Searching for new particles: Discovery of axions could help answer one of physics’ most puzzling questions

Searching for new particles: Discovery of axions could help answer one of physics' most puzzling questions

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The University of Minnesota researchers’ new hypothetical axion search method involves measuring the particle’s decay into two known muon particles that are essentially the heavier version of the electrons pictured in the image above. Credit: Raymond Co, University of Minnesota

One of the most high-profile mysteries in physics today is what scientists call the “strong CP problem.” Stemming from the puzzling phenomenon that neutrons do not interact with electric fields despite being made of quarks, the smallest fundamental particles that carry electric charges, the strong CP problem challenges the Standard Model of physics, or the set of theories that scientists have used to explain the laws of nature for years.

A team led by Twin Cities theoretical physicists at the University of Minnesota has discovered a new way to look for axions, hypothetical particles that could help solve this mystery. Working in collaboration with experimental researchers at the Fermilab National Accelerator Laboratory, the physicists’ new strategy opens up previously unexplored opportunities to detect axions in particle collider experiments.

The researchers’ paper is published and presented as an editor’s suggestion in Physical Review Letters.

“As particle physicists, we are trying to develop our best understanding of nature,” said Zhen Liu, co-author of the paper and an assistant professor in the School of Physics and Astronomy at the University of Minnesota. ‚ÄúScientists have had enormous success over the last century in finding elementary particles through established theoretical structures. Hence, it is extremely puzzling why neutrons do not couple to electric fields because in our known theory, we would expect them to. If discover the axion, it will be a great advance in our fundamental understanding of the structure of nature.”

One of the primary means of studying subatomic particles, and potentially discovering new ones, are collider experiments. Essentially, scientists force particle beams to collide, and when they hit each other, the energy they produce creates more particles that pass through a detector, allowing researchers to analyze their properties.

The method proposed by Liu and his team involves measuring the product of “decay” or what happens when an unstable heavy particle turns into multiple lighter particles than the hypothetical axion into two known particles of muons which are essentially the version heavier than electron. By working backwards from the muon tracks in the detector to reconstruct these decays, the researchers believe they have a chance to locate the axion and prove its existence.

“With this research, we’re expanding the ways we can look for the axion particle,” said Raymond Co, a co-author of the paper and a postdoctoral researcher at the University of Minnesota School of Physics and Astronomy and the William Fine Theoretical Physics Institute. “People have never used axion decay into muons as a way to look for the axion particle in neutrinos or collider experiments. This research opens up new possibilities to pave the way for future efforts in our field.”

Liu and Co, along with University of Minnesota physics and astronomy postdoctoral researcher Kun-Feng Lyu and University of California, Berkeley postdoctoral researcher Soubhik Kumar, are behind the theoretical part of the research. They are part of the ArgoNeuT collaboration, which brings together theorists and experimenters from across the country to study particles through experiments at Fermilab.

In this paper, the theoretical team led by the University of Minnesota worked with experimental researchers to perform an axion search using their new method and existing data from the ArgoNeuT experiment. The researchers plan to use the experimental results to further refine their theoretical calculations of the axion production rate in the future.

More information:
R. Acciarri et al, First Constraints on Heavy QCD Axions with a Liquid Argon Time Projection Chamber Using the ArgoNeuT Experiment, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.221802

About the magazine:
Physical Review Letters

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