Elementary particles
Page 1
The charge of a neutron is zero. Consequently, neutrons do not play a role in the amount of charge on the nucleus of an atom. The serial number of chromium is also equal to the same value.
The charge of the proton is qp e The charge of the neutron is zero.
It is easy to see that in this case the charge of the neutron is zero, and the charge of the proton is 1, as expected. We obtain all baryons included in two families - eight and ten. Mesons consist of a quark and an antiquark. The line denotes antiques; their electric charge differs in sign from the charge of the corresponding quark. The pi meson does not include a strange quark; pi mesons, as we have already said, are particles with strangeness and spin equal to zero.
Since the charge of a proton is equal to the charge of an electron and the charge of a neutron is equal to a bullet, then if you turn off the strong interaction, the interaction of a proton with electromagnetic field And it would be the usual interaction of a Dirac particle - Yp / V. The neutron would have no electromagnetic interaction.
Designations: 67 - charge difference between electron and proton; q - neutron charge; qg- absolute value electron charge.
The nucleus consists of positively charged elementary particles- protons and charge-free neutrons.
The basis of modern ideas about the structure of matter is the statement about the existence of atoms of matter, consisting of positively charged protons and uncharged neutrons, forming a positively charged nucleus, and negatively charged electrons rotating around the nucleus. The energy levels of electrons, according to this theory, are discrete in nature, and the loss or acquisition of some additional energy by them is considered as a transition from one allowed energy level to another. In this case, the discrete nature of electronic energy levels causes the same discrete absorption or emission of energy by an electron during the transition from one energy level to another.
We assumed that the charge of an atom or molecule is completely determined by the scalar sum q Z (q Nqn, where Z is the number of electron-proton pairs, (q qp - qe is the difference between the charges of an electron and a proton, A is the number of neutrons, and qn is the charge of a neutron.
The charge of a nucleus is determined only by the number of protons Z, and its mass number A coincides with the total number of protons and neutrons. Since the charge of a neutron is zero, there is no electrical interaction according to Coulomb's law between two neutrons, or between a proton and a neutron. At the same time, an electrical repulsive force acts between the two protons.
Further, within the limits of measurement accuracy, not a single collision process has ever been recorded in which the law of conservation of charge was not observed. For example, the inflexibility of neutrons in uniform electric fields allows us to consider the charge of a neutron as equal to zero with an accuracy of 1 (H7 of the charge of an electron.
We have already said that the difference in the magnetic moment of a proton from one nuclear magneton is an amazing result. Even more surprising (It seems that there is a magnetic moment in a neutron that has no charge.
It is easy to verify that these forces cannot be reduced to any of the types of forces discussed in the previous parts of the physics course. In fact, if we assume, for example, that between nucleons in nuclei there are gravitational forces, then it is easy to calculate from the known masses of the proton and neutron that the binding energy per particle will be negligible - it will be 1036 times less than that, which is observed experimentally. The assumption of an electrical character also disappears. nuclear forces. Indeed, in this case it is impossible to imagine a stable nucleus consisting of one charged proton and no neutron charge.
The strong bond that exists between nucleons in the nucleus indicates the presence of special, so-called nuclear forces in atomic nuclei. It is easy to verify that these forces cannot be reduced to any of the types of forces discussed in the previous parts of the physics course. In fact, if we assume, for example, that gravitational forces act between nucleons in nuclei, then it is easy to calculate from the known masses of the proton and neutron that the binding energy per particle will be negligible - it will be 1038 times less than that observed experimentally. The assumption about the electrical nature of nuclear forces also disappears. Indeed, in this case it is impossible to imagine a stable nucleus consisting of one charged proton and no neutron charge.
Let's talk about how to find protons, neutrons and electrons. There are three types of elementary particles in an atom, each with its own elementary charge and mass.
Core structure
In order to understand how to find protons, neutrons and electrons, imagine It is the main part of the atom. Inside the nucleus are protons and neutrons called nucleons. Inside the nucleus, these particles can transform into each other.
