Every second of every hour of every day, an inconceivable number of invisible particles passes through each one of us. Some of these particles could be axions. Current quantum field theories suggest that approximately 10¹⁴ axions are passing through each square centimetre of our bodies every second, with a rest mass potentially as low as 10⁻⁶ eV. However, no direct evidence of axions has yet been found.
If axions do exist, they could unlock two of science's most profound mysteries: the strong force and dark matter. To clarify, the strong force is the fundamental force that attracts matter on a quantum scale, and dark matter is the elusive substance that contributes to the universe's unseen mass.
Introducing the Strong CP Problem
The story of the axion particle begins with the "Strong CP Problem," a dilemma discovered in 1977 that poses a contradiction for particle physicists. Symmetry is a concept that arises in many areas of science. For example, in Newtonian physics, if time were reversed, all particles would still obey the laws of physics. The strong force, which governs quantum chromodynamics (QCD), does not require CP symmetry—meaning that if each particle were swapped with its anti-particle, and left- and right-handed particles were exchanged, the resulting physics would change. However, experimental observations have shown that CP symmetry is present, which creates the strong CP problem.
The Yang-Mills Lagrangian provides the mathematical description for this issue. For a general massive quark of mass meiθ′γ5 (for some arbitrary phase θ′), the Lagrangian is written as:
In this equation, the first and third terms represent the kinetic terms of the gauge and quark fields, the fourth term is the quark mass term, and the second term represents the vacuum angle.
The issue arises when θ (and therefore θ′) are non-zero, contradicting CP symmetry in the second and fourth terms, respectively. A chiral transformation can reduce this using an angle ααα, such that θ becomes the only conflicting term. This transformation is practically useful when dealing with real masses. But why not assume θ is always zero? There is no compelling reason why it should always be zero. While CP violation is observed in the weak force approximately 1 in 1,000 times, it has never been observed in the strong force, despite extensive experimental efforts.
The mathematical implications can be confirmed experimentally. For a non-contributing θ value, the CP-violating angle θ¯=θ−argdet(YuYd) becomes non-pertubative, leading to a neutron electric dipole moment:
The observed neutron electric dipole moment is an upper bound of dN<10−26e⋅cm, which implies that θ¯<10−10. Since theoretically, θ can take any value between 0 and 2π, this forms a key aspect of the Strong CP problem.
Axions to Clean Up the Strong CP Problem
A simple solution to the strong CP problem would be to assume one of the quarks is massless. However, experimental data has essentially ruled this out.
The leading solution was proposed by physicists Roberto Peccei and Helen Quinn in 1977. They introduced a new Peccei-Quinn symmetry, which, when spontaneously broken, transforms the θ-angle into a field and gives rise to a new particle. This particle, which replaces θ′, eliminates CP violation and effectively narrows the possibility of θ violating symmetry. Frank Wilczek named this particle the "axion" after a brand of laundry detergent because it "cleaned up" the CP problem. Steven Weinberg also identified the new particle at the same time but chose not to name it the "higglet."
Axions as Dark Matter Candidates
This theoretical solution to the strong CP problem led to an exciting new possibility: axions could also be the elusive dark matter that makes up around 85% of the universe's mass. Dark matter does not interact with light or electromagnetic forces, making it invisible to conventional detection methods. Axions, with a mass lower than a billionth of an electron's mass, could easily pass through most matter without being noticed, which is why they are incredibly difficult to detect.
Axions would have been produced in vast quantities during the QCD transition phase in the early universe, with even more being created through the decay of strings formed during the Peccei-Quinn phase transformation. Confirming the existence of axions could therefore provide solutions to a variety of cosmological mysteries, including the distribution of galaxies, the formation of galactic halos, and much more.
The Search: Pushing the Boundaries of Technology
Detecting axions to prove their existence is no easy task. The search spans a wide range of possibilities, making it difficult to pinpoint the exact parameters. However, the mass and charge of axions are believed to be directly proportional. The upper limit of the search is set by experiments that have not yet detected axions, while the lower limit is constrained by the understanding that the lightest axions would have been overproduced in the early universe. Even with these constraints, the search space remains vast.
Fortunately, axions have been theorised for nearly 50 years, allowing researchers to predict many of their properties and influences on other phenomena. Models such as the KSVZ (Kim-Shifman-Vainshtein-Zakharov) and DFSZ (Dine-Fischler-Srednicki-Zhitnitsky) models offer distinct predictions about how axions couple to quarks and leptons, guiding the experimental search. Current experiments include:
- Searches using the Primakoff effect, which allows axions to convert to photons and vice versa in strong electromagnetic fields.
- Investigations into changes in the polarization of light propagating through electromagnetic fields.
- Measuring the axion-photon and photon-axion transformations through successive electromagnetic ‘walls.’
- Searching for axion production via Higgs boson decays in particle colliders.
- Studying quasi-particle refraction in systems with strong magnetic gradients around compact astrophysical objects such as magnetars.
- Observing precession in nuclear spin rotations with oscillation frequencies in resonance with external electric fields.
Much of the current focus is on narrowing the potential masses of axions by ruling out certain ranges through experimental data.
Astrophysical observations have also provided indirect evidence of axions. White dwarfs and red giants have been found to cool at a faster rate than predicted, and axion emission could explain this discrepancy. While this evidence is tantalizing, definitive proof remains elusive, and the expanding scope of axion searches presents both opportunities and challenges for finding them—or ruling them out entirely.
The Unseen Yet Essential Role of Axions
While axions have yet to be definitively proven, they are likely an essential building block of the universe, shaping everything from subatomic interactions to the structure of galaxies. Their potential to illuminate these fundamental processes continues to drive the quest for discovery, pushing the limits of human ingenuity as experimental and theoretical research progresses.
Just as axions may be critical to understanding the cosmos, Axion Global plays an essential role in helping forward-facing businesses achieve tangible results. Much like axions, Axion Global’s influence may be invisible and difficult to measure externally, but its strategic insights and precision execution are integral to the success of global business operations.
So, the next time you find yourself gazing up at the night sky or confronting a business challenge, remember the role of axions—and how both science and business are driven by forces that shape our world, often unseen but undeniably powerful.