Protons and neutrons: a crowd within matter

In the center of each atom is a nucleus, a tiny set of particles called protons and neutrons. In this article we will study the nature of protons and neutrons, consisting of particles even smaller in size - quarks, gluons and antiquarks. (Gluons, like photons, are antiparticles themselves). Quarks and gluons, as far as we know, can be really elementary (indivisible and not consisting of something smaller in size). But to them later.

Surprisingly, protons and neutrons have almost the same mass - to the nearest percent:

This is the key to their nature - they are in fact very similar. Yes, there is one obvious difference between them: the proton has a positive electric charge, and the neutron has no charge (it is neutral, hence its name). Accordingly, electrical forces act on the first, but not on the second. At first glance, this distinction seems very important! But actually it is not. In all other senses, the proton with the neutron is almost twins. They have identical not only the masses, but also the internal structure.

Because they are so similar, and because of these particles are composed of nuclei, protons and neutrons are often called nucleons.

Protons were identified and described around 1920 (although they were discovered earlier; the nucleus of a hydrogen atom is just a separate proton), and neutrons were found somewhere in 1933. The fact that protons and neutrons are so similar to each other, understood almost immediately. But the fact that they have a measurable size, comparable to the size of the nucleus (approximately 100,000 times smaller than an atom in radius), was not known until 1954. The fact that they consist of quarks, antiquarks, and gluons was gradually understood from the mid-1960s to the mid-1970s. By the end of the 70s and the beginning of the 80s, our understanding of protons, neutrons, and what they are made of has mostly settled down, and since then has remained unchanged.

Nucleons are much more difficult to describe than atoms or nuclei. Not to say that atoms are in principle simple , but at least one can say without thinking that a helium atom consists of two electrons orbiting around a tiny helium nucleus; and the helium nucleus is a fairly simple group of two neutrons and two protons. But with the nucleons everything is not so simple. I already wrote in the article “ What is a proton, and what is inside it? ” That the atom looks like an elegant minuet, and a nucleon is like a wild party.

The complexity of the proton and neutron, apparently, is real, and does not result from incomplete physical knowledge. We have equations used to describe quarks, antiquarks, and gluons, as well as strong nuclear interactions occurring between them. These equations are called QCD, from " quantum chromodynamics ." The accuracy of the equations can be checked in various ways, including measuring the number of particles appearing at the Large Hadron Collider. Substituting the QCD equations into a computer and launching calculations of the properties of protons and neutrons, and other similar particles (with the general name "hadrons"), we obtain predictions of the properties of these particles, well approaching observations made in the real world. Therefore, we have reason to believe that the QCD equations do not lie, and that our knowledge of the proton and neutron is based on the correct equations. But just having the right equations is not enough, for:

As far as we can judge, this is exactly the case with nucleons: these are complex solutions to simple QCD equations, and it is not possible to describe them with a couple of words or pictures.

Because of the intrinsic complexity of the nucleons, you, the reader, will have to make a choice: how much do you want to know about the described complexity? No matter how far you go, it most likely will not bring you satisfaction: the more you learn, the clearer you will become the topic, but the final answer will remain the same - the proton and neutron are very complex. I can offer you three levels of understanding, with increased detail; You can stop after any level and go to other topics, or you can dive to the last. There are questions about each level, the answers to which I can partially give in the following, but new answers raise new questions. As a result - as I do in professional discussions with colleagues and advanced students - I can only refer you to the data obtained in real experiments, to various influential theoretical arguments, and computer simulations.

First level of understanding

What are the protons and neutrons?

Fig. 1: an oversimplified version of protons consisting of only two upper quarks and one lower, and neutrons consisting only of two lower quarks and one upper

To simplify things, in many books, articles and websites it is indicated that protons consist of three quarks (two upper and one lower) and draw something like fig. 1. The neutron is the same, only consisting of one upper and two lower quarks. This simple image illustrates what some scientists believed, mainly in the 1960s. But it soon became clear that this view was overly simplified to such an extent that it was no longer correct.

From more sophisticated sources of information, you will learn that protons consist of three quarks (two upper and one lower), held together by gluons - and there may appear a picture similar to fig. 2, where gluons are drawn in the form of springs or threads holding quarks. Neutrons are the same, with only one upper quark and two lower ones.

