First observation of the KS→π0γγ decay

Using the NA48 detector at the CERN SPS, 31 KS→π0γγ candidates with an estimated background of 13.7±3.2 events have been observed. This first observation leads to a branching ratio of BR(KS→π0γγ)z>0.2=(4.9±1.6stat±0.9syst)×10−8 in agreement with Chiral Perturbation Theory predictions.