ChPT tests at the NA48 and NA62 experiments at CERN

TL;DR

This paper reports on precise measurements of rare kaon decays from CERN experiments, providing new insights into low-energy strong interactions and testing predictions of Chiral Perturbation Theory and lattice QCD.

Contribution

It presents new experimental data on kaon decay form factors, branching ratios, and the observation of a cusp in the pion mass spectrum, supporting ChPT predictions.

Findings

Support for ChPT prediction of the cusp in the $\pi^0\pi^0$ spectrum

Model-independent decay rate measurements for rare kaon decays

Validation of ChPT and lattice QCD calculations with new data

Abstract

The NA48/2 Collaboration at CERN has accumulated unprecedented statistics of rare kaon decays in the Ke4 modes: Ke4(+-) ($K^\pm \to \pi^+ \pi^- e^\pm \nu$) and Ke4(00) ($K^\pm \to \pi^0 \pi^0 e^\pm \nu$) with nearly one percent background contamination. The detailed study of form factors and branching rates, based on these data, has been completed recently. The results brings new inputs to low energy strong interactions description and tests of Chiral Perturbation Theory (ChPT) and lattice QCD calculations. In particular, new data support the ChPT prediction for a cusp in the $\pi^0\pi^0$ invariant mass spectrum at the two charged pions threshold for Ke4(00) decay. New final results from an analysis of about 400 $K^\pm \to \pi^\pm \gamma \gamma$ rare decay candidates collected by the NA48/2 and NA62 experiments at CERN during low intensity runs with minimum bias trigger configurations are presented. The results include a model-independent decay rate measurement and fits to ChPT description.