All five letter elementary particles are listed below. A brief description is given for each definition.
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List of elementary particles
Photon
It is a quantum of electromagnetic radiation, for example light. Light, in turn, is a phenomenon that consists of streams of light. A photon is an elementary particle. A photon has a neutral charge and zero mass. The photon spin is equal to unity. The photon carries the electromagnetic interaction between charged particles. The term photon comes from the Greek phos, meaning light.
Phonon
It is a quasiparticle, a quantum of elastic vibrations and displacements of atoms and molecules of the crystal lattice from an equilibrium position. In crystal lattices, atoms and molecules constantly interact, sharing energy with each other. In this regard, it is almost impossible to study phenomena similar to vibrations of individual atoms in them. Therefore, random vibrations of atoms are usually considered according to the type of propagation of sound waves inside a crystal lattice. The quanta of these waves are phonons. The term phonon comes from the Greek phone - sound.
Phazon
The fluctuon phason is a quasiparticle, which is an excitation in alloys or in another heterophase system, forming a potential well (ferromagnetic region) around a charged particle, say an electron, and capturing it.
Roton
It is a quasiparticle that corresponds to elementary excitation in superfluid helium, in the region of high impulses, associated with the occurrence of vortex motion in a superfluid liquid. Roton, translated from Latin means - spinning, spinning. Roton appears at temperatures greater than 0.6 K and determines exponentially temperature-dependent properties of heat capacity, such as normal density entropy and others.
Meson
It is an unstable non-elementary particle. A meson is a heavy electron in cosmic rays.
The mass of a meson is greater than the mass of an electron and less than the mass of a proton.
Mesons have an even number of quarks and antiquarks. Mesons include Pions, Kaons and other heavy mesons.
Quark
It is an elementary particle of matter, but so far only hypothetically. Quarks are usually called six particles and their antiparticles (antiquarks), which in turn make up a group of special elementary particles hadrons.
It is believed that particles that participate in strong interactions, such as protons, neurons and some others, consist of quarks tightly connected to each other. Quarks constantly exist in different combinations. There is a theory that quarks could exist in a free form in the first moments after the big bang.
Gluon
Elementary particle. According to one theory, gluons seem to glue quarks together, which in turn form particles such as protons and neurons. In general, gluons are the smallest particles that form matter.
Boson
Boson-quasiparticle or Bose-particle. A boson has zero or integer spin. The name is given in honor of the physicist Shatyendranath Bose. A boson is different in that an unlimited number of them can have the same quantum state.
Hadron
A hadron is an elementary particle that is not truly elementary. Consists of quarks, antiquarks and gluons. The hadron has no color charge and participates in strong interactions, including nuclear ones. The term hadron, from the Greek adros, means large, massive.
- Translation
Rice. 1: hydrogen atom. Not to scale.
You know that the Large Hadron Collider basically smashes protons into each other. But what is a proton?
First of all, it’s a terrible and complete mess. As ugly and chaotic as the hydrogen atom is simple and elegant.
But what then is a hydrogen atom?
This is the simplest example of what physicists call a “bound state.” “State” essentially means something that has been around for quite some time, and “connected” means that its components are connected to each other, like spouses in a marriage. In fact, the example of a married couple in which one spouse is much heavier than the other fits very well here. The proton sits in the center, barely moving, and at the edges of the object there is an electron moving, moving faster than you and I, but much slower than the speed of light, the universal speed limit. A peaceful image of a marriage idyll.
Or it seems that way until we look into the proton itself. The insides of the proton itself are more like a commune, where many single adults and children are densely packed: pure chaos. This is also a bound state, but it does not connect something simple, like a proton with an electron, as in hydrogen, or at least several dozen electrons with an atomic nucleus, as in more complex atoms like gold - but a countless number (that is, there are too many of them and they change too quickly to be practically counted) lightweight particles called quarks, antiquarks and gluons. It is impossible to simply describe the structure of the proton, to draw simple pictures - it is extremely disorganized. All quarks, gluons, antiquarks rush around inside at the maximum possible speed, almost at the speed of light.
