previously described, and by rescaling to 68% the signal events when the peak is dominated by the experimental resolution. Nonetheless, our simulation does not account for Initial State Radiation (ISR) effects. ISR can have two main sources: QED-like ISR (i.e., photon emission), that has the effect of shifting the peak and of creating a tail towards smaller en- ergy, and QCD-like ISR (i.e., gluon emission), that has similar effects and might also induce trigger issues in the intent of removing backgrounds (e.g., by cutting on final state jets). Although we are aware of such effects, we believe that their analysis goes beyond the scope of this paper and it will be reported on separately. Altogether, we are confident that, while particular aspects of our analysis may be sensitive to such effects, the general picture will not depend upon these substantially (see section III A). Also, the only background considered here was the irreducible SM Drell-Yan. Reducible backgrounds, photon-to-electron conver- sion, efficiencies in reconstructing electrons/muons, jets faking leptons etc., whose overall effect is to deplete the signal, were neglected (being tt the most important source, at the level of ≲ 10%). However, for this analysis they are not quantitatively important [4, 20, 32]. The net effect of the factors above is usually regarded as an overall reduction of the total acceptance, being the lepton identification the most important source, about 80 ÷ 90% per each lepton. We comment on this in the conclusions. We can now quantitatively compare our simulation to the literature. Ref. [6] contains a comprehensive analysis of expected backgrounds for several Z ′ boson masses 7 at 3 . 6 fb − 1 of integrated luminosity. Table I shows the comparison of our expectations with theirs, based on table II in [6], where the compatibility is, as usual, defined as: A. Validation Ev − Ev$_{D}$$_{Ø}$ σ$_{D}$$_{Ø}$ . (20) It is clear that our simulation is reasonable despite the lack of detector simulation, since it reproduces the DØ backgrounds within two standard deviations. Nonetheless, the limits 7 The simulation therein is modelled using the PYTHIA [33] Monte Carlo event generator, with CTEQ6L1 PDFs, and then processed through the standard DØ detector simulation based on GEANT3 [34]. 9