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What Are Quarks?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.


In the middle of the 20th century, scientists were on the hunt for new particles in the standard Model of Particle Physics, and in an effort to do so they tried to arrange the known ones in an effort to uncover a pattern. Murray Gell-Mann (Caltech) and George Zweig independently of one another wondered if instead scientists should look at the subatomic and see what would be found there. And sure enough, there was: quarks, with fractional charges of +/- 1/3 or 2/3. Protons have 2 +2/3 and 1 -1/3 for a total of +1 charge, while neutrons combine to give zero. This alone is weird but it was favorable because it heled explain meson particle charges but for many years quarks were treated as a mathematical tool only, and not as a serious matter. And 20 years of experiments didn’t uncover them either. It wouldn’t be until 1968 that the SLAC experiment gave some evidence for their existence. It showed that the particle trails post collision of an electron and a proton were a total of three divergences, which is exactly the behavior the quarks would undergo! (Morris 113-4)

Quantum World

But quarks get stranger. The forces between quarks increase as distance does, not the inverse proportion we are used to. And energy that is poured into separating them can lead to new quarks being generated. Can anything hope to account for this strange behavior? Possibly, yes. Quantum electrodynamics (QED), the merging of quantum mechanics with electromagnetics, along with quantum chromodynamics (QCD), the theory behind the forces between quarks, were important tools in this quest. That QCD involves colors (not literally) in the form of red, blue, and green as ways to convey the exchange of gluons, which bind quarks together and therefore act as the force carrier for QED. On top of this, quarks also have spin up or spin down, so a total of 18 different quarks are known to exist (115-119).

Mass Issues

Protons and neutrons have a complicated structure that essentially amounts to quarks being held by binding energy. If one were to look at the mass profile for any of these, one would find that the mass would be 1% from the quarks and 99% from the binding energy holding the proton or neutron together! That is a nutty result, for it implies that most of the stuff that we are constituted from is just energy, with the “physical portion” consisting of just 1% of the total mass. But this is a consequence of the entropy that wants to be put into effect. We need a lot of energy to counteract this natural drive to disorder. We are more energy than quark or electron, and we have a preliminary answer as to the why but is there more to this? Like the relationship this energy has to inertia and gravity. Higgs Bosons and the hypothetical graviton are possible answers. But that Boson requires a Field to operate in and acts like inertia conceptually does. This viewpoint implies that it is inertia itself that causes mass instead of energy arguments! Different masses are just different interactions with the Higgs Field. But what differences would these be? (Cham 62-4, 68-71).

Quark-gluon plasma, visualized.

Quark-gluon plasma, visualized.

Quark-Gluon Plasma

And if one can get two particles to collide at the right speed and angle, one can get a quark-gluon plasma. Yes, the collision can be so energetic that is breaks the bonds holding the atomic particles together just like how the early Universe was. This plasma has many fascinating properties including being the lowest viscosity fluid known, the hottest known fluid known, and had a vorticity of 1021 per second (similar to frequency). This lattermost property is tough to measure because of the energy and complexity of the mix itself but scientists looked at the resulting particles that formed form the cooled off plasma to determine the overall spin. This is important because it allows scientists to test out QCD and see which symmetry theory works best for it. One is chiral magnetic (if a magnetic field is present) and the other is chiral vortical (if spin is present). Scientists want to see if these plasmas can go from one type to the other, but no known magnetic fields around quarks have been seen yet (Timmer "Taking").


What we haven’t talked about are quark pairings. Mesons can have two and baryons can have three, but four should be impossible. That is why scientists were surprised in 2013 when the KEKB accelerator found evidence for a tetraquark in a particle called Z(3900), which itself decayed from an exotic particle called Y(4260). At first the consensus was that it was two mesons orbiting each other while others felt it was two quarks and their antimatter counterparts in the same area. Just a few years later, another tetraquark (called X(5568)) was found at the Fermilab Tevatron, but with four different quarks present. The tetraquark could offer scientists new ways to test QCD and see if it still needs revision, such as color neutrality (Wolchover, Moskowitz, Timmer "Old").

Possible pentaquark configurations.

Possible pentaquark configurations.

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Surely that tetraquark should have been it in terms of interesting quark pairings, but think again. This time it was the LHCb detector at CERN that found evidence for it while looking at how certain baryons with an up, down and bottom quark behaved as it decayed. The rates where off from what theory predicted, and when scientists looked at models for the decay using computers, it showed a temporary pentaquark formation, with possible energies of 4449 MeV or 4380 MeV. As for the full structure of this, who knows. I'm sure like all these topics, it shall prove to be facinating… (CERN, Timmer “CERN”)

Works Cited

CERN. “Discovery of a new class of particles at the LHC.” Kalmbach Publishing Co., 15 Jul. 2015. Web. 24 Sept. 2018.

Cham, Jorge and Daniel Whiteson. We Have No Idea. Riverhead Press, New York, 2017. Print. 60-73.

Morris, Richard. The Universe, the Eleventh Dimension, and Everything. Four Walls Eight Windows, New York. 1999. Print. 113-9.

Moskowitz, Clara. “Four-Quark Subatomic Particle Seen in Japan and China May Be Entirely New Form of Matter.” Huffington Post, 19 Jun. 2013. Web. 16 Aug. 2018.

Timmer, John. “CERN experiment spots two different five-quark particles.” Conte Nast., 14 Jul. 2015. Web. 24 Sept. 2018.

---. "Old Tevatron data turns up new four-quark particle." Conte Nast., 29 Feb. 2016. Web. 10 Dec. 2019.

---. “Taking quark-gluon plasma for a spin may unbreak a fundamental symmetry." Conte Nast., 02 Aug. 2017. Web. 14 Aug. 2018.

Wolchover, Natalie. “Quark Quartet Fuels Quantum Feud.” Quanta, 27 Aug. 2014. Web. 15 Aug. 2018.

© 2019 Leonard Kelley

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