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Jun 9th 2020

The Axion: A Lean, Clean Answer to Dark Matter’s Mess?

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Physics has a problem. It has a host of them, actually, but that’s the literal nature of the game: Develop a theory, test it, then find out why it doesn’t work as predicted. Wash, rinse, repeat.

One of the biggest conundrums in particle phsyics stems from symmetry. More precisely, it addresses the issue of why charge-parity (CP) symmetry seemingly occurs in some cases but not others. A theoretical particle — the axion — could solve this problem, and incidentally clean up dark matter’s mess.

I Need to Axion You Something

The weak nuclear force — which governs radioactive decay — violates CP-symmetry all the time, churning out results that don’t match mirror predictions. No big deal; theories get updated, results are accounted for and everything’s fine, right? Not quite. According to Symmetry Magazine, the strong nuclear force — responsible for holding protons and neutrons together — seems to obey mirror symmetry perfectly. This causes a problem, since weak force violations should feed into similar strong force effects.

The culprit? Theta, or θ, an angle with an apparent constant value of zero. As noted by New Scientist, this value means neutrons have no electric dipole moment, in turn allowing them to remain in CP-symmetry. In 1977, physicists Helen Quinn and Roberto Peccei uncovered a solution: If θ got an upgrade from constant to dynamic field, it could allow for initial variations in charge that eventually settled down to zero. Quinn compared the field model to a tilted bowler hat, according to Quanta Magazine — while a ball can start at any angle around the rim, it always ends up at the bottom.

Soon after the Peccei-Quinn theory made the rounds as a CP-problem solver, scientists Frank Wilczek and Steven Weinberg realized the model creates room for a new particle: the axion. Named after a dish detergent, axions offer the potential to clean up symmetry issues — and may have the same scrubbing muscle for dark matter.

It’s a Photon-finish!

So far, detection hasn’t been forthcoming. Science News notes that they predict axions’ mass to be billionths or trillionths that of an electron — recent work from the Axion Dark Matter Experiment (ADMX) at the University of Washington searched for this elusive particle between 2.81 millionths and 3.31 millionths of an electron volt and came up empty-handed, but has helped narrow the range of possible mass.

There’s also the issue of interaction: In most cases, axions encountering typical matter would produce little to no effect. But efforts from the Max Planck Institute for Chemical Physics of Solids, meanwhile, looked to circumvent this limitation by using Weyl semimetal, which allows electrons to behave as if they have no mass. The result? “Collective vibrations in materials that behave and respond exactly as the particle would.”

Turns out there’s a natural action analog here: the photon. According to the International Axion Observatory (IAXO), the Peccei-Quinn model postulates that photons can transform into axions (and vice-versa) in the presence of an electromagnetic field. ADMX is leveraging this light-based relationship to scan increasingly broad regions of resonance — using a powerful magnet cooled to almost absolute zero, the University of Washington team is hoping to find the right frequency and detect the exact moment of axion creation, then extrapolate its mass and magnitude.

Fishing in the Dark

Dark matter doesn’t want to play nice. While observations tell us that normal or “baryonic” matter makes up 5% of the universe, its dark counterpart should account for a whopping 27% — but we can’t find any of it. There’s strong evidence for its existence; if normal matter produced all gravity in the universe, stars at the center of the Milky Way should orbit much faster than their more-distant counterparts. But surprise — they don’t! Non-baryonic matter is the likely cause.

The problem? As noted by NASA, scientists are far more certain about what this matter can’t be that what it can. Some popular but not-possible theories include:

  • Dark” clouds of regular matter While big clouds of baryonic matter might be hiding in plain sight, there’s no way they could avoid energy interactions. Radiation passing through them would cause both an absorption and emission spectrum — but there’s no sign of any such spectra.
  • AntimatterIf antimatter were the underlying source of our missing universal mass, it would produce gamma rays from near-constant annihilation collisions with regular matter.
  • Galaxy-sized black holesSupermassive black holes have also been suggested as the 27% potential owing to their huge gravitational output, but a lack of gravitational lensing — light being bent as it passes by these objects — rules out this possibility.

Are Axions the Answer?

Sneaky regular matter can’t manage, antimatter isn’t making the cut and big black holes don’t have much hope. So what’s left? Weakly interacting massive particles (WIMPs) were a strong candidate for distribution of dark matter, but repeated experiments suggest these potential particles aren’t living up to expectations.

Extremely lightweight and incredibly ubiquitous axions, meanwhile, suggest a way to settle this dark discussion once and for all. If these potential particles permeate the cosmos, they could account for the missing “dark” matter — since they’re easy to overlook and don’t exactly tip the scales, axions may offer the best-fit solution for the universe’s missing mass.

Bottom line? We can’t see them, can’t (currently) detect them and don’t know their exact properties. Still, axions may represent the best-fit solution to both solve CP-symmetry problems and clean up dark matter’s mess.

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