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Keeping the ATLAS Inner Detector in perfect alignment

The ATLAS Inner Detector can measure the position of charged particles that pass through it to better than a 100th of a millimetre! To reach that precision, the detector must be aligned to equal or better accuracy

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The ATLAS pixel detector, the most inner part of the inner detector installed in the cavern, June 2007
The innermost layer of the ATLAS Inner Detector: the Pixel sub-detector. (Image: CERN)

Precision is key to an experiment’s success. But how can you track a particle’s trajectory when your detector keeps moving? This was the challenge faced by the ATLAS experiment’s Inner Detector during Run 2 of the LHC (2015–2018).

Located at the heart of the experiment, the ATLAS Inner Detector can measure the position of charged particles that pass through it to better than a 100th of a millimetre! To reach that precision, the detector must be aligned to equal or better accuracy. In a recent paper, ATLAS physicists revealed the complex solutions they developed to ensure the accuracy of the experiment.

The 2 m tall and 6 m long ATLAS Inner Detector is composed of three sub-detectors made of highly granular silicon pixels, silicon strips and straw tubes. As charged particles pass through the detector, they leave behind a track of small energy deposits (or hits) in each sub-detector, allowing physicists to reconstruct their trajectory.

Yet, these detectors are far from stationary. During high-intensity LHC collisions, they can shift due to fluctuations in temperature or changes in the magnetic field strength. ATLAS researchers found that parts of the Inner Detector showed signs of short-timescale movements. As electronic chips on the sub-detectors would record data – up to 100 000 times per second! – they needed a significant amount of electrical power, causing a rise in temperature at the centre of the ATLAS detector.

In the innermost layer of the Inner Detector (the Pixel sub-detector), this effect was especially pronounced. The excess of heat would bring the Pixel’s cooling liquid to a boil, causing a rapid change in the coolant’s mass. The Pixel would experience significant displacements over the first hour of data-taking, until thermal equilibrium between the sub-detector and the cooling system could be reached. As the intensity of the collisions decreased, so did the Pixel’s heat dissipation. The Pixel’s coolant would gradually return to its liquid form, increasing the sub-detector’s overall mass and inducing it to drift slowly in the opposite direction.

To solve this issue, ATLAS physicists had to develop a new automated alignment scheme for the Inner Detector. This dynamic alignment would update throughout each operation of the LHC, correcting the data recorded by ATLAS accordingly. New alignment constants were derived every 20 minutes during the first hour of data-taking and every 100 minutes thereafter.

This novel alignment solution allowed ATLAS physicists to continue to record data with unprecedented precision throughout Run 2. Physicists are now preparing the Inner Detector for its next challenge: Run 3 of the LHC, which is scheduled to start in early 2022.

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The vertical position of the Pixel sub-detector during a single operation of the LHC. The blue dashed line shows the average vertical position. The instantaneous luminosity of the LHC fill is shown in green. (Image: ATLAS Collaboration/CERN)

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Read more on the ATLAS website.