Abstract
We discuss the track based alignment of the inner detector of the ATLAS experiment. After describing the main alignment method based on the minimization of hit residuals, we focus on alignment methods using information from the electromagnetic calorimeter as well as information obtained from physics quantities such as the invariant mass of the J/Ψ-resonance. We present an overview of the current performance of the ATLAS inner detector and conclude with some general remarks summarizing the experience of the commissioning of the detector from the alignment point of view.
Highlights
The main idea of the track-based alignment consists in the minimization of hit residuals with respect to the corresponding reconstructed tracks
After describing the main alignment method based on the minimization of hit residuals, we focus on alignment methods using information from the electromagnetic calorimeter as well as information obtained from physics quantities such as the invariant mass of the J/Ψ-resonance
The χ02 can be written as χ02 = ∑ [r(a, π)]TV −1[r(a, π)] tracks with the sum running over all tracks, r(a, π) being the hit residuals being dependent on the alignment parameters a as well as the track parameters π, and V being the the covariance matrix
Summary
The main idea of the track-based alignment consists in the minimization of hit residuals with respect to the corresponding reconstructed tracks. The contribution from hits on all selected tracks are accumulated in a single χ02-value, which is minimized with respect to the alignment parameters [3]. The calculation returns a which represents the required detector movement in order to bring the overall χ2 to its minimum This requirement is necessary but not sufficient to solve the alignment problem. The solution could return a local χ2-value only because of only weakly determined movements, the so called weak modes. This alignment step is repeated several times following the hierarchical structure of the detector. The residual distribution obtained after this alignment step almost matches the expected distribution obtained from simulated events with perfect geometry, as shown in Figure 2 [4]
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