Silicon vertex detector of the Belle II experiment
Authors: Mondal S. , Adamczyk K. , Aggarwal L. , Aihara H. , Aziz T. , Bacher S. , Bahinipati S. , Batignani G. , Baudot J. , Behera P.K. , Bettarini S. , Bilka T. , Bozek A. , Buchsteiner F. , Casarosa G. , Corona L. , Das S.B. , Dujany G. , Finck C. , Forti F. , Friedl M. , Gabrielli A. , Gobbo B. , Halder S. , Hara K. , Hazra S. , Higuchi T. , Irmler C. , Ishikawa A. , Jin Y. , Kaleta M. , Kaliyar A.B. , Kandra J. , Kang K.H. , Kodyš P. , Kohriki T. , Kumar R. , Lalwani K. , Lautenbach K. , Leboucher R. , Lee S.C. , Libby J. , Martel L. , Massaccesi L. , Mohanty G.B. , Nakamura K.R. , Natkaniec Z. , Onuki Y. , Otani F. , Paladino A. , Paoloni E. , Park H. , Polat L. , Rao K.K. , Ripp-Baudot I. , Rizzo G. , Sato Y. , Schwanda C. , Serrano J. , Shimasaki T. , Suzuki J. , Tanaka S. , Tanigawa H. , Tenchini F. , Thalmeier R. , Tiwary R. , Tsuboyama T. , Uematsu Y. , Vitale L. , Wang Z. , Webb J. , Werbycka O. , Wiechczynski J. , Yin H. , Zani L. ,   The Belle-II SVD collaboration
Published: February 2024

Abstract
The silicon vertex detector (SVD) is installed at the heart of the Belle II experiment,
taking data at the high-luminosity B-Factory SuperKEKB since 2019. The SVD is a four-layer
double-sided strip detector with tracking and particle-identification capabilities. In this paper,
we report on the performance of the reconstruction of SVD hits. The detector has shown a stable
and above-99% hit efficiency, with a large signal-to-noise in all sensors since the beginning of
data taking. Cluster position and time resolution have been measured with 2020 and 2022 data and
show excellent performance and stability. In particular, the cluster-position resolution is
between 7 and 12 μm for the small-pitch sensors, in reasonable agreement with the
expectations, while the cluster time resolution is measured to be below 3 ns. The effect of
radiation damage is visible, but not affecting the performance. As the luminosity increases,
higher machine backgrounds are expected and the excellent hit-time information in SVD can be
exploited for background rejection. In particular, we have recently developed a novel procedure to
select hits by grouping them event-by-event based on their time. This new procedure allows a
significant reduction of the fake rate, while preserving the tracking efficiency, and it has
therefore replaced the previous cut-based procedure. We have developed a method that uses the SVD
hits to estimate the track time (previously unavailable) and the collision time. It has a similar
precision to the estimate based on the drift chamber readout but its execution time is three
orders of magnitude smaller, allowing a faster online reconstruction that is crucial in a high
luminosity regime. The track time is a powerful information that allows, together with the
aforementioned grouping selection, to raise the occupancy limit above that expected at nominal
luminosity, leaving room for a safety factor. Finally, in June 2022 the data taking of the Belle
II experiment was stopped to install a new two-layer DEPFET detector (PXD) and upgrade components
of the accelerator. The whole silicon tracker (PXD+SVD) has been extracted from Belle II, the new
PXD installed, the detector closed and commissioned. We briefly describe the SVD results of this
upgrade.