Abstract
In April 2018, e+e- collisions of the SuperKEKB B-Factory have been recorded by the Belle II detector in Tsukuba (Japan) for the first time. The new accelerator and detector represent a major upgrade from the previous Belle experiment and will achieve a 40 times higher instantaneous luminosity. Special considerations and challenges arise for track reconstruction at Belle II due to multiple factors. This high luminosity configuration of the collider increases the beam-induced background by many factors compared to Belle and a new track reconstruction software has been developed from scratch to achieve an excellent physics performance in this busy environment. Even though on average only eleven signal tracks are present in one event, all of them need to be reconstructed down to a transverse momentum of 50 MeV and no fake tracks should be present in the event. Many analyses at Belle II rely on the advantage that the initial state in B-factories is well known and a clean event reconstruction is possible if no tracks are left after assigning all tracks to particle hypotheses. This contribution will introduce the concepts and algorithms of the Belle II tracking software. Special emphasis will be put on the mitigation techniques developed to perform track reconstruction in high-occupancy events. First results from the data-taking with the Belle II detector will be presented.
Highlights
The Belle II experiment is operated at the asymmetric electron-positron collider SuperKEKB and recorded its first hadronic collision event at the end of April 2018
It is the successor of the Belle experiment, which ran from 1998 to 2010, and uses the same operating principle as the former KEKB collider to confirm the violation of CP symmetry in the B meson system
This results in large parts of the hits recorded in the PXD, SVD and central drift chamber (CDC) detectors to originate from background radiation sources and not from the primary interaction
Summary
The Belle II experiment is operated at the asymmetric electron-positron collider SuperKEKB and recorded its first hadronic collision event at the end of April 2018. Belle II is expected to record 50 times more collisions than Belle and to significantly increase the sensitivity of various searches and measurements [1] Both the detector [2] and the software [3] used to process the data have been upgraded to take advantage of technological developments and to be able to handle the greatly increased data rate. The innermost silicon-based tracking system VXD is a new development and consists of two layers of pixel sensors (PXD) and four layers of strip sensors (SVD), which are arranged in ladders around the interaction point Following this is the central drift chamber (CDC) which uses the technologies proven. The electromagnetic calorimeter and the K-Long and Muon Detector (KLM) complete the instrumentation of Belle II
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