Finite-Volume Partially-Quenched Two-Pion Amplitudes in the I=0 Channel

TL;DR

This paper analyzes finite-volume two-pion matrix elements in the I=0 channel within (partially) quenched QCD using chiral perturbation theory, highlighting limitations and conditions for reliable amplitude extraction.

Contribution

It provides a detailed one-loop chiral perturbation theory analysis of two-pion amplitudes in finite volume, clarifying conditions under which matrix elements can be reliably determined in (partially) quenched QCD.

Findings

Matrix elements cannot be determined when sea and valence quark masses differ in partially quenched QCD.

Matching valence and sea quark masses for u and d quarks allows amplitude extraction below the two-kaon threshold.

Finite-volume states are mixtures of multiple two-meson states, complicating amplitude extraction.

Abstract

We present a study of the finite-volume two-pion matrix elements and correlation functions of the I=0 scalar operator, in full and partially quenched QCD, at one-loop order in chiral perturbation theory. In partially quenched QCD, when the sea and valence light quark masses are not equal, the lack of unitarity leads to the same inconsistencies as in quenched QCD and the matrix elements cannot be determined. It is possible, however, to overcome this problem by requiring the masses of the valence and sea quarks to be equal for the u and d quarks while keeping the strange quark (s) quenched (or partially quenched), but only in the kinematic region where the two-pion energy is below the two-kaon threshold. Although our results are obtained at NLO in chiral perturbation theory, they are more general and are also valid for non-leptonic kaon decays (we also study the matrix elements of (8,1) operators, such as the QCD penguin operator Q6). We point out that even in full QCD, where any problems caused by the lack of unitarity are clearly absent, there are practical difficulties in general, caused by the fact that finite-volume energy eigenstates are linear combination of two-pion, two-kaon and two-eta states. Our work implies that extracting Delta I=1/2, K-->pipi decay amplitudes from simulations with m_s=m_{d,u} is not possible in partially quenched QCD (and is very difficult in full QCD).