A new limit on the CP violating decay KS→3π0KS→3π0 with the KLOE experiment

Abstract We have carried out a new direct search for the CP violating decay KS→3π0KS→3π0 with 1.7 fb−1 of e+e−e+e− collisions collected by the KLOE detector at the Φ-factory DAΦNE. We have searched for this decay in a sample of about 5.9×1085.9×108KSKLKSKL events tagging the KSKS by means of the KLKL interaction in the calorimeter and requiring six prompt photons. With respect to our previous search, the analysis has been improved by increasing of a factor four the tagged sample and by a more effective background rejection of fake KSKS tags and spurious clusters. We find no candidates in data and simulated background samples, while we expect 0.12 standard model events. Normalizing to the number of KS→2π0KS→2π0 events in the same sample, we set the upper limit on BR(KS→3π0)⩽2.6×10−8BR(KS→3π0)⩽2.6×10−8 at 90% C.L., five times lower than the previous limit. We also set the upper limit on the η000η000 parameter, |η000|⩽0.0088|η000|⩽0.0088 at 90% C.L., improving by a factor two the latest direct measurement.