In the RICH technique, high-speed particles passing through a medium generate an optical shock wave ­ Cherenkov radiation ­ whose opening angle depends on particle velocity. If the momentum is known from the curvature in a magnetic field, then the particle's mass, and hence its identity is revealed. Over the years, this basic RICH idea has been improved and embellished by a series of technical innovations (June 1996, page 13). The newly installed RICH detector of the HERA-B experiment was recently tested successfully in the HERA proton beam with the experiment's wire target in place. There were several Cherenkov rings which perfectly matched the shapes that were expected.

In the HERA-B RICH 106 m3 gas tank filled with C4F10, charged particles radiate a cone of Cherenkov light. This is focused by a system of 80 spherical mirrors and 36 planar mirrors, and a special lens system boosting the light collection efficiency to 65%. Light photons are detected by a total of 2250 multi-anode photomultipliers, with an inner region using 16-channel devices and coarser 4-channel tubes outside. Most parts of the detector were installed during last winter's HERA shutdown.

For the first test using 820 GeV protons, the radiator vessel was filled with air and the wire target tuned to a nominal interaction rate in the range of 10 to 30 MHz. An unbiased, random trigger initiated readout of the lower photon detector, the upper awaiting additional data acquisition electronics.

Under these conditions, most photons were expected to come from relatively soft electrons and positrons from electromagnetic showers, a picture consistent with the observed events: many were blank or had a few isolated hits; others had a diffuse pattern of hundreds of hits. Some showed what appeared to be intersecting rings and a few had clear, isolated rings.

One particularly striking ring (see figure) was fitted to the expected shape, including optical corrections, yielding a Cherenkov angle of 23.5±0.2 mr, in perfect agreement with the expected value. Note that the number of Cerenkov photons is considerably larger than expected for a single track (7), indicating that the ring is probably due to an electron­positron pair from a high-energy photon.

HERA-B's goal is the investigation of CP violation in the B meson system. B mesons, plentifully produced when the halo of HERA's proton beam hits a wire target, will be detected in a large acceptance forward spectrometer.

HERA-B was approved in February 1995. After installation and test operation of various components, 1997 saw the installation of the electromagnetic calorimeter, the data acquisition system and the gas vessel for the RICH detector, which was completed during last winter's shutdown.

The HERA-B RICH group includes the University of Texas at Austin, Barcelona, LIP Coimbra, Northwestern, UT Houston, and University and J. Stefan Institute of Ljubljana. It is supported by the University of Hamburg and DESY.