The name atom comes from the ancient Greek word "ἄτομος", which translates as indivisible. On
The proton is one of three such stable subatomicparticles (the other two are electrons and neutrons), which are the building blocks of atoms. This name (from the ancient Greek "πρῶτος" - first) was proposed by Ernest Rutherford in 1920. In a series of experiments, a British physicist showed that the "nucleus of hydrogen" (the lightest chemical element) could be extracted from nitrogen by colliding with an alpha particle (the nucleus of a helium atom).
More than a century after Rutherforddiscovered the positively charged particle at the heart of every atom, physicists are still trying to understand what a proton is. School teachers usually describe them as featureless balls with one unit of positive electrical charge. At a more complex level, these particles are represented as a beam of three interconnected quarks: two up and one down.
But even this model is a strong simplification.Decades of research have uncovered and continue to explore a deeper picture that is too bizarre to be fully conveyed in words or pictures.
Artistic illustration of changing ideas about the composition of the proton. Image: Brookhaven National Laboratory
How to break a proton apart and study its composition
Proof that a proton containsmany particles, was first obtained at the SLAC National Accelerator Laboratory at Stanford in the late 60s of the last century. In previous experiments, researchers bombarded them with electrons and watched them ricochet like billiard balls.
At the SLAC particle accelerator, physicists succeeded for the first timeaccelerate the electrons strongly enough to change the results of observations. The electrons, in the process of deeply inelastic scattering, hit the proton hard enough to break it and bounced off point-like fragments of the proton called quarks.
The authors of this discovery, which was the firstproof of the existence of quarks, won the 1990 Nobel Prize in Physics. And scientists around the world have since conducted hundreds of scattering experiments. They draw conclusions about various aspects of the object's interior by adjusting the strength of the bombardment and choosing which scattered particles they study as a result of the experiment.
Using high energy electrons, physicistscan detect finer details of the proton. Thus, the electron energy sets the maximum resolution of the deep inelastic scattering experiment. The more powerful colliders, the more complete picture they give about the composition of the proton.
Higher energy colliders alsoproduce a wider range of collision results, allowing researchers to select different subsets for analysis. This flexibility turned out to be the key to understanding quarks, which move around inside the proton with different amounts of momentum.
By measuring the energy and trajectory of each scatteredelectron, researchers can tell which quark it bounced off. Statistical analysis of many repeated experiments, like a population census, "tells" researchers how the momentum of a proton is distributed or what kind of quarks it consists of.
More than just three quarks
First experiments at the SLAC colliderconfirmed the theory developed earlier by Murray Gell-Mann and George Zweig about the composition of the proton from three quarks. The electrons after the collision flew apart in such a way as if they crashed into three separate particles, each of which carries a third of the momentum of the proton.
The quark model of Gell-Mann and Zweig describesproton as a particle consisting of two “up” quarks with an electric charge of +2/3 each and one “down” quark with a charge of -1/3, which gives the total charge of the proton +1.
But the quark model is an oversimplification,has serious shortcomings. For example, it does not work when it comes to rotation (back) proton, a quantum property similar to angular momentum. This subatomic particle has a spin of ½, as do each of its up and down quarks.
Initially, physicists assumed that inIn calculations repeating simple charge arithmetic, half the units of the two up quarks minus the fraction of the down quark should equal half the unit of the proton as a whole. But in 1988, the European Muon Collaboration calculated that the quark spins add up to much less than half.
Similarly, studies have shown that massestwo up quarks and one down quark make up only about 1% of the total proton mass. This meant that something else must be hidden inside it - other elementary particles that would explain the properties of this subatomic particle.
Simplified model of the proton structure. Animation: Brookhaven National Laboratory
Many quarks and antiquarks in one particle
HERA Particle Accelerator in Germanresearch center DESY in Hamburg from 1992 to 2007 studied the collision of electrons and protons with a force about a thousand times higher than that achieved at SLAC. Although the experiment was completed over 10 years ago, physicists continue to analyze the collected data.
In the HERA experiments, physicists were able to studyelectrons bounced off extremely low momentum quarks, including those carrying only 0.005% of the proton's total momentum. The results of the observation confirmed that the composition of the proton is much more complicated than the quark model of Gell-Mann and Zweig: electrons bounced off the "whirlpool" of low-momentum quarks and their antimatter counterparts, antiquarks.
A complex structure of many quarks and antiquarks. Animation: MIT/Jefferson Lab/Sputnik Animation
The results confirmed the complex and outlandishtheory of quantum chromodynamics. This is the quantum theory of the strong force that binds quarks. This model endows quarks with a new property, tentatively called "color", and introduces new particles, gluons, which carry the strong interaction between quarks.
Every quark and every gluon has one of threetypes of "color" charges, designated red, green and blue. These color-charged particles naturally attract each other and form a group—like a proton—whose colors add up to neutral white.
According to the theory of quantum chromodynamics, gluonscan pick up instant bursts of energy. With this energy, these particles decay into a quark and an antiquark, each carrying only a small amount of momentum, before the pair annihilates and disappears. It is this "sea" of transition gluons, quarks and antiquarks that was discovered by the sensitive detectors of HERA.
Proton visualization. Animation: MIT
Charming new look
High Energy Extreme Collisionsshow a huge variety of quarks, anikirks and gluons into which protons decay. Collisions with less energy show only three valence quarks, which determine the quantum number of an elementary particle. But a new study shows that sometimes the proton acts as a structure made up of five quarks.
Research team led by Juan Rojofrom the National Institute for Subatomic Physics in the Netherlands and VU University of Amsterdam analyzed more than 5,000 images of protons taken over the past 50 years, using machine learning to determine the movement of quarks and gluons inside a proton.
New check found background blur onimages that had eluded previous researchers. In relatively mild collisions that barely ripped apart a proton, most of the momentum was in the usual three quarks: two up and one down. But a small amount of momentum, studies have shown, came from the "charm" quark and the charmed antiquark. These are large elementary particles, each of which is heavier than the entire proton by more than one third.
The study showed that although at highIn energetic collisions, gluons can split into any of six different types of quarks if they have enough energy, charmed quarks and antiquarks form much more frequently, making them noticeable even in relatively mild collisions.
In these collisions, the proton appears asquantum mixture or superposition of several states: an electron usually collides with three light quarks. But from time to time he will encounter a rarer "molecule" of five quarks, such as an up, down, and charm quark clustered on one side, and an up quark and a charmed antiquark on the other.
These results are not only theoreticalmeaning. For example, a sufficient amount of energy can be generated by the collision of cosmic radiation protons with elementary particles in the composition of the atoms of the earth's atmosphere. In the process of such collisions, protons can decay into charmed quarks and "spill" to the Earth in the form of high-energy neutrinos. This can confuse observers who use these particles to study the distant universe.
One hundred years after the discovery of proton physicscontinue to study the internal structure of these elementary particles. Next generation experiments will look for even more unknown features. For example, physicists at the Brookhaven National Laboratory in the US plan to launch the electron-ion collider in the 2030s and pick up where HERA left off. They will take higher resolution images that will enable the first 3D reconstructions of the proton.
This should help researchers definitivelydetermine the origin of the proton's rotation and answer other fundamental questions about the obscure particle that makes up most of the surrounding ordinary (baryonic) matter.
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On the cover: an artistic illustration of the many quarks, antiquarks and gluons inside a single proton. Image: D. Dominguez, CERN