We propose that the dynamics of a scalar $\varphi$ of mass $\mathcal{O}(10)\mathrm{MeV}$ that is weakly coupled to the Higgs can lead to a first-order electroweak phase transition, fulfilling a key requirement for baryogenesis. The 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 $\varphi$ via a dimension-five operator. We conjecture that the entire Standard Model flavor structure could turn on, mutatis mutandis, after the electroweak phase transition, via dimension-five interactions of $\varphi$ suppressed by scales ranging from $\mathcal{O}(10^3)\mathrm{TeV}$ to near the Planck mass. Due to its suppressed couplings, $\varphi$ is long lived and can lead to missing energy signals in rare kaon decays, which can be probed by the KOTO experiment.

• Received 25 January 2021
• Accepted 1 April 2021

DOI:https://doi.org/10.1103/PhysRevD.103.083534

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Published by the American Physical Society