Factorisation in higher-twist single-spin amplitudes

We analyse the twist-three amplitudes that can give rise to single-spin asy mmetries in hadron-hadron scattering; in so doing we bring to light a novel factorisation property. As already known, the requirement of an imaginary part leads to consideration of twist-three contributions that are also rela ted to transverse spin in deep-inelastic scattering. In particular, when an external line becomes soft in contributions arising from three-parton corr elators, the imaginary part of an internal propagator may be exposed. As sh own here, it is precisely this kinematical configuration that permits the f actorisation. An important feature is the resulting simplification: the cal culation of tens of Feynman diagrams normally contributing to such processe s is reduced to the evaluation of products of the simple factors derived he re and known two-body helicity amplitudes. We thus find clarifying relation s between the spin-dependent and spin-averaged cross-sections and formulate a series of selection rules. In addition, the kinematical dependence of su ch asymmetries, is rendered more transparent.