You’re probably familiar with the standard model, a theory of fundamental particles and
how they interact. These particles have counterparts that are mirror images, or opposite charges,
or both. But in the '60s, we discovered particles that were flipped- image and charge versions
of each other didn’t always behave how we expected. We’ve since adjusted our expectations,
but even so, some of these particles still behave in a way we can’t explain. It’s
what’s known as the "strong CP problem," and it’s a glaring flaw in the standard
model. In order to understand the strong CP problem, there’s a hierarchy of terms we
need to make clear so we’re all on the same page. First up, we need to review the four
fundamental forces. They are gravity, electromagnetism, the weak nuclear force, and the strong nuclear
force. With the exception of gravity, these forces are mediated by particles in the standard
model called bosons. The way these forces affect decaying particles starts to get complicated
when we talk about symmetry. Imagine an unstable particle that, through an electromagnetic
interaction mediated by photons, decays into “daughter” particles. If you were to take
that unstable particle and flip its charge, what’s known as charge conjugation or just
C, the charge-flipped particle undergoes electromagnetic interactions in the same way as its antiparticle.
The decay happens at the same rate and with the same properties, meaning electromagnetism
has what’s called "C-symmetry." The same is true if you were to take that unstable
particle and flip all its spatial coordinates to make a mirror image of it, what’s known
as parity, or P. A mirror particle will also undergo electromagnetic interactions
in the same way, or symmetrically, to its regular self. So electromagnetism has "P-symmetry."
And finally, electromagnetic interactions are the same whether we’re going forward
in time or back, so they exhibit "T-symmetry." They also are symmetrical with any combination
of C, P, and T, even all three together. So if you have a charge-flipped mirror image
of an unstable particle undergoing an electromagnetic interaction backward in time...you still know
what you’re going to get. Simple, right? Okay, stop, catch your breath. Let’s all
take a minute to sit with this new information, because I think you know what’s coming next.
That’s right, it gets more complicated. If our hypothetical unstable particle were
instead to undergo radioactive decay mediated by the weak force, then its mirror image version
wouldn’t behave symmetrically every time. It would violate P-symmetry. This was first
observed in 1956, back when we thought parity conservation was the law. So you can imagine
it was quite a shock when scientists observed two arrangements of cobalt-60 decaying differently.
Since then, it’s been observed that weak interactions can also violate C- and T-symmetry,
and any combination of any two, though not C, P, and T altogether. So, after reworking
the math, the standard model today allows for weak and strong interactions to violate
all symmetries except CPT altogether. Which gives rise to a new problem. We’ve observed
weak interactions that violate CP-symmetry. It doesn’t happen often, but it does happen
nonetheless. In fact, it happens a lot more than we’ve seen charge-parity violation
in interactions mediated by the strong force. We’ve seen that a grand total of, drumroll
please…. no times. Not once. Kind of disappointing, isn’t it? The fact that the strong force
should violate CP symmetry but hasn’t as far as we know is called the strong CP problem.
But in science, the unexplained is where the fun begins! Because the strong CP problem
is such a mathematical improbability, we think there must be something else at play here.
In the '70s, scientists Roberto Peccei and Helen Quinn proposed that maybe there’s
some undiscovered parameter, like a field that inhibits strong CP violation. If this
field exists, then there should be a particle called an axion to go with it. Axions should
be chargeless, very light, and incredibly abundant. Hmm, a particle that’s hard to
find and doesn’t interact with anything except through gravity? Sounds like another
candidate for dark matter to me. Indeed, since the 1980s, scientists have been hunting for
axions in labs. As you might have guessed, we haven’t found them yet, but we’re still
looking for them with research like the ADMX-G2 Experiment. Axions are not the only possible
solution to the strong CP problem, and when we eventually do figure out why this expected
unexpected event...isn’t...occurring, it’ll be exciting to see where physics takes us
next.
If the search for axions and their relation to dark matter has piqued your curiosity,
check out this Focal Point episode on how today’s scientists are attempting to hunt
them down. Don’t forget to subscribe, and keep coming back to Seeker for all of the
latest science news. Thanks for watching, and I’ll see you next time!