Supersymmetric Phase Transitions and Gravitational Waves at LISA

Abstract

Gravitational waves generated during a first-order electroweak phase transition have a typical frequency which today falls just within the band of the planned space interferometer LISA. Contrary to what happens in the Standard Model, in its supersymmetric extensions the electroweak phase transition may be strongly first order, providing a mechanism for generating the observed baryon asymmetry in the Universe. We show that during the same transition the production of gravitational waves can be rather sizable. While the energy density in gravitational waves can reach at most $h_0^2 Ω_{\rm gw}\simeq 10^{-16}$ in the Minimal Supersymmetric Standard Model, in the Next-to-Minimal Supersymmetric Model, in some parameter range, $h_0^2 Ω_{\rm gw}$ can be as high as $4\times 10^{-11}$. A stochastic background of gravitational waves of this intensity is within the reach of the planned sensitivity of LISA. Since in the Standard Model the background of gravitational waves is totally neglegible, its detection would also provide a rather unexpected experimental signal of supersymmetry and a tool to descriminate among supersymmetric models with different Higgs content.

Figures (5)

• Figure 1: The bounds from nucleosynthesis (horizontal dashed lines, for $N_{\nu}=4$ and for $N_{\nu}=3.2$), from COBE and from ms pulsars, together with the sensitivity of ground based detectors and of LISA. See ref. [17] for details.
• Figure 2: $h_0^2\Omega_{\rm gw}$ as a function of the stop mass for a 110 GeV Higgs mass.
• Figure 3: $h_0^2\Omega_{\rm gw}$ as a function of the Higgs mass for a 155 GeV stop mass.
• Figure 4: $h_0^2\Omega_{\rm gw}$ as a function of $A_\lambda$ for $A_k=480$ GeV, $x=350$ GeV, $\lambda=0.83$, $k=0.67$ and $\tan \beta_{{\rm MSSM}}=2$
• Figure 5: $h_0^2\Omega_{\rm gw}$ as a function of $A_k$ for $A_\lambda=450$ GeV, $x=350$ GeV, $\lambda=0.83$, $k=0.67$ and $\tan \beta_{{\rm MSSM}} =2$