The Jinxie Games and the Proton Spin Crisis · 11:52am Jul 23rd, 2023
Here is my spin on MYM Chapter 4 Episode 3. Will I manage to write a random blog post to accompany every new episode? At the current rate, I just might get Chapter 4 done before Chapter 5 comes out. Unless I get distracted by something else. Let’s go into a story of mystery, cryptic messages, relationships, and rules that don’t make sense; and tell the tale of the Proton Spin Crisis.
Okay, I think somepony's being overdramatic.
Our protagonist in this story is the proton, the particle at the heart of all atoms. The hadron in the collisions at the Large Hadron Collider. Anyone who knows anything about the quark model will know that the proton is made of two Up quarks and one Down quark. I explain this to young children using Lego bricks (two yellow, one red). They understand perfectly.
I'm gonna need some context for what all of this is.
Like many things we tell to young children, this simple model is not the full picture. The quarks are stuck together by the Strong Nuclear Interaction, which works by the exchange of a force particle: the Gluon. The quarks are radiating and absorbing gluons all the time, and the gluons can also create pairs of quarks and antiquarks, which only exist momentarily before annihilating into gluons, or radiating more gluons, which can also radiate even more gluons as the gluon interacts with itself. The actual picture of the proton is something like a messy sea of quarks and antiquarks stuck together with a lot of glue. But after you have cancelled all the antiquarks with quarks, and removed all the gluons, you should be left with the simple Up-Up-Down combo.
Games... Games... Games!
The quark model nicely explains the proton electric charge as the sum of the quark charges (+\frac{2}{3}) + (+\frac{2}{3}) + (-\frac{1}{3}) = +1 , just as neutral charge of the neutron is the sum of the charge of one Up quark and two Down quarks (+\frac{2}{3}) + (-\frac{1}{3}) + (-\frac{1}{3}) = 0.
Next question: Can it explain the proton spin? What is spin for that matter?
Spin is surprisingly tricky to explain. The basic idea that particles have an intrinsic angular momentum, just like gyroscopes, puppies, planets, and just about everything else in the universe, seems simple. And at one level it is. Except it isn’t.
You girls do love decoding things.
If the proton was a solid ball, then it would be spinning so fast it would break the laws of physics. At the microscopic scale, particle don’t follow classical mechanics. Quantum mechanics rules. Spin is a property of a particle, but doesn't mean it is rotating in the conventual sense. Particle spin is quantized in units of \frac{\hbar}{2} or \frac{1}{2} (“spin-half”). Spin-half particles like electrons and quarks have spins of magnitude \frac{1}{2}, which can be polarised in two possible directions +\frac{1}{2} or -\frac{1}{2}. As the proton is also spin-half, this seemed to fit the quark model as the two Up quarks must have opposite spin, so we have (+\frac{1}{2}) + (-\frac{1}{2}) + (+\frac{1}{2}) = (+\frac{1}{2}) or (+\frac{1}{2}) + (-\frac{1}{2}) + (-\frac{1}{2}) = (-\frac{1}{2}). Simple. That was what everyone expected. It was assumed that when the spin of the quarks inside the proton was measured, it would sum up to the spin of the proton.
This is an interesting ritual that they sometimes perform to "unjinx" themselves if they're confronted with anything magical.
Measuring the spin of a quark is not easy. You need to hit a quark in polarized proton with a polarized electron or muon, and you need a very high energy electron or muon beam so it isn’t deflected by the fuzzy electric field of the proton itself, but can penetrate inside. These Deep Inelastic Scattering experiments were done at the SLAC laboratory in Stanford in the 1960s, and gave the first evidence for quarks, but it look longer to get accurate measurements of the spin.
Fascinating, Skye. Truly superstitious.
Then in 1988, the European Muon Collaboration (EMC) at CERN shocked the particle physics world with the result that the spin of the quarks accounted for only somewhere between 4 and 24 percent of the spin of the proton. That was not what was expected. By this point the quark model was established as a super successful theory. What was going on? Was Quantum Chromodynamics all wrong? The resulting chaos and confusion became known as the Proton Spin Crisis.
It can only mean one thing... She's been pony-napped!