Rice. 2: Image of a proton. Imagine that all the quarks (up, down, strange - u,d,s), antiquarks (u,d,s with a dash), and gluons (g) scurry back and forth almost at the speed of light, collide with each other, appear and disappear
You may have heard that a proton consists of three quarks. But this is a lie – for the greater good, but still quite a big one. In fact, there are a myriad of gluons, antiquarks and quarks in the proton. The standard abbreviation “a proton is made up of two up quarks and one down quark” simply says that a proton has two more up quarks than up quarks and one more down quark than down quarks. For this reduction to be true, it is necessary to add to it “and countless more gluons and quark-antiquark pairs.” Without this phrase, the idea of a proton will be so simplified that it will be completely impossible to understand the operation of the LHC.
Rice. 3: Little White Lies in a Stereotypical Wikipedia Image
In general, atoms compared to protons are like a pas de deux in an elaborate ballet compared to a disco filled with drunk teenagers jumping up and down and waving at the DJ.
This is why if you are a theorist trying to understand what the LHC will see in proton collisions, you will have a hard time. It is very difficult to predict the results of collisions between objects that cannot be described in a simple way. But fortunately, since the 1970s, based on Bjorken's ideas from the 60s, theoretical physicists have found a relatively simple and working technology. But it still works up to certain limits, with an accuracy of about 10%. For this and some other reasons, the reliability of our calculations at the LHC is always limited.
Another thing about the proton is that it is tiny. Really tiny. If you blow up a hydrogen atom to the size of your bedroom, the proton will be the size of a grain of dust so small that it will be very difficult to notice. It is precisely because the proton is so small that we can ignore the chaos going on inside it, describing the hydrogen atom as simple. More precisely, the size of a proton is 100,000 times smaller than the size of a hydrogen atom.
For comparison, the size of the Sun is only 3000 times smaller than the size of the Solar System (measured by the orbit of Neptune). That's right - the atom is more empty than the solar system! Remember this when you look at the sky at night.
But you might ask, “Wait a second! Are you saying that the Large Hadron Collider somehow collides protons that are 100,000 times smaller than an atom? How is this even possible?
Great question.
Proton collisions versus mini-collisions of quarks, gluons and antiquarks
Proton collisions at the LHC occur with a certain energy. It was 7 TeV = 7000 GeV in 2011, and 8 TeV = 8000 GeV in 2012. But particle physicists are mainly interested in collisions of a quark of one proton with the antiquark of another proton, or collisions of two gluons, etc. – something that can lead to the emergence of a truly new physical phenomenon. These mini-collisions carry a small fraction of the total proton collision energy. How much of this energy can they carry, and why was it necessary to increase the collision energy from 7 TeV to 8 TeV?The answer is in Fig. 4. The graph shows the number of collisions detected by the ATLAS detector. Data from the summer of 2011 involve the scattering of quarks, antiquarks, and gluons from other quarks, antiquarks, and gluons. Such mini-collisions most often produce two jets (jet of hadrons, manifestations of high-energy quarks, gluons or antiquarks knocked out of the parent protons). The energies and directions of the jets are measured, and from this data the amount of energy that should have been involved in the mini-collision is determined. The graph shows the number of mini-collisions of this type as a function of energy. The vertical axis is logarithmic - each line denotes a 10-fold increase in quantity (10 n denotes 1 and n zeros after it). For example, the number of mini-collisions observed in the energy interval from 1550 to 1650 GeV was about 10 3 = 1000 (marked with blue lines). Note that the graph starts at 750 GeV, but the number of mini-collisions continues to increase as you study lower energy jets, up to the point where the jets become too weak to detect.
Rice. 4: number of collisions as a function of energy (m jj)
Consider that the total number of proton-proton collisions with an energy of 7 TeV = 7000 GeV approached 100,000,000,000,000. And of all these collisions, only two mini-collisions exceeded 3,500 GeV - half the energy of a proton collision. Theoretically, the energy of a mini-collision could increase to 7000 GeV, but the probability of this is decreasing all the time. We see 6000 GeV mini-collisions so rarely that we are unlikely to see 7000 GeV even if we collect 100 times more data.
