limit, giving rise to potentially large single spin asymmetries.