to misidentify a pion as a kaon. The related probabilities, which are the usual measures of signal detection and background rejection in real data, are plotted below to show their variation with momentum (simulation with 10% random background); Note that the ~30% π → k misidentification rate at 100 GeV is, for the most part, due to the presence of 10% background hits (faking photons) in the simulated detector; the 3-sigma separation in the mean Cherenkov angle (shown in the 4th plot above) would, by itself, only account for about 6% misidentification. More detailed analyses of the above type, for operational RICH detectors, can be found in the published literature. For example, the LHCb experiment at the CERN LHC studies, amongst other B-meson decays, the particular process B0 → π+π−. The following figure shows, on the left, the π+π− mass distribution without RICH identification, where all particles are assumed to be π ; the B0 → π+π− signal of interest is the turquoise-dotted line and is completely swamped by background due to B and Λ decays involving kaons and protons, and combinatorial background from particles not associated with the B0 decay. On the right are the same data with RICH identification used to select only pions and reject kaons and protons; the B0 → π+π− signal is preserved but all kaon- and proton-related backgrounds are greatly reduced, so that the overall B0 signal/background has improved by a factor ~ 6, allowing much more precise measurement of the decay process. RICH Types Both focusing and proximity-focusing detectors are in use. In a focusing RICH detector, the photons are collected by a spherical mirror with focal length and focused onto the photon detector placed at the focal plane. The result is a circle with a radius , independent of the emission point along the particle's track (). This scheme is suitable for low refractive index radiators (i.e., gases) with their larger radiator length needed to create enough photons. In the more compact proximity-focusing design a thin radiator volume emits a cone of Cherenkov light which traverses a small distance, the proximity gap, and is detected on the photon detector plane. The image is a ring of light the radius of which is defined by the Cherenkov emission angle and the proximity gap. The ring thickness is mainly determined by the thickness of the radiator. An example of a proximity gap RICH detector