Search for the Rare Decay KL --> pi0 ee

TL;DR

This paper reports a search for the rare decay KL --> pi0 ee using data from the KTeV experiment, setting a new upper limit on its branching ratio, which tests Standard Model predictions and probes for new physics.

Contribution

The study provides the most stringent upper limit on the branching ratio of KL --> pi0 ee to date, based on combined data from 1997 and 1999-2000.

Findings

Observed one event with expected background of 0.99 events.

Set an upper limit on the branching ratio at 2.8 x 10^(-10).

Results constrain theories predicting this rare decay.

Abstract

The KTeV/E799 experiment at Fermilab has searched for the rare kaon decay KL--> pi0ee. This mode is expected to have a significant CP violating component. The measurement of its branching ratio could support the Standard Model or could indicate the existence of new physics. This letter reports new results from the 1999-2000 data set. One event is observed with an expected background at 0.99 +/- 0.35 events. We set a limit on the branching ratio of 3.5 x 10^(-10) at the 90% confidence level. Combining the results with the dataset taken in 1997 yields the final KTeV result: BR(KL --> pi0 ee) < 2.8 x 10^(-10) at 90% CL.