Ricerc@Sapienza

This is the third out of five chapters of the final report [1] of the Workshop on Physics at HL-LHC, and perspectives on HE-LHC [2]. It is devoted to the study of the potential, in the search for Beyond the Standard Model (BSM) physics, of the High Luminosity (HL) phase of the LHC, defined as $3~mathrmab^-1$ of data taken at a centre-of-mass energy of $14~mathrmTeV$, and of a possible future upgrade, the High Energy (HE) LHC, defined as $15~mathrmab^-1$ of data at a centre-of-mass energy of $27~mathrmTeV$.

We investigate first order phase transitions arising from hidden sectors
which are in thermal equilibrium with the Standard Model bath in the Early
Universe. Focusing on two simplified scenarios, an higgsed U(1) and a two
scalar singlet model, we show the impact of friction effects acting on the
bubble walls on the gravitational wave spectra and on the consequences for
present and future interferometer experiments. We further comment on the
possibility of disentangling the properties of the underlying theory featuring