First Order Electroweak Phase Transition from Weakly Coupled sub-GeV Physics and Possible Connection to Fermion Flavor

TL;DR

This paper explores how a weakly coupled sub-GeV scalar field can induce a first order electroweak phase transition, potentially explaining baryogenesis and linking to fermion flavor structure, with testable signals in rare kaon decays.

Contribution

It introduces a model where a light scalar triggers a first order electroweak phase transition and connects flavor physics to high-scale dynamics through dimension-5 operators.

Findings

Scalar $$ can induce a first order phase transition.

Model predicts long-lived scalar detectable via rare kaon decays.

Flavor structure may emerge after the electroweak transition.

Abstract

We propose that the dynamics of a scalar $\phi$ of mass $O(10)$ MeV that is weakly coupled to the Higgs can lead to a first order electroweak phase transition, fulfilling a key requirement for baryogenesis. Stability of the model near the weak scale requires a suppressed - possibly vanishing - top Yukawa coupling to the Higgs before the transition which rises to the Standard Model value afterwards. This can be accomplished through the dynamics of $\phi$ via a dimension-5 operator. We conjecture that the entire Standard Model flavor structure could turn on, mutatis mutandis, after the electroweak phase transition, via dimension-5 interactions of $\phi$ suppressed by scales ranging from $O(10^3)$ TeV to near Planck mass. Due to its suppressed couplings, $\phi$ is long-lived and can lead to missing energy signals in rare kaon decays, which can be probed by the KOTO experiment.