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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), written by: Wood&Shermann, compiled by: 1Universe 137

The positively charged particle at the center of the atom is an indescribable complexity that changes its appearance according to the way it is detected, showing a variety of properties. Nowadays, people are trying to piece together the various forms of protons into the most complete picture.

Researchers have recently discovered that protons sometimes include charm quarks and charm antiquarks, and these huge particles are heavier than the protons themselves. [Samuel Velasco / Quanta Magazine] more than a century after Ernest Rutherford discovered that there are positively charged particles in the center of every atom, physicists are still trying to fully understand protons.

High school physics teachers portray them as featureless balls-each with a unit of positive charge-perfectly setting off the "buzzing" negatively charged electrons around them. College students know that the ball is actually made up of three elementary particles called quarks. But research in recent decades has revealed a deeper truth-a truth that is too bizarre to be fully captured by words or images. "it's the most complicated thing you can imagine," says physicist Mike Williams of the Massachusetts Institute of Technology. "in fact, you can't even imagine how complicated it is."

A proton is a substance described by quantum mechanics, and its existence is a probabilistic "haze" before experiments force it to become a specific form. Its appearance will vary greatly depending on how the researchers set up the experiment. Generations of physicists have tried to weave the many sides of protons together.

As the research work continues, the secrets of protons continue to be revealed. A landmark data analysis published in August 2022 found that protons contain traces of charm quarks (charm quark), which are heavier than the protons themselves. Protons "always put humans to shame," Williams said. "every time you think you're sure of it, it gives you a heavy hammer."

Recently, MIT nuclear physicist Milner worked with Rolf Ent of Jefferson Lab (Jefferson Lab), MIT producers Chris Boebel and Joe McMaster, and animator James Lapland to create a series of animations of proton "deformation" based on hundreds of experimental results. Turn the obscure experimental map into a proton story that is easy to watch. This article will use their wonderful animations to reveal the secrets inside the protons.

Evidence that knocking protons contain many components comes from the Stanford Linear Accelerator Center (Stanford Linear Accelerator Center,SLAC) in 1967. In early experiments, researchers bombarded protons with electrons and observed them bounce off like billiards balls. And when SLAC increased its energy and emitted electrons more violently, the researchers found that they bounced back differently. Electrons hit protons strong enough to break the latter-a process called deep inelastic scattering (Deep Inelastic Scattering,DIS)-and bounce back from the point-like fragments of protons, quarks. This is the first evidence of quark existence.

The observation of protons has become more rigorous since SLAC's discovery won three physicists the 1990 Nobel Prize in physics. So far, physicists have carried out hundreds of scattering experiments. They infer all aspects of the interior by adjusting the intensity of the bombarding protons and measuring the scattered particles.

By using higher-energy electrons, physicists can find out the finer characteristics of the target protons. In such experiments, the electron energy sets the maximum resolution of the deep inelastic scattering experiment, and the more powerful Particle Collider provides clearer images of protons.

The high-energy collider will also produce a wider range of collision results, and the researchers chose different parts of the emitted electrons to analyze. This flexibility turns out to be the key to understanding quarks, which sway in different momentum inside the proton.

By measuring the energy and trajectory of each scattered electron, the researchers can tell whether it skimmed the quark, whether it is a quark with a large total momentum or a quark carrying a small amount of momentum. Under repeated collisions, the researchers, like a census, determined whether the momentum of protons was mainly concentrated in a few quarks or distributed in many quarks.

Even SLAC's proton splitting collisions are mild by today's standards. In these scattering events, the ejection of electrons indicates that they hit quarks, which carry 1/3 of the total momentum of protons. This finding is consistent with the theory of Gherman and George Zweig, who assumed in 1964 that protons are made up of three quarks.

Gherman and Zweig's "quark model" is still an elegant way to imagine protons. It has two "up" quarks, each with a charge of + 2 up 3, and a "down" quark with a charge of-1 pound 3, and the total charge of protons is + 1.

However, the quark model is oversimplified and has serious defects.

For example, when it comes to the spin of a proton, it fails. Spin is a quantum property similar to angular momentum. A proton has half a unit of spin, and so does each of its upper and lower quarks. Physicists initially thought that half a unit of two upper quarks minus half of the lower quarks must be equal to half of the whole proton, which is basically the result of simple arithmetic. But in 1988, the European μ Sub-Cooperation Group (European Muon Collaboration) reported that quark spins add up to much less than 1 max 2. Similarly, the mass of two upper quarks and one lower quark accounts for only about 1% of the total mass of protons. These defects have made physicists realize that there are far more than three quarks in protons.

Proton "ocean" [Samuel Velasco / Quanta Magazine] more than three quarks the Hadron-Electron Ring Accelerator (Hadron-Electron Ring Accelerator,HERA), which operated in Hamburg, Germany, from 1992 to 2007, bombarded protons with electrons about a thousand times stronger than SLAC. In the HERA experiment, physicists can choose electrons that bounce off very low-momentum quarks, or even electrons that carry only 0.005% of the total momentum of protons. And they did find very low-momentum electrons: HERA electrons bounce back from the whirlpool of low-momentum quarks and their antimatter counterparts, antiquarks.

These results confirmed a complex and bizarre theory that had replaced Gherman and Zweig's quark model at the time. This theory, developed in the 1970s, is a "powerful" quantum theory that acts between quarks. The theory describes quarks as particles tied together in the bearing capacity called gluons (gluon). Each quark and gluon has one of three types of "color charge", marked red, green and blue; these charged particles naturally pull at each other and form a mass-such as protons-whose colors add up to neutral white. This is QCD's quark confinement (confinement). In popular terms, there are no free quarks. This colorful theory is called quantum chromodynamics (quantum chromodynamics), or QCD.

