First observation and study of the K ± → π ± π 0 e + e − decay with the NA48/2 experiment at CERN

B Bloch-Devaux

doi:10.1088/1742-6596/1526/1/012004

Abstract

The first observation of the rare decay K ± → π ± π 0 e + e − is reported by the NA48/2 experiment at CERN, based on 4919 candidates with 4.9% background contamination. From the analysis of 1.7 × 1011 kaon decays collected in 2003–2004, the branching ratio in the full kinematic region is measured to be BR(K ± → π ± π 0 e + e −)= (4.24 ± 0.14) × 10−6. A detailed study of the kinematic space shows evidence for a structure dependent contribution. Both measured BR and structure dependent contribution are in perfect agreement with theoretical predictions based on Chiral Perturbation Theory. Several P- and CP-violating asymmetries are also evaluated.

Highlights

Kaon decays have played a major role in establishing the quark mixing flavour structure of the Standard Model
The K ± → π ± π 0 e+ e− decay, never observed so far, proceeds through virtual photon exchange followed by internal conversion into an electron-positron pair, i.e. K ± → π ± π 0 γ ∗ → π ± π 0 e+ e−
The virtual γ ∗ can be produced by two different mechanisms: Inner Bremsstrahlung (IB), where the γ ∗ is emitted by one of the charged mesons in the initial or final state and Direct

Summary

Introduction

Published under licence by IOP Publishing Ltd doi:10.1088/1742-6596/1526/1/012004 decay rate consists of three terms: the dominant long-distance IB contribution The authors of [2] were able to predict, on the basis of the NA48/2 measurement of the magnetic and electric terms in the K ± → π ± π 0 γ decay mode [4], the contributions of the single components relative to IB. Including more experimental measurements from radiative kaon decays [5], the authors of [3] were able to refine their predictions. The momenta of charged decay products were measured in a magnetic spectrometer, housed in a tank filled with helium and placed downstream of the decay volume. A detailed description of the detector can be found in [7]

Analysis strategy and measurements

Candidate selections and background estimates

Branching ratio measurement

Kinematic space study and asymmetries

Findings

Summary