Interference effects in dilepton resonance searches for Z′ bosons and dark matter mediators

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.

Reinterpretation of LHC Results for New Physics: Status and recommendations after Run 2

“…Moreover, whenever the tested signal splits up into many different channels, as typically the case in complex models with several new particles, the derived limits tend to be highly conservative and often far too weak [88,89]. Tools which evaluate cross-section × branching ratio limits for specific models/signatures, such as HiggsBounds [90,91] for additional Higgs bosons, ZPEED [92] for Z resonances, and Dark-Cast [93] for dark photons, also fall into this class. 5 Reinterpretation by means of MC simulation is more generally applicable and more precise but also more difficult and much more time consuming.…”

“…Regarding spin-1 resonance searches, the recently released code ZPEED [92] provides fast likelihoods and exclusion bounds from dilepton resonance searches for general Z models. This is achieved by combining analytical expressions for leading-order differential cross-sections with tabulated functions that account for PDF effects, phase space cuts, detector efficiencies, energy resolution and higher-order corrections.…”

“…In the case of simplified or complete scenarios, the new degrees of freedom may be accessible at LHC energies, and resonant bumps or other striking final state features may appear. However, in this case, interference effects are typically not considered although they may be important in some cases [92].…”

Cross-fertilising extra gauge boson searches at the LHC

“…Replacing the Breit-Wigner (BW) distribution by a δ function is therefore an inaccurate way to characterise the production and decay of the new states [13][14][15][16]. Furthermore, in these circumstances, interference effects with the SM background become visible in the aforementioned mass spectra [14,17]. A combined correlation of W and Z analyses, including finite width effects, is so far lacking.…”

Unitarity Constraints and Collider Searches for Dark Photons

“…The invariant mass is related to the center-ofmass energy via m = √ ŝ = √ x 1 x 2 S, where √ ŝ and √ S are the partonic center-of-mass energy and center-of-mass energy, respectively. The cross section of dilepton production consists of terms arising from exchange of γ, Z, Z D , as well as the interference terms [52]:…”