Introduction to Charged Lepton Flavor Violation

Abstract

Neutrino masses are evidence of lepton flavor violation, but no violation in the interactions among the charged leptons has been observed yet. Many models of Physics Beyond the Standard Model (BSM) predict Charged Lepton Flavor Violation (CLFV) in a wide spectrum of processes with rates in reach of upcoming experiments. The experimental searches that provide the current best limits on the CLFV searches are reviewed, with a particular emphasis on the muon-based experiments that give the most stringent constraints on the BSM parameter space. The next generation of muon-based experiments (MEG-II, Mu2e, COMET, Mu3e) aims to reach improvements by many orders of magnitude with respect to the current best limits, thanks to several technological advancements. We review popular heavy BSM theories, and we present the calculations of the predicted CLFV branching ratios, focusing on the more sensitive μ→e sector.

Theory introduction to Lepton Flavour Violation

“…Currently, non-observation of this process constrains the branching ratio to be 𝐵𝑟 (𝜇 → 𝑒𝛾) < 3.1 × 10 −13 (90% CL) [5], while future searches aim to reach branching ratio sensitivity of around 𝐵𝑟 (𝜇 → 𝑒𝛾) ∼ 6 × 10 −14 [6]. The LFV radiative muon decay is predicted by various well-motivated extensions of the SM, including supersymmetry and grand-unified theories, extended scalar or gauge sectors, neutrino mass models, and many others (see [1,3,4] and references therein for more details). Regardless of the specific model, the decay amplitude of this process can…”

“…Therefore, LFV processes provide a crucial probe of leptonic NP addressing both theoretical and experimental limitations of the SM. Several reviews on this topic are available in the literature [1][2][3][4].…”

Leptons and other forces of nature

“…Conservation of lepton flavour (LF) or lepton family number is a consequence of the U(1) e ⊗ U(1) μ ⊗ U(1) τ symmetry of the SM, which is an accidental global symmetry [67,68]. An accidental symmetry is not a fundamental symmetry of nature that has to be conserved at all energy scales, but it is a symmetry that happens to be conserved at a given energy scale where relevant symmetry breaking operators are not allowed by theory.…”

Charged Lepton-Flavour Violation

“…For a more detailed overview of NP models predicting CLFV effects, we refer the reader to Refs. [8,9].…”

“…Several excellent reviews exist in the area of CLFV, among them Refs. [8][9][10][11][12][13]. In this review, we focus on experimental aspects and the most recent results on CLFV.…”