Protons and neutrons, the particles that form the nuclei of atoms, might seem to be really tiny. But scientists say that those subatomic particles themselves are made up of something even smaller — particles called quarks.
"Well, I think the simplest way of stating it is that quarks are the fundamental constituent of matter, of all the stuff that's around us," explains Geoffrey West. He's a theoretical physicist who founded the high energy physics group at the Los Alamos National Laboratory and now is Shannan Distinguished Professor at the Santa Fe Institute. (He's also author of the 2017 bestseller "Scale," about how the mathematical laws governing the structure and growth of the physical world apply to biological life and to human society.)
Quarks are so tiny that it's mind-boggling to even try to express their estimated size. University College London physics professor Jon Butterworth explained that the radius of a quark is roughly 2,000 times smaller than that of a proton, which in turn is 2.4 trillion times as small as a grain of sand.
Existence of Quarks First Proposed in 1964
The existence of quarks was first proposed in 1964 by California Institute of Technology theoretical physicist Murray Gell-Mann, one of the key figures in the development of the Standard Model of particle physics. Gell-Mann, winner of the of the 1969 Nobel Prize in Physics, figured out that explaining the properties of protons and neutrons required that they be made up of smaller particles. At the same time, another CalTech physicist, Georg Zweig, also independently came up with the idea as well.
The existence of quarks was confirmed by experiments conducted from 1967 to 1973 at the Stanford Linear Accelerator Center.
One of the odd things about quarks, as West explains, is that they can be observed, but they can't be isolated. "There's a subtle difference," he says. "They're like electrons in that electrons are fundamental, but with electrons we can observe and also isolate them. You can point to an electron. With quarks, you can't take one out of the nucleus and put it on the table and examine it."
Instead, by using gigantic particle accelerators, scientists speed up electrons and use them to probe the depth of the nucleus. If they go deep enough within, the electrons will scatter off the quarks, which can be measured using very sophisticated detectors. "We reconstruct what's in the target that protons and neutrons are made up of," West says. "You see these little point objects that we identify as quarks."
There Are Six Types of Quarks
Quarks have fractional charges compared to the protons that they form. There are six types of quarks based upon mass, and the particles also have a quality called color, which a way of describing how the strong force holds them together. Color is carried by gluons — a sort of messenger for the strong force that bind quarks together. (They're analogous to photons.)
A team of University of Kansas physicists plan to use a device installed at the Large Hadron Collider, a massive particle accelerator located in a 17-mile (27-kilometer) tunnel between France and Switzerland, to investigate the strong interaction between quarks and gluons.
"The idea is to get a better understanding of the proton and the heavy ion structure — such as Lead for instance — and to study a new phenomenon called saturation," Christophe Royon, a University of Kansas physics professor who is leading the research, explains in an email. "When two proton or two ions collide at very high energy, we are sensitive to their substructure — quarks and gluons — and we can probe some region where the density of gluons gets very large."
"An analogy would be the metro in New York at peak hours when the metro is completely congested," Royon continues. "In that case, the gluons do not behave as single identities but can show collective behavior, in the same way as a crowded metro, if somebody falls, everybody will feel it since people are so close to one another. At some point, the protons or heavy ion can behave like a solid object, like a glass, called color glass condensate. This is what we want to see at the LHC and also at the future Electron-Ion Collider in the U.S."
Royon says that finding proof of the existence of this dense gluon material would answer one of the biggest unanswered questions about quarks. "This is a new state of matter," he says. "Some hints already appeared at the Relativistic Heavy Ion Collider or the Large Hadron Collider but nothing is certain yet. It would be an important discovery, and both the Large Hadron Collider and the Electron-Ion Collider are ideal machines to see this."
Scientists also wonder whether there might be something even smaller than a quark. "It begs the question, is there another level yet?" West says. "We don't know the answer to that."