New and Standard Physics contributions to anomalous Z and gamma self-couplings
G. J. Gounaris, J. Layssac, F. M. Renard
TL;DR
This study analyzes CP-conserving neutral gauge self-couplings $ZZZ$, $ZZ\gamma$, and $Z\gamma\gamma$ arising at one loop from Standard Model fermions and from New Physics, including MSSM states. It demonstrates that bosonic loops do not generate these couplings at one loop and that the dominant contributions come from fermion triangles, with $h_3^V$ and $f_5^V$ related by $h_3^Z \approx -f_5^{\gamma}$ and $h_4^V$ vanishing at this order. The paper then provides quantitative results for SM and MSSM scenarios, and discusses how anomaly cancellation, mass spectra, and potential heavy-fermion NP affect the size of these couplings, including threshold-induced imaginary parts. It concludes that observable deviations from SM predictions are unlikely except in cases with light superpartners or enhanced couplings, and notes that higher-order or nonperturbative effects are generally too small to yield detectable signals at planned colliders. The findings inform expectations for future collider searches of neutral gauge self-interactions and the sensitivity to new electroweakly charged fermions.
Abstract
We examine the Standard and the New Physics (NP) contributions to the ZZZ, ZZgamma and Zgammagamma neutral gauge couplings. At the one-loop level, if we assume that there is no CP violation contained in NP beyond the Standard Model one, we find that only CP conserving neutral gauge couplings are generated, either from the standard quarks and leptons, or from possible New Physics (NP) fermions. Bosonic one-loop diagrams never contribute to these couplings, while the aforementioned fermionic contributions satisfy hZ3=-fgamma5, hZ4=hgamma4=0. We also study examples of two-loop NP effects that could generate non vanishing h4 couplings. We compare quantitative estimates from SM, MSSM and some specific examples of NP contributions, and we discuss their observability at future colliders.