Nuclei and particles. Nominal particle A b particles

1.2. Properties β -radiation

Beta radiation ( b -particles) is a stream of electrons (positrons), each of which has a charge equal to one elementary charge, 4.8 × 10 – 10 CGSE electrostatic units or 1.6 × 10 –19 coulombs. Rest mass b -particle is equal to 1/1840 of the elementary mass of a hydrogen atom (7000 times less than the mass α -particles) or in absolute units 9.1×10 –28 g. Since b -particles move at a speed much greater than α -particles equal to » 0.988 (Einstein mass) of the speed of light, then their mass should be calculated using the relativistic equation:

Where That – rest mass (9.1·10 -28 g);

V - speed β -particles;

C - speed of light.

For the fastest β -particles m ≈ 16 m o .

When emitting one b -particles, the atomic number of the element increases (electron emission) or decreases (positron emission) by one. Beta decay is usually accompanied by g - radiation. Each radioactive isotope emits an aggregate b -particles of very different energies, not exceeding, however, a certain maximum energy characteristic of a given isotope.

Energy spectra b - radiations are shown in Fig. 1.5, 1.6. In addition to a continuous energy spectrum, some radioelements are characterized by the presence of a line spectrum associated with the ejection of secondary electrons from the electron orbits of the atom by g-quanta (the phenomenon of internal conversion). This happens when β -decay occurs through an intermediate energy level, and excitation can be removed not only by emission γ -quantum, but also by knocking out an electron from the inner shell.

However, the number b -particles corresponding to these lines are small.

The continuity of the beta spectrum is explained by the simultaneous emission b -particles and neutrinos.

p = n + β + + η(neutrino)

n = p + β - + η(antineutrino)

The neutrino absorbs some of the beta decay energy.

Average energy b -particle is equal to 1/3. E max and fluctuates between 0.25–0.45 E max for various substances. Between the maximum energy value E max b -radiation and decay constant l element Sargent established a relation (for E max > 0.5 Mev),

l = k∙E 5 max (1.12)

Thus, for β -radiation energy β -particles are larger, the shorter the half-life. For example:

Pb 210 (RaD) T = 22 years, E max = 0.014 MeV;

Bi 214 (RaC) T = 19.7 months, E max = 3.2 MeV.

1.2.1. Interaction β - radiation with matter

When interacting β –particles with matter the following cases are possible:

a) Ionization of atoms. It is accompanied by characteristic radiation. Ionization capacity β -particles depends on their energy. Specific ionization is greater, the less energy β -particles. For example, with energy β -particles of 0.04 MeV, 200 pairs of ions are formed per 1 cm of path; 2 MeV – 25 pairs; 3 MeV – 4 pairs.

b) Excitation of atoms. It is typical for β -particles with high energy, when the interaction time β -there are few particles with an electron and the probability of ionization is low; in this case β -the particle excites an electron, the excitation energy is removed by emitting characteristic X-rays, and in scintillators, a significant part of the excitation energy appears in the form of a flash - scintium (i.e. in the visible region).

c) Elastic scattering. Occurs when the electric field of a nucleus (electron) deflects β -particle, while the energy β -particles do not change, only the direction changes (by a small angle);

d) Electron deceleration in the Coulomb field of the nucleus. In this case, electromagnetic radiation appears with more energy, the greater the acceleration the electron experiences. Since individual electrons experience different accelerations, the bremsstrahlung spectrum is continuous. Energy losses due to bremsstrahlung are determined by the expression: the ratio of energy losses due to bremsstrahlung to losses due to excitation and ionization:

Thus, losses and bremsstrahlung are significant only for high-energy electrons with large atomic numbers.

For most β -particles maximum energy lies in the range of 0.014–1.5 MeV, we can assume that per 1 cm of path β -particles, 100–200 pairs of ions are formed. α -a particle forms 25 - 60 thousand pairs of ions per 1 cm of path. Therefore, we can assume that the specific ionization capacity β- radiation is two orders of magnitude less than that of α-radiation. Less ionization - energy is lost more slowly, since ionization capacity (and probability of excitation) β -the particle is 2 orders of magnitude smaller, which means it decelerates 2 orders of magnitude slower, i.e., approximately the mileage β -particles are 2 orders of magnitude larger than for α- particles. 10 mg/cm2 ·100 = 1000 mg/cm2 ≈ 1 g/cm2.

