Electromagnetic calorimeter for the Heavy Photon Search Experiment at Jefferson Lab

[摘要] The Heavy Photon Search Experiment (HPS) seeks to detect a hypothesised hidden sector boson, the A', predicted to be produced in dark matter decay or annihilation. Theories suggest that the A' couples weakly to electric charge through kinetic mixing, allowing it, as a result, to decay to Standard Matter (SM) lepton pair, which may explain the electron and positron excess recently observed in cosmic rays. Measuring the lepton pair decay of the A' could lead to indirect detection of dark matter. The HPS experiment is a fixed target experiment that will utilize the electron beam produced at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The detector set-up includes a silicon vertex tracker (SVT) and an Electromagnetic Calorimeter (ECal). The ECal will provide the trigger and detect e+e- pairs and its construction and testing forms the focus of this thesis.The ECal consists of 442 PbWO4} tapered crystals with a length 16cm and a 1.6x1.6cm^2 cross-section, stacked into a rectangular array and are coupled to Large Area APDs and corresponding pre-amplifiers. Supplementary to the ECal is a Light Monitoring System (LMS) consisting of bi-coloured LEDs that will monitor changes in APD gain and crystal transparency due to radiation damage.Before construction of the ECal each of the components were required to be individually tested to determine a number of different characteristics. Irradiation tests were performed on PbWO4 ECal crystals and, as a comparison, one grown by a different manufacturer to determine their radiation hardness. A technique for annealing the radiation damage by optical bleaching, which involves injecting light of various wavelengths into the crystal, was tested using the blue LED from the LMS as apotential candidate. The light yield dependence on temperature was also measured for one of the PbWO4 crystal types.Each APD was individually tested to determine if they functioned correctly and within the requirements of the experiment, then arranged into groups of similar gain at chosen applied voltages, for connection to High Voltage (HV) supplies.Each bi-coloured LED was also tested to determine if they functioned within the specifications of the experiment; including their signal quality at high frequency and their radiation hardness.The HPS crystals were recycled from a previous Jefferson Lab detector, the Inner Calorimeter from CLAS, which needed to be dismantled and reconditioned using various removal and cleaning techniques. The HPS ECal was then constructed in a new formation using a combination of different gluing and construction techniques, and initial functionality tests were performed.

[发布机构] University:University of Glasgow;Department:School of Physics and Astronomy

[关键词] Jefferson Lab, Electromagnetic Calorimeter, Lead tungsten crystal, Heavy Photon Search, Dark matter, Light Monitoring System, Dark photon, Experimental Nuclear Physics.