CLFV

Charged Lepton Flavor Violation (CLFV) refers to processes in which the flavor of a charged lepton changes without the production of neutrinos, contrary to the expectations of the Standard Model (SM) in which such transitions are forbidden at tree level and highly suppressed by neutrino mixing. Typical CLFV signatures include the decay μ → eγ, the trilepton decay μ → eee, and coherent μ–e conversion in the field of a nucleus. In the SM extended with massive neutrinos, the branching ratios for these processes are proportional to (Δm²/ M_W²)² and are predicted to be of order 10⁻⁵⁰, far below experimental reach. Consequently, observation of CLFV would be a clear signal of physics beyond the SM.

Many extensions of the SM predict observable CLFV rates. Supersymmetric models with soft slepton mixing, models with heavy neutrinos or vector-like leptons, and theories with leptoquarks or extra gauge bosons all can enhance CLFV branching fractions to levels accessible to current experiments. Experimental searches have set stringent limits: the MEG experiment has placed an upper bound BR(μ → eγ) < 4.2 × 10⁻¹³, while the SINDRUM experiment limits BR(μ → eee) < 1.0 × 10⁻¹². Upcoming facilities such as Mu2e and COMET aim to improve the sensitivity to μ–e conversion by several orders of magnitude, and the Belle II experiment will probe τ lepton flavor violating decays such as τ → μγ and τ → 3μ. Continued progress in these searches is essential for probing the scale and nature of new physics.