In physics, elementary particles were physical objects on the scale of the atomic nucleus that cannot be divided into their component parts. However, today, scientists have managed to split some of them. The structure and properties of these tiny objects are studied by particle physics.

The smallest particles that make up all matter have been known since ancient times. However, the founders of the so-called “atomism” are considered to be the Ancient Greek philosopher Leucippus and his more famous student, Democritus. It is assumed that the latter coined the term “atom”. From the ancient Greek “atomos” is translated as “indivisible”, which determines the views of ancient philosophers.

Later it became known that the atom can still be divided into two physical objects - the nucleus and the electron. The latter subsequently became the first elementary particle, when in 1897 the Englishman Joseph Thomson conducted an experiment with cathode rays and discovered that they were a stream of identical particles with the same mass and charge.

In parallel with Thomson's work, Henri Becquerel, who studies x-rays, conducts experiments with uranium and discovers a new type of radiation. In 1898, a French pair of physicists, Marie and Pierre Curie, studied various radioactive substances, discovering the same radioactive radiation. It would later be found to consist of alpha particles (2 protons and 2 neutrons) and beta particles (electrons), and Becquerel and Curie would receive the Nobel Prize. While conducting her research with elements such as uranium, radium and polonium, Marie Sklodowska-Curie did not take any safety measures, including not even using gloves. As a result, in 1934 she was overtaken by leukemia. In memory of the achievements of the great scientist, the element discovered by the Curie couple, polonium, was named in honor of Mary’s homeland - Polonia, from Latin - Poland.

Photo from the V Solvay Congress 1927. Try to find all the scientists from this article in this photo.

Since 1905, Albert Einstein has devoted his publications to the imperfection of the wave theory of light, the postulates of which were at odds with the results of experiments. Which subsequently led the outstanding physicist to the idea of ​​a “light quantum” - a portion of light. Later, in 1926, it was named “photon”, translated from the Greek “phos” (“light”), by the American physical chemist Gilbert N. Lewis.

In 1913, Ernest Rutherford, a British physicist, based on the results of experiments already carried out at that time, noted that the masses of the nuclei of many chemical elements are multiples of the mass of the hydrogen nucleus. Therefore, he assumed that the hydrogen nucleus is a component of the nuclei of other elements. In his experiment, Rutherford irradiated a nitrogen atom with alpha particles, which as a result emitted a certain particle, named by Ernest as a “proton”, from the other Greek “protos” (first, main). Later it was experimentally confirmed that the proton is a hydrogen nucleus.

Obviously, the proton is not the only component of the nuclei of chemical elements. This idea is led by the fact that two protons in the nucleus would repel each other, and the atom would instantly disintegrate. Therefore, Rutherford hypothesized the presence of another particle, which has a mass equal to the mass of a proton, but is uncharged. Some experiments of scientists on the interaction of radioactive and lighter elements led them to the discovery of another new radiation. In 1932, James Chadwick determined that it consists of those very neutral particles that he called neutrons.

Thus, the most famous particles were discovered: photon, electron, proton and neutron.

Further, the discovery of new subnuclear objects became an increasingly frequent event, and at the moment about 350 particles are known, which are generally considered “elementary”. Those of them that have not yet been split are considered structureless and are called “fundamental.”

What is spin?

Before moving forward with further innovations in the field of physics, the characteristics of all particles must be determined. The most well-known, apart from mass and electric charge, also includes spin. This quantity is otherwise called “intrinsic angular momentum” and is in no way related to the movement of the subnuclear object as a whole. Scientists were able to detect particles with spin 0, ½, 1, 3/2 and 2. To visualize, albeit simplified, spin as a property of an object, consider the following example.

