A proton contains three quarks that are held together by particles called gluons.
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Protons, one of the building blocks of all matter, vary in size depending on how you look at them. If you look at the charge, it has one radius, but if you look at the mass, the radius is smaller because the mass is kept centered.
“We have a new image of the proton. It’s new in the sense that we didn’t remove information, we added information that wasn’t there.” Zane Edin Meziani at Argonne National Laboratory in Illinois.
In the 1960s, experiments in which electrons were fired at protons revealed that protons contained point-like charged particles. These are now called quarks. A proton has two up quarks and one down quark. These quarks were later discovered to be held together by particles called gluons.
We now know more about quarks and how far their electric fields, sometimes called proton radii, extend in space. However, little is known about gluons, which contain most of the proton mass in the form of energy. By understanding how they are distributed, we can learn about how the proton’s mass is arranged and about its internal structure.
Meziani and his colleagues now probe proton gluons using particles called J/psi mesons. This is because even though the gluon has no electric charge, he in the universe has a property called color charge that derives from the powerful nuclear force, one of the four fundamental forces.
The researchers fired a beam of photons into liquid hydrogen, which was mostly just protons, and the photons interacted with the protons. These collisions produce short-lived her J/psi mesons, each composed of a charm quark and its anti-quark, carrying a color charge and thus potentially interacting with gluons.
By measuring the number of J/psi mesons produced, Meziani and his team were able to calculate the proton mass distribution using a quantum mechanical model that describes the interaction of gluons and quarks.
Their results suggest that the mass of the gluon is confined to the dense core at the center of the proton, while the charge from the quark is spread over a second, larger radius.
They also compared their results to predictions from different models of protons, which agreed in some places and diverged in others.
“If confirmed, it would be a very interesting discovery because it would tell us something very profound about how the proton building blocks behave from a spatial perspective,” he said. Juan Rojo at the Free University of Amsterdam in the Netherlands.
Different internal structures can affect calculations of other proton properties, such as spin, angular momentum and energy distribution, says Rojo. However, some of the most recent findings are based on the models used to calculate them, which have not proven completely reliable in the past.
Meziani and his team’s results follow another revelation about the inner workings of protons. Last year, a team led by Rojo discovered that Yoko could contain three normal quarks, plus a much heavier charm, his quark. “If they account for charm quarks, it will be great to see what happens. Do the mass radii grow or shrink?” Rojo says.
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