These plots are the distributions of the reconstructed Kaon mass
in our online "filter code".
Top-Left: Mass of supposed K->PI+ PI- events, occuring in the vacuum beam.
Top-Right: K->PI+ PI- events, regenerator beam.
Bottom-Left: K->PI0 PI0 events, vacuum beam.
Bottom-Right: K->PI0 PI0 events, regenerator beam.
The reconstructed mass is used in our offline analysis to reject background (i.e., other than K->2PI) events. The better the reconstructed mass resolution (i.e., the more narrow the peak), the cleaner our separation of desirable "signal" events from undesirable "noise" events.