Let an object have a spin equal to 1. Then such an object, when rotated 360 degrees, will return to its original position. On a plane, this object can be a pencil, which, after a 360-degree turn, will end up in its original position. In the case of zero spin, no matter how the object rotates, it will always look the same, for example, a single-color ball.

For a ½ spin, you will need an object that retains its appearance when rotated 180 degrees. It can be the same pencil, only sharpened symmetrically on both sides. A spin of 2 will require the shape to be maintained when rotated 720 degrees, and a spin of 3/2 will require 540.

This characteristic is very important for particle physics.

Standard Model of Particles and Interactions

Having an impressive set of micro-objects that make up the world around us, scientists decided to structure them, and this is how the well-known theoretical structure called the “Standard Model” was formed. She describes three interactions and 61 particles using 17 fundamental ones, some of which she predicted long before the discovery.

The three interactions are:

• Electromagnetic. It occurs between electrically charged particles. In a simple case, known from school, oppositely charged objects attract, and similarly charged objects repel. This happens through the so-called carrier of electromagnetic interaction - the photon.
• Strong, otherwise known as nuclear interaction. As the name implies, its action extends to objects of the order of the atomic nucleus; it is responsible for the attraction of protons, neutrons and other particles also consisting of quarks. The strong interaction is carried by gluons.
• Weak. Effective at distances a thousand smaller than the size of the core. Leptons and quarks, as well as their antiparticles, take part in this interaction. Moreover, in the case of weak interaction, they can transform into each other. The carriers are the W+, W− and Z0 bosons.

So the Standard Model was formed as follows. It includes six quarks, from which all hadrons (particles subject to strong interaction) are composed:

• Upper(u);
• Enchanted (c);
• true(t);
• Lower (d);
• Strange(s);
• Adorable (b).

It is clear that physicists have plenty of epithets. The other 6 particles are leptons. These are fundamental particles with spin ½ that do not participate in the strong interaction.

• Electron;
• Electron neutrino;
• Muon;
• Muon neutrino;
• Tau lepton;
• Tau neutrino.

And the third group of the Standard Model are gauge bosons, which have a spin equal to 1 and are represented as carriers of interactions:

• Gluon – strong;
• Photon – electromagnetic;
• Z-boson - weak;
• The W boson is weak.

These also include the recently discovered spin-0 particle, which, simply put, imparts inert mass to all other subnuclear objects.

As a result, according to the Standard Model, our world looks like this: all matter consists of 6 quarks, forming hadrons, and 6 leptons; all these particles can participate in three interactions, the carriers of which are gauge bosons.

Disadvantages of the Standard Model

However, even before the discovery of the Higgs boson, the last particle predicted by the Standard Model, scientists had gone beyond its limits. A striking example of this is the so-called. “gravitational interaction”, which is on par with others today. Presumably, its carrier is a particle with spin 2, which has no mass, and which physicists have not yet been able to detect - the “graviton”.

Moreover, the Standard Model describes 61 particles, and today more than 350 particles are already known to humanity. This means that the work of theoretical physicists is not over.

Particle classification

To make their life easier, physicists have grouped all particles depending on their structural features and other characteristics. Classification is based on the following criteria:

• Lifetime.
  1. Stable. These include proton and antiproton, electron and positron, photon, and graviton. The existence of stable particles is not limited by time, as long as they are in a free state, i.e. don't interact with anything.
  2. Unstable. All other particles after some time disintegrate into their component parts, which is why they are called unstable. For example, a muon lives only 2.2 microseconds, and a proton - 2.9 10 * 29 years, after which it can decay into a positron and a neutral pion.
• Weight.
  1. Massless elementary particles, of which there are only three: photon, gluon and graviton.
  2. Massive particles are all the rest.
• Spin meaning.
  1. Whole spin, incl. zero, have particles called bosons.
  2. Particles with half-integer spin are fermions.
• Participation in interactions.
  1. Hadrons (structural particles) are subnuclear objects that take part in all four types of interactions. It was mentioned earlier that they are composed of quarks. Hadrons are divided into two subtypes: mesons (integer spin, bosons) and baryons (half-integer spin, fermions).
  2. Fundamental (structureless particles). These include leptons, quarks and gauge bosons (read earlier - “Standard Model..”).