What are the advantages of increasing the collision energy from 7 TeV in 2010-2011 to 8 TeV in 2012? Obviously, what you could do at the energy level E, you can now do at the energy level 8/7 E ≈ 1.14 E. So, if before you could hope to see in so much data signs of a certain type of hypothetical particle with mass of 1000 GeV/c 2, then we can now hope to achieve at least 1100 GeV/c 2 with the same set of data. The capabilities of the machine are increasing - you can search for particles of slightly larger mass. And if you collect three times more data in 2012 than in 2011, you will get more collisions for each energy level, and you will be able to see the signature of a hypothetical particle with a mass of, say, 1200 GeV/s 2 .
But that is not all. Look at the blue and green lines in Fig. 4: they show that they occur at energies of the order of 1400 and 1600 GeV - such that they correlate with each other like 7 to 8. At the proton collision energy level of 7 TeV, the number of mini-collisions of quarks with quarks, quarks with gluons, etc. P. with an energy of 1400 GeV is more than twice the number of collisions with an energy of 1600 GeV. But when the machine increases the energy by 8/7, what worked for 1400 starts to work for 1600. In other words, if you are interested in mini-collisions of fixed energy, their number increases - and much more than the 14% increase in proton collision energy! This means that for any process with a preferred energy, say the appearance of lightweight Higgs particles, which occurs at energies of the order of 100-200 GeV, you get more results for the same money. Going from 7 to 8 TeV means that for the same number of proton collisions you get more Higgs particles. Higgs particle production will increase by about 1.5. The number of up quarks and certain types of hypothetical particles will increase slightly more.
This means that although the number of proton collisions in 2012 is 3 times higher than in 2011, the total number of Higgs particles produced will increase by almost 4 times simply due to the increase in energy.
By the way, fig. Figure 4 also proves that protons do not simply consist of two up quarks and one down quark, as depicted in drawings like Fig. 3. If they were, then quarks would have to transfer about a third of the energy of protons, and most mini-collisions would occur at energies of about a third of the proton collision energy: around 2300 GeV. But the graph shows that nothing special happens in the region of 2300 GeV. At energies below 2300 GeV there are many more collisions, and the lower you go, the more collisions you see. This is because the proton contains a huge number of gluons, quarks and antiquarks, each of which transfers a small part of the proton’s energy, but there are so many of them that they participate in a huge number of mini-collisions. This property of the proton is shown in Fig. 2 – although in fact the number of low-energy gluons and quark-antiquark pairs is much greater than shown in the figure.
But what the graph does not show is the fraction that, in mini-collisions with a certain energy, falls on collisions of quarks with quarks, quarks with gluons, gluons with gluons, quarks with antiquarks, etc. In fact, this cannot be said directly from experiments at the LHC—the jets from quarks, antiquarks, and gluons look the same. How we know these shares is a complex story, involving many different past experiments and the theory that combines them. And from this we know that the highest energy mini-collisions usually occur between quarks and quarks and between quarks and gluons. Low energy collisions usually occur between gluons. Collisions between quarks and antiquarks are relatively rare, but they are very important for certain physical processes.
Distribution of particles inside a proton
Rice. 5
Two graphs, differing in the scale of the vertical axis, show the relative probability of a collision with a gluon, up or down quark, or antiquark carrying a fraction of the proton's energy equal to x. At small x, gluons dominate (and quarks and antiquarks become equally probable and numerous, although there are still fewer of them than gluons), and at medium x, quarks dominate (although they become extremely few in number).
Both graphs show the same thing, just at a different scale, so what is difficult to see on one of them is easier to see on the other. What they show is this: if a proton beam comes at you in the Large Hadron Collider, and you hit something inside the proton, how likely is it that you will hit an up quark, or a down quark, or a gluon, or an up antiquark, or a down quark? antiquark carrying a fraction of the proton energy equal to x? From these graphs it can be concluded that:
From the fact that all curves grow very quickly at small x (seen in the lower graph), it follows that most of the particles in the proton transfer less than 10% (x< 0,1) энергии протона, и вероятность столкнуться с частицей, переносящей мало энергии, гораздо больше вероятности столкнуться с частицей, переносящей много. При этом, 10% - не так уж и мало. В 2012 году лучи на БАК достигали энергий в 4 ТэВ, поэтому 10% означало 400 ГэВ. При этом для того, чтобы создать частицу хиггса энергией 124 ГэВ из двух глюонов требуется всего 62 ГэВ на глюон.
