Revisiting Kaon Physics in General $Z$ Scenario

TL;DR

This paper reevaluates the impact of new physics on Z penguin processes in light of recent CP violation discrepancies in K decays, highlighting the importance of interference effects and their implications for rare decay rates.

Contribution

It introduces the consideration of interference effects between standard model and new physics in Z penguin contributions, which tighten experimental bounds and influence predictions for rare kaon decays.

Findings

Interference effects significantly tighten experimental bounds.

New physics can enhance the K_L → π^0 ν ν̄ branching ratio up to 6×10^{-10}.

Tuning of new physics contributions is required to explain CP violation discrepancies.

Abstract

New physics contributions to the $Z$ penguin are revisited in the light of the recently-reported discrepancy of the direct CP violation in $K\to\pi\pi$. Interference effects between the standard model and new physics contributions to $\Delta S = 2$ observables are taken into account. Although the effects are overlooked in the literature, they make experimental bounds significantly severer. It is shown that the new physics contributions must be tuned to enhance $\mathcal{B}(K_L \to \pi^{0} \nu \bar{\nu})$, if the discrepancy of the direct CP violation is explained with satisfying the experimental constraints. The branching ratio can be as large as $6 \times 10^{-10}$ when the contributions are tuned at the 10 % level.