Isospin breaking in the phases of the Ke4 form factors

TL;DR

This paper analyzes isospin breaking effects in Kl4 form factors due to pion mass differences, providing two-loop analytical expressions and reinterpreting experimental data to refine low-energy constants and scattering lengths.

Contribution

It introduces a two-loop dispersive approach to quantify isospin-breaking corrections in Kl4 form factors, improving upon previous one-loop analyses and connecting experimental phase shifts with theoretical scattering parameters.

Findings

Derived analytical two-loop expressions for form factor phases.

Quantified isospin-breaking corrections and their components.

Reanalyzed experimental data to refine scattering lengths and low-energy constants.

Abstract

Isospin breaking in the Kl4 form factors induced by the difference between charged and neutral pion masses is studied. Starting from suitably subtracted dispersion representations, the form factors are constructed in an iterative way up to two loops in the low-energy expansion by implementing analyticity, crossing, and unitarity due to two-meson intermediate states. Analytical expressions for the phases of the two-loop form factors of the K\pm -> pi^+ pi^- e^\pm nu_e channel are given, allowing one to connect the difference of form-factor phase shifts measured experimentally (out of the isospin limit) and the difference of S- and P-wave pi-pi phase shifts studied theoretically (in the isospin limit). The isospin-breaking correction consists of the sum of a universal part, involving only pi-pi rescattering, and a process-dependent contribution, involving the form factors in the coupled channels. The dependence on the two S-wave scattering lengths a_0^0 and a_0^2 in the isospin limit is worked out in a general way, in contrast to previous analyses based on one-loop chiral perturbation theory. The latter is used only to assess the subtraction constants involved in the dispersive approach. The two-loop universal and process-dependent contributions are estimated and cancel partially to yield an isospin-breaking correction close to the one-loop case. The recent results on the phases of K^\pm -> pi^+ pi^- e^\pm nu_e form factors obtained by the NA48/2 collaboration at the CERN SPS are reanalysed including this isospin-breaking correction to extract values for the scattering lengths a_0^0 and a_0^2, as well as for low-energy constants and order parameters of two-flavour ChPT.