Ultralight scalars in leptonic observables

Self Cite

We propose a simple model of spontaneous lepton number violation with potentially large flavor violating decays, including the possibility that majoron emitting decays, such as μ → eJ, saturate the experimental bounds. In this model the majoron is a singlet-doublet admixture. It generates a type-I seesaw for neutrino masses and contains also a vector-like lepton. As a by-product, the model can explain the anomalous (g − 2)μ in parts of its parameter space, where one expects that the branching ratio of the Higgs to muons is changed with respect to Standard Model expectations. However, the explanation of the muon g − 2 anomaly would lead to tension with recent astrophysical bounds on the majoron coupling to muons.

Section: Jhep01(2022)098mentioning

confidence: 74%

Section: Jhep01(2022)098mentioning

confidence: 94%

“…The interaction of the majoron with charged leptons can be described in a model independent way as [7],…”

Section: Jhep01(2022)098mentioning

confidence: 99%

Section: Phenomenology Of the Modelmentioning

confidence: 99%

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Observable flavor violation from spontaneous lepton number breaking

Escribano

Hirsch

Nava

et al. 2022

Self Cite

“…But even if these are absent, flavour-changing neutral currents (FCNCs) appear at one loop through the diagrams depicted in figure 1. Rare decays induced by such transitions have been exploited in the literature to test the axion paradigm [59,60,63,64,[69][70][71], and lepton-flavour violation has been studied in similar detail [109,110].…”

Section: Jhep07(2021)059mentioning

confidence: 99%

Leading logs in QCD axion effective field theory

Alonso-Álvarez

Ertas

Jaeckel

et al. 2021

The axion is much lighter than all other degrees of freedom introduced by the Peccei-Quinn mechanism to solve the strong CP problem. It is therefore natural to use an effective field theory (EFT) to describe its interactions. Loop processes calculated in the EFT may however explicitly depend on the ultraviolet cutoff. In general, the UV cutoff is not uniquely defined, but the dimensionful couplings suggest to identify it with the Peccei-Quinn symmetry-breaking scale. An example are K+ → π+ + a decays that will soon be tested to improved precision in NA62 and KOTO and whose amplitude is dominated by the term logarithmically dependent on the cutoff. In this paper, we critically examine the adequacy of using such a naive EFT approach to study loop processes by comparing EFT calculations with ones performed in complete QCD axion models. In DFSZ models, for example, the cutoff is found to be set by additional Higgs degrees of freedom and to therefore be much closer to the electroweak scale than to the Peccei-Quinn scale. In fact, there are non-trivial requirements on axion models where the cutoff scale of loop processes is close to the Peccei-Quinn scale, such that the naive EFT result is reproduced. This suggests that the existence of a suitable UV embedding may impose restrictions on axion EFTs. We provide an explicit construction of a model with suitable fermion couplings and find promising prospects for NA62 and IAXO.

The majoron coupling to charged leptons

Herrero-Brocal,

Vicente

2024

The particle spectrum of all Majorana neutrino mass models with spontaneous violation of global lepton number include a Goldstone boson, the so-called majoron. The presence of this massless pseudoscalar changes the phenomenology dramatically. In this work we derive general analytical expressions for the 1-loop coupling of the majoron to charged leptons. These can be applied to any model featuring a majoron that have a clear hierarchy of energy scales, required for an expansion in powers of the low-energy scale to be valid. We show how to use our general results by applying them to some example models, finding full agreement with previous results in several popular scenarios and deriving novel ones in other setups.