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 Υ(4S) resonance, holding the world luminosity record of 5.1×10^34 cm^-2 s^−1, reached in 2024.
The Vertex Detector (VXD) provides a vertex resolution of approximately 10 µm, allowing highly precise tracking and vertex reconstruction, essential for time-dependent measurements.
The VXD is designed to achieve this performance through two complementary sub-systems: the Pixel Detector (PXD) closest to the interaction point (IP), consisting of two layers of thin DEpleted
P-channel Field Effect Transistor (DEPFET)-based silicon sensors, and the Silicon Vertex Detector (SVD), consisting of four outer layers of double-sided silicon strip sensors (DSSD).
During Run 1 (2019-2022), a single layer of PXD and the full SVD were installed and matched the excellent performance which included high hit efficiency (> 99%).
We describe the challenges during the installation of the complete two-layer PXD (PXD2) in 2023, during the first long shutdown (LS1): PXD2 has now 40 modules, each consisting of DEPFET sensors with an array of 250 × 768 pixels, allowing an average material budget of 0.21% X0/layer.
Since February 2024, Belle II has resumed data-taking for Run 2, the VXD performed within specifications and we report our operational experience.
In May 2024 PXD2 was temporarily shut down to prevent further damage from uncontrolled beam losses and it will be powered-up again as soon as stable beam conditions are recovered.
To reach the target instantaneous luminosity of 6×10^35 cm^−2 s^−1, with the goal of collecting a data set 50
times larger than the previous B-factories, a redesign of the interacion region (IR) is needed.
The new QCS and the the backgroung shields are constraining further the detector envelope in the region of backward and forward polar angles, making the mechnaical integration quite challenging.
In such a luminosity range harsh beam backgrounds are expected leading to a maximal hit rate of up to 120 MHz/cm^2 for the inner detection layer, placed at a radius of 14 mm from the I.P. .
The radiation tolerance also reaches levels up to 5x10^14 (1MeV) n_eq/cm^2 and 1 MGy.
An intensive R&D program has been established to develop a new pixelated vertex detector (VTX) entirely made from a single type of depleted monolithic pixel detector (DMAPS), the OBELIX sensor, as a replacement of the Belle II vertex detector. The VTX strategy entails higher space-time granularity, lighter overall structure and services compared to the current operating vertex detector.
The baseline layout consists of two identical layers composing the inner part (iVTX) and four outer layers (oVTX), all arranged in a barrel-shaped geometry, with minimal material budget.
The ladder design is now being optimized considering the expected background hit rate and related power consumption at the target luminosity. With a power density in the range of 200-300 mW/cm2 depending on the hit rate, the ladder cooling is especially challenging. The system should maintain the sensor at room temperature to preserve the sensor functionality after irradiation, while minimising the material budget. The iVTX ladder concept is using a post-process redistribution layer (RDL) for distributing power and read out signals. Passive cooling using high-performance heat-drain material (TPG) over the ladder or, in alternative, active liquid cooling in thin aluminum pipes are currently under consideration. The oVTX design involves a ladder support structure inherited from the Alice-ITS2, with thin kapton cooling pipes integrated in a CF cooling plate hosting the chips, with a more traditional Aluminum flex for electrical connections.
This new tracking system, designed to be more robust against background sources, will converge in the TDR of the upgraded Belle2 detector, scheduled for the beginning of 2027. A long shutdown is expected to start in 2032 and the installation of the new VTX will ensure high-performance vertexing for the Belle II experiment needed to improve its search for new physics at the intensity frontier.
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