Testing Seesaw and Leptogenesis with Gravitational Waves
  We present the possibility that the seesaw mechanism with thermal
leptogenesis can be tested using the stochastic gravitational background.
Achieving neutrino masses consistent with atmospheric and solar neutrino data,
while avoiding non-perturbative couplings, requires right-neutrinos lighter
than the typical scale of grand unification. This scale separation suggests a
symmetry protecting the right handed neutrinos from getting a mass. Thermal
leptogenesis would then require that such a symmetry be broken below the
reheating temperature. We enumerate all such possible symmetries consistent
with these minimal assumptions and their corresponding defects, finding that in
many cases, gravitational waves from the network of cosmic strings should be
detectable. Estimating the predicted gravitational wave background we find that
future space-borne missions could probe the entire range relevant for thermal
leptogenesis.