Having familiarized yourself with the classification of all particles, you can, for example, accurately identify some of them. So the neutron is a fermion, a hadron, or rather a baryon, and a nucleon, that is, it has a half-integer spin, consists of quarks and participates in 4 interactions. Nucleon is a common name for protons and neutrons.

• It is interesting that opponents of the atomism of Democritus, who predicted the existence of atoms, stated that any substance in the world is divided indefinitely. To some extent, they may turn out to be right, since scientists have already managed to divide the atom into a nucleus and an electron, the nucleus into a proton and a neutron, and these, in turn, into quarks.
• Democritus assumed that atoms have a clear geometric shape, and therefore the “sharp” atoms of fire burn, the rough atoms of solids are firmly held together by their protrusions, and the smooth atoms of water slip during interaction, otherwise they flow.
• Joseph Thomson compiled his own model of the atom, which he saw as a positively charged body into which electrons seemed to be “stuck.” His model was called the “Plum pudding model.”
• Quarks got their name thanks to the American physicist Murray Gell-Mann. The scientist wanted to use a word similar to the sound of a duck quack (kwork). But in James Joyce's novel Finnegans Wake he encountered the word “quark” in the line “Three quarks for Mr. Mark!”, the meaning of which is not precisely defined and it is possible that Joyce used it simply for rhyme. Murray decided to call the particles this word, since at that time only three quarks were known.
• Although photons, particles of light, are massless, near a black hole they appear to change their trajectory as they are attracted to it by gravitational forces. In fact, a supermassive body bends space-time, which is why any particles, including those without mass, change their trajectory towards the black hole (see).
• The Large Hadron Collider is “hadronic” precisely because it collides two directed beams of hadrons, particles with dimensions on the order of an atomic nucleus that participate in all interactions.

Alpha(a) rays- positively charged helium ions (He++), flying out of atomic nuclei at a speed of 14,000-20,000 km/h. The particle energy is 4-9 MeV. α-radiation is observed, as a rule, from heavy and predominantly natural radioactive elements (radium, thorium, etc.). The range of an alpha particle in air increases with increasing energy of alpha radiation.

For example, a-particles of thorium(Th232), having an energy of 3.9 MeV, travels 2.6 cm in the air, and a-particles of radium C with an energy of 7.68 MeV have a range of 6.97 cm. The minimum thickness of the absorber required for complete absorption of particles is called the range of these particles in a given substance. The ranges of alpha particles in water and fabric are 0.02-0.06 mm.

a-particles are completely absorbed by a piece of tissue paper or a thin layer of aluminum. One of the most important properties of a-radiation is its strong ionizing effect. Along the path of motion, an alpha particle in gases forms a huge number of ions. For example, in air at 15° and 750 mm of pressure, one alpha particle produces 150,000-250,000 pairs of ions, depending on its energy.

For example, specific ionization in air a-particles from radon, having an energy of 5.49 MeV, is 2500 ion pairs per 1 mm of path. The ionization density at the end of the path of α-particles increases, so the damage to cells at the end of the path is approximately 2 times greater than at the beginning of the path.

Physical properties of alpha particles determine the characteristics of their biological effect on the body and methods of protection against this type of radiation. External irradiation with a-rays does not pose a danger, since it is enough to move a few (10-20) centimeters away from the source or install a simple screen made of paper, fabric, aluminum and other ordinary materials so that the radiation is completely absorbed.

The greatest danger of a-rays represent when ingested and deposited inside radioactive a-emitting elements. In these cases, direct irradiation of the cells and tissues of the body occurs with a-rays.

