Beyond Universality: Probing Lepton Flavor in the SMEFT

TL;DR

This work performs a comprehensive Bayesian global fit of 17 dimension-six SMEFT operators focusing on lepton flavor, combining collider and flavor observables under MFV for the quark sector. By matching SMEFT onto LEFT and exploiting a diverse data set from Z/W decays, Drell–Yan, tt̄ℓℓ production, and a wide array of lepton and meson decays, it derives stringent bounds up to scales of order $ ext{O}(1000) ext{ TeV}$, with the strongest limits arising in the lepton-flavor democratic scenario for electroweak dipoles. The analysis reveals nontrivial MFV structures in the left-handed quark sector and shows how flavor assumptions crucially shape global SMEFT constraints, while providing posterior predictions for dineutrino modes that will be testable at Belle II. Overall, the results demonstrate the complementary power of collider and flavor data to constrain beyond-the-Standard-Model physics in the lepton sector and map out robust MFV-consistent limits on new interactions.

Abstract

We present a global analysis of lepton-flavor-specific operators in the Standard Model Effective Field Theory (SMEFT), combining data from collider and flavor physics experiments. We systematically explore various lepton-flavor scenarios, including flavor-specific, universal, and democratic patterns, while employing a minimal flavor violation (MFV) ansatz in the quark sector. We constrain a set of 17 dimension-six SMEFT operators using a Bayesian fitting approach. Our analysis yields stringent bounds on the Wilson coefficients, probing new physics scales up to $\mathcal{O}(1000)$\,TeV. The strongest constraints are obtained in lepton-flavor universal and lepton-flavor democratic scenarios, in particular for the electroweak dipole operators. We further provide posterior-based predictions for $B \to K^{(*)} ν\bar ν$ and $B \to (π, ρ) ν\bar ν$ decays, highlighting their role as probes of the MFV hypothesis in upcoming Belle II measurements. Our results demonstrate the complementarity of collider and flavor observables, the impact of flavor assumptions on global SMEFT fits, and establish robust limits on extensions of the Standard Model in the lepton sector.

Figures (24)

• Figure 1: Feynman diagrams illustrating double insertions of the SMEFT Higgs-current operators (left diagram) in the matching to the LEFT four-lepton operator (right diagram).
• Figure 2: 90% credible regions for the two-dimensional fit in the $\tilde{C}^{(1)}_{\varphi l}$--$\tilde{C}^{(3)}_{\varphi l}$ plane. The blue (orange) region is obtained from $Z$ decays to neutrinos (charged leptons $Z \to e \mu$). The red region presents a combined fit of both decays.
• Figure 3: 90% credible regions for the two-dimensional fit in the $\tilde{C}_{eB}$--$\tilde{C}_{eW}$ plane for a lepton flavor specific analysis with only the $e\mu$ coefficient switched on. The blue region is obtained from a fit to $W\to e\nu$ decays, while the orange region is obtained from $Z\to e\mu$. The red region shows the combined fit.
• Figure 4: 90% credible intervals of the Wilson coefficients $\tilde{C}_{\varphi l}^{(1)}$, $\tilde{C}_{\varphi l}^{(3)}$ and $\tilde{C}_{\varphi e}$ from lepton-flavor specific global SMEFT fits.
• Figure 5: Lower bounds on $\Lambda/\sqrt{C}$ at the 90% credible level for the Wilson coefficients $\tilde{C}_{\varphi l}^{(1)}$, $\tilde{C}_{\varphi l}^{(3)}$ and $\tilde{C}_{\varphi e}$ from lepton-flavor specific global SMEFT fits.