What is Leading Order for LFV in SMEFT?

Abstract

Upcoming searches for lepton flavour change (LFV) aim to probe New Physics (NP) scales up to ΛNP ∼ 104 TeV, implying that they will be sensitive to NP at lower scales that is suppressed by loops or small couplings. We suppose that the NP responsable for LFV is beyond the reach of the LHC and can be parametrised in Effective Field Theory, introduce a small power-counting parameter λ (à la Cabibbo-Wolfenstein), and assess whether the existing dimension six operator basis and one-loop RGEs provide a good approximation for LFV. We find that μ ↔ e observables can be sensitive to a few dozen dimension eight operators, and to some effects of two-loop anomalous dimensions, for ΛNP ≲ 20 − 100 TeV. We also explore the effect of some simplifying assumptions in the one-loop RGEs, such as neglecting flavour-changing effects.