Diffraction and correlations at the LHC: definitions and observables V.A. Khoze, F. Krauss, A.D. Martin, M.G. Ryskin and K.C. Zapp Eur. Phys. J. C, 69 1-2 (2010) 85-93 Published online: 12 August 2010 DOI: 10.1140/epjc/s10052-010-1392-5