Neutrino masses and gravitational wave background
  We consider the Standard Model with three right-handed neutrinos to generate
tiny neutrino masses by the seesaw mechanism. Especially, we investigate the
case when one right-handed neutrino has the suppressed Yukawa coupling
constants. Such a particle has a long lifetime and can produce an additional
entropy by the decay. It is then discussed the impact of the entropy production
on the gravitational wave background originated in the primordial inflation. We
show that the mass and the coupling constants of the long-lived right-handed
neutrino can be probed by the distortion of the gravitational wave spectrum,
leading to the information of the mass of the lightest active neutrino.
