The ATLAS experiment at the Large Hadron Collider (LHC) is on the threshold of a new data collection with the restart of the LHC for Run 3. The upcoming four-year run will provide a dataset that is almost twice as large as that collected in Run 2 (2015 –2018).
LHC proton collisions have already produced millions of billions of particles inside the ATLAS detector. Counting and determining particle properties through their interactions with the detector’s active material stock has resulted in a large number of physics results. Unfortunately, the same interactions that enable the ATLAS physics program also damage the detector.
The silicone Pixel detector, installed in the heart of ATLAS, measures charged particles passing through the sensors on four concentric layers. Because it is closest to the point of collision, it receives significant levels of radiation. At the end of run 3, the number of particles that will have hit the innermost pixel layers will be comparable to the number it would get if it were placed just a few kilometers from the sun during a solar flare. This radiation damage changes the crystal structure of the silicon wafers, making it more difficult to detect charged particles. By the end of run 2, the signals in the Pixel Detector had already decreased by ~ 25%. This is predicted to decrease further to ~ 50% at the end of run 3, as shown in the left plot below.
ATLAS researchers have developed a comprehensive strategy to address these effects and mitigate their impact. The first defense is to regularly adjust the detector’s operating parameters. Physicists can, for example, increase the voltage across a sensor and set the distinction between signal and noise. Then, using computer simulations of radiation damage effects in ATLAS, researchers can set up algorithms for rebuilding charged particle tracks to reduce the effect of this damage. This strategy is the result of years of collaboration between detector, software and analysis teams in ATLAS, including fruitful exchanges with the wider international detector community.
The first proton collisions in 2022 yielded one first test of this strategy in driving 3. In May 2022, the LHC collided with low-intensity proton beams at a mass center energy of 900 GeV. ATLAS recorded enough particles to compare detector and track reconstruction performance with predictions from simulated events. Once radiation damage effects were included in the detector simulation, the signals in the Pixel detector from charged particles matched those in the data. The number of pixel layers where each particle is detected also matched the data.
Despite being in operation for more than a decade, the Pixel detector is kept in shape by adjusting the detector’s operating parameters and radiation-conscious algorithms. The pixel efficiency and spatial resolution measured with the first collisions in run 3 are similar to those measured at the beginning of run 2. The remarkable similarity between data and simulation, highlighted in the right-hand plot above, is evidence of ATLAS ‘Run 3 software, which can accurately model radiation effects. ATLAS track reconstruction is ready for Run 3!
While these achievements are crucial to Run 3, they are even more critical LHC program with high brightness. A new silicone Inner Tracker (ITk) will replace it Internal detector after driving 3. The amount of radiation that the innermost ITk pixel layers will experience exceeds by an order of magnitude what will accumulate at the end of driving 3. Therefore, experience gained in the coming years is crucial to maximize the effect of high brightness ATLAS physics program .
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