For example, to find protons, neutrons and electrons in one, you need to know its serial number. If we take into account that it is this element that heads the periodic table, then its nucleus contains one proton.
Diameter atomic nucleus is a ten-thousandth of the total size of an atom. It contains the bulk of the entire atom. The mass of the nucleus is thousands of times greater than the sum of all the electrons present in the atom.
Particle characteristics
Let's look at how to find protons, neutrons and electrons in an atom, and learn about their features. A proton is what corresponds to the nucleus of a hydrogen atom. Its mass exceeds the electron by 1836 times. To determine the unit of electricity passing through a conductor with a given cross-section, electric charge is used.
Each atom has a certain number of protons in its nucleus. It is constant value, characterizes chemical and physical properties of this element.
How to find protons, neutrons and electrons in a carbon atom? Serial number of this chemical element 6, therefore, the nucleus contains six protons. According to the planetary system, six electrons move in orbits around the nucleus. To determine the number of neutrons from the carbon value (12), we subtract the number of protons (6), we get six neutrons.
For an iron atom, the number of protons corresponds to 26, that is, this element has the 26th atomic number in the periodic table.
A neutron is an electrically neutral particle, unstable in a free state. A neutron can spontaneously transform into a positively charged proton, emitting an antineutrino and an electron. Middle period its half-life is 12 minutes. Mass number is the total number of protons and neutrons inside the nucleus of an atom. Let's try to figure out how to find protons, neutrons and electrons in an ion? If an atom during chemical interaction with another element it acquires positive degree oxidation, the number of protons and neutrons in it does not change, only electrons become less.
Conclusion
There were several theories regarding the structure of the atom, but none of them were viable. Before the version created by Rutherford, there was no detailed explanation of the location of protons and neutrons inside the nucleus, as well as the rotation of electrons in circular orbits. After the emergence of the theory of the planetary structure of the atom, researchers had the opportunity not only to determine the number of elementary particles in an atom, but also to predict physical and Chemical properties specific chemical element.
- Translation
At the center of every atom is the nucleus, a tiny collection of particles called protons and neutrons. In this article we will study the nature of protons and neutrons, which consist of even smaller particles - quarks, gluons and antiquarks. (Gluons, like photons, are their own antiparticles.) Quarks and gluons, as far as we know, can be truly elementary (indivisible and not consisting of anything smaller in size). But to them later.
Surprisingly, protons and neutrons have almost the same mass - accurate to within a percentage:
- 0.93827 GeV/c 2 for the proton,
- 0.93957 GeV/c 2 for a neutron.
Because they are so similar, and because these particles make up 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 single proton), and neutrons were discovered around 1933. It was realized almost immediately that protons and neutrons are so similar to each other. But the fact that they have a measurable size comparable to the size of a nucleus (about 100,000 times smaller in radius than an atom) was not known until 1954. That they consist of quarks, antiquarks and gluons was gradually understood from the mid-1960s to the mid-1970s. By the late 70s and early 80s, our understanding of protons, neutrons, and what they are made of had largely settled down, and has remained unchanged ever since.
Nucleons are much more difficult to describe than atoms or nuclei. Not to say that atoms are fundamentally simple, but at least one can say without thinking that a helium atom consists of two electrons in orbit around a tiny helium nucleus; and the helium nucleus is a fairly simple group of two neutrons and two protons. But with nucleons everything is not so simple. I already wrote in the article “What is a proton and what is inside it?” that an atom is like an elegant minuet, and a nucleon is like a wild party.
The complexity of the proton and neutron appears to be genuine, and does not stem from incomplete knowledge of physics. We have equations used to describe quarks, antiquarks, and gluons, and the strong nuclear interactions that occur between them. These equations are called QCD, from quantum chromodynamics. The accuracy of the equations can be tested in a variety of ways, including measuring the number of particles produced at the Large Hadron Collider. Substituting the QCD equations into the computer and running calculations of the properties of protons and neutrons, and other similar particles (with common name"hadrons"), we obtain predictions of the properties of these particles that closely approximate the observations made in 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 have correct equations not enough, because:
- U simple equations decisions may be very difficult,
- Sometimes it is impossible to describe complex decisions in a simple way.
