Testing the Seesaw Mechanism 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 nonperturbative couplings, requires right handed 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.

Figure 1

The predicted GW background from cosmic strings for different symmetry breaking scales, assuming the particle production is subdominant. For comparison, we also display the sensitivity of current (solid) and future (dashed) experiments (from left to right) of the Square Kilometre Array (SKA), NANOGRAV (NANO), Laser Interferometer Space Antenna (LISA), Big Bang Observer (BBO), DECi-hertz Interferometer Gravitational wave Observatory (DECIGO), Einstein Telescope (ET), Cosmic Explorer (CE), and Laser Interferometer Gravitational-Wave Observatory (LIGO). Here, we made an approximation for the string tension $\mu =v^2$ where $v$ is the symmetry breaking scale.