Since the yellow curve (below) is much higher than the others, it follows that if you encounter something that carries less than 10% of the energy of a proton, it is most likely a gluon; and falling below 2% of the proton energy it is equally likely to be quarks or antiquarks.
Since the gluon curve (top) drops below the quark curves as x increases, it follows that if you encounter anything carrying more than 20% (x > 0.2) of the proton's energy - which is very, very rare - it , most likely a quark, and the probability that it is an up quark is twice as likely as the probability that it is a down quark. This is a remnant of the idea that “a proton is two up quarks and one down quark.”
All curves drop sharply as x increases; It is very unlikely that you will encounter anything carrying more than 50% of the proton's energy.
These observations are indirectly reflected in the graph in Fig. 4. Here are a couple more non-obvious things about the two graphs:
Most of the proton's energy is divided (about equally) between a small number of high-energy quarks and a huge number of low-energy gluons.
Among the particles, low-energy gluons predominate in number, followed by quarks and antiquarks of very low energies.
The number of quarks and antiquarks is huge, but: the total number of up quarks minus the total number of up antiquarks is two, and the total number of down quarks minus the total number of down antiquarks is one. As we saw above, extra quarks carry a significant (but not the majority) portion of the energy of the proton flying towards you. And only in this sense can we say that the proton basically consists of two up quarks and one down quark.
By the way, all this information was obtained from a fascinating combination of experiments (mainly on the scattering of electrons or neutrinos from protons or from the atomic nuclei of heavy hydrogen - deuterium, containing one proton and one neutron), put together using detailed equations describing electromagnetic, strong nuclear and weak nuclear interactions. This long story stretches back to the late 1960s and early 1970s. And it works great for predicting phenomena observed in colliders where protons collide with protons and protons with antiprotons, such as the Tevatron and the LHC.
Other evidence for the complex structure of the proton
Let's look at some of the data obtained at the LHC and how it supports claims about the structure of the proton (although the current understanding of the proton dates back 3-4 decades, thanks to many experiments).Graph in Fig. 4 is obtained from observations of collisions during which something like the one shown in Fig. 1 occurs. 6: a quark or antiquark or gluon of one proton collides with a quark or antiquark or gluon of another proton, scatters from it (or something more complex happens - for example, two gluons collide and turn into a quark and an antiquark), resulting in two particles (quarks, antiquarks or gluons) fly away from the point of collision. These two particles turn into jets (hadron jets). The energy and direction of the jets are observed in particle detectors surrounding the impact point. This information is used to understand how much energy was contained in the collision of the two original quarks/gluons/antiquarks. More precisely, the invariant mass of the two jets, multiplied by c 2, gives the energy of the collision of the two original quarks/gluons/antiquarks.
Rice. 6
The number of collisions of this type depending on the energy is shown in Fig. 4. The fact that at low energies the number of collisions is much greater is confirmed by the fact that most of the particles inside the proton transfer only a small fraction of its energy. The data starts at energies of 750 GeV.
Rice. 7: Data for lower energies taken from a smaller data set. Dijet mass – the same as m jj in Fig. 4.
Data for Fig. 7 are taken from the CMS experiment from 2010, on which they plotted flesh collisions up to energies of 220 GeV. The graph here is not the number of collisions, but a little more complicated: the number of collisions per GeV, that is, the number of collisions divided by the width of the histogram column. It can be seen that the same effect continues to work across the entire range of data. Collisions like those shown in Fig. 6, much more happens at low energies than at high energies. And this number continues to grow until it is no longer possible to distinguish the jets. A proton contains a lot of low-energy particles, and few of them carry a significant fraction of its energy.
What about the presence of antiquarks in the proton? Three of the most interesting processes that are not similar to the collision depicted in Fig. 6, sometimes occurring at the LHC (in one of several million proton-proton collisions) involves the process:
Quark + antiquark -> W+, W- or Z-particle.
They are shown in Fig. 8.
Rice. 8
The corresponding data from the CMS is given in Fig. 9 and 10. Fig. Figure 9 shows that the number of collisions that produce an electron or positron (left) and something undetectable (probably a neutrino or antineutrino), or a muon and an antimuon (right), is predicted correctly. The prediction is made by combining the Standard Model (equations that predict the behavior of known elementary particles) and the structure of the proton. The large peaks in the data are due to the appearance of W and Z particles. The theory fits the data perfectly.