It is still not entirely clear what is contributing to the proton spin. If it’s not the quarks, the next candidate was the gluons that hold them together. The gluons have no electric charge, so measuring their contribution required a high energy polarised proton beam. This measurement was done at the Relativistic Heavy Ion Collider at Brookhaven, New York, in 2008, and showed that the gluon contribution to the proton spin was also less than expected.
We've got to get across this balance beam before those birds drop stewed cabbage balls on us!
Another possible contribution to the proton spin is the orbital angular momentum – the relative motion of the quarks and gluons as they rotate about the proton axis. The picture which is now emerging is that all these processes contribute to the total spin. To fully understand what is going on, we will need more precision measurements.
They're pulling a boulder up a hill with, like, some sort of giant rubber band?
Fortunately, there is a planned new machine: the Electron Ion Collider planned, which will collide an intense beam of polarized electrons with a high-energy proton beam. Analysing the particles ejected from these collisions will let us further decipher the structure of quarks and gluons within the proton, and let us better understand quantum chromodynamics and how quarks are confined inside hadrons. I have now started working on this project, so I am trying to get my head around all this. My primary motivation is not so much a fascination with the science of the proton structure, but seeing an opportunity to get away from the tedium of coding component database protocols for the ATLAS detector and work on something more interesting, where I can shape the design of a new detector. If this works out, you will likely get more posts about this from me in the future.
Did that make any sense?
Is it me, or is the 24% in the same vicinty as the ratio between amount of visible matter that we measure with current technology, and dark matter that we can only infer from double indirect measurements and simulations?
What would happen if a Lorentz Invarient VSL behaviour was discovered that had at least three different overall outcomes depending on the combination of scale and energy, but was smoothly variable?
Given one description of electric charge Ive seen, could spin be the axial rotation along the 4th spacial dimention of donut quarks? But under that model, gluons are not only strange, but only exist as a result of energy stresses between seperated quarks? Like taking a slime ball and pulling it apart then claiming the thinning neck between is a totally different structure because its hperbolic instead of spherical?
Has there been any further news on taking Feynman diagrams in 2 D and wrapping them round to form edge vertex surfaces of polyhedra so their volume is a relative measure of total energy?
The biggest problem of all though, isnt it that if youre injecting massive amounts of energy into subatomic particles to examine them, that youre actually examining those particles as they existed during the big bang, not as they exist in the current universe?
I think so?
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I am glad that if it's anyone's job to comment on how to sensibly and logically measure the volumes of Feynman diagrams wrapped around to form polyhedra, it might be Pineta's job but surely not mine.
"Have you tried turning the universe off and back on again?"
Four words in and already a physics pun? That's a record to beat.
I always thought quark and antiquark annihilation turned them into photons, but odds are that was an oversimplification. Since photons and gluons are both gauge bosons, would I be right in correcting to that instead: that annihilation translates quarks and antiquarks into gauge bosons on collision?
Apologies if I've misunderstood: I ask this as an amateur.
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They can do both. Quarks have both electric charge and colour charge, so they can annihilate by both the strong nuclear interaction (to gluons) or the electroweak interaction (to photons, and the Z boson at high energy). As the strong interaction is stronger, gluons are most significant in the virtual particle environment inside the proton. If you are smashing protons together and looking for quark-antiquark annihilations producing new particles, then you also have to consider conservation laws and the electroweak process is often more significant.
Sounds very holistic. Would the solar system be a good analogy here? For instance, the angular momentum of the individual solar system objects (planets, stars, dwarf planets, asteroids, etc.) versus the total angular momentum of the whole system, e.g. treating the solar system as one big spherical object in its own right (such as the heliosphere).
Also, this might be a daft question, since it seems likely to me they've already been taken into account (or automatically cancel out anyway), but could the virtual particles mentioned early on in the OP be contributors to the proton spin?
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Yes, in a 2-body system the total angular momentum is the sum of the combined spin and the orbital angular momentum, but the way the spins add up is different in quantum mechanics and classical mechanics.
This goes back to the early day of quantum mechanics and Neil's Bohrs model of the hydrogen atom. It was clear that the classical picture of an electron orbitting a proton wouldn't work as it would lose all its energy by radiating electromagnetic wave, so it was postulated that the electrons in atoms exist in "orbitals" or "shells" with fixed energy and angular momentum. In quantum mechanics, the wavefunction, not particles, is fundamental.
Yes the virtual particles, or sea quarks also play a role.