Charged Lepton Flavour Violation: An Experimental and Theoretical Introduction (1709.00294v2)

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 $\mu \to e\gamma$ decay. In the next decade several experiments are planned to pursue the search for $\mu \to e\gamma$, $\mu \to eee$, $\mu \to e$ conversion in nuclei, as well as on processes involving the $\tau$, 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.

• The paper presents a comprehensive review of CLFV, focusing on muon and tau decay channels as probes for new physics.
• It details how SM extensions, such as the Two-Higgs-Doublet Model and SUSY, may induce observable lepton flavor violation via effective field theory operators.
• The study highlights advanced experimental techniques, emphasizing sensitivity improvements in experiments like MEG, Mu2e, and Belle II.

An Examination of Charged Lepton Flavour Violation: An Experimental and Theoretical Introduction

The paper of Charged Lepton Flavour Violation (CLFV) presents a promising avenue for probing new physics beyond the Standard Model (SM). The paper by Lorenzo Calibbi and Giovanni Signorelli offers a comprehensive review of experimental and theoretical approaches to CLFV, focusing particularly on processes involving muons and taus. This work is pivotal for addressing the apparent lack of observed CLFV, despite its theoretical plausibility in various SM extensions.

Theoretical Aspects of CLFV

In SM, the conservation of lepton flavor is a direct consequence of the model's structure, particularly the minimal particle content and specific Yukawa coupling interactions. Introducing additional particles or interactions, such as right-handed neutrinos or a second Higgs doublet, often leads to lepton flavor violation. In this context, the paper highlights several extensions like the Two-Higgs-Doublet Model and SUSY models where CLFV could occur at detectable levels.

The introduction of massive neutrinos, as confirmed by neutrino oscillation experiments, signals lepton flavor non-conservation. Despite the tiny neutrino masses suggesting negligible CLFV rates in the SM framework, their existence nudges theoretical exploration toward models suggesting new CLFV mechanisms. One notable mechanism is the seesaw model that explains small neutrino masses and potentially significant CLFV through heavy right-handed neutrinos, known to induce observable effects under specific parameter regimes.

The paper extends this discussion by integrating various model-independent approaches using effective field theories. By considering dimension-6 operators that violate lepton flavor, the authors present the potential impact of new physics at higher energy scales. This examination forms a key framework for understanding the constraints on new interactions using current experimental bounds.

Experimental Pursuits in CLFV

On the experimental frontier, muon decays such as $\mu \to e \gamma$, $\mu \to eee$, and $\mu \to e$ conversion in nuclei are under intense investigation. These processes, due to their simple kinematics and clear signals, offer a unique opportunity to paper deviations from the SM predictions. The MEG experiment, for example, has produced remarkable sensitivity limits on the $\mu \to e \gamma$ process, pushing constraints on CLFV processes significantly.

The paper outlines the detailed nature of experimental setups for these decay modes, describing the importance of resolving accidental backgrounds, including processes like internal conversion decays that could mimic CLFV signals. Enhancing experimental sensitivity often involves innovative detector technologies and higher-intensity muon beams, aims pursued by facilities like the Paul Scherrer Institute and the upcoming experiments at Fermilab (Mu2e) and J-PARC (COMET).

Prospects and Implications

In the tau sector, potential CLFV decays such as $\tau \to \mu \gamma$ and $\tau \to e \gamma$ are scrutinized for deviations in flavor universality, supported by data from B-factories like Belle and BaBar. The future holds much promise with the Belle II experiment, which is set to enhance current sensitivity limits and thus provide deeper insights into tau decay channels.

The implications of detecting CLFV are profound, extending beyond particle physics into cosmological considerations such as baryogenesis via leptogenesis, relying on CP violation and non-zero net lepton number produced in the early universe. A discovery in this field could illuminate the path for understanding dark matter candidates influenced by exotic particles in extended models.

As experimental sensitivities approach the anticipated theoretical predictions for various extensions of the SM, the research into CLFV stands at the cusp of potentially groundbreaking insights. Consequently, fostering collaborations between theoretical modeling, experimental endeavors, and technological innovations remains crucial in this expansive, investigative space within particle physics.