Factorization at fixed Q^2(1-x)

TL;DR

This paper explores QCD factorization in the limit where the parton's momentum fraction approaches unity with fixed Q^2(1-x), revealing coherence effects and their implications for processes like Drell-Yan and spin asymmetries.

Contribution

It introduces the BB limit, a new factorization regime where soft and hard subprocesses are coherent, connecting inclusive and exclusive phenomena at high x.

Findings

Longitudinal polarization of virtual photons in pion-induced Drell-Yan

Transverse polarization in proton-induced Drell-Yan

Relevance of the BB limit to high-x spin asymmetries

Abstract

We consider QCD factorization between hard and soft subprocesses in inclusive reactions where the momentum fraction x of one parton approaches unity as the hard scale Q^2 -> \infty, such that Q^2(1-x) is fixed. In this "BB limit" the entire (multi-parton) Fock state containing the high x parton is coherent with the hard subprocess. The soft contribution is given by a forward multiparton matrix element. The BB limit corresponds to a fixed (large or small) missing mass and is thus closely connected to exclusive production. We analyze the Drell-Yan process h + N -> \gamma^* + X in detail, explaining why the virtual photon is longitudinally polarized for h = \pi and transversely polarized for h = p. The BB limit may be relevant also for other phenomena observed at high x, such as the large single spin asymmetries of p p -> \Lambda^\uparrow X and in p p^\uparrow -> \pi X.