Rice. 9: black dots – data, yellow – predictions. The number of events is indicated in thousands. Left: The central peak is due to neutrinos in the W particles. On the right, the lepton and antilepton produced in the collision are combined and the mass of the particle from which they came is implied. The peak appears due to the resulting Z particles.
Even more details can be seen in Fig. 10, where it is shown that the theory, in terms of the number of not only these, but also many associated measurements - most of which are associated with collisions of quarks with antiquarks - matches the data perfectly. Data (red dots) and theory (blue bars) never match exactly due to statistical fluctuations, for the same reason that if you flip a coin ten times you won't necessarily get five heads and five tails. Therefore, the data points are placed within the “error bar,” the vertical red stripe. The size of the band is such that for 30% of measurements the error band should border on the theory, and for only 5% of measurements it should be two bands away from the theory. It can be seen that all the evidence confirms that the proton contains many antiquarks. And we correctly understand the number of antiquarks that carry a certain fraction of the proton’s energy.
Rice. 10
Then everything is a little more complicated. We even know how many up and down quarks we have depending on the energy they carry, since we correctly predict - with an error of less than 10% - how much more W + particles we get than W - particles (Fig. 11).
Rice. eleven
The ratio of up antiquarks to down quarks should be close to 1, but there should be more up quarks than down quarks, especially at high energies. In Fig. 6 we can see that the ratio of the resulting W + and W - particles should approximately give us the ratio of up quarks and down quarks involved in the production of W particles. But in Fig. Figure 11 shows that the measured ratio of W + to W - particles is 3 to 2, not 2 to 1. This also shows that the naive idea of a proton as consisting of two up quarks and one down quark is too simplistic. The simplified 2 to 1 ratio is blurred, since a proton contains many quark-antiquark pairs, of which the upper and lower ones are approximately equal. The degree of blurring is determined by the mass of the W particle of 80 GeV. If you make it lighter, there will be more blurring, and if it is heavier, there will be less blurring, since most of the quark-antiquark pairs in the proton carry little energy.
Finally, let's confirm the fact that most of the particles in a proton are gluons.
Rice. 12
To do this, we will use the fact that top quarks can be created in two ways: quark + antiquark -> top quark + top antiquark, or gluon + gluon -> top quark + top antiquark (Fig. 12). We know the number of quarks and antiquarks depending on the energy they carry based on the measurements illustrated in Fig. 9-11. From this, we can use the equations of the Standard Model to predict how many top quarks will be produced from collisions of only quarks and antiquarks. We also believe, based on previous data, that there are more gluons in a proton, so the process gluon + gluon -> top quark + top antiquark should occur at least 5 times more often. It's easy to check whether there are gluons there; if they are not, the data must lie well below theoretical predictions.
gluons Add tags
By studying the structure of matter, physicists found out what atoms are made of, got to the atomic nucleus and split it into protons and neutrons. All these steps were given quite easily - you just had to accelerate the particles to the required energy, push them against each other, and then they themselves would fall apart into their component parts.
But with protons and neutrons this trick no longer worked. Although they are composite particles, they cannot be “broken into pieces” in even the most violent collision. Therefore, it took physicists decades to come up with different ways to look inside the proton, see its structure and shape. Today, the study of the structure of the proton is one of the most active areas of particle physics.
Nature gives hints
The history of studying the structure of protons and neutrons dates back to the 1930s. When, in addition to protons, neutrons were discovered (1932), having measured their mass, physicists were surprised to find that it was very close to the mass of a proton. Moreover, it turned out that protons and neutrons “feel” nuclear interaction in exactly the same way. So identical that, from the point of view of nuclear forces, a proton and a neutron can be considered as two manifestations of the same particle - a nucleon: a proton is an electrically charged nucleon, and a neutron is a neutral nucleon. Swap protons for neutrons and nuclear forces will (almost) notice nothing.
Physicists express this property of nature as symmetry - nuclear interaction is symmetrical with respect to the replacement of protons with neutrons, just as a butterfly is symmetrical with respect to the replacement of left with right. This symmetry, in addition to playing an important role in nuclear physics, was actually the first hint that nucleons had an interesting internal structure. True, then, in the 30s, physicists did not realize this hint.
