Gravitational waves from seesaw assisted collapsing domain walls

TL;DR

This work links gravitational waves from domain-wall annihilation to the type-I seesaw scale by introducing a minimal model with a Z2-odd scalar φ and two heavy RHNs. Radiative corrections from RHN-φ couplings generate a Z2-breaking bias that triggers domain-wall decay, producing a stochastic GW spectrum whose peak properties depend on the seesaw parameters and φ’s vacuum expectation value. The predicted GW signals can fall within the reach of current and future GW detectors and CMB measurements of N_eff, offering an indirect probe of the seesaw scale. Additionally, the framework allows tiny RHN mass splittings to arise naturally, enabling resonant leptogenesis and the observed baryon asymmetry in UV-complete scenarios, thus connecting neutrino mass generation, baryogenesis, and gravitational-wave phenomenology in a testable way.

Abstract

Spontaneous breaking of discrete symmetries like $Z_2$ leads to the formation of stable topological defects such as domain walls which, if allowed to dominate, can potentially be in conflict with cosmological observations. Incorporating explicit $Z_2$-breaking bias terms can lead to annihilation of such walls while also emitting stochastic gravitational wave (GW). We study the role of heavy right-handed neutrinos present in type-I seesaw origin of light neutrino masses to generate such bias term via quantum corrections. This offers interesting correlation among the seesaw scale, GW peak amplitude and peak frequency which can be probed at present and future experiments related to GW as well as precision measurements of the cosmic microwave background (CMB). In flavor symmetric UV complete scenarios with degenerate RHNs at leading order, such tiny coupling of RHNs to a $Z_2$-odd scalar can also lead to small mass splittings suitable for explaining the observed baryon asymmetry of the universe via resonant leptogenesis.

Figures (3)

• Figure 1: GW spectrum with $M_1$=500 GeV and $y v_\phi\sim 1$ keV (top left), $v_\phi=4 \times 10^6$ GeV and $yv_\phi=1$ keV (top right), $M_1$=500 GeV and $v_\phi=3 \times 10^6$ GeV (bottom left), different combinations of $M_1, v_\phi$ and fixed $y v_\phi\sim 1$ keV (bottom right). The shaded regions correspond to future GW experiments' sensitivities with orange colored data points correspond to NANOGrav 2023 data. The horizontal black dashed line correspond to upper limit on GW contribution to dark radiation or $\Delta N_{\rm eff}$.
• Figure 2: Parameter space in $M_1$-$v_\phi$ plane, with $y v_\phi =1$ keV (left), $y v_\phi =1$ MeV (middle) and $y v_\phi =1$ GeV (right). The white region towards the right of the solid purple contour remains allowed part of which can be probed at future CMB and GW experiments.
• Figure 3: Evolution of comoving number densities for $m_{N_1} = 500$ GeV and $m_{N_2}-m_{N_1}= y v_\phi =1$ keV. The complex angle in CI parametrisation is chosen as $z_1=0.25+0.15i$.