Developments of Hadron Blind Detector for $\phi \to $ e$^{+ }$e$^{- }$measurements at J-PARC

We are proposing a new experiment at J-PARK to measure mass modifications of $\phi$ meson in nucleus using $\phi \quad \to $ e$^{+}$e$^{-}$ decays. In the experiment, a Hadron Blind Detector (HBD) is proposed for the electron identification. The HBD is originally developed for the PHENIX experiment at BNL and modified for the experiment. Current design has a 50cm Cerenkov radiator operated with pure CF$_{4}$ directory coupled to a 3layers-GEM detector with CsI photocathode and a pad-readout is used. CsI is directory evaporated on the top of GEM foil and used as a photocathode. In our case, the GEM foil has 100$\mu $m thickness and this is a highly new feature done by our group. The main advantage of this scheme is that 100$\mu $m thickness GEM has larger effective gain than 50$\mu$m thickness GEM. By virtue of this, photoelectrons from Cerenkov radiation to be amplified higher by the first photocathode-GEM foil. Thus, we carry out a stable operation. We measured the quantum efficiency of CsI photocathode GEM in pure CF$_{4}$ and a test experiment using electron-beam at LNS GeV-$\gamma $ beam-line at Tohoku University is done to evaluate performance of the HBD. As a result of the test, we succeeded in detecting Cerenkov radiation using the HBD.