[Paper Review] Charged Lepton Flavour Violation: An Experimental and Theoretical Introduction

Lorenzo Calibbi, G. Signorelli|arXiv (Cornell University)|Aug 31, 2017

Particle physics theoretical and experimental studies360 references83 citations

TL;DR

This review surveys theoretical motivations and experimental searches for charged lepton flavour violation (CLFV), detailing SM extensions, EFT approaches, SUSY, and muon/tau experiments, with MEG limits and future prospects.

ABSTRACT

Charged lepton flavour violating transitions would be a clear signal of new physics beyond the Standard Model. Their search has been carried out in a variety of channels, the most sensitive being those involving a muon: however no positive evidence has been found so far. The MEG experiment has recently set the best limit on such processes by investigating the existence of the $μ o eγ$ decay. In the next decade several experiments are planned to pursue the search for $μ o eγ$, $μ o eee$, $μ o e$ conversion in nuclei, as well as on processes involving the $τ$, to an unprecedented level of precision. In this review we want to give a pedagogical introduction on the theoretical motivations for such searches as well as on the experimental aspects upon which they are based.

Motivation & Objective

• Motivate CLFV searches as probes of new physics beyond the Standard Model.
• Explain how lepton flavour conservation is accidental in the SM and how it can be violated in extensions.
• Present model-independent EFT approaches to CLFV and discuss specific models (notably SUSY).
• Outline experimental strategies and key current limits for muon and tau CLFV channels.

Proposed method

• Review the lepton sector of the SM and its extensions to identify sources of CLFV.
• Discuss neutrino masses and PMNS mixing as a source of loop-induced CLFV in the SM with massive neutrinos.
• Introduce non-renormalisable effective operators as a model-independent CLFV framework.
• Discuss SUSY-based realizations of CLFV, including simplified models and seesaw scenarios.
• Examine Higgs-mediated CLFV and the constraints on off-diagonal Higgs couplings from experiments.

Experimental results

Research questions

• RQ1 What theoretical mechanisms can generate observable charged lepton flavour violation beyond the Standard Model?
• RQ2 How do current and planned experiments constrain CLFV across muon, tau, and heavy boson channels?
• RQ3 What is the role of neutrino masses and mixing in driving (or suppressing) CLFV signals in various models?
• RQ4 How do Higgs-sector and SUSY-sector contributions compare in setting CLFV limits?

Key findings

• The MEG experiment sets the best current limit on mu+ -> e+ gamma with BR < 4.2 x 10^-13 (90% C.L., 2016).
• Existing limits also constrain mu+ -> e+ e- e+ (BR < 1.0 x 10^-12, 90% C.L., SINDRUM, 1988) and mu- Ti -> e- Ti (BR < 6.1 x 10^-13, 90% C.L., SINDRUM II, 1998).
• In the SM with massive neutrinos, CLFV in charged currents is present only at loop level and is highly suppressed; observable CLFV points to new physics (e.g., SUSY or extended Higgs sectors).
• Two-Higgs-doublet models and beyond-SM Higgs sectors can mediate CLFV at tree level or via loops, leading to strong bounds on off-diagonal Higgs couplings (|Y^h_{mu e}|, |Y^h_{e mu}|) from muon processes and LHC Higgs decays.
• The review connects CLFV to lepton flavour universality studies and outlines current and future experiments targeting muon channels (mu -> e gamma, mu -> e e e, mu -> e conversion) and tau channels.