Preprint number: CP3-Origins-2013-13 DNRF90 and DIAS-2013-13
The interference effects between an extra neutral spin-1 Z’-boson and the Standard Model background in the Drell-Yan channel at the LHC are studied in detail. The final state with two oppositely charged leptons is considered. The interference contribution to the new physics signal, currently neglected by experimental collaborations in Z’-searches and in the interpretation of the results, can be substantial. It may affect limits or discovery prospects of Z’ at the LHC. As the Z’-boson interference is model-dependent, a proper treatment would a priori require a dedicated experimental analysis for each particular model. Doing so could potentially improve the sensitivity to new physics, but would require a much bigger effort from the experimental side.
At the same time, it is shown that one can define an invariant mass window, valid for a wide range of models, for which the contribution of the model-dependent interference to the Beyond the Standard Model signal is reduced down to O(10%) level, comparable to the level of the combined uncertainty from parton densities and higher order corrections. This quasi-model independent “magic cut” does not scale with the mass of the Z’-boson and is approximately constant over a large range of masses. Such a control of the interference effects relies on not-too-small branching ratios of Z’ to leptons which can be suppressed, however, by an additional new decay channels of the Z’ increasing the interference effect. Under the general assumption that these new decay chanels of Z’ are not dominant, one can perform quasi-model independent analyses, preserving the current scheme used by the experimental collaborations for the Z’-boson search using the suggested invariant mass window cut.