Was there an electroweak phase transition?

A illustrative scalar potential in the presence of an extended Higgs sector with a new scalar field F . Today’s universe lives in the Higgs phase. At high temperatures in the early universe the maximum at the zero-field origin was the global minimum. The cooling universe could have reached the Higgs phase either directly in a single step through a multiple step process involving an intermediate phase. Our research explores the landscape of such potentials and thermal histories; develops state-of-the-art methods for computing their thermodynamics and early universe dynamics; and delineates the experimental signatures in collider studies and gravitational wave probes.

We know from lattice studies that the electroweak symmetry breaking transition in a purely Standard Model universe is a smooth crossover transition. The presence of beyond Standard Model physics, particularly extended Higgs sectors, can dramatically alter this thermal history, leading to the occurrence of a first order electroweak phase transition. This possibility is both intrinsically fascinating and consequential. If a first order electroweak phase transition took place, it would provide needed preconditions for generating the cosmic matter-antimatter asymmetry and lead to generation of potentially detectable gravitational waves.

Theoretical electroweak phase transition research presents several compelling challenges:

  • What is the landscape of extended Higgs sector potentials and what are the corresponding early universe symmetry-breaking dynamics?
  • What experimental signatures in high energy collider studies and gravitational wave searches could discover and characterize the particle physics model responsible for an electroweak phase transition?
  • How reliably can we compute phase transition thermodynamics and out-of-equilibrium dynamics?

Thermodynamic phase diagram for the real triplet scalar extension of the Standard Model Higgs sector. Vertical and horizontal axes give the triplet mass and coupling to the Higgs, respectively. Differently colored regions indicate thermal histories as identified through a combination of thermal effective field theory and lattice computations. Here O, S , and f denote the electroweak symmetric, triplet, and Higgs phases. The latter two phases break electroweak symmetry in distinct ways. Results taken from L. Niemi, M. J. Ramsey-Musolf, T. V. I. Tenkanen, and D. Weir, Phys. Rev. Lett. 126 (2021) 171802.

In addressing these questions, our team pursues a variety of avenues: particle physics model building; finite temperature quantum field theory using effective field theory methods, lattice computations, and quantum transport theory developments; phenomenological studies pertinent to the Large Hadron Collider, prospective future lepton Higgs factories, a next generation hadron collider, and gravitational wave probes.

While our colleagues in other groups around the world are making important contributions in elements of this quest, our research ties them all together in an integrated theory-model building-phenomenology program. Read more in the Current Research, Accomplishments, and What’s Hot pages.