Baryogenesis and gravitational waves in the Zee-Babu Model
To explain the matter-antimatter asymmetry in the Zee-Babu (ZB) model, the sphaleron process in the baryogenesis scenario is calculated. It always satisfies the de-coupling condition and the strength of phase transition ($S$) is always greater than $1$ in the presence of triggers other than that in the Standard Model, which are singly ($h^{\pm}$) and doubly ($k^{\pm\pm}$) charged scalar bosons. Sphaleron energies are in the range of 5-10 TeV, in calculation with bubble profiles containing free parameters and assuming nuclear bubbles of $h^{\pm}$ and $k^{\pm\pm}$ are very small. We tested the scaling law of sphaleron again with an average error of $10\%$. When the temperature is close to the critical one ($T_c$), the density of nuclear bubble is produced very large and decreases as the temperature decreases. The key parameter is $α$ which results in the gravitational wave density parameter ($Ωh^2$) in the range of $10^{-14}$ to $10^{-12}$ when $β/H^*=22.5$, this is not enough to detect gravitational waves from electroweak phase transition (EWPT) according to the present LISA data but may be detected in the future. As the larger strength of phase transition is, the more $α$ increases (this increase is almost linear with $S$), the larger the gravitational wave density parameter is. Also in the context of considering the generation of gravitational waves, in the ZB model we calculated $α\sim \text{a few} \times 10^{-2}\ll 1$, so rigorously conclude that the EWPT is not strong even though $S>1$. We also suggest that, for a model with a lot of extra scalar particles and particles which play a role in mass generation, the stronger the EWPT process and the larger $Ωh^2$ can be.