"The idea that matter and antimatter particles behave slightly differently is not new and has been observed previously in studies of particles containing strange quarks and bottom quarks. What makes this study unique is that it is the first time this asymmetry has been observed in particles containing charm quarks."
So we have these matter particles that are defined by three numbers, a weak hypercharge Y in the set {1, 1/3, -1/3, -1}, a weak isospin T in the set {1/2, -1/2}, and a ‘generation’ in the set {0, 1, 2}. The electric charge is a derived quantity Q = T + Y/2 that does not depend on generation. For generation 0 we have the particles,
T Y Q name
-----|-----|-----|----------
-1/2 -1 -1 electron
-1/2 -1/3 -2/3 antiüp quark
-1/2 1/3 -1/3 down quark
-1/2 1 0 antineutrino
1/2 -1 0 neutrino
1/2 -1/3 1/3 antidown quark
1/2 1/3 2/3 up quark
1/2 1 1 positron
The weak interaction has to preserve these quantum numbers, so for example when a free neutron (up-down-down) turns into a proton (up-up-down) and an electron, as it will do if you leave it alone for about 15 minutes, then one of the down quarks is turning into an up quark and an electron. This preserves electric charge but it does not preserve the underlying quantum numbers, so it requires emitting an antineutrino. [The fact that you need 4 particles total is part of why it takes a long time on the order of minutes; in this case the Feynman diagram vertexes only have three lines going in/out and so creating a 4-particle state requires two of them, in the middle you have a W- boson, (T=-1, Y=0).]
And then there are some things which are not present but fit the quantum numbers, for example many grand unified theories predict something which is spectacularly unobserved called “proton decay” where an up (1/2, 1/3) could hypothetically annihilate with a down (-1/2, 1/3) to generate an antiüp (-1/2, -1/3) plus a positron (1/2, 1)—this would manifest as a proton decaying into a neutral pion (up-antiüp) plus a positron, which would presumably be hugely energetically favored (protons have several times the mass of pions and elecron/positron masses are negligible)... this sort of decay does not have a way to happen in the standard model because there is no interim (0, 2/3) particle to sit between the two vertices.
Anyway, the next generations up are basically copies of the same 8 matter particles, with "electron" replaced by "muon" and then "tau", "neutrino" replaced by "mu neutrino" and then "tau neutrino," "down" replaced by "strange" and then "bottom", and "up" replaced by "charm" and then "top". The down and up quarks typically cannot decay into anything without some antidown or antiüp quarks sitting around to annihilate with them, though again, this is not 100% obvious from the table above, as the case of proton decay shows. So that we have observed this with strange and bottom quarks is two out of our four possibilities.
So what this makes very clear is that the CP-violations are not something specific to the (-1/2, 1/3) / (1/2, -1/3) antiparticle pairs that are called (anti-)down, (anti-)strange, (anti-)bottom in the three generations. It is not some sort of physics phenomenon that requires these two signs to be opposite; it has now been observed in the (-1/2, -1/3) / (1/2, 1/3) antiparticle pairs, too. Assuming that the presence in the bottom quark means that this asymmetry crosses generation lines, then we are all but assured that the much harder to measure top quarks would also display the asymmetry, and it is something very fundamental, rather than some as-yet-unappreciated aspect of the coupling of isospin to hypercharge.
Thank you so much for this. Not only did this description help me understand what this discovery actually meant, it clarified and tied together a lot of what I have been trying to learn through occasional reading and youtube videos on the subject. The table of particles and associated values and the description of the rules of decay bridged a huge gap I've had in understanding all of this for a really long time.
When you say "The fact that you need 4 particles total is part of why it takes a long time on the order of minutes...", does it take a long time because there are significantly fewer decays (described by Feynman diagrams?) from a lone neutron that result in a proton, anti-neutrino, and electron than there are decays that end up back at a neutron?
So like it wouldn't be a decay if it went neutron → neutron, if that makes sense. There is one “main” diagram which goes neutron → neutron and it looks like a straight line with no vertices and it is by far the most probable thing, most neutrons just stay neutrons.
So there are two reasons that a free neutron outside of a nucleus takes so long to become a proton, and you can kind of visualize it like pulling a molecule of air through an air filter or so, the first reason that this particular setup takes so long is that this particular air filter is really thick, and the second reason is that the fan you're using is not very strong.
The “wall being thick” has to do with this intermediate particle, and that’s what I was alluding to above. The wall is thick because you need to create this W- boson. The problem is that this boson has about twice the mass of the neutron itself, call it Bohb because it’s a Big Ol’ Honking Boson. There's just nowhere near the energy in the system to create this thing directly. And in quantum mechanics that is okay because quantum systems can “tunnel” through states that they cannot directly actually occupy: but it generally takes longer and longer the more and more energy you need to borrow, and this is a lot of energy to borrow.
The other thing is the weak blower, and that has to do with what “pressure” or “energy difference” drives the decay. In this case the driver is the mass difference: down-quarks are just intrinsically about 2 MeV heavier than up-quarks and that is enough to cover the 0.5 MeV of an electron and a neutrino., so you have something like 1.5 MeV left over to spread across the universe. By itself that number doesn't mean anything, though—what means something is the ratio of the initial to the final masses, which is something like 939.57 MeV : 938.78 MeV, so the final mass is only 0.08% lighter than the initial mass. The reaction rate goes like some high power—a fifth or sixth power—of this ratio, so when one side has like half the mass of the other side then the reaction happens very very fast because there is so much pressure driving it. But in this case the masses are so close to equal that the reaction takes something like hundreds of times longer than you might otherwise expect from just the thickness of the barrier alone.
Just getting back to follow up on my question now, and I have to agree, this was very thoughtful. If hacker news had the equivalent of reddit gold I would give you some.
