What is an axion?

Axions are hypothetical elementary particles which, if they exist, would eliminate a problematic exception to a rule involving what is known as CP symmetry.

As a bonus, their existence could also blow away another puzzling mystery in science, the invisible added mass that helps galaxies cling together, known as dark matter.

While there have been some tantalizing signs of their existence, physics has yet to confirm whether they’re the fix we’re looking for, or a simple bubble of wishful thinking.

What is CP Symmetry?

The C in this case stands for charge, like the negativity of electrons and the positivity of protons, while P stands for parity, a term that describes the spatial coordinates of a particle, such as whether it curves to the left or hurtles to the right.

Symmetry refers to changes that don’t make much difference to a particular feature, just like rotating a square 90 degrees doesn’t change whether it’s a square.

Normally, flipping a particle’s charge from positive to negative, or vice versa, would change the way it responds to another particle’s charge.

However, if we flipped all the charges involved in a process that transforms them all into their antiparticles, nothing would change. The same rules would still apply. We could say that a “charge reversed” universe is symmetrical.

Likewise, flipping the spatial coordinates so that our Universe looks like it’s in a mirror, where left is now right and right is now left, would also work on the same physics. This “parity-inverted” version of the Universe would also be symmetrical.

Incidentally, time also has its own symmetry. By most rules, rewinding a process should simply bring you back to square one, with nothing feeling different.

If we look at a particular rule in physics, we could say it has CP symmetry if the rule works the same after flipping both the charge and the spatial coordinates together. Most rules seem to follow this trend well until they do.

In 1964, a team including American particle physicist James Cronin and nuclear physicist Val Fitch saw something in the decay of two different types of quark-antiquark pairings called kaons that would only be seen if CP symmetry were not true for the weak interactions.

An observation is annoying, but it could still be dismissed as an experimental quirk. In recent decades, more direct evidence of this breach has also emerged in particles made up of quarks, and perhaps among other particles, reinforcing confidence that bona fide disruption exists, at least in a small percentage of interactions.

What is the problem of strong PC?

What is frustrating is that we don’t see CP violation happening when quarks are bound into groups by the strong force, as they are inside particles such as protons and neutrons.

Physicists call it the strong CP problem, and it appears to be an annoying gap in our observations of when we would expect a combination of charge symmetry and parity to be violated (and when it isn’t).

The Standard Model suggests that if we saw it in one, we should see it in the other as well. So something isn’t right.

Perhaps there is something wrong with the Standard Model. Or maybe some clever tweaking is needed to make everything fit.

In 1977, physicists Roberto Peccei and Helen Quinn came up with a solution that could explain the difference simply by introducing a new type of field.

Problem solved, right? The only problem is that the new field means a new particle, one with no charge and tiny mass.

They called it an axion, after the funny name of a laundry detergent in hopes it would clean up their mess.

Unfortunately he left physics with a stain that doesn’t wash off so easily.

Where are all these axes?

Despite decades of searching for particles with the characteristics of an axion, nothing concrete has been found.

However, physicists haven’t stopped looking.

One method is to look for particles that turn into photons as they pass through a magnetic field. While some experiments have found no trace of this phenomenon, others continue to hint at its possibility.

In 2020, an experiment called XENON1T yielded results that are tempting to interpret as axions, given their perfect match. The glow of axions in magnetic fields could also explain an unexpected flare of distant darkness observed by the New Horizons spacecraft in 2022.

Physics is a conservative field, however, and much more data would be needed before anyone could celebrate it as an axion discovery.

What would happen if we found the axions?

First of all, the strong CP problem would be a minor problem.

Axions must also have existed in abundance in the early Universe, ejected from the field that was thought to have rapidly inflated space to gigantic proportions. The theory also predicts that they would drag against this field, slowing them down so they wouldn’t drift away from each other so fast.

If they have appeared, they shouldn’t interact with other particles in any obvious way, making them harder to spot.

Despite their tiny mass, massive numbers, and relatively slow motion, they would still cluster into huge bodies that would act uncannily like dark matter.

The existence of axions would fill in some incredible gaps in our understanding of the Universe, just as the Higgs boson filled a disturbing gap in our knowledge of mass.

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