Lattice extraction of $ K \to \pi \pi $ amplitudes to NLO in partially quenched and in full chiral perturbation theory

TL;DR

This paper demonstrates how to compute $K o \pi\pi$ amplitudes to NLO in partially quenched and full chiral perturbation theory using lattice methods, addressing finite volume effects and operator ambiguities.

Contribution

It introduces a method to construct $\epsilon'/\epsilon$ at NLO from lattice-computable amplitudes without requiring three-momentum insertion, and clarifies the treatment of penguin operators in PQChPT.

Findings

NLO $K o \pi\pi$ amplitudes can be obtained without three-momentum on the lattice.

Enhanced finite volume effects vanish at NLO when sea and valence quark masses are equal.

Explicit finite logarithm expressions for the amplitudes are provided.

Abstract

We show that it is possible to construct $\epsilon^\prime/\epsilon$ to NLO using partially quenched chiral perturbation theory (PQChPT) from amplitudes that are computable on the lattice. We demonstrate that none of the needed amplitudes require three-momentum on the lattice for either the full theory or the partially quenched theory; non-degenerate quark masses suffice. Furthermore, we find that the electro-weak penguin ($\Delta I=3/2$ and 1/2) contributions to $\epsilon^\prime/\epsilon$ in PQChPT can be determined to NLO using only degenerate ($m_K=m_\pi$) $K\to\pi$ computations without momentum insertion. Issues pertaining to power divergent contributions, originating from mixing with lower dimensional operators, are addressed. Direct calculations of $K\to\pi\pi$ at unphysical kinematics are plagued with enhanced finite volume effects in the (partially) quenched theory, but in simulations when the sea quark mass is equal to the up and down quark mass the enhanced finite volume effects vanish to NLO in PQChPT. In embedding the QCD penguin left-right operator onto PQChPT an ambiguity arises, as first emphasized by Golterman and Pallante. With one version (the "PQS") of the QCD penguin, the inputs needed from the lattice for constructing $K\to\pi\pi$ at NLO in PQChPT coincide with those needed for the full theory. Explicit expressions for the finite logarithms emerging from our NLO analysis to the above amplitudes are also given.