Observation of the rare decay KS→π0e+e−

J.R Batley,G.E Kalmus,C Lazzeroni,D.J Munday,M Patel,M.W Slater,S.A Wotton

doi:10.1016/j.physletb.2003.10.001

Abstract

A search for the decay KS→π0e+e− has been made by the NA48/1 experiment at the CERN SPS accelerator. Using data collected during 89 days in 2002 with a high-intensity KS beam, 7 events were found with a background of 0.15 events. The branching fraction BR(KS→π0e+e−,mee>0.165 GeV/c2)=(3.0+1.5−1.2(stat)±0.2(syst))×10−9 has been measured. Using a vector matrix element and a form factor equal to one, the measurement gives BR(KS→π0e+e−)=(5.8+2.9−2.4)×10−9.

Highlights

When not forbidden by CP-conservation, the K → πee decay can proceed via single photon exchange
The theoretical aspects of the decay KS → π0e+e− were studied to leading order in the chiral expansion in ref. [2, 3] and the implications of this decay with respect to the search for CP-violation in rare kaon decays were investigated in ref. [4] and re-examined in ref
A background source is from the decay KS → π0π0 where two photons from different π0’s converted either internally (i.e. KS → πD0 πD0 ) or externally and one electron and one positron from different π0’s were outside the detector acceptance

Summary

Introduction

When not forbidden by CP-conservation, the K → πee decay can proceed via single photon exchange. – The hardware trigger [12] selected events satisfying the following conditions: - hit multiplicity in the first drift chamber compatible with one or more tracks; - hadron calorimeter energy less than 15 GeV; - electromagnetic calorimeter energy greater than 30 GeV; - the centre of energy of the electromagnetic clusters (see eq 5 below) less than 15 cm from the beam axis; - the decay occurring within six KS lifetimes from the end of the collimator; - no hits in the two ring scintillator counters farthest downstream. Assuming the observed event to be a kaon decay, the proper lifetime was computed from the position of the neutral vertex, taking the end of the final collimator as the origin. Both the signal and the control regions were kept masked while cuts to reject the background were studied

Background

Result

Discussion