k_T factorization is violated in production of high-transverse-momentum particles in hadron-hadron collisions

TL;DR

This paper demonstrates that k_T factorization is violated in high-transverse-momentum hadron production during hadron-hadron collisions, challenging the validity of standard factorization assumptions in QCD for such processes.

Contribution

It provides an explicit counterexample showing the breakdown of k_T factorization in back-to-back hadron production with polarized beams, highlighting limitations of existing factorization proofs.

Findings

k_T factorization fails in certain hadron-hadron collision scenarios

Counterexample involving single-spin asymmetry and Sivers function

Implications for the validity of factorization in QCD processes

Abstract

We show that hard-scattering factorization is violated in the production of high-p_T hadrons in hadron-hadron collisions, in the case that the hadrons are back-to-back, so that k_T factorization is to be used. The explicit counterexample that we construct is for the single-spin asymmetry with one beam transversely polarized. The Sivers function needed here has particular sensitivity to the Wilson lines in the parton densities. We use a greatly simplified model theory to make the breakdown of factorization easy to check explicitly. But the counterexample implies that standard arguments for factorization fail not just for the single-spin asymmetry but for the unpolarized cross section for back-to-back hadron production in QCD in hadron-hadron collisions. This is unlike corresponding cases in e^+e^- annihilation, Drell-Yan, and deeply inelastic scattering. Moreover, the result endangers factorization for more general hadroproduction processes.