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 $\ensuremath{\tau}$-leptons to probe untested directions in the SMEFT parameter space.
Highlights
The search for physics beyond the Standard Model (SM) at the Large Hadron Collider (LHC) and other experiments has so far yielded no new particles
The search for new particles will continue in the future at the high-luminosity LHC, it is becoming increasingly important to search for potentially small and subtle indirect signatures of new physics, and to understand the constraints imposed by current data on high-scale new physics
We review aspects of the SM effective field theory (SMEFT) needed for our analysis in Sec
Summary
The search for physics beyond the Standard Model (SM) at the Large Hadron Collider (LHC) and other experiments has so far yielded no new particles. Assuming MFV implies that the flavor structure of the SMEFT Wilson coefficients are carried by combinations of Yukawa matrices This leads to several familiar intuitions [19,20]: for example, that the coefficients of scalar and dipole operators are suppressed by small fermion masses for the lighter generations, and that vector four-fermion interactions are generation-independent. We point out that differential measurements can completely determine all four-muon Wilson coefficients, which motivates their study with future high-luminosity data. III we study the constraints imposed by inclusive LHC measurements of the Z → 4μ decay rate on the SMEFT four-muon operators We discuss their complementarity with constraints from neutrino-trident production.
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