Fig. 2: Fig. Improvement. 1 due to the emphasis on the important role of strong nuclear interaction keeping quarks in a proton

Not such a bad way of describing nucleons, since it focuses on the important role of a strong nuclear interaction that keeps quarks in a proton due to gluons (just like a photon is associated with electromagnetic interaction, the particle that makes up light). But this is also confusing, since it does not really explain what gluons are and what they do.

There are reasons to move on and describe things as I have done in other articles : a proton consists of three quarks (two upper and one lower), heaps of gluons, and mountains of quark-antiquark pairs (mostly upper and lower quarks, but there are several strange). They all fly to and fro with very high speed (approaching the speed of light); this whole set is held by strong nuclear interaction. I showed it in pic. 3. Neutrons are the same again, but with one upper and two lower quarks; the quark that changed its identity is indicated by the arrow.

Fig. 3: more realistic, although still imperfect image of protons and neutrons

These quarks, antiquarks and gluons not only rush hither and thither, but also collide with each other and turn into each other through processes such as particle annihilation (in which a quark and an antiquark of the same type turn into two gluons, or vice versa) or absorption and emission of a gluon (in which a quark and a gluon can collide and produce a quark and two gluons, or vice versa).

What these three descriptions have in common:

And, since:

Each figure relates the electric charge of the proton to the two upper and one lower quark, and “something else” adds to the charge 0. Similarly, the neutron charge is zero due to one upper and two lower quarks:

These descriptions differ as follows:

Since most of the mass of the atom, and, therefore, of all ordinary matter, is contained in protons and neutrons, the last point is extremely important for a correct understanding of our nature.

Fig. 1 says that quarks, in fact, represent a third of a nucleon - approximately as a proton or neutron represents a quarter of a helium nucleus or 1/12 of a carbon nucleus. If this pattern were true, the quarks in the nucleon would move relatively slowly (with speeds much lower than light) with relatively weak interactions between them (although with some powerful force holding them in place). The mass of the upper and lower quarks would then be about 0.3 GeV / s 2 , about a third of the proton mass. But this simple image and the ideas it imposes are simply wrong.

Fig. 3. gives a completely different idea of ​​the proton, as a cauldron of particles scurrying in it with speeds close to the light. These particles collide with each other, and in these collisions some of them annihilate, while others are created in their place. Gluons do not have a mass, the masses of the upper quarks are about 0.004 GeV / s 2 , and the lower ones - about 0.008 GeV / s 2 - are hundreds of times smaller than a proton. Where does the energy of the proton mass come from, the question is complicated: some of it comes from the energy of the mass of quarks and antiquarks, some from the energy of motion of quarks, antiquarks and gluons, and some (possibly positive, possibly negative) of energy stored in strong nuclear interactions holding quarks, antiquarks and gluons together.

In a sense, rice. 2 tries to eliminate the difference between pic. 1 and fig. 3. It simplifies rice. 3, removing many quark-antiquark pairs, which, in principle, can be called ephemeral, since they constantly appear and disappear, and are not necessary. But it gives the impression that gluons in nucleons are a direct part of the strong nuclear interaction that holds protons. And it does not explain where the mass of the proton comes from.

At fig. 1 there is another drawback, besides the narrow framework of the proton and neutron. It does not explain some properties of other hadrons, for example, pion and ro-meson . Rice has the same problems. 2

These restrictions and led to the fact that my students and on my website, I give the picture from Fig. 3. But I want to warn you that she has many limitations, which I will consider later.

It is worth noting that the extreme complexity of the structure, the implied figure. 3, it was worth expecting from an object that holds together such a powerful force as strong nuclear interaction. And one more thing: three quarks (two upper and one lower near the proton), which are not part of the group of quark-antiquark pairs, are often called “valent quarks”, and the quark-antiquark pairs are called “sea of ​​quark pairs”. Such a language is in many cases technically convenient. But it gives the false impression that if you could look inside the proton, and look at a certain quark, you could immediately tell if it is part of the sea or valence. This can not be done, there is simply no way.

Proton mass and neutron mass

Since the proton and neutron masses are so similar, and since the proton and neutron differ only in replacing the upper quark by the lower one, it seems likely that their masses are provided in the same way, come from the same source, and their difference lies in the slight difference between the upper and lower quarks . But the three cited figures indicate the presence of three very different views on the origin of the mass of the proton.

