$\epsilon_K$ and $\epsilon'/\epsilon$ in a diquark model

TL;DR

This paper explores how a diquark model extension of the Standard Model can potentially explain the discrepancy in the CP violation parameter '/, by analyzing new box diagrams and their impact on Kaon decay processes.

Contribution

It introduces a diquark model with novel box diagrams, including those with W-bosons, to account for '/ and examines their effects under _K constraints.

Findings

Diquark effects can enhance '/ to ^{-3}

Box diagrams with W-bosons are significant in '/ calculations

The model remains consistent with _K constraints.

Abstract

Based on the calculations using the lattice QCD by RBC-UKQCD collaboration and a large $N_c$ dual QCD, the resulted $\epsilon'/\epsilon$, which is less than the experimental data by more than a $2\sigma$ in the standard model (SM), suggests the necessity of a new physics effect. In order to complement the insufficient $\epsilon'/\epsilon$, we study the extension of the SM with a colored scalar in a diquark model. In addition to the pure diquark box diagrams, it is found that the box diagrams with one $W$-boson and one diquark, ignored in the literature, play an important role in the $\Delta S=2$ process. The mass difference between $K_L$ and $K_S$ in the diquark model is well below the current data, whereas the Kaon indirect CP violation $\epsilon_K$ gives a strict constraint on the new parameters. Three mechanisms are classified in the study of $\epsilon'/\epsilon$, and they include: tree-level diagram, QCD and electroweak penguins, and chromomagnetic dipole. Taking the Kobayashi-Maskawa phase as the unique CP source, we analyze each contribution of the three mechanisms in detail and conclude that these diquark-mediated effects can singly enhance $\epsilon'/\epsilon$ to be of ${\cal O}(10^{-3})$, depending on the values of free parameters, when the bound from $\epsilon_K$ is satisfied.