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• 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 explainIt'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 intodaughterparticles. 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 altogetherWhich 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 abundantHmm, 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!

You're probably familiar with the standard model, a theory of fundamental particles and

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# This Missing Force Field Could Lead to a Dark Matter Breakthrough

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Summer posted on 2020/08/31
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