A simplest modular $S_3$ model for leptons

Abstract: We present minimalist constructions for lepton masses and mixing based on flavour symmetry under the modular group $\Gamma_N$ of lowest level $N=2$. As opposed to the only existing model of $\Gamma_2\cong S_3$ formulated in a SUSY framework, the only non-SM field is the modulus $\tau$, and a generalised CP symmetry is implemented. Charged-leptons masses are reproduced through symmetry arguments, without requiring fine-tuning of the free parameters. As a result, all lepton observables (masses and mixing) are reproduced within $1\sigma$ experimental range using a minimum of nine free real parameters (including the real and imaginary parts of the modulus). A normal ordering for the neutrino masses is predicted. We also obtain predictions for the CP violating phases: the Dirac CP phase is predicted around $1.6\pi$, the Majorana phases lie in narrow regions near $\pm \pi$. The sum of neutrino masses is within the current bound at $\sim 0.09\,\text{eV}$. Furthermore, we provide predictions for the neutrinoless double beta decay and tritium decay effective masses, around $20\,\text{meV}$. Given the reduced number of free input parameters as compared to the existing literature on modular $S_3$, this work renews interest for a unified predictive model of quark-lepton sectors based on $\Gamma_2\cong S_3$.