Factorization at fixed Q^2(1-x)

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 -> γ^* + X in detail, explaining why the virtual photon is longitudinally polarized for h = πand 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 -> Λ^\uparrow X and in p p^\uparrow -> πX.

18 pages, 6 figures. v2: Version published in JHEP. Text modified as suggested by the referee

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