Limitations in the use of RICH counters to detect tau-neutrino appearance

A comprehensive simulation study of the feasibility of an earlier proposed tau-neutrino appearance experiment based on a RICH with liquid radiator (C//6F//1//4) has been performed. Tau leptons, that are created in charged current interactions of incident tau neutrinos with the same momentum spectrum as that of the planned Gran Sasso beam (average 17 GeV/c), have an average track length of about 0.5 mm. Cherenkov photons emitted in the liquid radiator from the short track of the tau lepton provide a characteristic pattern for tau- neutrino appearance. In the present report it is shown, on the basis of simulations performed with the GEANT4 code, that Cherenkov radiation from delta electrons, originating from the interaction of the long-lived tau decay products with the Cherenkov radiator, constitutes a severe background that renders the detection of the millimeter-short tau track unfeasible. Increasing the incident neutrino beam momentum results in longer tau tracks and therefore more signal photons, but this does not solve the background problem since the kink angle between the tau track and the secondary decay particle decreases with increasing neutrino momentum, leading to an increased overlap between the Cherenkov rings of these two particles.