Abstract
The alignment of the Belle II tracking system, composed of a pixel and strip vertex detectors and central drift chamber, is described by approximately sixty thousand parameters; from local alignment of sensors and wires to relative global alignment of the sub-detectors. In the next data reprocessing, scheduled since Spring 2021, we aim to determine all parameters in a simultaneous fit by Millepede II, where recent developments allow to achieve a direct solution of the full problem in about one hour and make it practically feasible for regular detector alignment. The tracking detectors and the alignment technique are described and the alignment strategy is discussed in the context of studies on simulations and experience obtained from recorded data. Preliminary results and further refinements based on studies of real Belle II data are presented.
Highlights
The Belle II detector at the SuperKEKB accelerator (KEK, Tsukuba, Japan) [1] has accumulated approximately 90 fb−1 of collision data by the end of 2020
4 Solution Methods and Computing Requirements In Millepede II [4], the alignment problem is solved by means of the linearized least squares method
For the large problem with wires, we have investigated an approximate solution method based on generalized minimization of residuals (GMRES, with about 1 hour solution time) as well as an exact solution via decomposition [5]
Summary
The Belle II detector at the SuperKEKB accelerator (KEK, Tsukuba, Japan) [1] has accumulated approximately 90 fb−1 of collision data by the end of 2020. At the beginning of Spring 2021, a reprocessing and recalibration of the whole dataset is planned to deliver the best available performance for physics analyses on top of a preliminary, so-called prompt calibration. The prompt calibration is performed regularly after data taking in blocks spanning several weeks, called buckets, and is mostly automated [2]. The alignment of the Belle II vertex detector and the drift chamber is unified [3] and solved simultaneously at local and global level. Local level alignment degrees of freedom are parameters of sensors and (optionally) wires. Global level alignment describes larger structures, like layers of the drift chamber. The main motivation for this approach is to avoid systematic biases, which naturally occur when the alignment of (correlated) local and global degrees of freedom, especially for different sub-detectors, is performed separately
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