Fig. 1 says that the upper and lower quarks are simply 1/3 of the mass of the proton and neutron: about 0.313 GeV / s 2 , or because of the energy required to keep the quarks in the proton. And since the difference between the proton and neutron masses is a fraction of a percent, the difference between the masses of the upper and lower quarks should also be a fraction of a percent.

Fig. 2 is less clear. What part of the proton mass exists due to gluons? But, in principle, it follows from the figure that most of the proton mass still comes from the quark mass, as in Fig. one.

Fig. 3 reflects a more subtle approach to how a proton mass actually appears (as we can verify directly through computer calculations of a proton, and not directly using other mathematical methods). It is very different from the ideas presented in fig. 1 and 2, and is not so simple.

To understand how this works, you need to think not in terms of the mass m of the proton, but in terms of its mass energy E = mc 2 , the energy associated with the mass. Conceptually, the correct question is not “where did the proton mass m come from,” after which you can calculate E by multiplying m by c 2 , and vice versa: “where does the energy of proton mass E come from”, after which you can calculate mass m, dividing E by c 2 .

It is useful to classify the contributions to the energy of the proton mass into three groups:

A) Mass energy (rest energy) of quarks and antiquarks contained in it (gluons, massless particles, they do not make any contribution).
B) The energy of motion (kinetic energy) of quarks, antiquarks and gluons.
B) The interaction energy (binding energy or potential energy) stored in strong nuclear interactions (more precisely, in gluon fields) that hold the proton.

Fig. 3 says that the particles inside the proton move with great speed, and that it contains a lot of massless gluons, therefore the contribution B) is greater than A). Usually, in most physical systems B) and C) are comparable, while C) is often negative. So the energy of the mass of the proton (and the neutron) is mainly obtained from a combination of B) and C), and A) introduces a small fraction. Therefore, the masses of the proton and neutron appear mainly not because of the masses of the particles contained in them, but because of the energies of motion of these particles and the energy of their interaction associated with the gluon fields generating the forces holding the proton. In most of the other systems we are familiar with, the energy balance is distributed differently. For example, in atoms and in the solar system A) dominates, and B) and C) are much less, and comparable in magnitude.

Summing up, we point out that:

We know that rice is true. 3. To test it, we can conduct computer simulations, and, more importantly, thanks to various convincing theoretical arguments, we know that if the masses of the upper and lower quarks were zero (and everything else remained as it is), the mass of the proton is practically not would change. So, apparently, the masses of quarks cannot make important contributions to the proton mass.

If rice. 3 does not lie, the masses of quark and antiquark are very small. What are they really? The mass of the upper quark (as well as the antiquark) does not exceed 0.005 GeV / s 2 , which is much less than 0.313 GeV / s 2 , which follows from fig. 1. (The mass of the upper quark is hard to measure, and this value changes due to subtle effects, so it can be much less than 0.005 GeV / s 2 ). The mass of the lower quark is about 0.004 GeV / s 2 more than the mass of the upper one. This means that the mass of any quark or antiquark does not exceed one percent of the mass of a proton.

Note that this means (contrary to Fig. 1) that the ratio of the mass of the lower quark to the upper does not approach unity! The mass of the lower quark is at least twice the mass of the upper one. The reason that the masses of the neutron and proton are so similar is not because the masses of the upper and lower quarks are similar, but because the masses of the upper and lower quarks are very small - and the difference between them is small relative to the masses of the proton and neutron. Recall that in order to transform a proton into a neutron, you just need to replace one of its upper quarks with a lower one (Fig. 3). This replacement is enough to make the neutron a little heavier than the proton, and change its charge from + e to 0.

By the way, the fact that different particles inside a proton collide with each other, and constantly appear and disappear, does not affect the things we are discussing - energy is saved in any collision. The energy of mass and the energy of motion of quarks and gluons can vary, as well as the energy of their interaction, but the total energy of the proton does not change, although everything inside it is constantly changing. So the proton mass remains constant, despite its internal vortex.

At this point, you can stop and absorb the information received. Amazing Virtually all mass contained in ordinary matter comes from the mass of nucleons in atoms. And most of this mass comes from the chaos inherent in the proton and neutron - from the energy of motion of quarks, gluons and antiquarks in nucleons, and from the energy of the work of strong nuclear interactions that keep the nucleon in a whole state. Yes: our planet, our bodies, our breathing are the result of such a quiet, and, until recently, unimaginable crowds.


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