Analysis of the scintillation light production and propagation in the WA105 Dual-Phase demonstrator

The next-generation long-baseline neutrino oscillation experiments aim to solve the main unanswered questions in neutrino physics, being sensitive to the observation of the CP violation in the lepton sector and the neutrino mass order. The Deep Underground Neutrino Experiment (DUNE) is one of these projects; the final design, still to be defined, consists of a far detector located 1300 km far away from the neutrino beam production at Fermilab and made of four 10-kton liquid argon (LAr) time projection chambers (TPCs), in single and dual-phase (DP) configurations. Although the LAr is a conventional medium for neutrino detectors, none of the two technologies have been worked with such a large volume. Consequently, the operation of small prototypes is crucial for demonstrating their scalability toward the giant scale. In DP LAr-TPCs, a pocket of gas argon (GAr) is kept above the LAr mass, allowing the extraction and amplification of ionization electrons before their collection at the anode plane. This configuration is particularly advantageous in large detectors because the charge amplification helps cover longer drift paths. In order to validate the suitability of the DP technology for DUNE, two prototypes with an increasing LAr active volume have been operated: the WA105-DP demonstrator, of 3x1x1 m3 (~4.2 tons), and ProtoDUNE-DP, of 6x6x6 m3 (~300 tons); both have been operated at CERN and exposed to cosmic muons for several months. The WA105-DP detector demonstrated the operability of the DP LAr-TPC at the ton scale, allowing to achieve important technological milestones, such as the extraction and amplification of ionization electrons over a 3m2 surface. In these kinds of detectors, the photon detection system (PDS) is used for triggering and it is expected to be valuable for discriminating non-beam or low-energy events. This thesis is dedicated to studying the scintillation light collected during the whole operation period of the WA105-DP prototype. The first part of the dissertation is focused on the study of the PDS performance; the main outcomes have been crucial to validating the design of this system for bigger DP LAr-TPC, as ProtoDUNE-DP and the DUNE DP far detector module. The second part of the thesis is devoted to the detailed characterization of the two light signals respectively produced in the liquid (primary scintillation light, S1) and gas (electroluminescence light, S2) Ar phases. Pioneering results have been achieved from the study of the primary scintillation light, which allowed to deepen the understanding of the LAr micro-physics. In particular, a detailed characterization of the scintillation light produced by cosmic muons crossing a DP LAr-TPC operated at the ton scale is presented. For the first time, the dependence of the scintillation time profile parameters on the electric drift field strength has been reported. The thorough knowledge of the S1 production and propagation in the LAr bulk contributed to a good improvement of the current simulations for physics sensitivity studies in big LAr detectors. Ultimately, the development of an algorithm dedicated to the reconstruction of the electroluminescence light signal led to its characterization, allowing the measurement of the drift velocity of ionization electrons, under low electric fields. Finally, for the first time, the impact of the amplification field on the S2 signal has been studied.