Parton

He'd been trying to understand violent hadron collisions under a model where the proton was built from small, independent constituents he called partons.

Under this picture, when a hard-hitting electron struck a proton, it was scattering off of these individual partons cleanly, almost as if that parton was just sitting there.

But it's important to be precise about what parton meant, because it's not simply another word for quark.

Feynman chose the name deliberately to avoid committing to any specific identity for these constituents.

It was a deliberately vague term for whatever was scattering the electrons, but it was a physical, real particle he proposed, in line with Zweig's view of his aces, and in contrast to Gell-Mann's quarks, which Gell-Mann treated strictly as a mathematical tool rather than a physical constituent of matter.

Even with the parton picture in hand, connecting it specifically to quarks was a strong hypothesis rather than a settled fact for a few years afterward, and the physicists doing the work were quite candid about that.

A separate 1968 calculation by Kurt Callan and David Gross showed that a particular ratio buried in the scattering data could reveal the spin of the charged partons, and the result came back consistent with spin one-half, the same spin quarks were predicted to have.

But Kendall himself later recalled that some of the very theorists building the case for the parton theory, like Bjorken and his colleague Kurt Gottfried, said outright that they'd constructed some of their tests hoping to disprove the quark model rather than confirm it.

It has to do with something physicists call nuclear parton distribution functions, or NPDFs for short.

Okay, let's break that down.

Partons are just the collective term for the quarks and the gluons, right? The inmates and the chains.

Correct.

But those quarks and gluons inside an oxygen nucleus are not static.

What actually pops out the other side? Like what does a map of the pion look like?

Well, the key deliverable from the Polaris calculations was something called the quark generalized parton distribution, or GPD for short.

Generalized parton distribution.

Again, physicists naming things-

Yeah

Showing the actual terrain?

Right.

A generalized parton distribution maps the exact spatial distribution of quarks inside the pion, but it also cross-references that with how much of the pion's total momentum each individual quark is carrying.