Belle II Track Reconstruction and Results from first Collisions

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.

Belle II Vertex Detector Performance

“…Belle II Vertex Detector Performance T. Bilka in the parameter space of plausible tangents to the drift circles. In comparison, the local approach looks for compatible neighbouring hits forming a track using a cellular automaton approach [4]. Track candidates found by global and local approach are merged and extrapolated using a Combinatorial Kalman Filter (CKF) [4].…”

“…In comparison, the local approach looks for compatible neighbouring hits forming a track using a cellular automaton approach [4]. Track candidates found by global and local approach are merged and extrapolated using a Combinatorial Kalman Filter (CKF) [4]. The CKF algorithm extrapolates the tracks inwards and looks for hits to attach.…”

Alignment for the first precision measurements at Belle II

“…The alignment procedure is integrated in CAF and largely profits from standard tools of basf2, using the same methods as standard Belle II reconstruction [7], from extrapolation in inhomogeneous magnetic field to track fitting and vertexing. For track fitting, the GENFIT toolkit [8] is used, which allows for generic treatment of various measurement types, extrapolation in detector material and integration of different track fitting methods.…”