Silvia De Benedictis ; Giulia Casarosa ; Francesco Forti

2022
University of Pisa Pisa

Abstract: The SuperKEKB B factory, home of the Belle II experiment, is a e+e− col- √ lider with a center-of-mass energy of Υ(4S) resonance. Its predecessor is KEKB, hosting the Belle experiment cred- ited, together with BABAR at PEP-II (SLAC), for measuring the CP violation in the BB ̄ system, among many other results. Belle II goal is to reach an inte- grated luminosity of 50 ab−1 that will allow to provide precision measurements in different sectors and search for deviations from the Standard Model that may be interpreted in the context of New Physics models. To obtain this huge data sample, the nominal instantaneous luminosity goal is set at 6.5 · 1035cm−2s−1, around 30 times the one at KEKB, resulting in a high beam background rate: to perform high precision measurements, an efficient rejection of the background is necessary. An important feature that can be exploited to distinguish the beam background hits from signals is their timing. The Belle II sub-detectors collect different time information of an event, such as the time the e+e− collision, or the time of a hit in the Silicon Vertex Detector: if a hit is temporally near to the time of the collision, it is more likely to be signal, while if the hit is off-time with respect to the time of the event, it is more likely to be a beam background hit. A fast and efficient timing system is therefore fundamental to provide a good selection on data. Belle II has a two-stage trigger system: a hardware trigger based on FPGA logic (Level 1 trigger), and a software trigger (High Level Trig- ger), that runs the full reconstruction online. There are stringent requirements on the execution time of the full reconstruction to avoid stopping the data ac- quisition because of the full trigger processing capacity (trigger busy). Among the processes in the online full event reconstruction, the estimate of the event time is one that takes the longest. My thesis work focuses on the estimate of the event time with the Silicon Vertex Detector (SVD), which is a 4-layer device based on double-sided strip detector readout by fast electronics (the APV25 chip developed for the CMS experiment). It has a high hit detection efficiency (>99% for most sensors) and a time resolution on crossing particles of 2-4ns. Its properties are briefly discussed, starting from the functioning principles of strip detectors, and going through the data acquisition modality, controlled by the APV25 front-end electronics, and the cluster (set of adjacent strips, crossed by a particle, that pass precise selection criteria) reconstruction. The event time is estimated with an algorithm, that I developed, that com- putes the event time as the average of the signal cluster time, measured by the SVD, weighted by the number of signal clusters. An optimization on different variables is carried out to provide a convenient estimate that keeps high absolute efficiency and resolution: among the cuts that are tried out on the variables, the chosen one is the selection on the transverse momentum of the particles, pT >250MeV/c. The performance of the SVD event time estimator can be compared to the estimates provided by other sub-detectors in terms of efficiency and resolution, evaluated both on simulated events with nominal back- ground, and on data, acquired in 2021, for different types of events (hadronic, muonic, bhabha). The performance is very good: ∼99.8% absolute efficiency, higher than the one of other detectors, and 8.6ns standard deviation on hadronic events. The resolution on the residuals between the event time computed by the Silicon Vertex Detector and by the other sub-detectors is O(1ns); moreover, the execution time in the online reconstruction of the module that computes the event time in the Silicon Vertex Detector is around 2000 times shorter than those of the other sub-detectors: thanks to this excellent result, the algorithm will be implemented in the next release of the Belle II software. The improved event time resolution and the high absolute efficiency should allow a better beam background rejection, and should make it easier to retain good physics performance while increasing the luminosity.

Note: Presented on 24 05 2022
Note: MSc