Na48 Results On Rare K

Abstract

lected during a 2-day test run in 1999 using a K S beam only at high intensity (# 610 9 ppp). 2 Measurement of the Branching Ratio K S The study of the decay K S ## is interesting for the understanding of the low energy hadronic dynamics in Chiral Perturbation Theory (#PT ), since it is sensitive to higher order loop e#ects. Cagliari-Cambridge-CERN-Dubna-Edinburgh-Ferrara-Firenze-Mainz-Orsay-Perugia-Pisa-Saclay-SiegenTorino -Warsaw-Wien There are no short distance contributions and theoretical predictions for the branching ratio are finite and unambiguous (2.25 10 -6 with an error of less than 10 %); the measured value is (2.4 0.9) 10 -6 . The kaon energy and decay vertex are reconstructed from the energies and the positions of the electromagnetic showers, measured in the LKr. In order to select K ## candidates, the data sample is restricted to events with 2 energy clusters. Pairs of clusters that are in time within 5 ns are then selected, with the addit