Factorisation, parton entanglement and the Drell-Yan process

Abstract.

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