ZZ and Zgamma still haven't found what they are looking for

TL;DR

The work analyzes high-energy neutral diboson production to probe physics beyond the SM via an effective field theory with a focus on dimension-8 operators that survive in $pp\to ZZ$ and $pp\to Z\gamma$. It identifies a concrete set of $d=8$ operators that modify the dominant SM helicity amplitudes, yielding energy-growing contributions that interfere with SM processes and can be detected in TT and LL helicity configurations. The authors discuss UV motivations from weakly-coupled spin-2 resonances and non-linear SUSY (Pseudo-Goldstini), and derive model-independent positivity constraints from dispersion relations that substantially restrict the allowed operator coefficients. They show that positivity reduces the naive EFT parameter space and outline an experimental strategy to test these effects with current and future LHC data, while highlighting links to resonance searches and novel BSM scenarios. The work provides a unified framework to interpret neutral diboson measurements in terms of high-energy EFT structures, guiding both phenomenology and model-building.

Abstract

Neutral diboson processes are precise probes of the Standard Model (SM) of particle physics, which entail high sensitivity to new physics effects. We identify in terms of dimension-8 effective operators the leading departures from the SM that survive in neutral diboson processes at high-energy, and which interfere with the unsuppressed SM helicity contributions. We describe symmetries and selections rules that single out those operators, both for weakly and strongly coupled physics beyond the SM. Finally, we show that unitarity and causality enforce, via dispersion relations, positivity constraints on the coefficients of these effective operators, reducing the parameter space which is theoretically allowed.

Figures (2)

• Figure 1: Differential $\bar{\psi}\psi\to ZZ,Z\gamma$ cross-section for different helicities, in units of the $\gamma\psi,Z\psi$ couplings, $g_{\gamma\psi}, g_{Z\psi}$ ($n_V=1$ for $Z\gamma$ and $n_V=2$ for $ZZ$). Dashed coloured lines correspond to the SM-only contribution (solid black, the sum over helicities), while solid blue to the BSM-only TT polarisations, with an arbitrarily chosen normalisation, to be shown in the same plot as the SM.
• Figure 2: Parameter space for the operators with $\psi=Q$ (we have neglected superscripts for clarity). The blue region is allowed by positivity constraints Eqs. (\ref{['posconst']},\ref{['poscone']}).The black segment correspond to the explicit scenarios of section \ref{['sec:BSM']}.