Speaker
Description
Since 2019, the Belle II experiment at the SuperKEKB ($B$-factory) in Tsukuba, Japan has been collecting data from asymmetric-energy $e^-e^+$ collisions at the $\Upsilon({4S})$ resonance. It holds the world luminosity record of $5.1 \times 10^{34}\ \mathrm{cm}^{-2}\ \mathrm{s}^{-1}$ recorded in 2024, with the goal of collecting a data set 50 times larger than the predecessor experiments Belle and BaBar.
The Vertex Detector (VXD) provides a vertex resolution of approximately 10 $\mu$m, allowing highly precise tracking and vertex reconstruction, which is essential for time-dependent measurements. The VXD is designed to achieve this performance through two complementary subsystems: closest to the interaction point (IP), the Pixel Detector (PXD), which consists of two layers of thin DEpleted P-channel Field Effect Transistor (DEPFET)-based silicon sensors, and four outer layers of double-sided silicon strip sensors (DSSD), known as the Silicon Vertex Detector (SVD).
During Run 1 (2019-2022), a single layer of PXD and the full SVD, consisting of 172 DSSD modules, were installed and delivered the excellent performance which included high hit efficiency $(> 99 \%)$. In 2023, during the first long shutdown (LS1), a new fully populated two-layer PXD (PXD2) was installed with the same SVD. PXD2 has 40 modules, each consisting of an array of 250 × 768 pixels, with pixel sizes ranging from 50 $\mu$m × 55 $\mu$m to 50 $\mu$m × 85 $\mu$m. PXD utilizes DEPFET sensors, allowing an average material budget of 0.21% $X_0$ per layer.
Since February 2024, Belle II has resumed data-taking for Run 2, and the VXD performed within specifications until May 2024, when PXD2 was temporarily shut down due to partial damage from two uncontrolled beam losses. The following talk will discuss the details and performance of both the PXD and SVD during Runs 1 and 2. In addition, we present the development of a next-generation pixelated vertex detector (VTX), based entirely on depleted monolithic active pixel sensors (DMAPS), specifically the OBELIX sensor, which aims to collect 50 $\mathrm{ab}^{-1}$ of data and increase the luminosity up to $6 \times 10^{35}\ \mathrm{cm}^{-2}\ \mathrm{s}^{-1}$ during upcoming run periods. This new system will replace the full VXD and promises improved spatial resolution, reduced readout integration time, reduced material budget, and lower power consumption. It is designed to operate at high hit rates and high radiation levels. This transition marks a strategic shift toward ensuring continued high-performance vertexing as Belle II advances its search for new physics at the intensity frontier.
