Performance of the endcap RPC in the Belle detector under high luminosity operation of the KEKB accelerator

The endcap RPCs in the Belle detector at KEK B factory have operated stably with high detection efficiency since the start of the experiment, June 1999. In contrast with high luminosity operation of the KEKB accelerator, however, efficiency reduction due to high background rate has been observed in the outermost RPC layers in the endcap part of the KLM detector. To identify the background source, the background data is compared with simulation result using Geant4 simulation toolkit. Further, for higher luminosity operation of the accelerator, simulation result on the shield design for reducing the background rate is also reported.