Ultralight scalars in leptonic observables

TL;DR

This paper investigates how ultralight scalars like axions and majorons influence low-energy leptonic processes, providing model-independent constraints and exploring their effects on leptonic observables and moments.

Contribution

It offers a comprehensive, model-independent analysis of ultralight scalars' effects on leptonic observables, including production, mediation, and magnetic/electric moments.

Findings

Current limits on scalar-lepton couplings are established.

Ultralight scalars can be produced in leptonic decays such as μ → e φ.

Scalars can mediate processes like τ → μ μ μ, affecting decay rates.

Abstract

Many new physics scenarios contain ultralight scalars, states which are either exactly massless or much lighter than any other massive particle in the model. Axions and majorons constitute well-motivated examples of this type of particle. In this work, we explore the phenomenology of these states in low-energy leptonic observables. After adopting a model independent approach that includes both scalar and pseudoscalar interactions, we briefly discuss the current limits on the diagonal couplings to charged leptons and consider processes in which the ultralight scalar $\phi$ is directly produced, such as $\mu \to e \, \phi$, or acts as a mediator, as in $\tau \to \mu \mu \mu$. Contributions to the charged leptons magnetic and electric moments are studied as well.