Riccardo Manfredi ; Diego Tonelli ; Lorenzo Vitale

2019
Università di Trieste Trieste

Abstract: This is an experimental particle-physics thesis aimed at preserving and optimizing the charged-particle tracking performance of the Belle II experiment. Belle II is a detector designed to reconstruct billions of decays of heavy mesons and tau leptons from 10 GeV electron-positron collisions in search for indirect indications of non-Standard-Model physics. The first part of the work focuses on the data-commissioning of the radiation-monitoring system based on synthetic-diamond sensors. Diamonds protect the inner subdetectors from radiation due to the intense colliding beams. As a local expert in Japan during the critical phase of early data taking, I optimized diamond operations by providing daily maintenance, analysis and troubleshooting of unexpected events, operation improvements, and beambackground analyses. The second part describes a novel method I proposed for determining the charged-particle finding efficiency, based on D*+->D0[->K0S(->pi+pi-)pi+pi-]pi+ decays. The key idea is that the small (Q < 7MeV) kinetic energy available in the D*+->D0pi+ transition imposes sufficient kinematic constraints to distinguish a signal in the D*+ - D0 mass-difference even if one pion from the D0 decay is not reconstructed. The ratio between yields of fully reconstructed decays and decays reconstructed without that pion determines the effciency for reconstructing the unbiased pion. My work resulted in significantly streamlined and improved operations of the diamond system, ensuring smoother, more efficient data taking, and significantly safer conditions for the tracking detectors. In addition, it introduces in Belle II an innovative approach to determine the track-finding efficiency that relies only on data and probes previously inaccessible ranges in momentum, complementing and supplementing existing methods.

Note: Presented on 20 09 2019
Note: MSc