It appears that the mass radius of the proton is shorter than the radius of the charge – Ars Technica

If you ask how much an object like a bicycle weighs, there is a simple answer. But if you ask where the bike’s mass is, things get a little more complicated. A bicycle has a lot of parts—some of which move—which are all different sizes, shapes, and densities, so its mass is distributed irregularly around its shape.

To some extent, this is analogous to the question of where is the mass of a proton. A proton is a collection of quarks and gluons moving at relativistic speeds around a central point. It would be hard to tell where its group lifetime would be even without the fact that the analogy with bicycles completely breaks down due to one embarrassing fact: a proton weighs much more than its constituent quarks, and the gluons holding the quarks together are massless. In fact, the mass of the particles involved is somewhat irrelevant. “If you do calculations where you set the quark’s mass to zero, a proton is pretty much the same thing,” physicist Sylvester Johannes Josten told Ars.

Instead, much of a proton’s mass comes from the incredibly high energy density generated by the gluons’ strong force interactions. So, to understand the mass of a proton, we have to understand what gluons have to do. Which, being massless and having no charge, is extremely difficult to do. But some experimental work has created a value for the proton’s mass radius, which describes the mass distribution inside the particle. And it turns out that the value is significantly different from the radius of the charge of the proton.

Inhalation of gluons

Gluons without mass or charge are very difficult to detect; We primarily infer where they must be from the debris they help create in particle collisions. To some extent, we can model their behavior, but this is primarily described by quantum chromodynamics, which is legendary for its ability to bring massive computational resources to their knees. So even our best models of gluon behavior are approximations.

If the mass of a proton depends primarily on its gluons, then, and we can’t know what the gluons are doing, how can we know what’s going on?

The trick was to identify a process that would be detectable, but sensitive to the presence of gluons. This process is the conversion of energy (in the form of light) into matter. Specifically, a photon of sufficient energy can be converted into a so-called J/meson, consisting of a charm quark and a charm antiquark through a delicate process of forming gluons into any nearby protons. By measuring the production of J/mesons, it is possible to determine something called gluon gravitational form factors, which describe where mass is located in a proton.

How to do that is, well, almost as complex as the process description in the paragraph above. It starts with a beam of high-energy electrons, produced at the Thomas Jefferson National Accelerator Facility. These electrons are then moved back and forth across their direction of travel in a process with a very technical name: wiggle. This causes them to lose energy, which they release in the form of high-energy photons.

These photons are then sent through a chamber containing a container of liquid hydrogen. While crossing the basin some photons will be converted into J/ mesons which will decay instantly. Two of the products of this decay are the electron and the positron, which can be captured by detectors, allowing the production of J/ mesons to be recorded. Based on these discoveries, it is possible to re-work and find out the shape factors of gluon gravity.

(Note that I’m not even trying to find an analogy for the gluon gravitational form factors that would help you understand them. They are described in the paper as “matrix elements of the energy-momentum tensor of a proton and encode the mechanical properties of the proton, while the anomaly effect of the energy-momentum tensor is a major component of the origin of mass according to quantum chromodynamics.” You just have to trust everyone involved that it’s a terminology that helps describe the relationship between gluons and mass.)

Here and there

With the data available, all the researchers needed was a model of gluons’ behavior to relate their findings to what’s going on inside the proton. Unfortunately, physicists have produced several different models, again because direct computing of anything involving quantum chromodynamics is nearly impossible on current hardware. So, the available models are mostly alternative ways of making approximations that allow computers to produce something useful.

For the most part, a variety of estimates produced fairly similar answers, though things got a bit worse with lower energy photons, which barely have enough energy to convert into something with J/m mass. However, different approaches provide rough agreement on where the proton’s mass is located, and thus the proton’s mass radius.

The amazing thing about the result is that it differs from the radius of the charge of a proton. While there are some differences between the different methods of measuring the radius of a charge (though), the differences are relatively minor. And all measurements put the charge radius apart from the mass radius. Since the charge is a product of the quarks, this indicates that these particles are regularly orbiting outside the region frequented by gluons that are busy holding the entire proton together.

But Justine said there are hints that the situation is more complicated. The reactions that formed J mesons required the exchange of gluons that had their spins aligned. It is also possible to exchange gluons that have opposite spins, called scalar interaction. And there are some hints that the standard radius is different as well.

“You should expect to see some standard radii,” Justin said. “We see something huge. It’s much bigger – like this big halo going around the proton.”

One caveat here is that Joosten said the findings are very preliminary: “This isn’t something we found, this is something we’d like to investigate in future trials.” The other is that his use of the term “huge” is relative; All of this happens inside a subatomic particle.

But if the results hold up, they indicate that a proton has at least three distinct radii—charge, mass, and gauge—and they are all of different lengths.

And this is where the bicycle analogy might start to make sense again. After all, you don’t expect a bike’s center of mass to be in the same location as its pedals or where the steering occurs. Although it is a single organism, its composite nature means that these different aspects of its behavior are not necessarily local in common. The same seems to be true of the proton.

Nature, 2023. DOI: 10.1038 / s41586-023-05730-4 (about DOIs).