Cosmological Consequences of a Highly Decoupled Right-handed Neutrino
Author(s)
King, Stephen F., Roshan, Rishav, Show, Sudipta
Abstract
In the minimal type-I seesaw, one right-handed neutrino (RHN) can decouple from neutrino mass generation. Such a state is usually neglected, yet its feeble couplings make it long-lived, and it can dominate the energy density of the Universe before decaying. We study this in a minimal $U(1)_{B-L}$ model, where the RHNs are produced through $B-L$ gauge interactions, for the cases of a decoupled lightest RHN $N_1$ and a decoupled heaviest RHN $N_3$. The resulting early matter-dominated era dilutes the baryon asymmetry, and for decoupled $N_3$ it also delays the equilibration of the charged-lepton Yukawa interactions, allowing a lighter $N_1$ to generate the asymmetry in the unflavoured regime. Breaking $U(1)_{B-L}$ forms cosmic strings whose gravitational-wave background acquires a spectral break from the early matter-dominated era, with a frequency set by the decoupled RHN mass and the lightest neutrino mass, within reach of future detectors such as SKA, THEIA, LISA, $μ$ARES, DECIGO and ET.
Figures
Caption
Evolution of the various energy densities, $\rho_R$ (black) and $\rho_{N_1}$ (red) as a function of the temperature $T$.Caption
Evolution of the baryon asymmetry with the expansion of the Universe for a fixed value of parameters: $M_1=10^{8}~\rm{GeV},~M_2=10^{12}~\rm{GeV},~r=10,~a=0.48,~b=-0.9$ for three different values of $m_{l}$.Caption
Evolution of the various energy densities,$\rho_R$ (black) and $\rho_{N_3}$ (red), as a function of the temperature $T$.Caption
Variation of $\langle \Gamma_\alpha \rangle/\mathcal{H}$ vs. $T$ for standard (solid lines) and modified (dot-dashed and dashed) scenarios.Caption
Variation of $\langle \Gamma_\tau \rangle/\mathcal{H}$ vs. $T$ for standard (solid lines) and modified (dot-dashed and dashed) scenarios.Caption
Evolution of the baryon asymmetry with the expansion of the Universe for a fixed value of parameters: $M_3=10^{14}~\rm{GeV},~M_1=5\times10^{11}~\rm{GeV},~r=10,~a=0.12,~b=-0.1$ for three different values of $m_{l}$.Caption
The GW spectral shapes for local strings are shown with a period of eMD for a fixed value of parameters: $M_1=10^{8}~{\rm{GeV}},~v_{\rm BL}=10^{13}{\rm{GeV}}$ for three different values of $m_{l}$.Caption
The GW spectral shapes for local strings are shown with a period of eMD for a fixed value of parameters:$M_3=10^{14}~{\rm{GeV}},~v_{\rm BL}=10^{14}{\rm{GeV}}$ for three different values of $m_{l}$.Caption
Variation of $\langle \Gamma_\alpha \rangle/\mathcal{H}$ vs. $T$ for standard (solid lines) and modified (dot-dashed and dashed) scenarios.References
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