Kaon Matrix Elements and CP-violation from Quenched Lattice QCD: (I) the 3-flavor case

TL;DR

This study computes kaon decay matrix elements and CP-violation parameters using quenched lattice QCD with domain wall fermions, providing results close to experimental values but with notable approximations and limitations.

Contribution

First quenched lattice QCD calculation of K to pi pi matrix elements for CP-violation, using domain wall fermions and non-perturbative renormalization, with results comparable to experiment.

Findings

Ratio |A_0|/|A_2| = 25.3 ± 1.8, close to experimental 22.2

Calculated ε'/ε ≈ -4.0 × 10^{-4}, significantly below experimental value

Determined B_K parameter as 0.532(11) at 2 GeV

Abstract

We report the results of a calculation of the K --> pi pi matrix elements relevant for the $\DIhalf$ rule and $\epe$ in quenched lattice QCD using domain wall fermions at a fixed lattice spacing $a^{-1} \sim 2$ GeV. Working in the three-quark effective theory, where only the u, d and s quarks enter and which is known perturbatively to next-to-leading order, we calculate the lattice K --> pi and K --> |0> matrix elements of dimension six, four-fermion operators. Through lowest order chiral perturbation theory these yield K --> pi pi matrix elements, which we then normalize to continuum values through a non-perturbative renormalization technique. For the ratio of isospin amplitudes |A_0|/|A_2| we find a value of $25.3 \pm 1.8$ (statistical error only) compared to the experimental value of 22.2, with individual isospin amplitudes 10-20% below the experimental values. For $\epe$, using known central values for standard model parameters, we calculate $(-4.0 \pm 2.3) \times 10^{-4}$ (statistical error only) compared to the current experimental average of $(17.2 \pm 1.8) \times 10^{-4}$. Because we find a large cancellation between the I = 0 and I = 2 contributions to $\epe$, the result may be very sensitive to the approximations employed. Among these are the use of: quenched QCD, lowest order chiral perturbation theory and continuum perturbation theory below 1.3 GeV. We have also calculated the kaon B parameter, B_K and find $B_{K,\bar{MS}}(2 {\rm GeV}) = 0.532(11)$. Although currently unable to give a reliable systematic error, we have control over statistical errors and more simulations will yield information about the effects of the approximations on this first-principles determination of these important quantities.