Belle II track finding and hit filtering using precise timing information

Abstract

The SuperKEKB accelerator and the Belle II experiment constitute the second-generation asymmetric energy B-factory. SuperKEKB has recently set a new world record in instantaneous luminosity, which is anticipated to further increase during the upcoming run periods up to $6\cdot 10^{35}\,\mathrm{cm}^{-2}\mathrm{s}^{-1}$. An increase in luminosity is challenging for the track finding as it comes at the cost of a significant increase of the number of background hits. The Belle II experiment aims at testing the Standard Model of particle physics and searching for new physics by performing precision measurements. To achieve these physics goals, including e.g. time-dependent measurements, the track finding and fitting has to deliver tracks with high precision and efficiency. As the track reconstruction is part of the online high level trigger system of Belle II there are also stringent requirements on the resource usage. The Belle II tracking system consists of 2 layers of pixelated silicon detectors, 4 layers of double sided silicon strip detectors (SVD), and the central drift chamber. We will present the general performance and working of the track reconstruction algorithm of Belle II. In particular we will focus on the usage of hit time information from the silicon strip detector. The SVD has a very precise determination of the hit time, which will be used for the first time in the Belle II track finding in the next data taking period. These hit times are used for hit filtering, estimation of the time of collision, and the determination of the time of individual tracks. All of these are important tools to help to cope with the anticipated increase in background hits caused by the increase in luminosity.