Because of the inherent complexity of nucleons, you, the reader, will have to make a choice: how much do you want to know about the complexity described? No matter how far you go, it will most likely not bring you satisfaction: the more you learn, the clearer the topic will become, but the final answer will remain the same - the proton and neutron are very complex. I can offer you three levels of understanding, with increasing detail; you can stop after any level and move on to other topics, or you can dive in until the last one. Each level raises questions that I can partially answer in the next one, but new answers raise new questions. In the end - as I do in professional discussions with colleagues and advanced students - I can only refer you to data obtained in real experiments, to various influential theoretical arguments, and computer simulations.
First level of understanding
What are protons and neutrons made of?Rice. 1: an overly simplified version of protons, consisting of only two up quarks and one down quark, and neutrons, consisting of only two down quarks and one up quark
To simplify matters, many books, articles and websites indicate that protons consist of three quarks (two up quarks and one down quark) and draw something like Fig. 1. The neutron is the same, only consisting of one up and two down quarks. This simple image illustrates what some scientists believed, mostly in the 1960s. But it soon became clear that this point of view was oversimplified to the point that it was no longer correct.
From more sophisticated sources of information, you will learn that protons are made up of three quarks (two up and one down) held together by gluons - and a picture similar to Fig. 1 may appear. 2, where gluons are drawn as springs or strings holding quarks. Neutrons are the same, only with one up quark and two down quarks.
Rice. 2: improvement fig. 1 due to the emphasis on the important role of the strong nuclear force, which holds quarks in the proton
This is not such a bad way to describe nucleons, since it emphasizes the important role of the strong nuclear force, which holds quarks in a proton at the expense of gluons (just as the photon, the particle that makes up light, is associated with the electromagnetic force). But this is also confusing because it doesn't really explain what gluons are or what they do.
There are reasons to go ahead and describe things the way I did in: a proton consists of three quarks (two up and one down), a bunch of gluons, and a mountain of quark-antiquark pairs (mostly up and down quarks, but there are a few weird ones as well) . They all fly back and forth at very high speeds (approaching the speed of light); this entire set is held together by the strong nuclear force. I demonstrated this in Fig. 3. Neutrons are again the same, but with one up and two down quarks; The quark that changed its identity is indicated by an arrow.
Rice. 3: more realistic, although still imperfect, representation of protons and neutrons
These quarks, antiquarks and gluons not only rush back and forth wildly, 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 do these three descriptions general:
- Two up quarks and a down quark (plus something else) for a proton.
- The neutron has one up quark and two down quarks (plus something else).
- The “something else” of neutrons coincides with the “something else” of protons. That is, the nucleons have the same “something else”.
- The small difference in mass between the proton and the neutron appears due to the difference in the masses of the down quark and the up quark.
- for top quarks the electric charge is equal to 2/3 e (where e is the charge of a proton, -e is the charge of an electron),
- bottom quarks have a charge of -1/3e,
- gluons have a charge of 0,
- any quark and its corresponding antiquark have a total charge of 0 (for example, an antidown quark has a charge of +1/3e, so a down quark and a down quark will have a charge of –1/3 e +1/3 e = 0),
- the total electric charge of the proton is 2/3 e + 2/3 e – 1/3 e = e,
- the total electric charge of the neutron is 2/3 e – 1/3 e – 1/3 e = 0.
- how much “something else” is inside the nucleon,
- what is it doing there
- where does the mass and mass energy (E = mc 2, the energy present there even when the particle is at rest) of the nucleon come from.
