Operational Experience and Performance of the Belle II Pixel Detector

Abstract

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 µm2. 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% X0 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.