Belle II Vertex Detector Performance

The Belle II experiment at the SuperKEKB accelerator (KEK, Tsukuba, Japan) collected its first e+e− collision data in the spring 2019. The aim of accumulating a 50 times larger data sample than Belle at KEKB, a first generation B-Factory, presents substantial challenges to both the collider and the detector, requiring not only state-of-the-art hardware, but also modern software algorithms for tracking and alignment. The broad physics program requires excellent performance of the vertex detector, which is composed of two layers of DEPFET pixels and four layers of double sided-strip sensors. In this contribution, an overview of the vertex detector of Belle II and our methods to ensure its optimal performance, are described, and the first results and experiences from the first physics run are presented

Thermal mock-up studies of the Belle II vertex detector

The ongoing upgrade of the asymmetric electron–positron collider SuperKEKB at the KEK laboratory, Japan aims at a 40-fold increase of the peak luminosity to $8  ×  10^{35}  cm^ {−2} s ^{−1}$ . At the same time the complex Belle II detector is being significantly upgraded to be able to cope with the higher background level and trigger rates and to improve overall performance. The goal of the experiment is to explore physics beyond the standard model with a target integrated luminosity of 50 ab$^{−1}$ in the next decade. The new vertex detector (VXD), comprising two layers of DEPFET pixel detectors (PXD) surrounded by 4 layers of double sided silicon strip detectors (SVD), is indispensable for vertex determination as well as for reconstruction of low momentum tracks that do not reach the central drift chamber (CDC). Within the confined VXD volume the front-end electronics of the two detectors will dissipate about 1 kW of heat. The VXD cooling system has been designed to remove this heat with the constraint to minimize extra dead material in the physics acceptance region. Taking into account additional heat intake from the environment the cooling system must have a cooling capacity of 2–3 kW. To achieve this goal evaporative two-phase CO$_2$ cooling in combination with forced $N_2$ flow is used in the VXD cooling system. In order to verify and optimize the cooling concept and to demonstrate that acceptable operation conditions for the VXD system as well as the surrounding CDC can be obtained, a full size VXD thermal mock-up has been built at DESY. Various thermal and mechanical tests carried out with this mock-up are reported

Thermal mock-up studies of the DEPFET pixel vertex detector for Belle II

The Belle II experiment currently under construction at the $e^+e^-$-collider SuperKEKB in Japan is designed to explore new physics beyond the standard model with an approximately 50 times larger data sample compared to its predecessor. The vertex detector (VXD), comprising a two layer DEPFET pixel detector (PXD) surrounded by four layers of double sided silicon strip detector (SVD), is indispensable for the accurate determination of the decay point of $B$ or $D$ meson as well as track reconstruction of low momentum particles. The DEPFET sensors in Belle II are thinned down to 75 $\mu$m with low power consumption and low intrinsic noise. In the DEPFET concept, the front-end electronics is placed outside of the sensitive area, and thus no cooling components are necessary inside the physics acceptance of the detector. Evaporative two-phase CO$_2$ cooling in combination with forced air flow has been chosen as the scheme for the PXD cooling. To guarantee the DEPFET detector operation condition and verify the cooling concept, a PXD mockup is constructed at DESY. Studies of the thermal and mechanical performance are presented in this paper