Collider Probes of Electroweak Phase Transition in an Extended Higgs Sector
15 July 2024

Photo: Kateryna Radchenko Serdula
Is the Higgs boson solitary or part of an extended sector of Higgs bosons waiting to be discovered? Could that explain the cosmic matter-antimatter asymmetry? And what signatures of the extended Higgs sector can be predicted and searched for at the upcoming high luminosity Large Hadron Collider (HL-LHC) at CERN?
The best theory we have so far to describe subatomic particles and their interactions is the Standard Model (SM). However, it cannot be the ultimate theory of our Universe. Many physical phenomena cannot be explained within SM physics, and this model also gives rise to theoretical anomalies that puzzle physicists and motivate theories beyond the SM. One of the phenomena that cannot be explained is why we are made of matter (baryons), and not antimatter, which was created in the same abundance during the Big Bang. We know that three conditions need to be satisfied in a theory to allow for this asymmetry to be generated in an early stage of the Universe. The mechanism that explains the generation of this asymmetry is called baryogenesis and the Higgs boson plays an essential role in many of the models that try to explain it.
To study the role of the Higgs, we need to understand the Higgs potential, which describes how the vacuum energy configuration of the Higgs field behaves. Imagine a skateboarder (the Higgs field) placed at the peak of a Mexican hat. The center peak is unstable, so the skateboarder rolls down towards the circular trough where it can remain stably. Settling into a point in this circular trough represents choosing a ground energy state, and the rolling down is the spontaneous symmetry breaking, as the skateboarder is no longer at the center of the hat.
We still don't know the exact form of this function and how it evolved from an early Universe phase, in which the vacuum energy was zero everywhere and particles were massless. At some point, as the Universe was expanding and cooling down, the mechanism of electroweak symmetry breaking must have occurred, and if it was violent enough, this transition would have been a strong first order phase transition (SFOPT), similar to the transition from liquid to gas at room temperature and pressure. Thus, the Universe could have been literally boiling when the electroweak symmetry broke spontaneously and particles acquired their masses. If this was indeed the case, we hope to observe the echoes of this violent process by measuring a stochastic background of gravitational waves emitted during the collision of bubbles of the real electroweak vacuum. This possibility is fascinating because it would provide the out-of-thermal-equilibrium conditions needed to explain the baryon asymmetry.
The SFOPT is excluded in the Standard Model because all the parameters of the potential are set by the measured Higgs mass, and the strength of the transition is not enough for it to be first order. The presence of additional scalar states can completely alter the shape of the Higgs potential and its evolution with temperature, making this an exciting research avenue. In the specific extension of the Standard Model that we considered, which adds a Higgs doublet, we assume four extra scalar bosons exist in our Universe besides the one discovered at a mass of 125 GeV in 2012.
The most fascinating part of our research is that it can be probed at colliders, for instance during the high luminosity phase of the LHC, even before we can measure the gravitational wave background. Many searches are currently being conducted to find such new particles, but some searches are specifically targeted at probing the possibility of an SFOPT. For example, the search for a pseudoscalar Higgs (A) decaying into another heavy extra scalar (H) and an electroweak interaction boson (Z). This process is kinematically allowed when the mass of A is at least the mass of H plus the mass of Z. This necessary mass splitting favors a large barrier in the Higgs potential, which is compatible with an SFOPT. Further decays of H into a pair of top-antitop quarks (the heavy "brother" of the up quark that co-forms the proton together with the down quark) are also the most dominant when this Higgs is at least as massive as two tops. The final state of such a process is very challenging experimentally but provides a unique signature of the particle spectrum of models with extended Higgs sectors. Our investigation aims to probe a large part of the parameter region where such transitions are favored if this search is conducted experimentally.