Development of a Hadron Blind Detector for the J-PARC E16 experiment

The J-PARC E16 experiment was proposed to investigate the origin of mass through the mass modification of light vector mesons in a finite density environment. The masses of vector mesons, which are produced in p+A reactions, are measured through the electron-positron decays. A Čerenkov detector with a large acceptance and with fine segmentation is required for electron identification. Hadron Blind Detector (HBD) is ideal for purpose. It is a mirror-less, windowless Čerenkov detector with a triple stack of GEMs on top of which CsI is evaporated. CF₄ acts as amplification gas and as Čerenkov radiator. We have developed a prototype of HBD. A beam test was performed with a 1 GeV/c beam of negative particles (mainly pions) containing a few tens of percent of electrons. The number of photoelectrons observed was about 7. We achieved a pion rejection factor of 100 with the electron detection efficiency of 80 %, which is required for the E16 experiment.