##### Introduction

The Hadron Blind Detector (HBD) was a conceptually novel Cherenkov detector. Its primary aim was to recognize and reject tracks originating from π0 Dalitz decays and gamma-conversions, thus allowing to measure low mass (me+e- ⋜1GeV/c2) electron-positron pairs produced in central Au+Au collisions at RHIC energies. The main idea is to exploit the fact that the opening angle of electron pairs from these sources is very small compared to pairs from the vector mesons. The HBD was therefore located in a field-free region, where the pair opening angle was preserved. The field free region was created by an inner coil, installed in the central arms of PHENIX. This coil counteracted the main field of the outer coils and created an almost field-free region close to the vertex and extending to ≅50-60 cm in the radial direction.

##### Detector Design

The Hadron Blind Detector was a windowless Cherenkov detector with a 50cm long radiator operated with pure CF4, in a proximity focus configuration. For an introduction to GEM detectors please see

A triple GEM detector element avalanches the photoelectrons produced in a 350nm CsI photocathode, which is evaporated on the topmost Au plated GEM surface and produce a blob on the pad readout plane. The use of CF4 as a radiator and detector gas in a windowless geometry results in a very broad bandwidth (from 6 to 11.5 eV) and a very large figure of merit (N0∽800cm-1). A bias voltage is applied between the top GEM and the mesh. Depending on the direction of the bias field, charge produced by ionizing particles in the upper gap can either be collected by the GEM (FB = Forward Bias)(right panel), or by the mesh (RB = Reverse Bias)(left panel). In either configuration, photoelectrons produced on the photocathode are collected with good efficiency into the GEM due to the strong electric field near the holes. In the RB mode, only a very small amount of ionization charge produced very near the photocathode (within ∽150μm) is collected by the GEM. The FB mode is therefore sensitive to hadrons and other charged particles, while the RB mode is essentially sensitive only to the Cherenkov light produced by electrons and hence the term “Hadron Blind”. A comprehensive R&D program was carried out to demonstrate the concept validity including studies in the lab and also a beam test at KEK. The results are published in the two NIM papers.

The design and construction of the detector vessel as well as assembly and preliminary test of the GEM foils were carried out at the WIS whereas $\it{CsI}$ evaporation, final assembly and test of detector modules were done at the Stony Brook University. The analog and digital electronics were developed and built by BNL Instrumentation and Columbia University.

##### Select Theses
•    Measurements of di-electron production in Au+Au collisions at √sNN=200GeV (Yosuke Watanabe)
•    ω- and φ-meson production in p+p and d+Au collisions at RHIC energies, using the PHENIX Detector (Deepali Sharma)
•    Low Momentum Direct Photons as a Probe of Heavy Ion Collisions (Richard Petti)
•    Φ meson production in p+p, d+Au and Au+Au collisions at RHIC using the PHENIX detector (Maxim Naglis)
##### Internal Presentations
•    PHENIX Focus: Hadron Blind Detector (Jason Kamin)
##### Conference Presentations
•    Performance of PHENIX HBD in Au+Au central collisions (QM2011 poster) (Yosuke Watanabe)