Beta(b) rays- a stream of electrons ejected from atomic nuclei at a speed of approximately 100,000-300,000 km/sec. The maximum energy of p particles ranges from 0.01 to 10 MeV. The charge of a b-particle is equal in sign and magnitude to the charge of an electron. Radioactive transformations such as b-decay are widespread among natural and artificial radioactive elements.

b-rays have significantly greater penetrating power compared to a-rays. Depending on the energy of b-rays, their range in the air ranges from fractions of a millimeter to several meters. Thus, the range of b-particles with an energy of 2-3 MeV in air is 10-15 m, and in water and fabric it is measured in millimeters. For example, the range of b-particles emitted by radioactive phosphorus (P32) with a maximum energy of 1.7 MeV in tissue is 8 mm.

b-particle with energy, equal to 1 MeV, can form about 30,000 ion pairs along its path in the air. The ionizing ability of b-particles is several times less than that of a-particles of the same energy.

Exposure to b-rays on the body can manifest itself both during external and internal irradiation, if active substances emitting b-particles enter the body. To protect against b-rays during external irradiation, it is necessary to use screens made of materials (glass, aluminum, lead, etc.). The radiation intensity can be reduced by increasing the distance from the source.

They have been trying to find the Higgs boson for decades, but so far without success. Meanwhile, without it, the key provisions of the modern theory of the microworld hang in the air.

The study of particles began not so long ago. In 1897, Joseph John Thomson discovered the electron, and 20 years later Ernest Rutherford proved that hydrogen nuclei are part of the nuclei of other elements, and later called them protons. In the 1930s, the neutron, muon and positron were discovered and the existence of neutrinos was predicted. At the same time, Hideki Yukawa built a theory of nuclear forces carried by hypothetical particles hundreds of times heavier than an electron, but much lighter than a proton (mesons). In 1947, traces of decays of pi-mesons (pions) were found on photographic plates exposed to cosmic rays. Later, other mesons were discovered, some of them heavier not only than the proton, but also the helium nucleus. Physicists have also discovered many baryons, heavy and therefore unstable relatives of the proton and neutron. Once upon a time, all these particles were called elementary, but such terminology has long been outdated. Nowadays, only non-composite particles are considered elementary - fermions (with half spin - leptons and quarks) and bosons (with integer spin - carriers of fundamental interactions).

Elementary particles of the Standard Model

The fermion group (with half-integer spin) consists of leptons and quarks of the so-called three generations. Charged leptons are the electron and its massive counterparts, the muon and tau particle (and their antiparticles). Each lepton has a neutral partner in the form of one of three types of neutrinos (also with antiparticles). The spin-1 family of bosons are particles that carry interactions between quarks and leptons. Some of them have no mass and no electric charge - these are gluons, which provide interquark connections in mesons and baryons, and photons, quanta of the electromagnetic field. Weak interactions, which manifest themselves in beta decay processes, are provided by a trio of massive particles - two charged and one neutral.

Individual names of elementary and composite particles are usually not associated with the names of specific scientists. However, almost 40 years ago, another elementary particle was predicted, which was named after a living person, Scottish physicist Peter Higgs. Like the carriers of fundamental interactions, it has an integer spin and belongs to the class of bosons. However, its spin is not 1, but 0, and in this respect it has no analogues. For decades now, they have been looking for it at the largest accelerators - the American Tevatron, which was closed last year and the Large Hadron Collider, which is now functioning under the close attention of the world media. After all, the Higgs boson is very necessary for the modern theory of the microworld - the Standard Model of elementary particles. If it cannot be discovered, the key tenets of this theory will remain up in the air.

Gauge symmetries

The beginning of the path to the Higgs boson can be counted from a short paper published in 1954 by Chinese physicist Yang Zhenning, who moved to the United States, and his colleague at Brookhaven National Laboratory, Robert Mills. In those years, experimenters discovered more and more new particles, the abundance of which could not be explained in any way. In search of promising ideas, Young and Mills decided to test the possibilities of a very interesting symmetry that governs quantum electrodynamics. By that time, this theory had proven its ability to produce results that were in excellent agreement with experiment. True, in the course of some calculations infinities appear there, but they can be eliminated using a mathematical procedure called renormalization.

