Gravitational waves from the high temperature electroweak symmetry nonrestoration

The high temperature electroweak symmetry nonrestoration (SNR) opens a window to realize baryogenesis at high energy scales, which can alleviate the tight phenomenological constraints. An effective way to realize the electroweak SNR is to extend the Standard Model (SM) by adding a second Higgs doublet and a few new singlet fermions which interact with the second Higgs through a five-dimension operator. In this scenario, the second Higgs has a nonvanishing vacuum expectation value above the electroweak scale and thus breaks the electroweak symmetry. As the temperature decreases, the SM electroweak phase transition occurs and the Universe is located at SM electroweak vacuum. We show that the electroweak phase transition is the first order and the gravitational waves (GWs) generated during this process can be detected by the future GW experiments such as LISA, TianQin, DECIGO, and BBO.