A development of HBD for the J-PARC E16 experiment

Spontaneous breaking of the chiral symmetry is considered to be the origin of hadron mass. The chiral symmetry is expected to be (partially) restored in finite density and the hadron mass is predicted to decrease, even at the normal nuclear density. The J-PARC E16 experiment was proposed to investigate the origin of the mass through the mass modification of 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 Cherenkov detector with a large acceptance and with fine segmentations is required for the electron identification. HBD is ideal for the purpose; it is a mirror-less, windowless Cherenkov detector with a stack of Gas Electron Multipliers (GEMs) on top of which CsI is evaporated. CF4 acts as amplification gas and as Cherenkov radiator. We have developed a prototype of HBD for the J-PARC E16 experiment. It is constructed with two layers of Liquid-Crystal-Polymer (LCP) GEMs with a thickness of 100μm each, on the uppermost surface CsI is evaporated. The use of thicker GEMs leads to a larger gain at the first stage and to tolerance to sparks, compared to regular 50μm Kapton-GEMs. With a threshold at ∼16 electrons, a pion rejection factor of 100 can be achieved, which is required for the E16 experiment. A beam test was performed with a positron beam and the prototype successfully detected the Cherenkov radiation. The number of photoelectrons observed was about 5-6. Although these results were not satisfactory to reach a pion rejection factor of 100 with a reasonable efficiency at this point, improvements towards the goal are expected from photocathode and gas purity.