A near-minimal leptoquark model for reconciling flavour anomalies and generating radiative neutrino masses

Innes Bigaran,Raymond R Volkas,John Gargalionis

doi:10.1007/jhep10(2019)106

Abstract

We introduce two scalar leptoquarks, the SU(2)L isosinglet denoted ϕ ∼ (3,1, −1/3) and the isotriplet φ ∼ (3,3, −1/3), to explain observed deviations from the standard model in semi-leptonic B-meson decays. We explore the regions of parameter space in which this model accommodates the persistent tensions in the decay observables RD(∗), RK (∗) , and angular observables in b → sμμ transitions. Additionally, we exploit the role of these exotics in existing models for one-loop neutrino mass generation derived from ∆L = 2 effective operators. Introducing the vector-like quark χ ∼ (3,2, −5/6) necessary for lepton-number violation, we consider the contribution of both leptoquarks to the generation of radiative neutrino mass. We find that constraints permit simultaneously accommodating the flavour anomalies while also explaining the relative smallness of neutrino mass without the need for cancellation between leptoquark contributions. A characteristic prediction of our model is a rate of muon-electron conversion in nuclei fixed by the anoma- lies in b → sμμ and neutrino mass; the COMET and Mu2e experiments will thus test and potentially falsify our scenario. The model also predicts signatures that will be tested at the LHC and Belle II.

Highlights

The detection of neutrino oscillations establishes that neutrinos have small, but nonzero, masses and that the flavour and mass eigenstates do not coincide
16Although COMET and Mu2e will measure muon-electron conversion in Aluminium, we display the result on the same plot since we find that the calculations in Gold and Aluminium differ by less than an order of magnitude
Combining two existing completions of D7 ∆L = 2 effective operators, this model consists of two scalar LQs, φ ∼ (3, 3, −1/3) and φ ∼ (3, 1, −1/3), and the vector-like quark doublet χ ∼ (3, 2, −5/6)

Summary

Introduction

The detection of neutrino oscillations establishes that neutrinos have small, but nonzero, masses and that the flavour and mass eigenstates do not coincide. The dynamical origin of the tiny scale of neutrino masses remains a mystery, as does any potential impact on the flavour structure of the Standard Model (SM). A plausible explanation for the lightness of neutrinos is to not explicitly introduce a tree-level mass term, but rather to engineer the generation of mass at loop-level: radiative models. We restrict our attention to models that induce a Majorana mass term and violate lepton-number by two units (∆L = 2). The magnitudes of these masses are naturally loop-suppressed.. The magnitudes of these masses are naturally loop-suppressed.1 As such, this mechanism gives a neat explanation for the disparity between the sizes of neutrino masses relative to those of other SM fermions The magnitudes of these masses are naturally loop-suppressed. As such, this mechanism gives a neat explanation for the disparity between the sizes of neutrino masses relative to those of other SM fermions

Objectives

Results

Conclusion