Understanding came later. It began with the fact that in the 1940–50s, in the reactions of collisions of protons with the nuclei of various elements, scientists were surprised to discover more and more new particles. Not protons, not neutrons, not the pi-mesons discovered by that time, which hold nucleons in nuclei, but some completely new particles. For all their diversity, these new particles had two common properties. Firstly, they, like nucleons, very willingly participated in nuclear interactions - now such particles are called hadrons. And secondly, they were extremely unstable. The most unstable of them decayed into other particles in just a trillionth of a nanosecond, not even having time to fly the size of an atomic nucleus!
For a long time, the hadron “zoo” was a complete mess. At the end of the 1950s, physicists had already learned quite a lot of different types of hadrons, began to compare them with each other, and suddenly saw a certain general symmetry, even periodicity, in their properties. It was suggested that inside all hadrons (including nucleons) there are some simple objects called “quarks”. By combining quarks in different ways, it is possible to obtain different hadrons, and of exactly the same type and with the same properties that were discovered in the experiment.
What makes a proton a proton?
After physicists discovered the quark structure of hadrons and learned that quarks come in several different varieties, it became clear that many different particles could be constructed from quarks. So no one was surprised when subsequent experiments continued to find new hadrons one after another. But among all the hadrons, a whole family of particles was discovered, consisting, just like the proton, of only two u-quarks and one d-quark. A sort of “brother” of the proton. And here the physicists were in for a surprise.
Let's first make one simple observation. If we have several objects consisting of the same “bricks”, then heavier objects contain more “bricks”, and lighter ones contain fewer. This is a very natural principle, which can be called the principle of combination or the principle of superstructure, and it works perfectly both in everyday life and in physics. It even manifests itself in the structure of atomic nuclei - after all, heavier nuclei simply consist of a larger number of protons and neutrons.
However, at the level of quarks this principle does not work at all, and, admittedly, physicists have not yet fully figured out why. It turns out that the heavy brothers of the proton also consist of the same quarks as the proton, although they are one and a half or even two times heavier than the proton. They differ from the proton (and differ from each other) not composition, and mutual location quarks, by the state in which these quarks are relative to each other. It is enough to change the relative position of the quarks - and from the proton we will get another, noticeably heavier, particle.
What will happen if you still take and collect more than three quarks together? Will a new heavy particle be produced? Surprisingly, it won’t work - the quarks will break up in threes and turn into several scattered particles. For some reason, nature “does not like” combining many quarks into one whole! Only very recently, literally in recent years, hints began to appear that some multi-quark particles do exist, but this only emphasizes how much nature does not like them.
A very important and deep conclusion follows from this combinatorics - the mass of hadrons does not at all consist of the mass of quarks. But if the mass of a hadron can be increased or decreased by simply recombining its constituent bricks, then it is not the quarks themselves that are responsible for the mass of hadrons. And indeed, in subsequent experiments it was possible to find out that the mass of the quarks themselves is only about two percent of the mass of the proton, and the rest of the gravity arises due to the force field (special particles - gluons) that bind the quarks together. By changing the relative position of quarks, for example, moving them further away from each other, we thereby change the gluon cloud, making it more massive, which is why the hadron mass increases (Fig. 1).
What's going on inside a fast-moving proton?
Everything described above concerns a stationary proton; in the language of physicists, this is the structure of the proton in its rest frame. However, in the experiment, the structure of the proton was first discovered under other conditions - inside fast flying proton.
In the late 1960s, in experiments on particle collisions at accelerators, it was noticed that protons traveling at near-light speed behaved as if the energy inside them was not distributed evenly, but was concentrated in individual compact objects. The famous physicist Richard Feynman proposed to call these clumps of matter inside protons partons(from English part - Part).
Subsequent experiments examined many of the properties of partons—for example, their electrical charge, their number, and the fraction of proton energy each carries. It turns out that charged partons are quarks, and neutral partons are gluons. Yes, those same gluons, which in the proton’s rest frame simply “served” the quarks, attracting them to each other, are now independent partons and, along with quarks, carry the “matter” and energy of a fast-moving proton. Experiments have shown that approximately half of the energy is stored in quarks, and half in gluons.
