[Paper Review] Lepton number and lepton flavour violation in left-right symmetric theories

James Barry, Werner Rodejohann|arXiv (Cornell University)|Mar 25, 2013

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper investigates neutrinoless double beta decay in left-right symmetric models, analyzing contributions from lambda- and eta-diagrams when type I seesaw dominates at the TeV scale. It identifies parameter regions where these diagrams significantly enhance the decay half-life, while rigorously constraining them via lepton flavour violation observables.

ABSTRACT

The various diagrams leading to neutrinoless double beta decay in the left-right symmetric model have different relative magnitudes, depending on the scale of new physics. Neutrinos acquire mass from both type I and/or type II seesaw terms, making an unambiguous analysis difficult. We study the half-life for double beta decay in the case of type II and type I dominance, in the former case including interference terms. If the heavy neutrinos of the type I seesaw model are at the TeV scale, certain processes can be enhanced. In particular, there are regions of parameter space in which the so-called lambda- and eta-diagrams can give sizable contributions to the half-life for the decay. We perform a detailed study of one such scenario, paying careful attention to constraints from lepton flavour violation.

Motivation & Objective

• To analyze the relative contributions of various Feynman diagrams—particularly lambda- and eta-diagrams—to neutrinoless double beta decay in left-right symmetric theories.
• To investigate the impact of type I seesaw dominance at the TeV scale on the half-life of neutrinoless double beta decay.
• To examine interference effects between type I and type II seesaw mechanisms in the context of lepton number violation.
• To constrain the model parameters using experimental limits on lepton flavour violation processes such as μ→eγ.
• To identify viable parameter regions where lambda- and eta-diagrams yield significant contributions to the decay rate

Proposed method

• Constructing the effective Hamiltonian for neutrinoless double beta decay in the left-right symmetric model with both type I and type II seesaw contributions.
• Evaluating the contributions of lambda- and eta-diagrams to the decay amplitude, including interference terms between type I and type II seesaw mechanisms.
• Applying the seesaw mechanism to compute the light neutrino mass matrix, assuming type I seesaw dominance with heavy Majorana neutrinos at the TeV scale.
• Using the GIM mechanism and GIM-like suppression to estimate the size of lepton flavour violating processes such as μ→eγ.
• Applying stringent bounds from lepton flavour violation to constrain the model parameters, especially the mixing between light and heavy neutrinos.
• Performing a numerical scan over the parameter space to identify regions where lambda- and eta-diagrams dominate the decay amplitude

Experimental results

Research questions

• RQ1 What is the relative contribution of lambda- and eta-diagrams to the half-life of neutrinoless double beta decay in the left-right symmetric model?
• RQ2 How do interference terms between type I and type II seesaw mechanisms affect the decay amplitude and half-life?
• RQ3 In what parameter space regions do lambda- and eta-diagrams dominate the decay amplitude when heavy neutrinos are at the TeV scale?
• RQ4 How do experimental limits on lepton flavour violation, such as μ→eγ, constrain the model parameters in scenarios with TeV-scale type I seesaw?
• RQ5 Can the half-life for neutrinoless double beta decay be significantly enhanced by lambda- and eta-diagrams under realistic constraints from lepton flavour violation?

Key findings

• Lambda- and eta-diagrams can yield sizable contributions to the half-life of neutrinoless double beta decay when type I seesaw dominates at the TeV scale.
• In certain parameter regions, the interference between type I and type II seesaw terms enhances the decay amplitude, leading to a reduced half-life.
• The half-life can be significantly shorter than in the standard type II seesaw scenario due to constructive interference and enhanced diagram contributions.
• Constraints from lepton flavour violation processes, particularly μ→eγ, strongly limit the allowed mixing between light and heavy neutrinos, reducing the viable parameter space.
• Despite these constraints, there exist regions of parameter space where lambda- and eta-diagrams dominate and lead to observable half-lives within the reach of next-generation double beta decay experiments.
• The model remains viable under current experimental bounds, provided the heavy neutrino mass scale is near the TeV scale and mixing is sufficiently suppressed