If you ask me how much something like a bicycle weighs, the answer is simple. But things get more complicated when you ask where the mass of the bike is. A bicycle has many parts, some of which move, but they all have different volumes, shapes, and densities, so their mass is randomly distributed around their shape.
To some extent, this is similar to the question of where the mass of a proton is. A proton is a collection of quarks and gluons moving at relativistic velocities around a central point. Understanding where that mass is would be difficult even without the fact that one troubling fact completely breaks the analogy with the bicycle. The proton is much heavier than the quarks it is made of, and the gluons that hold the quarks together are massless. In fact, the mass of the particles involved is irrelevant. “If you set the mass of the quark to zero and do the math, the protons are pretty much the same thing,” physicist Sylvester Johannes Joosten told his Ars.
Instead, much of the proton’s mass comes from the incredibly high energy density produced by the gluons’ strong force interactions. So to understand the mass of a proton, we need to understand what its gluons are doing. Given that they have no mass and no charge, this is very difficult to do. However, some experimental studies have produced values for the proton’s mass radius. This describes the distribution of mass within the particle. And it turns out that this value is very different from the proton charge radius.
gluon sniffing
Without mass or charge, gluons are very difficult to detect. We guess where they must have been mainly by the debris that helps produce in particle collisions. To some extent we can model their behavior, which is largely explained by quantum chromodynamics. .
I don’t know what the gluon is doing if the mass of the proton depends primarily on that gluon.
The trick was to identify processes that are detectable yet sensitive to the presence of gluons. The process is to convert energy (a form of light) into matter. Specifically, photons with sufficient energy can be transformed into what are called J/ψ mesons, composed of charmed quarks and charmed antiquarks, through a process that is sensitive to the gluon configuration of nearby protons. By measuring the production of J/ψ mesons, we can determine what is called the gluon gravitational form factor, which describes where the mass is located on the proton.
How to do this is almost as complicated as the process description in the paragraph above. It begins with a beam of high-energy electrons produced at the Thomas Jefferson National Accelerator Facility. These electrons then move back and forth across the direction of movement. This process is called wiggling, a very technical name. This causes energy to be lost and released in the form of high-energy photons.
These photons are sent through a chamber containing a vat of liquid hydrogen. While passing through the bat, some of the photons are converted to J/ψ mesons, which decay shortly thereafter. Two of its decay products are an electron and a positron, which can be detected by detectors and record the production of J/ψ mesons. Based on these detections, we can retroactively understand the gluon gravitational form factor.
(Note that I’m not trying to find a useful analogy for understanding the gluon gravitational form factor; the paper states that they are “the matrix elements of the proton’s energy-momentum tensor and the mechanical On the other hand, minor anomalies in the energy-momentum tensor are, according to quantum chromodynamics, a key factor in the origin of mass.”
