Factorisation, parton entanglement and the Drell-Yan process D. Boer, A. Brandenburg, O. Nachtmann and A. Utermann Eur. Phys. J. C, 40 1 (2005) 55-61 Published online: 15 March 2005 DOI: 10.1140/epjc/s2005-02126-0