Rice. 1 says that quarks are essentially a third of a nucleon, much like a proton or neutron is a quarter of a helium nucleus or 1/12 of a carbon nucleus. If this figure were true, the quarks in the nucleon would move relatively slowly (at speeds much lower than light) with relatively weak interactions, acting between them (albeit in the presence of some powerful force holding them in place). The mass of the quark, up and down, would then be on the order of 0.3 GeV/c 2 , about a third of the mass of the proton. But this simple image and the ideas it imposes are simply wrong.
Rice. 3. gives a completely different idea of the proton, as a cauldron of particles scurrying around in it at speeds close to light. These particles collide with each other, and in these collisions, some of them are annihilated and others are created in their place. Gluons have no mass, the masses of the top quarks are on the order of 0.004 GeV/c 2 , and the masses of the bottom quarks are on the order of 0.008 GeV/c 2 - hundreds of times less than a proton. Where the energy of the proton mass comes from is a complex question: part of it comes from the energy of the mass of quarks and antiquarks, part from the energy of motion of quarks, antiquarks and gluons, and part (possibly positive, perhaps negative) from the energy stored in the strong nuclear interaction, holding quarks, antiquarks and gluons together.
In a sense, Fig. 2 attempts to resolve the difference between Fig. 1 and fig. 3. It simplifies the figure. 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 the gluons in the nucleons are a direct part of the strong nuclear force that holds the protons together. And it doesn't explain where the proton's mass comes from.
In Fig. 1 there is another drawback, in addition to the narrow frames of the proton and neutron. It does not explain some properties of other hadrons, for example, pion and rho meson. Fig. has the same problems. 2.
These restrictions led to the fact that I give my students and on my website the picture from Fig. 3. But I want to warn you that it also has many limitations, which I will discuss later.
It is worth noting that the extreme complexity of the structure implied by Fig. 3 would be expected from an object held together by a force as powerful as the strong nuclear force. And one more thing: three quarks (two up and one down for a proton) that are not part of a group of quark-antiquark pairs are often called “valence quarks”, and quark-antiquark pairs are called a “sea of quark pairs”. Such a language is technically convenient in many cases. But it gives the false impression that if you could look inside a proton, and look at a particular quark, you could immediately tell whether it was part of the sea or a valence one. This cannot be done, there is simply no such way.
Proton mass and neutron mass
Since the masses of the proton and neutron are so similar, and since the proton and neutron differ only in the replacement of the up quark by the down quark, 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 up and down quarks . But the three figures above indicate the presence of three very different views on the origin of the proton mass.Rice. 1 says that the up and down quarks simply make up 1/3 of the mass of the proton and neutron: on the order of 0.313 GeV/c 2, or because of the energy required to hold the quarks in the proton. And since the difference between the masses of a proton and a neutron is a fraction of a percent, the difference between the masses of an up and down quark must also be a fraction of a percent.
Rice. 2 is less clear. How much of a proton's mass is due to gluons? But, in principle, it follows from the figure that most of the proton mass still comes from the mass of quarks, as in Fig. 1.
Rice. 3 reflects a more nuanced approach to how the proton's mass actually comes about (as we can test directly through computer calculations of the proton, and indirectly using other mathematical methods). It is very different from the ideas presented in Fig. 1 and 2, and it turns out not so simple.
To understand how this works, you need to think not in terms of the proton's mass m, but in terms of its mass energy E = mc 2 , the energy associated with mass. Conceptually, the correct question is not “where does the proton mass m come from,” after which you can calculate E by multiplying m by c 2 , but vice versa: “where does the energy of the proton mass E come from,” after which you can calculate the mass m by dividing E by c 2 .
It is useful to classify contributions to the proton mass energy into three groups:
A) Mass energy (rest energy) of the quarks and antiquarks contained in it (gluons, massless particles, do not make any contribution).
B) Energy of motion ( kinetic energy) quarks, antiquarks and gluons.
C) Interaction energy (binding energy or potential energy) stored in the strong nuclear interaction (more precisely, in the gluon fields) holding the proton.
