Radial tail resolution in the SELEX RICH

We use a seven million event data sample of 600 GeV / c single-track pion events, where the pion track is reconstructed upstream and downstream of the SELEX RICH. We build the RICH ring radius histogram distribution and count the tail events that fall outside 5 σ , giving a fraction of 4 × 10 - 5 events outside the Gaussian tails. This control of events establishes the ability of using the RICH as a velocity spectrometer for high-precision searches of the K + → π + ν ν ¯ decay like it is planned in the CKM experiment.