Construction, Commissioning, and Installation of the Cylindrical GEM Inner Tracker of the BESIII Experiment

Abstract: BESIII (BEijing Spectrometer III) is a particle physics experiment with a vast physics program centered around the study of charmonium and the $\tau$ lepton. The performance of the spectrometer's inner tracker, the innermost part of a large drift chamber, has been degrading due to aging phenomena related to the large particle rate. Planned upgrades to the BEPCII (Beijing Electron Positron Collider II) collider, servicing the experiment, may further aggravate the problem, with the risk of disrupting the data taking. The Italian component of the BESIII collaboration proposed a detector based on cylindrical GEM (Gas Electron Multiplier) technology to replace the aging inner tracker. The new detector aims to improve the current tracker's spatial resolution in the beam direction at least a factor 2 and to ensure the continuation of BESIII's data taking until its end in 2030. After more than 10 years of design, development, and construction the three layers of the CGEM-IT (Cylindrical GEM Inner Tracker) are finally being installed in the spectrometer. This thesis describes the three final years of the detector's development, which led from diagnosing and resolving mechanical issues preventing the largest layer from powering on to securing the approval of the experiment's internal review committee for installation. Particular focus is given to the technological solutions adopted to overcome the challenges encountered during the development process, which often required a complete rethinking of previous methods and procedures. The thesis concludes with a snapshot of the ongoing installation of the detector, commenting on the results of the preparatory work undertaken to ensure its success.