Modular $A_4$ symmetry in 3+1 active-sterile neutrino masses and mixings
Motivated by the significance of modular symmetry in generating neutrino masses and flavor mixings, we apply the modular $A_4$ symmetry in a 3+1 scheme of active-sterile neutrino mixings. Neutrino oscillation observables in the 3$σ$ range are successfully reproduced through the vacuum expectation value of the modulus $τ$. We also study phenomenologies related to the effective neutrino masses $m_β$ in tritium beta decay and $m_{ββ}$ in neutrinoless double beta decay. Mixings between active neutrinos and eV scale sterile neutrino are analyzed in detail. The model also predicts the Dirac CP-violating phase $δ_{CP}$ and Majonara phases $α$ and $β$. The best-fit values of the neutrino mixing angles and ratios of two mass-squared differences are determined using minimum $χ^2$ analysis. The best-fit values of the neutrino oscillation observables are predicted as $\sin^2θ_{23}=0.573,$ $\sin^2θ_{12}=0.300,$ $ \sin^2θ_{13}=0.022$ and $r = 0.172$ for NH while $\sin^2θ_{23}=0.550,$ $\sin^2θ_{12}=0.303,$ $ \sin^2θ_{13}=0.022$ and $r = 0.171$ for IH. We also observe that the predictions of effective neutrino mass parameters in the 3+1 scheme are significantly different from the three neutrino paradigm.