Quarks, Colour, and Quantum Pony Dynamics: Explaining QCD with MLP · 2:30pm Sep 13th, 2020
As my smart readers may have guessed, my last post on rainbows, pink, and colour theory was just an introduction to what I really want to talk about. Now you have been suitably primed with red, green, and blue; you are ready to learn about Quarks and Quantum Chromodynamics. Get ready for some primary-coloured pony particle physics!
Quarks are one type of fundamental particle of matter. Quantum chromodynamics (QCD) is the theory that explains how the Strong Force glues them together. Let’s begin with the context. There are four fundamental forces of nature: Gravity, Electromagnetism, the Weak Interaction, and the Strong Interaction.
Gravity makes all massive things attract each other. Very important if you want to raise the moon, but it is mostly ignored by particle physicists as it is too difficult to understand in terms of fundamental particles, and our particles weigh so little that we can’t see its effects in high energy particle collisions anyway.
Electromagnetism makes particles with a positive electric charge (like protons) attract particles with a negative electric charge (like electrons), while repelling particles with the same charge. All described by the brilliant theory of quantum electrodynamics (QED).
The Weak Interaction describes the alchemy that can turn one type of particle into another. Fascinating stuff, but it’s a story for another time.
The Strong Interaction is the strong force that lets protons overcome electromagnetic repulsion and stick together with neutrons at the heart of atoms. And there is more to it. When we probe deeper we find that protons and neutrons are not fundamental, but are made from smaller fundamental particles – quarks.
Quarks have an electrical charge of either +2/3 or −1/3 that of a proton, so they feel the electromagnetic force. There is another type of charge associated with the strong force. As with electromagnetism, like-charges repel each other and unlike-charges attract each other, but instead of just being positive or negative, the strong interaction charge has three components, which we label red, green, and blue. As quarks are millions of times smaller than the wavelength of light they don’t actually have a real optical colour. Colour-charge is just a colourful analogy.
Over the years, teachers and science writers have developed analogies to explain this analogy. For example, Ben Still explains particle physics with the aid of coloured Lego Bricks. Each analogy works to illustrate some points and make a good story, but they all break down at some point, just as early models of fundamental particles fail to explain new data, and need to be developed further, or replaced. Meanwhile the creatives in our fandom have happily reimagined ponies as cats, dogs, planes, and other things, so I think it is time to explore the idea of ponies as quarks. Canon ponies are a bit too restricted for this exercise. We have specific colour requirements. It is, however, a task ideally suited to using an OC generator. These were created using PonyLumen’s powerful Pony Creator.
Introducing the six quark ponies:
[Yes, there is a Quantum Chromodynamics Trading Card Game.]
We have the light-weight pegasi Up and Down, intermediate unicorns Charm and Strange, and the heavy earth quark-ponies Top and Bottom. All six quarks can come in all three colours. Next we need to meet the anti-quark ponies, who have the opposite electrical charge (−2/3 or +1/3) to their corresponding particles; and the opposite colour charge, making them anti-red (cyan), anti-green (magenta), and anti-blue (yellow).
Now we can now investigate their friendships. Quarks join together to make hadrons. There are two types of hadron: baryons, made from three or more quarks, and mesons, made from two. All friendships must be colour neutral, with equal amounts of red, green, and blue (or the equivalent anti-colours). Fortunately, this law does not hinder friend-making as quark ponies can change their colour by interacting with other quarks. This is possible because the gluon—the particle which mediates the strong force carries a colour and an anti-colour. This is different to the electromagnetic force, mediated by the photon, which carries no electrical charge, so particles can’t change their electrical charge.
[Here The gluon takes red and anti-green away from the Up quark, making her green, and changing the Down quark from green to red.]
We can create a baryon by combining red, green, and blue quarks; or an anti-baryon with cyan, magenta, and yellow anti-quarks. Up and Down form the most stable friendship we know, the proton.
While Anti-Up and Anti-Down form an Anti-Proton.
[Antiparticles are just as stable in the antimatter world, but if an antiproton meets a proton then we have a rather messy annihilation.]
