Physics

Particle identification (PID) for hadrons and leptons over a large range of solid angle and momenta is an essential requirement for meeting the physics objectives at the HESR. Charged particles in a medium with index of refraction n propagating with velocity βc > 1/n emit radiation at an angle θc = arccos(1/βn). Thus, the mass of the detected particle can be determined by combining the velocity information determined from θc with momentum information from the tracking detectors. An example of the θc versus momentum correlation is presented in the following figure for the relevant particles at HESR traversing a medium with n = 1.47. Due to the strong variation of the typical particle momentum with polar angle, the particle identification can be achieved by 2 types of ring imaging Cherenkov counters in the TS with different values of n. Within the TS, at polar angles between 22°-140°, particle identification can be performed by the detection of internally reflected Cherenkov (DIRC) light as realized in the BaBar detector [1]. It will be produced in 1.7 cm thick quartz slabs surrounding the beam line at a radial distance of 48 cm. The main costs of the BABAR setup arises from the 11000 photomultiplier tubes, which allow timing information to be used to suppress the large background resulting from the electron beam environment. At HESR this cost could be reduced by employing gas chambers with multi-pad read-out. The DIRC also serves for the distinction between gammas and relativistic charged particles entering the EM-calorimeter behind and is, therefore, part of the trigger.