Caratterizzazione del sistema di calibrazione del rivelatore TOP a BELLE II

The TOP detector is one of the elements of the Belle II experiment, an ongoing upgrade of the Belle experiment located at the KEK facility in Japan. Its goal is to investigate phenomena beyond the Standard Model of fundamental interactions; in this context, the TOP detector is a crucial tool providing a reliable particle identification and fast trigger signal for the other subdetectors. It is based on production and reflection of Cherenkov radiation in quartz bars, gathered and detected by Micro-Channel-Plate PhotoMultiplier Tubes (MCP-PMTs). This detector is equipped with a calibration system used both for initial testing and for subsequent re-calibration of the electronics during the run. It is composed of bundles of Single Mode and Multi Mode optical fibers which carry the light produced by laser sources and illuminate the arrays of PMTs, acting as an emulation of the Cherenkov light produced by charged particles. The goal of this work was to study this calibration system, reproducing it and analyzing how the light propagates inside SM and MM fibers, how the MCP-PMTs detect it, and how the time resolution changes depending on a series of external parameters; so we reproduced this set-up in our laboratory on a simpler scale, we then studied the dependence of time resolution on external parameters such as the employment of an external trigger for the laser. We then analyzed the laser beam profiles using a commercial camera, after studying it response and calibrate its sensor. After that, we studied the beam spot sub-structures that appear after the light travels through MM fibers by doing a series of measurements for different positions and calculating the average time resolution for each of them. We found the time resolution not being dependent on the optical path inside fibers: this result had been predicted when the calibration system was built, but this is the first experimental proof of it. These measurements provided a way to characterize the whole calibration system without having access to it at the TOP location, and their results will be useful for a better understanding of the optical fibers chain, and consequently for the final testing of the whole calibration system. This has been the first end-to-end test of that entire system, and a proof of its correct design.