The other baryon with Up and Down is the neutron:
Free neutrons have an average lifetime of a bit less than fifteen minutes. However, they can live forever if they are bound with one or more protons. Protons and neutrons form wider friendship networks in atomic nuclei, from deuterium to uranium, which gives the whole field of nuclear physics.
There are plenty more baryonic friendships with Greek names such as Lambda, Sigma, and Xi. See the Particle Data Group for the full list. There are baryons for all combination of three quarks, except those involving Top, who sadly, due to her huge mass, lives for such a short time that she never has a chance to make friends with any quark.
Now moving on to mesons. These are two-pony ships combining a quark and an anti-quark with opposite colours (red and cyan, green and magenta, or blue and yellow). The lightest mesons are pions, made from up and down quarks and anti-quarks; followed by kaons, which include strange quarks.
Pions and kaons were discovered in cosmic ray showers in the days before big particle accelerator experiments. At the other end of the discovery timeline are exotic baryons: tetraquarks and pentaquarks, which have only been seen in the last few years at detectors like the LHCb experiment at CERN.
[See: CERN’s LHCb experiment reports observation of exotic pentaquark particles, World’s most precise measurements and search for the X(5568) tetraquark candidate]
Let’s finishing by stretching the analogy as far as we can. Quarks ponies are so friendly that they are never found as isolated particles. They only exist inside hadrons. What happens when we tear them apart? Unlike the electromagnetic force, the strong force between two quarks does not become weaker with distance. As you pull them apart, you need more and more energy. Very quickly the potential energy is large enough to create a new quark-anti-quark pair, so our two separated ponies have new friends to keep them happy.
This way, energetic quarks, for example those produced when a heavy particle like a Higgs Boson decays, will quickly turn into a shower of hadrons, producing a particle jet in a high energy experiment.
Thanks for reading this far. That took rather longer than I expected to explain, although there's a lot of details I skipped. Feel free to ask questions in the comments. If any physics nerds want a challenge, other than finding mistakes in what I've written, see if you can figure out what particle physics discovery is illustrated in the picture at the top of this post.
I ship Up and Down. Their relationship is just so positive!
5354584
@Pineta Fascinating stuff, can't say I'm a real science nerd, but I do like to read about stuff like this every now and again.
The ones with Strange's eyes are freaking me out. Good Job!
Typical how the colour, charge mass doesnt like to folow simple mathematical relationships. Or maybe they do but the simple function is too complex for current modeling, but will be reducible to a 5 term function eventually, then again Grahams number only has 6 terms?
Maybe all the particle complexities will end up being due to trying to fit everything into only 4 dimentions.
then again, maybe that will end up taking the existing particle character set, and creating every possibly equivalent combination text with them. 
5354584
They are besties
5354609
Thanks. I like to feel I am also reaching those who aren't science nerds. Or at least not yet science nerds.
5354618
While strange particles like kaons and hyperons are really not as strange now as they were when they were first discovered, we still picture them as the freaky ones.
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The quark model predicts the charge and spin of hadrons, but the precise masses are more difficult to predict. It depends on a lot more than just the masses of the quarks. The maths in lattice QCD gets pretty complicated and there are a lot of uncertainties.
5354855
Thats why Im really hoping the n dimentional polytope mesh model equivalents of the Feynam Diagrams turns out to work, as it seems to force renormalisation through just making the subdivisions of a given surface finer, but without altering the end value of the volume of the structure which I think is meant to represent the total energy of the sytem, giving mass?
The talk of quarks with pretty ponies reminded me about this scene in the movie Roxanne:
Roxanne explains quarks to Cyrano de Bergerac.
That reminded me about the song "Brains, Body, both":
That reminded me about the 3 marriages of Professor Carl Sagan:
Carl was an Astronomer. He figured that he should marry a great mind. He found that he could not maintain arousal
He figured that attraction must be a primitive urge, so married a physically attractive woman. He found her boring.
He realized that people had both an intellectual and animalistic side and married his 3rd wife, who is both intelligent and attractive, with whom he stayed until his death.
