RICH contains two different radiators in which charged particles with
speed exceeding that of light emit light. In vacuum no particle can go faster
than light, but in a medium like a gas or a solid it is possible. In that case
the charged particle emits light in a well-defined direction relative to the
particle trajectory. The particle gives rise to a cone of light which is
detected as a ring. The size of the ring depends on the particle type. With a
precise measurement of the light ring and knowledge of the momentum the
identity of the particle can be determined.
