The gluon cloud is exactly what QCD predicts.

“The HERA data are direct experimental proof that QCD describes nature,” Milner said.

But the young theory’s victory came with a bitter pill:

While QCD beautifully described the dance of short-lived quarks and gluons revealed by HERA’s extreme collisions,

the theory is useless for understanding the three long-lasting quarks seen in SLAC’s gentle bombardment.

QCD’s predictions are easy to understand only when the strong force is relatively weak.

And the strong force weakens only when quarks are extremely close together,
as they are in short-lived quark-antiquark pairs.

#Frank #Wilczek, #David #Gross and #David #Politzer identified this defining feature of QCD in 1973,
winning the Nobel Prize for it 31 years later.

But for gentler collisions like SLAC’s, where the proton acts like three quarks that mutually keep their distance,
these quarks pull on each other strongly enough that QCD calculations become impossible.

Thus, the task of further demystifying the three-quark view of the proton has fallen largely to experimentalists.
(Researchers who run “digital experiments,” in which QCD predictions are simulated on supercomputers,
have also made key contributions.)

And it’s in this low-resolution picture that physicists keep finding surprises.

Recently, a team led by #Juan #Rojo of the National Institute for Subatomic Physics in the Netherlands and VU University Amsterdam
analyzed more than 5,000 proton snapshots taken over the last 50 years,
using machine learning
to infer the motions of quarks and gluons inside the proton
in a way that sidesteps theoretical guesswork.

The new scrutiny picked up a background blur in the images that had escaped past researchers.

In relatively soft collisions just barely breaking the proton open,
most of the momentum was locked up in the usual three quarks:
two ups and a down.

But a small amount of momentum appeared to come from a “#charm” #quark and charm #antiquark
— colossal elementary particles that each outweigh the entire proton by more than
one-third❗️

Short-lived charms frequently show up in the “quark sea” view of the proton
(gluons can split into any of six different quark types if they have enough energy).

But the results from Rojo and colleagues suggest that the charms have a more permanent presence,
making them detectable in gentler collisions.

In these collisions, the proton appears as a quantum mixture,
or superposition,
of multiple states:

An electron usually encounters the three lightweight quarks.

But it will occasionally encounter a rarer “molecule” of five quarks,
such as an up, down and charm quark grouped on one side and an up quark and charm antiquark on the other.

Such subtle details about the proton’s makeup could prove consequential.

At the Large Hadron Collider, physicists search for new elementary particles by bashing high-speed protons together and seeing what pops out;

to understand the results, researchers need to know what’s in a proton to begin with.

The occasional apparition of giant charm quarks would throw off the odds of making more exotic particles.

And when protons called #cosmic #rays hurtle here from outer space and slam into protons in Earth’s atmosphere,
charm quarks popping up at the right moments would shower Earth with extra-energetic #neutrinos, researchers calculated in 2021.

These could confound observers searching for high-energy neutrinos coming from across the cosmos.

Rojo’s collaboration plans to continue exploring the proton by searching for an imbalance between charm quarks and antiquarks.

And heavier constituents,
such as the #top quark, could make even rarer and harder-to-detect appearances.

Next-generation experiments will seek still more unknown features.

Physicists at Brookhaven National Laboratory hope to fire up the
"Electron-Ion Collider"
in the 2030s
and pick up where HERA left off,
taking higher-resolution snapshots that will enable the first 3D reconstructions of the proton.

The #EIC will also use spinning electrons to create detailed maps of the spins of the internal quarks and gluons,
just as SLAC and HERA mapped out their momentums.

This should help researchers to finally pin down the origin of the proton’s spin,
and to address other fundamental questions about the baffling particle that makes up most of our everyday world.

https://www.quantamagazine.org/inside-the-proton-the-most-complicated-thing-imaginable-20221019/