Hat tip to BeninSC
Anyone who knows about atomic structure at the level of chemistry knows that ordinary matter is made up of just three particles: Protons, neutrons, and electrons. Protons have a unit positive electrical charge, electrons have a unit negative charge, and neutrons have no charge. The electron is roughly 1/1200 the mass of either the proton and the neutron, while the neutron is ever-so-slightly more massive than the proton. While the neutron is stable within an atom, an isolated neutron will decay into a proton, a electron, and an antineutrino, with a half-life of about 15 minutes.
You might think that, in some sense, the proton and the electron were mirror images of each other, since in equal numbers they produce neutral atoms, but this is not so. Intensive study of the electron has shown no internal structure whatsoever. The electron is essentially defines by its mass, its charge, and its spin. The best anyone can do defining its size is to provide an upper limit (about 10-18 meters)
By contrast, the protons is known to have internal structure, yet as study of the proton has proceeded over the years, the picture of its makeup and dynamics has become ever more complicated. The quark theory of protons and neutrons (and other less stable baryons), as devised independently by Murray Gall-Mann and George Zweig, proposed that such particles are made up of three smaller particles, termed quarks. The first (and most stable) generation of quarks consists of the up quark (spin = ½, charge = +2/3) and the down quark (spin = ½, charge = -1/3), and their antiparticles (reverse the charges). In this model, the proton is made up of two up quarks and one down quark; the overall charge is 2/3 + 2/3 -1/3 = +1, and the spins can arrange themselves to give an overall particle spin of ½, consistent with what is known about the proton. The first experiments probing the internal structure of the proton confirmed that there appear to be three particles in its interior. These particular experiments are ones that bounce electrons off of quarks carrying much of the momentum within the proton. An animated simulation based on data from such electron scattering experiments can be found in the linked article. Above is a screenshot of the simulation.
However, there are problems with the three quark model. Specifically, careful consideration of the mass and spin of the proton predicted by this model miss the mark by quite a bit. The reason is that there is far more going on inside the proton than three quarks dancing around. The reason is what’s called pair creation and annihilation. For short periods of time, the uncertainty principle allows the creation of pairs of a particle and its antiparticle. It turns out that this process is happening in empty space all the time; empty space is actually seething with these so-called virtual particles, which implies that empty space is not so empty. When this happens in the presence of a force, it’s possible for the members of the pair to escape from each other for a time. (This is the source of Hawking radiation, where a pair of particles is created near the event horizon of a black hole, and while one falls into the the black hole, the other escapes as radiation.) Under the influence of the strong force, the interior of the proton is actually seething with quarks. In experiment, this becomes apparent when observing electron scattering from low-momentum quarks. Again, the linked article shows an animation of this seething; the above image shows a screenshot.
Going down to electron scattering from the very lowest momentum quarks produces even more seething, but not so much the quarks themselves, but rather the force particles that hold them together. The force particle for the strong force are somewhat whimsically called gluons. The picture that emerges from these measurements corresponds very closely to the theory that emerged to describe the strong force, called quantum chromodynamics (or QCD), but it has lost any apparent relation to that first picture of just three quarks bound together. This has to do with the fact that the strong force is very weak when quarks are close together (the picture provided by low-momentum quarks) and become much stronger as they become farther apart (the high-momentum quarks that show distinct particles). QCD becomes extremely difficult to calculate when the force is strong, so as yet, the theory has not yielded this picture.
Finally, when data were sifted very carefully, it was apparent that the proton was hosting some highly unexpected visitors: a pair of charmed and anticharmed quarks. The charmed quarks are from the second generation of quarks, and are much less stable than the up and down quarks of the first generation. They are also much heavier; in fact they are each much heavier than the proton itself. This is the same sort of pair-creation causing the seething observed in low-momentum electron scattering, but on a much higher energy scale. I find it strange that the charmed quark-antiquark pair is favored over, say, the analogous strange quark pair, since the strange quarks have lower mass (and hence lower energy) than the charmed ones, but there must be a reason why charm is favored. The charmed pair might also be responsible for providing the mass that the three first-generation quarks can’t provide on their own, perhaps? I’m just spitballing here.
Anyway, it’s an interesting article.
Comments are below the fold.
Top Comments (December 10-11, 2022):
From reflectammt:
I'd like to nominate this comment from Jeff Y. It's from Rebekah Sager's diary on Brittney Griner's release causing the GOP's heads to explode, which was met with some unfortunate muddying here at DK in the comments. I appreciated how Jeff just cut to the chase with this comment. Jeez.
If Republicans are upset about an athlete protesting our National Anthem, wait until they find about the time a bunch of rabid Trumpers attacked our Capitol and beat police officers with flagpoles flying the American flag. And then followed that up by carrying a traitorous Confederate flag into the Capitol.
Top Mojo (December 9, 2022):
Top Mojo is courtesy of mik! Click here for more on how Top Mojo works.
Top Mojo (December 10, 2022):
Top Photos (December 9, 2022):
Thanks to jotter (RIP) for creating it and elfling for restoring it.
Top Photos (December 10, 2022):