Updated constraints on sterile neutrino mixing in the OPERA experiment using a new ${\nu }_e$ identification method

This paper describes a new ${\nu }_e$ identification method specifically designed to improve the low-energy ($<30\mathrm{G}\mathrm{e}\mathrm{V}$) ${\nu }_e$ identification efficiency attained by enlarging the emulsion film scanning volume with the next generation emulsion readout system. A relative increase of 25-70% in the ${\nu }_e$ low-energy region is expected, leading to improvements in the OPERA sensitivity to neutrino oscillations in the framework of the 3 + 1 model. The method is applied to a subset of data where the detection efficiency increase is expected to be more relevant, and one additional ${\nu }_e$ candidate is found. The analysis combined with the ${\nu }_{\tau }$ appearance results improves the upper limit on ${\sin }^{2}2{\theta }_{\mu e}$ to 0.016 at 90% C.L. in the MiniBooNE allowed region $\mathrm{\Delta }{m}_{41}^2\sim 0.3{\mathrm{e}\mathrm{V}}^2$.