Study of a proximity focusing RICH with a silica aerogel radiator

A proximity focusing RICH based on a silica aerogel Cherenkov radiator has been developed for a new particle identification device in the upgraded Belle detector. To further improve the detector performance, a new concept for Cherenkov ring imaging has been introduced, where multiple aerogel layers with different indices are accumulated to increase the number of detected photons without making the Cherenkov photon angle resolution worse. By constructing a prototype counter, this idea was validated in a test beam experiment, and a 5.5σπ/K separation at 4GeV/c with more than nine photoelectrons was achieved. We will present new results from a beam test as well as recent improvements on aerogel radiators.