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.

Drell-Yan production of multi Z ′-bosons at the LHC within Non-Universal ED and 4D Composite Higgs Models

“…As shown in ref. [11], within this mass interval and for a big enough BR(Z → l + l − ), the model dependent features of the Z -boson signal can be negleted up to an O(10%) theoretical accuracy in all theories predicting a narrow Z -boson.…”

“…This is explicitly shown in ref. [11], where these depend on the model being considered and can significantly affect the theoretical predictions. FW and interference effects have been studied by a number of authors also in different processes (see for example refs.…”

“…For heavy neutral vector boson production in Drell-Yan, only within narrow width Z -boson scenarios can the deviations from the NWA due to the above two phenomena be safely neglected at least to some degree. Following the recommendation of [11], when interpreting the derived 95% C.L. upper bound on the BSM cross section to extract the mass limits within a specific model, the CMS collaboration restricts the integration range of the differential cross section to the invariant mass of the dilepton pair in the window |M ll − M Z | ≤ 0.05 × E LHC where E LHC is the collider energy and M Z the hypothetical pole mass of the new Z -boson.…”

“…This result is common to a large class of narrow single Z -boson scenarios, as extensively discussed in ref. [11]. As to the SM background, the CMS collaboration represents its shape by a functional form whose parameters are fixed via a fit to the Monte Carlo (MC) SM background estimate.…”

“…The vertical red line represents the dilepton invariant mass interval proposed in ref. [11] for computing the total theoretical cross section. This shows that, if one restricts the integration region, the difference between complete cross section and NWA result, normalized to the NWA cross section, is indeed below O(10%).…”