A New bound on CP Violation in the $\tau$ Lepton Yukawa Coupling and electroweak baryogenesis

TL;DR

This paper demonstrates that the electron dipole moment constraints significantly limit the role of the tau lepton Yukawa coupling in electroweak baryogenesis, challenging its viability as a source of CP violation.

Contribution

It provides a new, stronger bound on the tau lepton Yukawa coupling within the Standard Model Effective Field Theory framework, assuming flavor symmetry.

Findings

Electron dipole moment bounds are two orders of magnitude stronger than previous estimates.

The tau lepton Yukawa coupling's role in electroweak baryogenesis is highly constrained.

The results make electroweak baryogenesis explanations more difficult.

Abstract

The origin of the matter-antimatter asymmetry in the Universe is a fundamental question of physics. Electroweak baryogenesis is a compelling scenario for explaining it but it requires beyond the Standard Model sources of the CP symmetry violation. The simplest possibility is CP violation in the third generation fermion Higgs couplings, widely investigated theoretically and searched for experimentally. It has been found that the experimental bounds on the CP violation in the quark Yukawa couplings exclude their significant role in the electroweak baryogenesis, but it can be still played by the $\tau$ lepton Yukawa coupling. It is shown in this paper that, within the context of the Standard Model Effective Field Theory and assuming an underlying flavour symmetry of the Wilson coefficients, the electron dipole moment bound on the $\tau$ lepton Yukawa coupling is two orders of magnitude stronger than previously reported. This sheds strong doubts on its role in the electroweak baryogenesis, further stimulates the interest in its experimental verification and makes electroweak baryogenesis even more difficult to explain.