Observation of the rare decay K_S -> pi^0mu^+mu^-

TL;DR

This paper reports the first observation of the rare decay K_S -> pi^0mu^+mu^- by the NA48/1 Collaboration, measuring its branching ratio with a small number of observed events and low background.

Contribution

The paper presents the first experimental evidence and measurement of the branching ratio for the rare decay K_S -> pi^0mu^+mu^- using data from CERN SPS.

Findings

Six candidate events observed with low background expectation.

Measured branching ratio: (2.9 +1.5 -1.2) x 10^{-9}.

Provides new data on rare kaon decay processes.

Abstract

A search for the decay K_S -> pi^0mu^+mu^- has been made by the NA48/1 Collaboration at the CERN SPS accelerator. The data were collected during 2002 with a high-intensity K_S beam. Six events were found with a background expectation of 0.22^+0.18_-0.11 event. Using a vector matrix element and unit form factor, the measured branching ratio is B(K_S -> pi^0mu^+mu^-)=[2.9^+1.5_-1.2(stat)+/-0.2(syst)]x10^{-9}.