Operational experience and commissioning of the Belle II vertex detector

Abstract

The Belle II experiment has completed the Phase 2, with a subset of the final Belle II vertex detector comprising six layers with a single ladder per layer. First physics collisions have been successfully recorded at the end of April 2018 and and a total luminosity of 500 pb−1 have been collected for physics studies. All BEAST II detectors were operational and provided valuable online feedback for machine commissioning and safe operation of Belle II during Phase 2. SVD key operation features like signal to noise ratio and hit time resolution are within or exceeding TDR expectation. PXD demonstrated stable operation of four half-ladders with low noise and an excellent signal to noise ratio. Background from soft photons in the energy range 7-10 keV was observed in the PXD, FANGS and radio-chromic foils and is in qualitative agreement with simulation of synchrotron radiation. Single beam background from the low energy ring is in reasonable agreement with MC prediction within a factor 3-10 for SVD and PXD while the the background from the high energy ring is underpredicted. The observed HER and LER background rates are much more similar than predicted and the overall sum of backgrounds is about 10 times too high for efficient operation of the VXD at full luminosity. Ways for mitigating the backgrounds by tuning the accelerator and installing additional collimators are currently under study. The Phase 2 run confirmed that the full VXD could be safely installed for the early part of Phase 3 in March 2019.