Factorisation, Parton Entanglement and the Drell-Yan Process

Abstract

We discuss the angular distribution of the lepton pair in the Drell-Yan process, hadron+hadron -> γ^* X -> l^+ l^- X. This process gives information on the spin-density matrix ρ^{(q,\bar{q})} of the annihilating quark-antiquark pair in q+\bar{q} -> l^+ l^-. There is strong experimental evidence that even for unpolarised initial hadrons ρ^{(q,\bar{q})} is nontrivial, and therefore the quark-antiquark system is polarised. We discuss the possibilities of a general ρ^{(q,\bar{q})} -which could be entangled- and a factorising ρ^{(q,\bar{q})}. We argue that instantons may lead to a nontrivial ρ^{(q,\bar{q})} of the type indicated by experiments.

14 pages, 2 figures, comments and references added; to appear in EPJC

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