The new and complete Belle II DEPFET pixel detector: Commissioning and previous operational experience

Abstract

The Belle II experiment at the SuperKEKB collider in Tsukuba, Japan, has collected e+e− collision data between 2019 and 2022. After reaching a record-breaking instantaneous luminosity of 4.71 × 1034cm-2s-1 and recording a dataset corresponding to 424fb-1, it completed its first planned long shutdown phase in December 2023. Aside from upgrades of the collider and detector maintenance, the shutdown was used for the installation of the two-layer Pixel VerteX Detector (PXD). As the innermost sub-detector, multiple scattering effects need to be reduced. PXD utilizes the Depleted P-channel Field Effect Transistor (DEPFET) technology, allowing for a material budget of 0.21% X0 per layer. Each of the tracker’s 40 modules consists of an array of 250 × 768 pixels with a pitch ranging from 50µm × 55µm for the inner to 85µm × 55µm for the outer layer yielding high gain and high signal-to-noise ratio while retaining about 99% hit efficiency. This article discusses the experience of the 4-year operation of the previous single-layer PXD in harsh background conditions as well as commissioning and testing of the fully-populated PXD2 during Long Shutdown 1.