Constraining models for keV sterile neutrinos by quasi-degenerate active neutrinos

Understand

We present a No-Go theorem for keV sterile neutrino Dark Matter: if sterile neutrinos at the keV scale play the role of Dark Matter, they are typically unstable and their decay produces an astrophysical monoenergetic X-ray line.

• It turns out that the observational bound on this line is so strong that it contradicts the existence of a quasi-degenerate spectrum of active neutrinos in a seesaw type I framework where the Casas-Ibarra matrix R is real.
• This is the case in particular for models without CP violation.
• We give a general proof of this theorem.