Abstract
CMS Silicon Tracker alignment consists of three key components: survey during Tracker construction, measurements with the Laser Alignment System during operation and track based alignment. Methods and results are explained in detail, with a special focus on track based alignment due to its enormous complexity and numerical challenges.
Highlights
The CMS Tracker [1] comprises both a silicon pixel vertex and a silicon strip tracker (Fig. 1)
The Pixel Tracker is built from 1440 pixel modules with a pixel size of 100 μm(rφ) × 150 μm(z), amounting to a total of 66 million readout channels
With analogue signal interpolation of charge sharing induced by the large Lorentz angle αL ≈ 23◦, a single hit resolution of 10 μm(rφ) × 20 μm(z) can be reached
Summary
The CMS Tracker [1] comprises both a silicon pixel vertex and a silicon strip tracker (Fig. 1). In order to not deteriorate the track parameter resolution and the CMS physics reach significantly, the position1) of all modules needs to be known to a level of better than 10 μm for the sensitive coordinate This is beyond the accuracy with which the huge cylindrical Tracker, 5.6 m long and 2.4 m in diameter, is constructed. One example is the measured position of sensors in modules (Fig. 2) that shows an overall accuracy of σx < 10 μm in the sensitive coordinate Another example is the survey of the Tracker Endcaps by photogrammetry (Fig. 3). The Laser Alignment System (LAS) uses infrared laser beams with a wavelength λ = 1075 nm to monitor the position of selected Tracker modules It operates globally on Tracker substructures: Inner Barrel (TIB), Outer Barrel (TOB), and Endcap (TEC) discs). Typical examples of constraints are a vertex and/or mass constraint for decay particles e. g. from Z → μ+μ− or jets, “tracks” from the Laser Alignment System, and survey constraints
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