Observation of a cusp-like structure in the pizero-pizero invariant mass distribution from K+- ==> pi+- pizero pizero decay and determination of the pi-pi scattering lengths

TL;DR

This paper reports the observation of a cusp-like anomaly in the pi0 pi0 invariant mass distribution from kaon decays, providing a new precise measurement of the difference in pion-pion scattering lengths, which is crucial for understanding low-energy QCD.

Contribution

The study presents the first observation of a cusp in the pi0 pi0 invariant mass distribution from kaon decays and determines the pi-pi scattering length difference with high precision.

Findings

Observation of a cusp-like structure in the invariant mass distribution.

Precise determination of the pi-pi scattering length difference a0 - a2.

First experimental evidence linking the anomaly to pi+ pi- to pi0 pi0 charge exchange scattering.

Abstract

We report the results from a study of ~23 Million K+- ==> pi+- pizero pizero decays recorded by the NA48/2 experiment at the CERN SPS, showing an anomaly in the pizero pizero invariant mass distribution in the region around 2m+, where m+ is the charged pion mass. This anomaly, never observed in previous experiments, can be interpreted as an effect due mainly to the final state charge exchange scattering process pi+ pi- ==> pizero pizero in K+- ==> pi+- pi+ pi- decay. It provides a precise determination of a0 - a2, the difference between the pi-pi scattering lengths in the isospin I=0 and I=2 states.