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Aligning the ATLAS muon spectrometer

In new results, ATLAS physicists describe novel techniques used to accurately align the muon spectrometer

ATLAS Experiment Briefing: aligning the ATLAS muon spectrometer
A schematic view of the muon spectrometer and its optical alignment system in the barrel region of the ATLAS experiment. The optical lines (red) form a network of alignment detectors that continuously monitors the positions of the chambers relative to each other and their deformations. (Image: ATLAS collaboration/CERN)

The muon spectrometer is made up of several thousand chambers and is the outermost layer of the ATLAS detector. It identifies and measures the momentum of muons that fly out of the collision point. Key to this is a precise understanding of the muon spectrometer’s geometry.

At small scales, the geometry of the muon spectrometer is almost constantly changing, albeit slowly. Small temperature variations make the chambers and their support structures contract, expand and deform. Further, some of the chambers are mounted on the ATLAS toroid magnets, which themselves can occasionally move and deform.

The muon spectrometer is therefore equipped with an optical alignment system that monitors in real time the positions of chambers relative to each other and to calibrated reference objects in the detector, as well as their deformations. This information can be combined with data from muon tracks in order to fully understand the muon spectrometer’s position.

But when new chambers are added or existing ones repaired, the spatial relationship between the alignment sensors and active detector elements is altered. Such changes require the entire muon spectrometer to be realigned.

ATLAS physicists implemented a new alignment procedure for the data-taking periods of 2017 and 2018. The resulting alignment is almost – but not quite – perfect. Judging from observed deviations, the alignment is accurate to around 50 μm in a large part of the spectrometer volume, with some slightly poorer regions being closer to 100 μm.

In other words: the entire muon spectrometer has been kept aligned to better than the diameter of a human hair. Such incredible precision is key to an experiment’s success, as evidenced by the excellent ATLAS results from Run 2 data.


Read the full ATLAS Experiment Briefing to learn more.