The Belle II Pixel Vertex Detector

Sumitted to PubDB: 2024-11-27

Category: Proceeding, Visibility: Public

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Authors Anselm Baur
Non-Belle II authors PXD Collaboration
Date June 25, 2024
Belle II Number BELLE2-CONF-PROC-2024-032
Abstract The Belle II experiment at the SuperKEKB e+e−collider took data from 2019 to 2022 (Run 1) and is currently undergoing its planned first long shutdown (LS1). During its operational period, SuperKEKB achieved a record-breaking instantaneous luminosity of 4.7 x 1034 cm−2s−1[1] and Belle II recorded a dataset corresponding to 428 fb−1. The Belle II Pixel Vertex Detector (PXD), which is the innermost sub-detector, is based on the DEpleted P-channel Field Effect Transistor (DEPFET) technology [2]. Along with the Silicon Vertex Detector (SVD) [3], utilizing double-sided silicon strips, this forms the Belle II Vertex Detector system (VXD), enabling precise reconstruction of primary and decay vertices. The PXD module features a 75 μm-thin DEPFET sensor area with varying pixel sizes from 50 x 55 μm to 50 x 85 μm while maintaining a hit efficiency of about 99 %. Its average material budget inside the physics acceptance corresponds to 0.2 % X0 per layer. PXD consist of 20 ladders arranged in two cylindrical layers around the beam axis. The Run 1 PXD was installed in a reduced configuration comprising the full inner layer (L1, eight ladders) and only two out of twelve outer layer (L2) ladders. In this article, we will illuminate the performance and operational challenges observed during its 4-year operation in a harsh environment characterized by a high beam background level. A fully populated detector can compensate acceptance losses by redundancy and reduce the probability of wrong hit assignment introduced by higher background levels resulting from increased instantaneous luminosity. During LS1 PXD was replaced by a new, fully populated PXD2. Its commissioning and testing phase during LS1 will be described. A hit efficiency of >98 % was measured in most regions using cosmic particles. The current installation schedule foresees beam operation to resume in winter 23/24.

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