Commissioning of a hadron blind detector for dielectron measurement in pA reactions at J-PARC

Abstract The J-PARC E16 experiment measures mass spectra of low-mass vector mesons, ρ , ω , and ϕ , in nuclei via e + e - decays. Vector mesons are produced in p A reactions. A spectrometer with a large acceptance and a high-intensity beam up to 1 × 10 10 protons per ∼2-sec duration pulse are utilized to collect a sufficient number of vector mesons. We have developed a spectrometer, which has two types of electron identification detectors: a hadron blind detector (HBD) and a lead-glass calorimeter. Commissioning runs have been performed at the J-PARC high-momentum beam line. The HBD successfully observed on average 11 ± 1 photoelectrons for an incident electron. We achieved the pion rejection of 0.9 ± 0.2% with the electron detection efficiency of 61 ± 4% for the HBD, which was consistent of the expected performance.