Lepton Flavor and Number Conservation, and Physics Beyond the Standard Model (1303.4097v2)

Abstract: The physics responsible for neutrino masses and lepton mixing remains unknown. More experimental data are needed to constrain and guide possible generalizations of the standard model of particle physics, and reveal the mechanism behind nonzero neutrino masses. Here, the physics associated with searches for the violation of lepton-flavor conservation in charged-lepton processes and the violation of lepton-number conservation in nuclear physics processes is summarized. In the first part, several aspects of charged-lepton flavor violation are discussed, especially its sensitivity to new particles and interactions beyond the standard model of particle physics. The discussion concentrates mostly on rare processes involving muons and electrons. In the second part, the status of the conservation of total lepton number is discussed. The discussion here concentrates on current and future probes of this apparent law of Nature via searches for neutrinoless double beta decay, which is also the most sensitive probe of the potential Majorana nature of neutrinos.

Overview of Lepton Flavor and Number Conservation and Its Implications Beyond the Standard Model

This paper, authored by André de Gouvêa and Petr Vogel, addresses the intricate physics concerning neutrino masses and lepton mixing, exploring aspects that point to potential physics phenomena beyond the established Standard Model. The focus is on two primary areas: the violation of lepton-flavor conservation in charged-lepton processes and the breakdown of lepton-number conservation via nuclear processes. The discourse in this paper is divided into two major sections, which together construct a comprehensive assessment of these topics.

Charged-Lepton Flavor Violation (CLFV)

The paper explores the implications of charged-lepton flavor violation, emphasizing processes predominantly involving muons and electrons. Notably, it highlights rare processes such as muon decay modes, including $\mu^+\to e^+\gamma$, $\mu^+\to e^+e^-e^+$ decays, and $\mu^-\to e^-$ conversion in nuclei.

Current experimental bounds are tight, with the MEG experiment setting a limit on $\mu^+\to e^+\gamma$ decay branching ratios to below $2.4\times 10^{-12}$. These limits are expected to strengthen as new experimental results are anticipated, potentially probing even further. The paper articulates these experiments' difficulties in surpassing sensitivities of $10^{-14}$ due to dominant accidental backgrounds, suggesting that improving the experimental design could theoretically achieve sensitivities up to $10^{-18}$.

The concept of how CLFV rates could illuminate underlying new physics, particularly by comparing the results of different CLFV observables, is underscored. An important aspect of the paper is the comparative paper of theoretical expectations versus experimental outcomes within models such as the seesaw mechanism, illustrating the nuanced relationships and bounds that these frameworks impose on CLFV.

Lepton Number Violation (LNV)

In the second part, the paper scrutinizes the status of total lepton-number conservation, focusing on neutrinoless double beta decay. Such decays are pivotal as they serve as the most sensitive probe to potential Majorana characteristics of neutrinos, offering profound insights into the nature of neutrino masses.

The analysis includes a historical overview of experimental searches and their progressively improved sensitivities to the effective neutrino Majorana mass, reaching as low as $<m_{\beta\beta}> < 0.11$ eV in recent experiments such as EXO-200 and KamLAND-Zen. The elucidation of the nuclear matrix elements and their complexities across different nuclei and models reinforces the challenges these experiments confront.

The paper discusses the multi-faceted testing methodologies for LNV, emphasizing the critical significance of these experiments in validating or invalidating the Majorana hypothesis for neutrinos, and asserts the necessity of diversified experimental approaches to substantiate the claims.

Implications and Prospects

The implications extrapolated from this paper are significant for the field of particle physics, as they potentially signal new physics beyond the Standard Model. The constraints on CLFV processes are indicative not only of the boundaries of current theories but point towards a richer understanding of the neutrino sector and the fundamental symmetries at play.

The prospect of discovering new particles and interactions via these avenues has the potential to reshape our understanding of particle physics, pushing towards a "New Standard Model" incorporating neutrino masses and their nature. This paper invites further exploration and experimental scrutiny in the pursuit of uncovering these elusive interactions and validates the necessity of such investigations in advancing the field.