Completeness and complementarity for μ → eγ, μ → e e ¯ e $$ e\overline{e}e $$ and μA → eA

• Main Author: S. Davidson
• Format: Article
• Language: English
• Published: SpringerOpen 2021-02-01
• Series: Journal of High Energy Physics
• Subjects: Beyond Standard Model; Effective Field Theories
• Online Access: https://doi.org/10.1007/JHEP02(2021)172

Abstract Lepton Flavour Violation (LFV) is New Physics that must occur, but is stringently constrained by experiments searching for μ ↔ e flavour change, such as μ → eγ, μ → e e ¯ e $$ e\overline{e}e $$ or μ → e conversion. However, in an Effective Field Theory(EFT) parametrisation, there are many more μ ↔ e operators than restrictive constraints, so determining operator coefficients from data is a remote dream. It is nonetheless interesting to learn about New Physics from data, so this manuscript introduces “observable-vectors” in the space of operator coefficients, which identify at any scale the combination of coefficients probed by the observable. These vectors have an overlap ≳ 10 −3 with most of the coefficients, and are used to study whether μ → eγ, μ → e e ¯ e $$ e\overline{e}e $$ and μ → e conversion give complementary information about New Physics. The appendix gives updated sensitivities of these processes, (and a subset of τ → ℓ decays), to operator coefficients at the weak scale in the SMEFT and in the EFT below m W .