$K_L$ detection

In order to identify $K_L$, a cluster must be observed in KLM. Then, tracks of charged particles measured in CDC are extrapolated into KLM. Clusters within 15 degrees of an extrapolated charged particle track are excluded from $K_L$ cluster candidates. For an isolated cluster, the center of gravity of the hits is calculated and used to determine the direction of the cluster from the interaction point. Fig. shows a histogram of the difference between the direction of the $K_L$ cluster candidate and the missing momentum direction. The data was obtained during the summer 1999 commissioning run of the KEK B-factory. The missing momentum vector is calculated using all the other measured particles in the event. The histogram shows a clear peak where the direction of the neutral cluster measured in KLM is consistent with the missing momentum in the event. A large deviation of the missing momentum direction from the neutral cluster direction is mainly due to undetected neutrinos and particles escaping the detector acceptance. Fig. shows the number of neutral clusters per event and a Monte Carlo simulation of the predicted number of $K_L$ clusters per event. The average number of $K_L$ clusters per event is 0.5. The agreement with the prediction gives us the confidence that the detector and our reconstruction software are performing correctly.

Figure: Difference between the neutral cluster and the direction of missing momentum in KLM.

Figure: Number of neutral clusters per event in KLM.