Inside the Proton, the ‘Most Complicated Thing’ Imaginable

Physicists are still trying to understand the protons more than 100 years after Ernest Rutherford discovered them.

The perfect foils for the negatively charged electrons that buzz around them were described by high school physics teachers. The ball is a bundle of quarks. There is a deeper truth that is too strange to be captured with words or images.

Mike Williams is a physicist at the Massachusetts Institute of Technology. You can't imagine how complex it is.

There is a haze of probabilities until an experiment forces it to take a concrete form. Researchers set up their experiment in a variety of ways. Generations of people have worked to connect the particle's faces. Richard Milner is a nuclear physicist at MIT.

The secrets of the proton keep falling out. There are traces of particles called charm quarks that are heavier than the protons.

Williams said that the protons had been "humble to humans." It throws you some surprises whenever you think you have a handle on it.

The result of hundreds of experiments was transformed into a series of animations by a group of people. We put their animations into our attempt to reveal its secrets.

Cracking Open the Proton

The SLAC had proof that the protons contain many people. Researchers watched as the electrons bounced off like billiard balls. Researchers saw that the electrons bounced back differently. The electrons were hitting the protons hard enough to cause deep inelastic scattering, and then they were bouncing from point to point. The first proof that quarks exist was found by Xiaochao Zheng, a physicist at the University of Virginia.

The scrutiny of the protons increased after the discovery of the SLAC. Hundreds of scattering experiments have been carried out by physicists. They deduce various aspects of the object's interior by adjusting how forcefully they bombard it and by choosing which scattered particles they collect after the event.

Physicists can use higher-energy electrons to look at the target protons. The resolving power of a deep inelastic scattering experiment is set by the electron energy. Particle colliders give a better view of the protons.

Researchers can use different subsets of the electrons to analyze the collision outcomes. Understanding quarks has been made possible by this flexibility.

Researchers can tell if an electron has glanced off a quark by measuring its energy and trajectory. They can use repeated collisions to determine if the protons are bound up in a few quarks or spread over many.

Today's standards are not as strict as they were a few years ago. In the scattering events, electrons shot out in ways that suggested they had crashed into quarks. Murray Gell-Mann and George Zweig came up with the idea that a protons consists of three quarks.

The quark model is an elegant way to imagine the protons. There are two up quarks with electric charges of +2/3 each and one down quark with a charge of 1/3.

The quark model has flaws.

When it comes to a protons spin, it fails. The up and down quarks have the same amount of spin as the protons. The half-units of the two up quarks minus the down quark should equal half a unit for the protons as a whole, according to a calculation by physicists. The European Muon Collaboration said that the quark spins add up to less than half. Two up quarks and one down quark make up less than 1% of the total mass. Physicists were already aware of the fact that the protons are more than three quarks.

Much More Than Three Quarks

TheHERA slammed electrons into protons a thousand times more forcefully than the SLAC did. Physicists were able to pick the electrons that bounced off of extremely low-momentum quarks. HERA's electrons came back from a maelstrom of quarks and antimatter.

The results showed that the theory that replaced Gell-Mann and Zweig's quark model was correct. The strong force theory was developed in the 70s. quarks are being pulled together by force-carrying particles. Each quark and each gluon has three types of charge, labeled red, green and blue, which naturally tug on each other and form a group, such as a protons. The theory was called quantum chromodynamics.

Surges of energy can be picked up by gluons. With this energy, a gluon splits into a quark and an antiquark before they destroy each other and disappear. The lower the energy spike, the shorter the lives. HERA, with its greater sensitivity to lower-momentum particles, was able to detect this "sea" of gluons, quarks and antiquarks.

HERA found out what the protons would look like in bigger colliders. Lower-momentum quarks showed up in larger and larger numbers as physicists adjusted HERA to look for them. The results suggested that in a high-energy collision, the protons would appear as a cloud.

The gluon dandelion is what it is predicted to be. The HERA data are proof that nature is described.

The young theory was able to describe the dance of short-lived quarks and gluons, but it was useless for understanding the three long- lasting quarks.

Only when the strong force is not strong can the predictions be understood. The strong force doesn't weaken when quarks are very close together. This feature was identified by Frank Wilczek, David Gross and David Politzer in 1973.

The quarks pull on each other so strongly that they become impossible to calculate. Experimentalists have been tasked with further demystifying the three-quark view of the protons. Key contributions have been made by researchers who run "digital experiments." Physicists find surprises when they look at low-resolution pictures.

A Charming New View

A team led by Juan Rojo of the National Institute for Subatomic physics in the Netherlands and VU University Amsterdam used machine learning to infer the motions of quarks and gluons inside the protons.

There was a blur in the images that had escaped past researchers. Most of the momentum was locked up in the three quarks in a relatively soft collision. The charm quark and charm antiquark were thought to be the source of a small amount of movement.

There are short-lived charms found in the "quark sea" view of the protons. The results from Rojo and colleagues show that the charms are more permanent. The electron usually encounters the three lightweight quarks in these collisions. It will occasionally come across an up, down and charm quark on one side and an up quark and charm antiquark on the other.

The makeup of the protons could be consequential. Physicists search for new elementary particles by bashing high-speed protons together and seeing what pops out, but they need to know what is in a protons to understand the results. The chance of making more exotic particles was thrown off by the occasional appearance of charm quarks.

Charm quarks popping up at the right moments would shower Earth with extra-energetic neutrinos, according to researchers. Observers are looking for high-energy neutrinos.

Rojo and his team plan to look for an imbalance between charm quarks and antiquarks in the protons. The heavier the quark, the harder it would be to detect.

More unknown features will be sought by next- generation experiments. The first 3D reconstructions of the protons will be made using higher-resolution snapshots that will be taken at the Electron-Ion collider. The EIC will use spinning electrons to create maps of the spins of the internal quarks and gluons. It will help researchers to finally pin down the origin of the protons spin, and to address other fundamental questions about the particle that makes up most of our everyday world.

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