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Newswise Scientists used a neural network, a type of brain-inspired machine learning algorithm, to sift through large volumes of particle collision data.

For over two decades, theATLASparticle detector has recorded the highest energy particle collisions in the world within the Large Hadron Collider (LHC) located atCERN, the European Organization for Nuclear Research. Beams of protons are accelerated around theLHCat close to the speed of light, and upon their collision atATLAS, they produce a cascade of new particles, resulting in over a billion particle interactions per second.

Particle physicists are tasked with mining this massive and growing store of collision data for evidence of undiscovered particles. In particular, theyre searching for particles not included in theStandard Modelof particle physics, our current understanding of the universes makeup that scientists suspect is incomplete.

As part of theATLAScollaboration, scientists at the U.S. Department of Energys (DOE) Argonne National Laboratory and their colleagues recently used a machine learning approach called anomaly detection to analyze large volumes ofATLASdata. The method has never before been applied to data from a collider experiment. It has the potential to improve the efficiency of the collaborations search for something new. The collaboration involves scientists from 172 research organizations.

The team leveraged a brain-inspired type of machine learning algorithm called a neural network to search the data for abnormal features, or anomalies. The technique breaks from more traditional methods of searching for new physics. It is independent of and therefore unconstrained by the preconceptions of scientists.

Rather than looking for very specific deviations, the goal is to find unusual signatures in the data that are completely unexplored, and that may look different from what our theories predict. Physicist Sergei Chekanov

Traditionally,ATLASscientists have relied on theoretical models to help guide their experiment and analysis in the directions most promising for discovery. This often involves performing complex computer simulations to determine how certain aspects of collision data would look according to the Standard Model. Scientists compare these Standard Model predictions to real data fromATLAS. They also compare them to predictions made by new physics models, like those attempting to explaindark matterand other phenomena unaccounted for by the Standard Model.

But so far, no deviations from the Standard Model have been observed in the billions of billions of collisions recorded atATLAS. And since the discovery of theHiggs bosonin 2012, theATLASexperiment has yet to find any new particles.

Anomaly detection is a very different way of approaching this search, said Sergei Chekanov, a physicist in Argonnes High Energy Physics division and a lead author on the study.Rather than looking for very specific deviations, the goal is to find unusual signatures in the data that are completely unexplored and that may look different from what our theories predict.

To perform this type of analysis, the scientists represented each particle interaction in the data as an image that resembles aQRcode. Then, the team trained their neural network by exposing it to 1% of the images.

The network consists of around 2 million interconnected nodes, which are analogous to neurons in the brain. Without human guidance or intervention, it identified and remembered correlations between pixels in the images that characterize Standard Model interactions. In other words, it learned to recognize typical events that fit within Standard Model predictions.

After training, the scientists fed the other 99% of the images through the neural network to detect any anomalies. When given an image as input, the neural network is tasked with recreating the image using its understanding of the data as a whole.

If the neural network encounters something new or unusual, it gets confused and has a hard time reconstructing the image, said Chekanov.If there is a large difference between the input image and the output it produces, it lets us know that there might be something interesting to explore in that direction.

Using computational resources at Argonnes Laboratory Computing Resource Center, the neural network analyzed around 160 million events withinLHCRun-2 data collected from 2015 to 2018.

Although the neural network didnt find any glaring signs of new physics in this data set, it did spot one anomaly that the scientists think is worth further study. An exotic particle decay at an energy of around 4.8 teraelectronvolts results in a muon (a type of fundamental particle) and a jet of other particles in a way that does not fit with the neural networks understanding of Standard Model interactions.

Well have to do more investigation, said Chekanov.It is likely a statistical fluctuation, but theres a chance this decay could indicate the existence of an undiscovered particle.

The team plans to apply this technique to data collected during theLHCRun-3 period, which began in 2022.ATLASscientists will continue to explore the potential of machine learning and anomaly detection as tools for charting unknown territory in particle physics.

The results of the study were published inPhysical Review Letters. This work was funded in part by theDOEOffice of Sciences Office of High Energy Physics and the National Science Foundation.

Argonne National Laboratoryseeks solutions to pressing national problems in science and technology. The nations first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance Americas scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed byUChicago Argonne,LLCfor theU.S. Department of Energys Office of Science.

The U.S. Department of Energys Office of Scienceis the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visithttps://energy.gov/science.

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Machine learning could help reveal undiscovered particles within data from the Large Hadron Collider - Newswise