According to QCD, gluons can absorb instantaneous peak energy. With this energy, gluons break up into a quark and an antiquark-each carrying only a little momentum-and then they annihilate and disappear. Because HERA is more sensitive to low-momentum particles, it directly detects this fleeting ocean of gluons, quarks and antiquarks.

What would protons look like in a more powerful collider? HERA found some signs. As physicists adjust HERA to look for low-momentum quarks, these quarks from gluons are emerging more and more. These results show that in higher energy collisions, protons will behave almost as gluon clouds. In the following animation, it looks like a dandelion.

The gluon "dandelion" is just like QCD's prediction. "HERA data are direct experimental evidence of QCD's description of nature," Milner said.

But the triumph of this young theory came with a bitter fruit: although QCD beautifully described the short-lived quark and gluon dance revealed by HERA's extreme collisions, the theory did nothing to understand the three persistent quarks seen in SLAC's mild bombardment.

QCD's prediction is easy to understand only when the strong interaction is relatively weak. This strong force weakens only when the quark is extremely close, as in the short-lived quark-antiquark pair (quark-antiquark pair), that is, the asymptotic freedom of QCD (Asymptotic freedom). Frank Wilchek (Frank Wilczek), David David Gross (David Gross) and David Politzer (David Politzer) discovered this decisive feature of QCD in 1973 and won the Nobel Prize 31 years later.

But for milder collisions like SLAC, protons are like three quarks at a distance from each other, and these quarks pull each other strong enough to make QCD calculations impossible. Therefore, the task of further unraveling the mystery of the proton three-quark image falls mainly on the experimenter. Researchers who conducted "digital experiments" (digital experiments) also made important contributions because QCD's predictions were simulated on supercomputers. It is in this low-resolution image that physicists keep discovering surprises.

Recently, a team led by Juan Rojo of the National Institute of subatomic Physics (National Institute for Subatomic Physics) in the Netherlands and the Free University of Amsterdam (VU University Amsterdam) analyzed more than 5000 proton snapshots taken over the past 50 years, using machine learning to infer the motion of quarks and gluons inside the protons, avoiding theoretical speculation.

A new detailed survey found blurred background images that researchers had not noticed in the past. In relatively soft collisions, protons are almost unbroken, and most of the momentum is frozen in the usual three quarks: two upper quarks and one lower quark. But a small portion of momentum seems to come from a "charm" quark and "charm" antiquark-giant elementary particles, each more than 1/3 heavier than the entire proton.

Protons are sometimes like "molecules" of five quarks.

Short-lived charm quarks appear frequently in protons'"quark sea" (quark sea) images (gluons can split into any of six different quark types if they have enough energy). But the findings of Roho and his colleagues show that the charm quarks are longer-lasting and can be detected in minor collisions. In these collisions, protons behave as quantum mixed states (quantum mixture) or superposition states (superposition) of multiple states: an electron usually encounters three light quarks. But it occasionally encounters a rarer "molecule" of five quarks, such as a combination of upper quark, lower quark and charm quark on one side, upper quark and charm antiquark on the other.

These subtle details about the composition of protons may prove important. In the large Hadron Collider (Large Hadron Collider,LHC), physicists look for new elementary particles by bumping high-speed protons together; to understand the experimental results, researchers need to know the initial composition inside the protons. The occasional giant charm quark phantom reduces the likelihood of producing more exotic particles.

Researchers calculated in 2021 that when protons in cosmic rays (cosmic rays) burst from outer space and slammed into protons in Earth's atmosphere, the timely charm quarks would spray showers of ultra-high-energy neutrinos on Earth. This may confuse some experimenters who have been looking for high-energy neutrinos from the other side of the universe.

Roho's team plans to continue to explore protons by looking for dissonances between charm quarks and antiquarks. Heavier components, such as top quarks, may produce rarer and harder-to-detect phenomena.

The next generation of experiments will look for more unknown features. Physicists at Brookhaven National Laboratory (Brookhaven National Laboratory) hope to move on from where HERA stopped and start the Electron Ion Collider (Electron-Ion Collider,EIC) around 2030 to take higher-resolution snapshots to achieve the first 3D reconstruction of protons. EIC will also use spintrons to create detailed maps of internal quark and gluon spins, just as SLAC and HERA plotted their momentum. This should help researchers finally determine the origin of proton spins and solve other basic problems that make up most of the everyday particles of matter, which are always confusing and fascinating.

This article is compiled from Inside the Proton, the 'Most Complicated Thing You Could Possibly Imagine', original text link: https://www.quantamagazine.org/inside-the-proton-the-most-complicated-thing-imaginable-20221019/

Annotation

[1] https://www.nature.com/articles/s41586-022-04998-2.

[2] the 2004 Nobel Committee awarded Frank Wilczek, David Gross and David Politzer the Nobel Prize in Physics for "finding asymptotic freedom in strong interactions" ("For the discovery of asymptotic freedom in the strong interaction.").

[3] at present, the non-perturbative calculation of strongly coupled QCD is still extremely difficult.

[4] https://www.quantamagazine.org/impossible-particle-discovery-adds-key-piece-to-the-strong-force-puzzle-20210927/.

[5] https://arxiv.org/abs/1512.06666.

[6] https://arxiv.org/abs/2107.13852.

[7] https://www.quantamagazine.org/cosmic-map-of-ultrahigh-energy-particles-points-to-long-hidden-treasures-20210427/.

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