Measurements of low mass image pairs in p+p and Au+Au collisions with the HBD upgrade of the PHENIX detector

The Hadron Blind Detector (HBD) was developed, installed and successfully operated in the PHENIX detector during RHIC Runs in 2009 (p+p) and 2010 (Au+Au). The HBD is a windowless Cherenkov detector, operated with CF4 gas in proximity focus configuration. It uses triple GEM elements for signal amplification with CsI photocathode evaporated on top of the first GEM. The purpose of the HBD is to reduce the combinatorial background from the dielectron invariant mass spectrum by recognizing and rejecting image or image originating from image Dalitz decays and conversion. This article reports on the in-beam performance of the HBD.