Protons seem to be a different size depending on how you look at them

The proton, one of the building blocks for all matter, has a variable size depending on how you look at it. If you are looking at its charge, it will have one radius, but if you look at its mass, you will see a smaller radius because its mass is kept at the centre.

“We have a new picture of the proton. It’s not that we removed information, it’s new in the sense that we’ve added information that wasn’t there,” says Zein-Eddine Meziani at Argonne National Laboratory in Illinois.

In the 1960s, experiments that fired electrons at protons revealed that the latter contained point-like, electrically charged particles, which we now call quarks. A proton has two up quarks and a down one. These quarks were later found to be bound together by particles called gluons.

We now know much more about quarks and how far their electric field extends in space, which is sometimes called the radius of the proton. But we know less about gluons, which contain most of the mass of the proton in the form of energy, because they are chargeless, and so harder to investigate. Understanding how they are distributed can tell us about how the proton’s mass is arranged and its internal structure.

Now, Meziani and his colleagues have probed the proton’s gluons with a particle called a J/psi meson. This is possible because even though gluons don’t have electric charge, they have a property called colour charge, which comes from the strong nuclear force, one of the four fundamental forces in the universe.

The researchers fired a beam of photons at liquid hydrogen, which is mainly just protons, and the photons interacted with the protons. These collisions produced short-lived J/psi mesons, each one made up of a charm quark and its antiquark, which also have colour charge and so could interact with gluons.

By measuring how many J/psi mesons were produced, Meziani and his team could calculate the proton’s mass distribution using quantum mechanical models that describe gluon-quark interactions.

Their results suggested that the gluons’ mass is confined to a dense core in the proton’s centre, while the charge from the quarks extends to a second, larger radius.

They also compared their results with predictions from another model of the proton, which agreed in some places and diverged at others, suggesting that these figures need validating with more precise experiments or ones that use different quarks to probe the proton’s structure, says Meziani.

“If it is confirmed, it is a very interesting finding because it tells us something quite deep about how the proton’s constituents behave from a spatial point of view,” says Juan Rojo at the Free University of Amsterdam in the Netherlands.

A different internal structure could have implications for calculating other proton properties, such as spin, angular momentum and energy distribution, says Rojo, which many other sensitive experiments rely on. But some of the latest findings rest on the models used to calculate them, which haven’t proved entirely reliable in the past, he adds.

Meziani and his team’s results follow another revelation about the proton’s internal structure. Last year, a team led by Rojo found that the proton can contain a much heavier charm quark, in addition to the three regular quarks. “It would be nice to see what happens if they account for a charm quark. Does the mass radius become larger or smaller?” says Rojo.