Hunting down elusive axion particles: Experiments suggest better ways to explore the dark sector

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The 90% predicted and effective CL from CCM120 for ALP-photon coupling gA. Also included is the projection region for the three-year run of the CCM200 using background taken from the CCM120 spectrum reduced by two orders of magnitude for various conservative improvements (dashed green line) and a no-background hypothesis (extension of the shaded green region). The parameter space of the QCD axion model for the reference scenario KSVZ extends in the region indicated by the arrows. Credit: Physical review D (2023). DOI: 10.1103/PhysRevD.107.095036

Since axions were first predicted by theory nearly half a century ago, researchers have searched for evidence of the elusive particle, which may exist outside the visible universe, in the dark sector. But how to find particles that cannot be seen?

The first physics results of the Coherent CAPTAIN-Mills experiment at Los Alamos just described in a publication in the journal Physical review Dsuggest that liquid argon accelerator-based experimentation, initially designed to search for similar hypothetical particles such as sterile neutrinos, could also be an ideal setup for stealthy axion searches.

“Confirmation of dark sector particles would have a profound impact on understanding the Standard Model of particle physics, as well as the origin and evolution of the universe,” said physicist Richard Van de Water. “A major focus of the physics community is exploring ways to detect and confirm these particles. The Coherent CAPTAIN-Mills experiment pairs existing predictions of dark matter particles such as axions with high-intensity particle accelerators capable of producing this obscure hard to find question.”

Demystifying the dark sector

Physics theory suggests that only 5% of the universe is made up of visible matter atoms that form things we can see, touch and feel, and that the remaining 95% is the combination of matter and energy known as the dark sector. Axions, sterile neutrinos, and others can account for and explain all or part of that missing energy density.

The existence of axions could also solve a long-standing problem in the Standard Model, which delineates the known behavior of the subatomic world. Sometimes referred to as “fossils” of the universe, hypothesized to have originated just a second after the Big Bang, axions could also tell us a lot about the universe’s founding moments.

The Coherent CAPTAIN-Mills experiment was one of several projects to receive funding from the Department of Energy for dark sector research in 2019, along with substantial funding from the laboratory-led research and development program at Los Alamos. A prototype detector dubbed CCM120 was built and operated during the 2019 Los Alamos Neutron Science Center (LANSCE) beam cycle. Physical review D publication describes the results of the initial engineering run of the CCM120.

“Based on the first round of CAPTAIN-Mills searches, the experiment demonstrated the ability to perform axion search,” said Bill Louis, also a project physicist at Los Alamos. ‘We are realizing that the energy regime provided by the LANSCE proton beam and liquid argon detector design offers an unexplored paradigm for axion-like particle research.’

Experiment design

Stationed in the Lujan Center adjacent to LANSCE, the Coherent CAPTAIN-Mills experiment is a 10-ton, supercooled, liquid argon detector. (CAPTAIN stands for Cryogenic Apparatus for Precision Tests of Argon Reactions with Neutrinos.)

High-intensity 800 megaelectron-volt protons generated by the LANSCE accelerator strike a tungsten target in the Lujan Center, then travel 23 meters through a large steel-and-concrete shield to the detector to interact with liquid argon.

The prototype detector’s inner walls are lined with 120 sensitive eight-inch photomultiplier tubes (hence the CCM120 nickname) that detect single-photon light flashes that occur when a normal or dark sector particle pushes an atom into the argon tank. liquid.

A special material coating on the inner walls converts the light output of the argon into visible light which can be detected by the photomultiplier tubes. The rapid timing of the detector and beam helps remove the effects of background particles such as beam neutrons, cosmic rays and gamma rays from radioactive decays.

Pieces of the puzzle

Axions are of great interest because they are “highly motivated”; that is, their existence is strongly implied by theories beyond the Standard Model. Developed over more than 70 years, the Standard Model explains three of the four known fundamental forces: electromagnetism, the weak nuclear force and the strong nuclear force that govern the behavior of atoms, the building blocks of matter. (The fourth force, gravity, is explained by Einsteinian relativity.) But the model is not necessarily complete.

An unsolved problem in Standard Model physics is known as the “strong CP problem”, where “CP” stands for charge-parity symmetry. Essentially, particles and their antiparticle counterparts are similarly affected by the laws of physics. However, nothing in Standard Model physics mandates that behavior, so physicists should see at least occasional violations of that symmetry.

In weak force interactions, charge parity symmetry violations occur. But no similar violations were observed in strong-force interactions. That puzzling absence of theoretically possible behavior poses a problem for Standard Model theory. What prevents charge parity symmetry violations from occurring in strong-force interactions?

Abundant, nearly weightless, and electrically neutral, axions can be an important part of the puzzle. Axion earned its nickname in 1978, coined by physicist Frank Wilczek after a brand of laundry detergent because such a particle could “clean up” the strong CP problem. Physicists speculate that they are components of a dark matter force that preserves charge parity symmetry and that they can pair or interact with photons and electrons.

Next steps

If axions exist, finding them may be a matter of devising the right experimental setup.

“As a result of this initial run with our CCM120 detector, we have a much better understanding of the signatures associated with axion-like particles coupled to photons and electrons as they move through liquid argon,” said Louis. “This data gives us the insight to upgrade the detector to be an order of magnitude more sensitive.”

More information:
AA Aguilar-Arevalo et al, Prospects for the detection of axion-like particles in the Coherent CAPTAIN-Mills experiment, Physical review D (2023). DOI: 10.1103/PhysRevD.107.095036

About the magazine:
Physical review D

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