Z' at the LHC: Interference and Finite Width Effects in Drell-Yan

Abstract

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 not fully taken into account 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 significantly more experimental effort. 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 to O(10%), 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 control of the interference effects relies on not-too-small branching ratios of Z' to leptons (typically of at least a few percent) which can be suppressed, however, by additional new decay channels of the Z'; a small width-to-mass ratio alone does not guarantee the interference to be small over an arbitrary kinematic range. Under the general assumption that these new decay channels 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.