The Beam-Dump eXperiment

Hadronic matter makes about 14% of the known universe. The remaining 86% is Dark Matter (DM). Since it does not interact with the ordinary matter via electromagnetic force, DM is not visible and, to date, it escaped detection. The search for DM is one of the hottest topic in modern physics. Despite the increasing number of astrophysical and cosmological observations proving the existence, so far no particle physics experiment has detected DM yet. In a popular class of models, DM is composed of particles with mass in the MeV-GeV range, interacting with the Standard Model via a new force, mediated by a massive vector boson, the Dark Photon or A'. BDX (Beam Dump eXperiment) is an approved experiment searching for DM in the Dark Photon theoretical scenario. The experiment makes use of CEBAF (Continuous Electron Beam Accelerator Facility) 11 GeV electron beam impinging on the Jefferson Lab (Jlab) Hall-A beam-dump. The interaction of the energetic electrons in the beam-dump may lead to the production of Dark Photons through a bremsstrahlung-like radiative process. Subsequently, the A's decay to DM particle-antiparticle pairs, which travel almost unaltered through the length of the dump. A ~ 1 cubic meter detector made of CsI(Tl) (Thallium doped Cesium Iodide) crystals is located in the trajectory of the DM beam, 20 m downstream of the beam-dump. A fraction of the DM particles scatter off atomic electrons in the detector giving rise to a detectable electromagnetic shower of approximately 100 MeV. Thanks to the cosmic background suppression and the high intensity of the electron beam (up to 65 ?A at 11 GeV), BDX will be able, with a 280 days run, to exceed by up to two orders of magnitude the sensitivity of current competitor experiments. A primary concern for BDX is in the estimation of beam related backgrounds from penetrating particles such as muons and neutrinos produced in the electron beam dump. A hodoscope detector, called BDX-HODO was placed downstream of the dump to measure the rates of these beam related processes and validate the necessary Monte Carlo simulations. Currently, the BDX collaboration is focused on the deployment and operation of a small detector, called BDX-MINI, built to perform a preliminary physics measurement searching for LDM at JLab. This test will pave the way to the realization of the full BDX experiment. This thesis is focused on the following topics: the muon flux measurement behind Jlab Hall A beam-dump with BDX-HODO, the evaluation of the BDX experiment reach, including an optimization study for the detector, the construction, commissioning and sensitivity estimate of the BDX-mini detector and the evaluation of the secondary positrons contribution to the sensitivity of BDX and other electron-beam thick-target experiments searching for DM in the Dark Photon paradigm.