Non-zero $\theta_{13}$ with Unbroken $\mu-\tau$ Symmetry of the Active   Neutrino Mass Matrix in the Presence of a Light Sterile Neutrino
  We revisit the possibility of generating non-zero reactor mixing angle in a scenario where there is a sterile neutrino at the eV scale apart from the usual three sub-eV scale active neutrinos. We show that the $3\times3$ active neutrino mass matrix can possess a $\mu-\tau$ symmetry and can still be consistent with non-zero value of the reactor mixing angle $\theta_{13}$, if this $\mu-\tau$ symmetry is broken in the sterile neutrino sector. We first propose a simple model based on the discrete flavour symmetry $A_4 \times Z_3 \times Z^{\prime}_3$ to realise such a scenario and then numerically evaluate the complete $3+1$ neutrino parameter space that allows such a possibility. We show that, such a possibility of generating non-zero $\theta_{13}$ can in general, remain valid even if the present $3+1$ neutrino global fit data get ruled out by future experiments. We also discuss the possible implications at neutrinoless double beta decay $(0\nu \beta \beta)$ experiments in view of the latest results from KamLAND-Zen experiment. 
Debasish Borah
hep-ph