Abstract
In extensions of the Standard Model with no dimensionful parameters the electroweak phase transition can be delayed to temperatures of order 100 MeV. Then the chiral phase transition of QCD can proceed with 6 massless quarks. The top-quark condensate destabilizes the Higgs potential through the Yukawa interaction, triggering the electroweak transition. Based on the symmetries of massless QCD, it has been argued that the chiral phase transition is first order. We point out that the top-Higgs Yukawa interaction is nonperturbatively large at the QCD scale, and that its effect on the chiral phase transition may not be negligible, violating some of the symmetries of massless QCD. The remaining symmetries indicate that top quarks condense in a second-order phase transition, but top condensation might also be triggered by a first-order symmetry-breaking transition in the light-quark sector.
Highlights
First-order phase transitions are interesting for cosmology because they can leave observable relics such as the baryon asymmetry of the Universe [1,2], gravitational waves [3], or large-scale magnetic fields [4]
In the Standard Model there is neither an electroweak, nor a QCD, or quark-hadron phase transition: both the electroweak symmetry [5,6] and the approximate symmetries of QCD get broken in a smooth crossover [7,8]
In some cases the temperature becomes as low as Oð100Þ MeV while the electroweak symmetry is still unbroken, and all six quark flavors are still massless
Summary
Fakultät für Physik, Universität Bielefeld, 33501 Bielefeld, Germany (Received 2 September 2021; accepted 1 November 2021; published 7 December 2021). In extensions of the Standard Model with no dimensionful parameters the electroweak phase transition can be delayed to temperatures of order 100 MeV. The chiral phase transition of QCD can proceed with six massless quarks. The top-quark condensate destabilizes the Higgs potential through the Yukawa interaction, triggering the electroweak transition. Based on the symmetries of massless QCD, it has been argued that the chiral phase transition is first order. We point out that the top-Higgs Yukawa interaction is nonperturbatively large at the QCD scale, and that its effect on the chiral phase transition may not be negligible, violating some of the symmetries of massless QCD. The remaining symmetries indicate that top quarks condense in a second-order phase transition, but top condensation might be triggered by a firstorder symmetry-breaking transition in the light-quark sector
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