Partons are most conveniently studied in collisions of protons with electrons. The fact is that, unlike a proton, an electron does not participate in strong nuclear interactions and its collision with a proton looks very simple: the electron emits a virtual photon for a very short time, which crashes into a charged parton and ultimately generates a large number of particles ( Fig. 2). We can say that the electron is an excellent scalpel for “opening” the proton and dividing it into separate parts - however, only for a very short time. Knowing how often such processes occur at an accelerator, one can measure the number of partons inside a proton and their charges.
Who are the Partons really?
And here we come to another amazing discovery that physicists made while studying collisions of elementary particles at high energies.
Under normal conditions, the question of what this or that object consists of has a universal answer for all reference systems. For example, a water molecule consists of two hydrogen atoms and one oxygen atom - and it does not matter whether we are looking at a stationary or moving molecule. However, this rule seems so natural! - is violated if we are talking about elementary particles moving at speeds close to the speed of light. In one frame of reference, a complex particle may consist of one set of subparticles, and in another frame of reference, of another. It turns out that composition is a relative concept!
How can this be? The key here is one important property: the number of particles in our world is not fixed - particles can be born and disappear. For example, if you push together two electrons with a sufficiently high energy, then in addition to these two electrons, either a photon, or an electron-positron pair, or some other particles can be born. All this is allowed by quantum laws, and this is exactly what happens in real experiments.
But this “law of non-conservation” of particles works in case of collisions particles. How does it happen that the same proton from different points of view looks like it consists of a different set of particles? The point is that a proton is not just three quarks put together. There is a gluon force field between the quarks. In general, a force field (such as a gravitational or electric field) is a kind of material “entity” that permeates space and allows particles to exert a forceful influence on each other. In quantum theory, the field also consists of particles, albeit special ones - virtual ones. The number of these particles is not fixed; they are constantly “budding off” from quarks and being absorbed by other quarks.
Resting A proton can really be thought of as three quarks with gluons jumping between them. But if we look at the same proton from a different frame of reference, as if from the window of a “relativistic train” passing by, we will see a completely different picture. Those virtual gluons that glued the quarks together will seem less virtual, “more real” particles. They, of course, are still born and absorbed by quarks, but at the same time they live on their own for some time, flying next to the quarks, like real particles. What looks like a simple force field in one frame of reference turns into a stream of particles in another frame! Note that we do not touch the proton itself, but only look at it from a different frame of reference.
Further more. The closer the speed of our “relativistic train” is to the speed of light, the more amazing the picture we will see inside the proton. As we approach the speed of light, we will notice that there are more and more gluons inside the proton. Moreover, they sometimes split into quark-antiquark pairs, which also fly nearby and are also considered partons. As a result, an ultrarelativistic proton, i.e. a proton moving relative to us at a speed very close to the speed of light, appears in the form of interpenetrating clouds of quarks, antiquarks and gluons that fly together and seem to support each other (Fig. 3).
A reader familiar with the theory of relativity may be concerned. All physics is based on the principle that any process proceeds the same way in all inertial frames of reference. But it turns out that the composition of the proton depends on the frame of reference from which we observe it?!
Yes, exactly, but this in no way violates the principle of relativity. The results of physical processes - for example, which particles and how many are produced as a result of a collision - do turn out to be invariant, although the composition of the proton depends on the frame of reference.
This situation, unusual at first glance, but satisfying all the laws of physics, is schematically illustrated in Figure 4. It shows how the collision of two protons with high energy looks in different frames of reference: in the rest frame of one proton, in the center of mass frame, in the rest frame of another proton . The interaction between protons is carried out through a cascade of splitting gluons, but only in one case is this cascade considered the “inside” of one proton, in another case it is considered part of another proton, and in the third it is simply some object that is exchanged between two protons. This cascade exists, it is real, but to which part of the process it should be attributed depends on the frame of reference.
3D portrait of a proton
All the results that we just talked about were based on experiments performed quite a long time ago - in the 60–70s of the last century. It would seem that since then everything should have been studied and all questions should have found their answers. But no - the structure of the proton still remains one of the most interesting topics in particle physics. Moreover, in recent years, interest in it has increased again because physicists have figured out how to obtain a “three-dimensional” portrait of a fast-moving proton, which turned out to be much more difficult than a portrait of a stationary proton.
