Search for the CP symmetry violation in the decays of Ks mesons using the KLOE detector

TL;DR

This study searched for CP violation in KS meson decays to 3pi0 using the KLOE detector, setting a new upper limit on the decay branching ratio and eta000 parameter, thus constraining CP symmetry violation.

Contribution

The paper provides the most stringent upper limit to date on the KS -> 3pi0 decay branching ratio and eta000 parameter, improving previous measurements significantly.

Findings

Upper limit on BR(KS -> 3pi0) < 2.7 x 10^-8 at 90% CL

No statistically significant signal detected

Set a new limit on the eta000 parameter |eta000| < 0.009

Abstract

The aim of this work was to determine the KS -> 3pi0 decay branching ratio and a modulus of the eta000 parameter, defined as the ratio of amplitudes for KS -> 3 pi0 to KL -> 3pi0 decays, which characterizes the CP symmetry violation in this decay. The measurement has been carried out with the KLOE detector operating at the phi-factory DAFNE in the Italian National Center for Nuclear Physics in Frascati. The KS mesons were identified with high efficiency via registration of these KL mesons which crossed the drift chamber without decaying and then interacted with the KLOE electromagnetic calorimeter. The KS four-momentum vector was then determined using the registered position of the KL meson and the known momentum of the phi meson. Next, the search for the KS -> 3pi0 -> 6gamma decay was carried out by the selection of events with six gamma quanta which momenta were reconstructed using time and energy measured by the electromagnetic calorimeter. To increase the signal over background ratio after identification of the KS meson and requiring six reconstructed photons a discriminant analysis is performed. It is based on kinematical fit, testing of the signal and background hypotheses and exploiting of the differences in kinematics of the KS decays into 2pi0 and 3pi0. Hence, we have obtained the upper limit on the KS -> 3pi0 branching ratio at the 90% confidence level BR(KS -> 3pi0)< 2.7 x 10^-8, which is almost five times lower than the latest published result. The search for the KS -> 3pi0 decay presented in this work failed to detect a signal of sufficient statistical significance. This upper limit can be translated into a limit on the modulus of the eta000 parameter amounting to: |eta000| < 0.009 at the 90% confidence level.