Abstract
On March 25th 2019, the Belle II detector recorded the first collisions delivered by the SuperKEKB accelerator. This marked the beginning of the physics run with vertex detector. The vertex detector was aligned initially with cosmic ray tracks without magnetic field simultaneously with the drift chamber. The alignment method is based on Millepede II and the General Broken Lines track model and includes also the muon system or primary vertex position alignment. To control weak modes, we employ sensitive validation tools and various track samples can be used as alignment input, from straight cosmic tracks to mass-constrained decays. With increasing luminosity and experience, the alignment is approaching the target performance, crucial for the first physics analyses in the era of Super-BFactories. We will present the software framework for the detector calibration and alignment, the results from the first physics run and the prospects in view of the experience with the first data.
Highlights
The Belle II detector at the SuperKEKB e+e− accelerator (KEK, Tsukuba, Japan) [1] recorded its first collision data in the full configuration 1 on March 25th 2019
To reduce possible systematic effects, a single alignment procedure is in place for interaction region (IR), vertex detector (VXD) and central drift chamber (CDC), which involves simultaneous internal and relative alignment of all the components
Alignment of the KL and muon system (KLM) is integrated in the common procedure and several calibrations related to the CDC are considered or under development to be included as well
Summary
The Belle II detector at the SuperKEKB e+e− accelerator (KEK, Tsukuba, Japan) [1] recorded its first collision data in the full configuration 1 on March 25th 2019 It accumulated about 6.5 fb−1 in the Spring run and in total about 10.57 fb−1 in 2019. Good alignment is of utmost importance namely for precision measurements of time-dependent CP violation – silicon sensors must be aligned significantly better than 10 μm to prevent degradation of resolution Reaching this goal involves alignment of the primary interaction region (IR) position and internal alignment of the vertex detector (VXD), and relative alignment of the vertex detector with respect to the rest of the tracking system - the central drift chamber (CDC), as well as internal alignment of CDC itself. Results primarily from a reduced problem applied to the first cosmic and collision data are used to demonstrate the performance of the method
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