Classic experiments on proton collisions tell only about the number of partons and their energy distribution. In such experiments, partons participate as independent objects, which means that it is impossible to find out from them how the partons are located relative to each other, or how exactly they add up to a proton. We can say that for a long time only a “one-dimensional” portrait of a fast-moving proton was available to physicists.
In order to construct a real, three-dimensional portrait of a proton and find out the distribution of partons in space, much more subtle experiments are required than those that were possible 40 years ago. Physicists learned to carry out such experiments quite recently, literally in the last decade. They realized that among the huge number of different reactions that occur when an electron collides with a proton, there is one special reaction - deep virtual Compton scattering, - which can tell us about the three-dimensional structure of the proton.
In general, Compton scattering, or the Compton effect, is the elastic collision of a photon with a particle, for example a proton. It looks like this: a photon arrives, is absorbed by a proton, which goes into an excited state for a short time, and then returns to its original state, emitting a photon in some direction.
Compton scattering of ordinary light photons does not lead to anything interesting - it is simply the reflection of light from a proton. In order for the internal structure of the proton to “come into play” and the distribution of quarks to be “felt,” it is necessary to use photons of very high energy - billions of times more than in ordinary light. And just such photons - albeit virtual ones - are easily generated by an incident electron. If we now combine one with the other, we get deep virtual Compton scattering (Fig. 5).
The main feature of this reaction is that it does not destroy the proton. The incident photon does not just hit the proton, but, as it were, carefully feels it and then flies away. The direction in which it flies away and what part of the energy the proton takes from it depends on the structure of the proton, on the relative arrangement of the partons inside it. That is why, by studying this process, it is possible to restore the three-dimensional appearance of the proton, as if to “sculpt its sculpture.”
True, this is very difficult for an experimental physicist to do. The required process occurs quite rarely, and it is difficult to register it. The first experimental data on this reaction were obtained only in 2001 at the HERA accelerator at the German DESY accelerator complex in Hamburg; a new series of data is now being processed by experimenters. However, already today, based on the first data, theorists are drawing three-dimensional distributions of quarks and gluons in the proton. A physical quantity, about which physicists had previously only made assumptions, finally began to “emerge” from the experiment.
Are there any unexpected discoveries awaiting us in this area? It is likely that yes. As an illustration, let's say that in November 2008 an interesting theoretical paper appeared, which states that a fast-moving proton should not look like a flat disk, but a biconcave lens. This happens because the partons sitting in the central region of the proton are compressed more strongly in the longitudinal direction than the partons sitting at the edges. It would be very interesting to test these theoretical predictions experimentally!
Why is all this interesting to physicists?
Why do physicists even need to know exactly how matter is distributed inside protons and neutrons?
Firstly, this is required by the very logic of the development of physics. There are many amazingly complex systems in the world that modern theoretical physics cannot yet fully cope with. Hadrons are one such system. By understanding the structure of hadrons, we are honing the abilities of theoretical physics, which may well turn out to be universal and, perhaps, will help in something completely different, for example, in the study of superconductors or other materials with unusual properties.
Secondly, there is direct benefit for nuclear physics. Despite the almost century-long history of studying atomic nuclei, theorists still do not know the exact law of interaction between protons and neutrons.
They have to partly guess this law based on experimental data, and partly construct it based on knowledge about the structure of nucleons. This is where new data on the three-dimensional structure of nucleons will help.
Thirdly, several years ago physicists were able to obtain no less than a new aggregate state of matter - quark-gluon plasma. In this state, quarks do not sit inside individual protons and neutrons, but roam freely throughout the entire clump of nuclear matter. This can be achieved, for example, like this: heavy nuclei are accelerated in an accelerator to a speed very close to the speed of light, and then collide head-on. In this collision, temperatures of trillions of degrees arise for a very short time, which melts the nuclei into quark-gluon plasma. So, it turns out that theoretical calculations of this nuclear melting require a good knowledge of the three-dimensional structure of nucleons.
Finally, these data are very necessary for astrophysics. When heavy stars explode at the end of their lives, they often leave behind extremely compact objects - neutron and possibly quark stars. The core of these stars consists entirely of neutrons, and maybe even cold quark-gluon plasma. Such stars have long been discovered, but one can only guess what is happening inside them. So a good understanding of quark distributions can lead to progress in astrophysics.
