The role of soft quarks in next-to-leading power threshold effects

Cross sections in perturbative QCD are plagued by large corrections from soft and collinear radiation. The most singular terms are known to be universal, which allows their resummation to all orders in the coupling. We have examined the structure of the next-to-singular contributions, which can originate from the emission of both soft quarks and gluons. We show that we can derive a next-to-soft amplitude for both types of emissions. The numerical impact of these contributions on the transverse momentum distribution of the single-photon production process are also discussed.