The Estimation of the Effective Centre of Mass Energy in q-qbar-gamma   Events from DELPHI
  The photon radiation in the initial state lowers the energy available for the e$^+$e$^-$ collisions; this effect is particularly important at LEP2 energies (above the mass of the Z boson). Being aligned to the beam direction, such initial state radiation is mostly undetected. This article describes the procedure used by the DELPHI experiment at LEP to estimate the effective centre-of-mass energy in hadronic events collected at energies above the Z peak. Typical resolutions ranging from 2 to 3 GeV on the effective center-of-mass energy are achieved, depending on the event topology. 
P. Abreu, A. De Angelis, G. Della Ricca, D. Fassouliotis, A. Grefrath,   N. Kjaer, R. Henriques, M. Mulders, M. Pimenta and ...