Abstract
New Z′ gauge bosons arise in many extensions of the Standard Model and predict resonances in the dilepton invariant mass spectrum. Searches for such resonances therefore provide important constraints on many models of new physics, but the resulting bounds are often calculated without interference effects. In this work we show that the effect of interference is significant and cannot be neglected whenever the Z′ width is large (for example because of an invisible contribution). To illustrate this point, we implement and validate the most recent 139 fb−1 dilepton search from ATLAS and obtain exclusion limits on general Z′ models as well as on simplified dark matter models with spin-1 mediators. We find that interference can substantially strengthen the bound on the Z′ couplings and push exclusion limits for dark matter simplified models to higher values of the Z′ mass. Together with this study we release the open-source code ZPEED, which provides fast likelihoods and exclusion bounds for general Z′ models.
Highlights
In models where the Z couples differently to left- and right-handed quarks, the presence of lepton couplings is imposed both by considerations of gauge invariance and by the requirement that there are no gauge anomalies (assuming no exotic SU(2) fermions or additional Higgs doublets)
We show in figure 3 our bound on the cross section as a function of the Z mass, compared to that published by ATLAS [5]
For fixed total width ΓZ the Z production cross section is proportional to g4, while interference effects scale as g2
Summary
We describe the calculation of the cross section for pp → Z → + − (see figure 1) at leading order, including the effect of interference with the SM background processes mediated by the Z boson and the photon This issue has previously been studied in the context of specific Z models in ref. If on the other hand ΓZ is comparable to the bin size (for example because of an invisible decay mode), dilepton resonance searches are potentially sensitive to signals with dσsignal/dm dσbackground/dm for all values of m For such small signal cross sections, interference effects can potentially be very important. We will restrict ourselves to vector mediators in the following
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