Carl came to think of people as consisting of aggressive territorial sexual urges going back to our reptillian-ancestors of the Paleozoic, The nurturing loving, prosocial empathetic sympathetic instincts of Mesozoic mammals, and the intellectual prowess of Cenozoic primates. Whether we let the inner reptile or mammal rule our intellect will determine whether we destroy ourself or travel among the stars.
Interesting; thanks.
On one hand, I don't think it takes a physics nerd to do this¹; on the other, that line did just get me to look up more science facts.
Now I know that omega baryons exist and I'm pretty certain that's what you're getting at (the sss group a little below the center of the image). However, as this blog post is the most thorough explanation of quarks that I have thus far been exposed to (And good job with that! I don't have any questions about what you've covered².), I suspect the significance is still lost on me, at least for now.
¹I'm not a physics nerd.
²Specifically only the contents of the blog post.
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You win. Yes this is the Omega Minus Baryon, the discovery of which was predicted by the quark model. The image shows the tracks in a bubble chamber at the Brookhaven lab in 1964 when a
K^-hits a proton and produces a\Omega^-,K^+andK^0.images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/f/fb087425-b3fb-4dc6-9c6f-5d3b403d7979/de5kb43-17412b65-e494-4af7-a46a-077b8cd6a558.png/v1/fill/w_839,h_952,q_70,strp/omega_minus_labels_by_pinetapony_de5kb43-pre.jpg?token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ1cm46YXBwOiIsImlzcyI6InVybjphcHA6Iiwib2JqIjpbW3siaGVpZ2h0IjoiPD0xMTM1IiwicGF0aCI6IlwvZlwvZmIwODc0MjUtYjNmYi00ZGM2LTljNmYtNWQzYjQwM2Q3OTc5XC9kZTVrYjQzLTE3NDEyYjY1LWU0OTQtNGFmNy1hNDZhLTA3N2I4Y2Q2YTU1OC5wbmciLCJ3aWR0aCI6Ijw9MTAwMCJ9XV0sImF1ZCI6WyJ1cm46c2VydmljZTppbWFnZS5vcGVyYXRpb25zIl19.FGW3ChisGSJc89Gom9Tm_ntpNVE6MSRwtfVaNOYB48M
See: http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/omega.html for full details of this important discovery.
So gluons have six states? Three "normal" states plus three anti-states that oppose each corresponding "normal" state?
For the most part, I'm personally pushing past the colour analogy because I'm finding it more distracting than anything. The electromagnetic properties of up and down quarks and how they add up to make a neutral neutron or a positive proton is collectively way easier for me to grasp (two times two-thirds [up, up], minus one third [down], bingo: +1 proton).
I get how, say, a "green" and an "anti-green" are polar opposites, but what's a "green" to a "blue"? Neutral? Something more like the thirds of up and down quarks in terms of electromagnetic charge (which when combined determine the total electric charge of protons and neutrons)?
And why does a combo have to balance out into a "colourless" hadron? Does it just fall apart as an unstable mess if it doesn't?
5402936
Thanks for the question. Yes there are six coloured gluon states for every combination of a colour and anti-colour. You can also get colourless gluons made from some combination of red-antired, green-antigreen, or blue-antiblue, but as they are colourless they don't interact with quarks.
Just as positive electrical charges attract negative electrical charges, unlike colour charges attract, so red, green and blue are needed to form a neutral baryon. The strong force is much stronger than the electromagnetic force, so this overcomes any electromagnetic repulsion between quarks with the same sign charge.
Quarks only exist in colourless hadrons. We don't see it fall apart if it isn't colourless, rather it is impossible to pull a single colour out of it.
Mathematically this is described by the SU(3) Lie group.
What's so charming about the charm quark?
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I guess it charmed its discoverers as it fitted the theory so nicely. At first there were just three quarks: Up, Down, and Strange. Up and Down formed a nice pair, but Strange seemed a bit strange as it was just on its own. When Charm came along, the theory that quarks come in pairs was restored.