Testing the Standard Model with signals from Cosmological Phase Transitions

Abstract

The Standard Model (MP) of Particle Physics successfully describes the interactions of all elementary particles known so far. In particular, with the discovery of the Higgs boson in 2012, the MP spectrum was complete. The Higgs sector and its interactions are being tested at the Large Hadron Collider (LHC) with increasing precision. However, these tests have not yet reached the precision with which fermion gauge interactions have been tested over the last thirty years. Precision testing of the MP's Higgs sector will be the frontier of particle physics for years to come, particularly in the high-luminosity era at the High Luminosity LHC (HL-LHC). Given that the energy of the HL-LHC will be practically the same as that of the LHC, the new frontier will not be new energy, but greater precision. From a theoretical point of view, there are many reasons to expect deviations in the Higgs sector from the MP predictions. To begin with, the Higgs sector is included in the MP as a way to solve the problem of the incompatibility of gauge invariance with the masses of fermions and gauge bosons. The Higgs mechanism implies the existence of a potential where the electroweak scale is introduced in an ad hoc manner. In fact, in the entire theory there is only one term with a mass scale and it is the quadratic term in the Higgs field in the potential. The origin of the electroweak scale is then one of the central questions in particle physics today. Furthermore, the electroweak scale appears unstable under radiative corrections (hierarchy problem). Among other MP problems, we have the origin of the mass of neutrinos, which requires either a particle outside the MP (right-hand neutrino) or a mechanism with energies above the electroweak scale (see saw); the origin of the dark matter of the universe and the origin of the baryon-anti-baryon asymmetry. On the other hand, the breaking of electroweak symmetry implies the existence of a phase transition in the thermal history of the universe from the trivial vacuum (disordered) to the current electroweak vacuum (ordered). This phase transition would occur at a temperature of approximately 150 GeV. The electroweak transition in the MP is of second order. However, extensions of the MP, such as those proposed to try to explain the issues mentioned above, can modify the order of the phase transition, potentially changing it to first order with important observable consequences. In this project, the effects of MP extensions in the Higgs sector on cosmological phase transitions will be studied. In many extensions of the MP, and in particular those where the Higgs is a (pseudo) Nambu-Goldstone boson (pNGB), other phase transitions must exist. We propose to study the details of these phase transitions and their possible correlations with experimental signals, both at the LHC and in gravitational wave detection experiments.