Minimal active-sterile neutrino mixing in seesaw type I mechanism with sterile neutrinos at GeV scale

Renewed interest in GeV-scale sterile neutrinos capable of explaining active neutrino oscillations via a seesaw type I mechanism has been expressed in several proposals of direct searches. Given this activity we estimate the minimal values of sterile-active mixing angles provided one, two, or three sterile neutrinos are lighter than the $D$ meson.

Figure 1

The minimal values of mixings for $M_1=M_2=500\mathrm{MeV}$ and (a) normal hierarchy, (b) inverted hierarchy of active neutrino masses. The different lines correspond to different values of the lightest active neutrino mass $m_{\text{lightest}}$. On both plots the dashed line refers to the mixing in $\nu \text{MSM}$ [3, 16] as explained in the main text.

Figure 2

The regions below the lines will be excluded by an experiment achieving the sensitivity to mixing parameters of $U_c^2=2.5\times 10^{-11}$ for (a) normal hierarchy and (b) inverted hierarchy of active neutrino masses. The different lines correspond to different values of the lightest active neutrino mass $m_{\text{lightest}}$.