Studies of a proximity focusing aerogel RICH for the Belle upgrade

For future upgrade of the Belle detector at the KEK B-factory we are developing a proximity focusing ring imaging Cherenkov detector using aerogel as a radiator. We present results from recent beam tests using 4times4 array of the flat-panel multi-anode PMTs. We introduce a new design, so called "dual radiator" scheme to improve the performance. In this design, we image Cherenkov photons from two aerogel radiators with different refractive indices. The results of the beam tests show that the photoelectron yield can be increased without significant deterioration in the angular resolution, leading to 4.8sigma K/pi separation at 4 GeV/c. We also discuss multi-radiator RICH, which consists of more than two radiators.