# Naturally Light Dirac Neutrino in Left-Right Symmetric Model

## TL;DR

This paper proposes a model where tiny Dirac neutrino masses are generated via a one-loop process in a Left-Right Symmetric Model without extra symmetries, linking neutrino mass, dark matter, and experimental constraints.

## Contribution

It introduces a novel mechanism for Dirac neutrino mass generation in LRSM using vector-like fermions without additional symmetries, and explores its implications for dark matter and experimental bounds.

## Key findings

- Right sector gauge bosons must be heavier than 3.548 TeV.
- Dark matter candidate masses are constrained by Planck data.
- Model predicts potential signals in charged lepton flavor violation.

## Abstract

We study the possibility of generating tiny Dirac masses of neutrinos in Left-Right Symmetric Model (LRSM) without requiring the existence of any additional symmetries. The charged fermions acquire masses through a universal seesaw mechanism due to the presence of additional vector like fermions. The neutrinos acquire a one-loop Dirac mass from the same additional vector like charged leptons without requiring any additional discrete symmetries. The model can also be extended by an additional $Z_2$ symmetry in order to have a scotogenic version of this scenario predicting a stable dark matter candidate. We show that the latest Planck upper bound on the effective number of relativistic degrees of freedom $N_{\text{eff}}=3.15 \pm 0.23$ tightly constrains the right sector gauge boson masses to be heavier than 3.548 TeV. This bound on gauge boson mass also affects the allowed values of right scalar doublet dark matter mass from the requirement of satisfying the Planck bound on dark matter relic abundance. We also discuss the possible implications of such a scenario in charged lepton flavour violation and generating observable electric dipole moment of leptons.