Belle II Vertex Detector Performance

The Belle II experiment at the SuperKEKB accelerator (KEK, Tsukuba, Japan) collected its first e+e− collision data in the spring 2019. The aim of accumulating a 50 times larger data sample than Belle at KEKB, a first generation B-Factory, presents substantial challenges to both the collider and the detector, requiring not only state-of-the-art hardware, but also modern software algorithms for tracking and alignment. The broad physics program requires excellent performance of the vertex detector, which is composed of two layers of DEPFET pixels and four layers of double sided-strip sensors. In this contribution, an overview of the vertex detector of Belle II and our methods to ensure its optimal performance, are described, and the first results and experiences from the first physics run are presented

Operational Experience and Performance of the Belle II Pixel Detector

The Belle II experiment at the super KEK B factory (SuperKEKB) started its physics operation with the full detector setup in March 2019, and it aims at collecting 50 ab$^{−1}$ of $e^+e^−$ collision data. The vertex detector (VXD) of Belle II contains a 4-layer silicon vertex detector (SVD) using double sided silicon strips and an inner 2-layer pixel detector (PXD) that is based on the depleted P-channel Field Effect Transistor (DEPFET) technology. The signal generation and amplification are combined in pixels with a minimum pitch of 55 × 50 µm$^2$. The sensors are thinned down to 75 µm, and each module has interconnects and ASICs integrated on the sensor with silicon frames for mechanical support. This approach led to a material budget of around 0.21% X$_0$ per layer including the cooling structure in the acceptance region. The PXD has an integration time of around 20 µs, a signal-to-noise ratio of around 50 and a detecting efficiency of better than 99%. Its two layers are arranged at the radii of 14 and 22 mm around the interaction point, and an impact parameter resolution of better than 15 µm has been achieved. Due to its close proximity to the beam line and its sensitivity to few-keV photons, the PXD also plays an important role in background studies

Operational experience of the Belle II pixel detector

The Belle II experiment at the SuperKEKB accelerator has started its physics data taking with the full detector setup in March 2019. It aims to collect 40 times more e+e− collision data compared with its predecessor Belle experiment. The Belle II pixel detector (PXD) is based on the Depleted P-channel Field Effect Transistor (DEPFET) technology. The PXD plays an important role in the tracking and vertexing of the Belle II detector. Its two layers are arranged at radii of 14 mm and 22 mm around the interaction point. The sensors are thinned down to 75 μm to minimize multiple scattering, and each module has interconnects and ASICs integrated on the sensor with silicon frames for mechanical support. PXD showed good performance during data taking. It also faces several operational challenges due to the high background level from the SuperKEKB accelerator, such as the damage from beam loss events, the drift in the HV working point due to radiation effect, and the impact of the high background

DEPFET pixel detector in the Belle II experiment

The Belle II experiment will run with a reduced beam asymmetry and a factor of 40 higher instantaneous luminosity compared to the Belle experiment. To cope with this and to be able to perform high precision vertex measurements for charge conjugation parity violating processes, a pixel detector based on DEPFET technology will be installed in the center of Belle II. Its basic properties and the DAQ chain are presented in this article

Commissioning and performance of the Belle II pixel detector

The Belle II experiment at the SuperKEKB energy-asymmetric $e^+ e^−$ collider has completed a series of substantial upgrades and started collecting data in 2019. The experiment is expected to accumulate a data set of 50 ab$^{−1}$ to explore new physics beyond the Standard Model at the intensity frontier. The pixel detector (PXD) of Belle II plays a key role in vertex determination. It has been developed using the DEpleted P-channel Field Effect Transistor (DEPFET) technology, which combines low power consumption in the active pixel area and low intrinsic noise with a very small material budget. In this paper, commissioning and performance of the PXD measured with first collision data are presented

Belle II pixel detector: Performance of final DEPFET modules

A DEpleted P-channel Field Effect Transistor (DEPFET) based pixel detector was developed for the Belle II VerteX Detector (VXD). It is designed to achieve a good impact parameter resolution better than 15μm at the very high luminosity conditions of this experiment. In the first half of 2018 four final production modules have been deployed in the commissioning run of the detector and their performance is discussed

The Belle II vertex detector integration

The Belle II experiment comes with a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric collider with energies tuned to (4 ) resonance sqrt() = 10.588 GeV. The accelerator has successfully completed the first phase of commissioning in 2016 and the first electron\u2013positron collisions in Belle II took place in April 2018. Belle II features a newly designed silicon vertex detector based on DEPFET pixel and double-sided strip layers. Currently, a subset of the vertex detector is installed (Phase 2 of the experiment). Installation of the full detector (Phase 3) will be completed by the end of 2018. This paper describes the Phase 2 arrangement of the Belle II silicon vertex detector, with focus on the interconnection of detectors and their integration with the software framework of Belle II. Alignment issues are discussed based on detector simulations and first acquired data

Operational experience and commissioning of theBelle II vertex detector

The construction of the new accelerator at the Super Flavor Factory in Tsukuba, Japan, has been finalized and the commissioning of its detector (Belle II) has started. This new e+e− machine (SuperKEKB) will deliver an instantaneous luminosity of 8×1035 cm−2s−1, which is 40 times higher than the world record set by KEKB. In order to be able to fully exploit the increased number of events and provide high precision measurements of the decay vertex of the B meson systems in such a harsh environment, the Belle II detector will include a new 6 layer silicon vertex detector. Close to the beam pipe, 2 pixel and 4 double-sided strip detector layers will be installed. During its first data taking period in 2018, the inner volume of the Belle II detector was only partially equipped with the final vertex detector technologies. The remaining volume was covered with dedicated radiation monitors, collectively called BEAST II, in order to investigate the particle and synchrotron radiation backgrounds near the interaction point. In this note, the milestones of the commissioning of the Belle II vertex detector and BEAST II are reviewed and the detector performance and selected background measurements will be presented