Four-lepton Z-boson decay constraints on the SMEFT

TL;DR

This paper demonstrates how four-lepton Z-boson decays can constrain previously unconstrained parameters in the SMEFT, with future high-luminosity data promising to fully resolve flat directions in the four-muon sector.

Contribution

It derives new constraints from Z-boson four-lepton decays on SMEFT operators and highlights the potential of future measurements to eliminate flat directions.

Findings

Current Z-decay data constrains some SMEFT parameters.

Future high-luminosity measurements can fully constrain four-lepton operators.

Rare Z-decays to tau leptons could probe additional SMEFT directions.

Abstract

We discuss how four-lepton decays of the Z-boson probe currently unconstrained flat directions in the parameter space of the Standard Model Effective Field Theory (SMEFT). We derive the constraints from these decays on four-lepton operators in the SMEFT and show how the LHC data for this process complements probes from neutrino-trident production. Future differential measurements with high-luminosity data can strongly constrain four-lepton operators and remove all flat directions in the four-muon sector of the SMEFT. We comment briefly on the possibility of using rare Z-decays to tau-leptons to probe untested directions in the SMEFT parameter space.