Electroweak Phase Transition in the Standard Model Effective Field Theory

Abstract: At least two Cosmological phase transitions are very probable to have happened since the beginning of the universe, one of them being the electroweak phase transition responsible for the breaking of the EW symmetry. It is possible that the EW phase transition could have caused the observed baryon asymmetry of the universe and therefore provide an explanation for the baryon asymmetry problem. Furthermore, it could also have generated observable gravitational waves. Both of these possibilities however hinge on the fact that the EW phase transition had been a first-order phase transition, which it is not according to the standard model. The SM predicts a crossover transition. In this work, the EW phase transition is studied in the simplest extension to the SM in The StandardModel Effective Field Theory, by adding a φ6 operator to the scalar sector. Calculations show that it is indeed possible to have a first-order EW phase transition in this extension. Characteristic parameters of the phase transition are then calculated to generate a GW power spectrum to see if they are detectable by LISA or not. The generated GW signatures lie just outside of the LISA sensitivity region. The theoretical uncertainties in the calculations are however possibly large enough that one cannot yet exclude that the GW signals can be observed by LISA.