Rice. 3 says that the particles inside the proton move at high speed, and that it is full of massless gluons, so the contribution of B) is greater than A). Typically, in most physical systems B) and C) turn out to be comparable, while C) is often negative. So the mass energy of the proton (and neutron) mainly comes from the combination of B) and C), with A) contributing a small fraction. Therefore, the masses of the proton and neutron appear mainly not because of the masses of the particles they contain, but because of the energies of motion of these particles and the energy of their interaction associated with the gluon fields that generate the forces that hold the proton. In most other systems familiar to us, the energy balance is distributed differently. For example, in atoms and in solar system A) dominates, and B) and C) are much smaller and comparable in magnitude.
To summarize, we point out that:
- Rice. 1 assumes that the proton mass energy comes from contribution A).
- Rice. 2 assumes that both contributions A) and B) are important, with B) making a small contribution.
- Rice. 3 suggests that B) and C) are important, and the contribution of A) turns out to be insignificant.
If fig. 3 does not lie, the masses of the quark and antiquark are very small. What are they really like? The mass of the top quark (as well as the antiquark) does not exceed 0.005 GeV/c 2, which is much less than 0.313 GeV/c 2, which follows from Fig. 1. (The mass of the up quark is difficult to measure and varies due to subtle effects, so it may be much less than 0.005 GeV/c2). The mass of the bottom quark is approximately 0.004 GeV/s 2 greater than the mass of the top quark. 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 down quark to up quark mass does not approach unity! The mass of the down quark is at least twice the mass of the up quark. The reason that the masses of the neutron and proton are so similar is not because the masses of the up and down quarks are similar, but because the masses of the up and down quarks are very small - and the difference between them is small, relative to the masses of the proton and neutron. Remember that to turn a proton into a neutron, you simply need to replace one of its up quarks with a down quark (Figure 3). This replacement is enough to make the neutron slightly heavier than the proton, and change its charge from +e to 0.
By the way, the fact that the various particles inside the proton collide with each other, and are constantly appearing and disappearing, does not affect the things we are discussing - energy is conserved in any collision. The mass energy and energy of motion of quarks and gluons can change, as can the energy of their interaction, but the total energy of the proton does not change, although everything inside it is constantly changing. So the mass of the proton remains constant, despite its internal vortex.
At this point you can stop and absorb the information received. Amazing! Virtually all the 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 strong nuclear interactions that hold the nucleon in its entire state. Yes: our planet, our bodies, our breath are the result of such quiet, and, until recently, unimaginable pandemonium.
As soon as you happen to encounter an unknown object, the mercantile and everyday question inevitably arises - how much does it weigh? But if this unknown is an elementary particle, what then? But nothing, the question remains the same: what is the mass of this particle. If someone were to start counting the costs incurred by humanity to satisfy their curiosity in researching, or rather, measuring, the mass of elementary particles, we would find out that, for example, the mass of a neutron in kilograms with a mind-boggling number of zeros after the decimal point cost humanity more than than the most expensive construction with the same number of zeros before the decimal point.
And it all started very routinely: in 1897, in the laboratory headed by J. J. Thomson, studies of cathode rays were carried out. As a result, a universal constant for the Universe was determined - the ratio of the mass of an electron to its charge. There is very little left to determine the mass of the electron - to determine its charge. After 12 years I managed to do it. He conducted experiments with oil droplets falling in an electric field, and he managed not only to balance their weight with the magnitude of the field, but also to carry out the necessary and extremely subtle measurements. Their result is the numerical value of the electron mass:
me = 9.10938215(15) * 10-31kg.
Research into the structure, where Ernest Rutherford was a pioneer, also dates back to this time. It was he who, observing the scattering of charged particles, proposed a model of an atom with an outer electron shell and a positive nucleus. The particle, which was proposed to play the role of the nucleus of the simplest atom, was obtained by bombarding nitrogen. This was the first nuclear reaction, obtained in the laboratory - as a result, oxygen and nuclei of future ones called protons were obtained from nitrogen. However, alpha rays consist of complex particles: in addition to two protons, they also contain two neutrons. The mass of the neutron is almost equal and the total mass of the alpha particle turns out to be quite substantial in order to destroy the oncoming nucleus and break off a “piece” from it, which is what happened.
