Abstract
The Belle II experiment has started to take data in 2018, studying e+e- collisions at the KEK facility in Tsukuba (Japan), in a center of mass energy range of the Bottomonium states. The tracking system includes a combination of hit measurements coming from the vertex detector, made of pixel detectors and double-sided silicon strip detectors, and acentral drift chamber, inside a solenoid of 1.5 T magnetic field. Once the pattern recognition routines have identified the track candidates, hit measurements are fitted taking into account the different information coming from different detectors, the energy loss in the materials and the inhomogeneity of the magnetic field. Track fitting is performed by the generic track-fitting software GENFIT, which includes a Kalman filter improved by a deterministic annealing filter, in order to reject outlier hits coming from not correctly associated hits by the pattern recognition. Several mass hypotheses are used in the fit, in order to achieve the best track parameter estimation for each particle kind. This article presents the design of the track fitting in the Belle II software, showing results in terms of track parameter estimation as well as computing performances.
Highlights
The Belle II experiment has started to take data in 2018, studying e+e− collisions at the KEK facility in Tsukuba (Japan), in a center of mass energy range of the Bottomonium states
The vertex detector (VXD) is the innermost part and surrounds the interaction region; it consists of two sub-detectors: a Pixel Vertex Detector (PXD), which includes two layers of pixelated sensors based on Depleted P-channel Field Effect Transistor (DEPFET) technology, and a double-sided Silicon strip Vertex Detector (SVD), with four layers of silicon strip sensors
In the Central Drift Chamber (CDC), since a large number of hits is produced by machine-induced background, first a boosted decision tree classifier removes noisy hits and cleans the hit sample; the surviving hits are used by two algorithms, one based on a Legendre track finder, the other based on Cellular Automaton, and the found track candidates are merged together
Summary
The Belle II experiment [1] aims to study e+e− collisions at the SuperKEKB accelerator in Tsukuba, Japan, with unprecedented design beam luminosity of 8 × 1035cm−1 s−1. The physics program is focused on precise determinations of fundamental physical constants and the search for physics beyond the Standard Model. To achieve these goals a very high precision tracking system is mandatory, in order to obtain particle momenta and decay vertices with high accuracy. The Belle II experiment features three different tracking detectors. The information coming from the different systems must be merged together and the position provided by the detector hits must be fitted in order to extract track parameters of each. This paper will focus on the strategies adopted by Belle II for track fitting, showing preliminary results coming from the first collisions which were recorded in 2018 with a partial setup
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