Distortions of CMB temperature and polarization maps caused by gravitational lensing , observable with high angular resolution and sensitivity , can be used to constrain the sterile neutrino mass , m _ { s } , from CMB data alone .
We forecast m _ { s } > 1.75 keV from Planck and m _ { s } > 4.97 keV from Inflation Probe at 95 % CL , by using the CMB weak lensing extraction .