The flow of positive protons was deflected electric field, compensating for its deviation caused by In these experiments, determining the mass of the proton was no longer difficult. But the most interesting question was what the ratio of the mass of a proton and an electron is. The riddle was immediately solved: the mass of a proton exceeds the mass of an electron a little more than 1836 times.
So, initially, the model of the atom was assumed, according to Rutherford, to be an electron-proton set with the same number of protons and electrons. However, it soon turned out that the primary nuclear model does not fully describe all the observed effects in the interactions of elementary particles. Only in 1932 did he confirm the hypothesis of additional particles in the nucleus. They were called neutrons, neutral protons, because. they had no charge. It is this circumstance that determines their greater penetrating ability - they do not spend their energy on ionizing oncoming atoms. The mass of a neutron is very slightly greater than the mass of a proton - only about 2.6 electron masses more.
The chemical properties of substances and compounds that are formed by a given element are determined by the number of protons in the nucleus of the atom. Over time, the participation of the proton in strong and other fundamental interactions: electromagnetic, gravitational and weak. Moreover, despite the fact that the neutron has no charge, in strong interactions the proton and neutron are considered as an elementary particle, the nucleon, in various quantum states. The similarity in the behavior of these particles is partly explained by the fact that the mass of a neutron differs very little from the mass of a proton. The stability of protons allows them to be used, after being previously accelerated to high speeds, as bombarding particles to carry out nuclear reactions.
§1. Meet the electron, proton, neutron
Atoms - tiny particles substances.
If you enlarge an apple to the size of the globe average size, then the atoms will become only the size of an apple. Despite such small dimensions, the atom consists of even smaller physical particles.
You should already be familiar with the structure of the atom from school course physics. And yet, let us recall that the atom contains a nucleus and electrons, which rotate around the nucleus so quickly that they become indistinguishable - they form " electron cloud", or electron shell atom.
Electrons usually denoted as follows: e − . Electrons e− very light, almost weightless, but they have negative electric charge. It is equal to −1. Electricity, which we all use, is a stream of electrons running in wires.
Atomic nucleus, in which almost all of its mass is concentrated, consists of particles of two types - neutrons and protons.
Neutrons denoted as follows: n 0
, A protons So: p +
.
In terms of mass, neutrons and protons are almost the same - 1.675 10−24 g and 1.673 10−24 g.
True, it is very inconvenient to count the mass of such small particles in grams, so it is expressed in carbon units, each of which is equal to 1.673 10 −24 g.
For each particle we get relative atomic mass, equal to the quotient of the mass of an atom (in grams) divided by the mass of a carbon unit. The relative atomic masses of a proton and a neutron are equal to 1, but the charge of protons is positive and equal to +1, while neutrons have no charge.
. Riddles about the atom
An atom can be assembled “in the mind” from particles, like a toy or a car from parts of a children’s construction set. It is only necessary to observe two important conditions.
- First condition: each type of atom has its own own set"details" - elementary particles. For example, a hydrogen atom will definitely have a nucleus with a positive charge of +1, which means it must certainly have one proton (and no more).
A hydrogen atom can also contain neutrons. More on this in the next paragraph.
Oxygen atom (atomic number in Periodic table is equal to 8) will have a nucleus charged eight positive charges (+8), which means there are eight protons. Since the mass of an oxygen atom is 16 relative units, to get an oxygen nucleus, we add another 8 neutrons. - Second condition is that each atom should be electrically neutral. To do this, it must have enough electrons to balance the charge of the nucleus. In other words, the number of electrons in an atom is equal to the number of protons in its core, and also the serial number of this element in the Periodic Table.
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