Alignment of resistive MicroMegas detectors for T2K’s near detector high-angle time projection chambers

Sammanfattning

This Master’s degree project report studies through cosmic ray data the alignment of Encapsulated Resistive Anode MicroMegas (ERAM) detectors inside the newly updated High-Angle Time Projection Chambers (HA-TPC) in the near detector ND280 of the T2K experiment. By alignment, we mean here the determination of the geometrical values of the parameters describing the horizontal and vertical translations but also the rotation angle of a top ERAM with respect to its bottom neighbor right underneath it in the same HA-TPC module. Such a determination is pivotal as misalignment degrades the near detector’s spatial resolution and thus its momentum resolution (i.e. its ability to reconstruct the momentum of a crossing particle and, therefore, its energy in the case of quasi-massless neutrinos). To perform ERAM column alignment, two main studies have been conducted: firstly, a geometrical misalignment analysis never done before, which permitted to describe key proxies justifying the misalignment and the relationships between these proxies and the alignment parameters at stake, and secondly the application of this new understanding of geometric misalignment to the actual ERAM detectors inside ND280 through the study of cosmic rays collected in 2024 and their approximation through Least Squares Method (LSM). As the checking of the geometrical analysis has been performed through a Monte-Carlo data analysis, this toy work also permitted to unveil a very visual methodology permitting to find the alignment parameters from a misaligned geometry. This Monte Carlo phase successfully cross-verified the preceding analytical study, enabling a data-driven alignment which, through numerical optimization algorithms, allowed to define precise alignment parameters to apply to future detector simulations in order not to suffer from the geometric misalignment of ERAM units. As an order of magnitude, top ERAMs are generally displaced of about 100 μm horizontally, 1 mm vertically but also rotated of about 1 mrad from their nominal position above theirbottom neighbors.