Symmetry, which interested Young and Mills, was introduced into physics in 1918 by the German mathematician Hermann Weyl. He called it gauge, and this name has survived to this day. In quantum electrodynamics, gauge symmetry manifests itself in the fact that the wave function of a free electron, which is a vector with a real and an imaginary part, can be continuously rotated at each point in spacetime (which is why the symmetry is called local). This operation (in formal language - changing the phase of the wave function) leads to the fact that additives appear in the equation of motion of the electron, which must be compensated for it to remain valid. To do this, an additional term is introduced there, which describes the electromagnetic field interacting with the electron. The quantum of this field turns out to be a photon, a massless particle with unit spin. Thus, from the local gauge symmetry of the free electron equation, the existence of photons (as well as the constancy of the electron charge) follows. We can say that this symmetry instructs the electron to interact with the electromagnetic field. Any phase shift becomes an act of such interaction - for example, the emission or absorption of a photon.

The connection between gauge symmetry and electromagnetism was identified back in the 1920s, but did not attract much interest. Young and Mills were the first to try to use this symmetry to construct equations describing particles of a nature other than the electron. They studied the two “oldest” baryons - the proton and the neutron. Although these particles are not identical, with respect to nuclear forces they behave almost identically and have almost the same mass. In 1932, Werner Heisenberg showed that the proton and neutron can be formally considered different states of the same particle. To describe them, he introduced a new quantum number - isotopic spin. Because the strong force does not differentiate between protons and neutrons, it preserves full isotopic spin, just as the electromagnetic force preserves electric charge.

Young and Mills asked which local gauge transformations preserve isospin symmetry. It was clear that they could not coincide with the gauge transformations of quantum electrodynamics - if only because we were talking about two particles. Young and Mills analyzed a set of such transformations and found that they generate fields whose quanta presumably transfer interactions between protons and neutrons. In this case there were three quanta: two charged (positively and negatively) and one neutral. They had zero mass and unit spin (that is, they were vector bosons) and moved at the speed of light.

The theory of B-fields, as the co-authors dubbed them, was very beautiful, but did not stand the test of experiment. The neutral B boson could be identified with the photon, but its charged brothers remained out of action. According to quantum mechanics, only sufficiently massive virtual particles can mediate the transfer of short-range forces. The radius of nuclear forces does not exceed 10–13 cm, and the massless Yang and Mills bosons clearly could not claim to be their carriers. In addition, experimenters have never detected such particles, although in principle charged massless bosons are easy to detect. Young and Mills proved that local gauge symmetries “on paper” could generate force fields of a non-electromagnetic nature, but the physical reality of these fields was purely a hypothesis.

Electroweak duality

The next step towards the Higgs boson was made in 1957. By that time, theorists (the same Yang and Li Zongdao) suggested, and experimenters proved, that parity is not conserved during beta decays (in other words, mirror symmetry is broken). This unexpected result interested many physicists, among whom was Julian Schwinger, one of the creators of quantum electrodynamics. He hypothesized that weak interactions between leptons (science had not yet reached quarks!) are carried by three vector bosons - a photon and a pair of charged particles similar to B-bosons. It followed that these interactions were in partnership with electromagnetic forces. Schwinger did not pursue this problem further, but suggested it to his graduate student Sheldon Glashow.

The work lasted for four years. After a number of unsuccessful attempts, Glashow constructed a model of the weak and electromagnetic interactions, based on the unification of the gauge symmetries of the electromagnetic field and the Yang and Mills fields. In addition to the photon, three more vector bosons appeared in it - two charged and one neutral. However, these particles again had zero mass, which created a problem. The weak interaction has a radius two orders of magnitude smaller than the strong interaction, and all the more so it requires very massive intermediaries. In addition, the presence of a neutral carrier required the possibility of beta transitions that did not change the electric charge, and at that time such transitions were not known. Because of this, after publishing his model in late 1961, Glashow lost interest in unifying the weak and electromagnetic forces and moved on to other topics.

Schwinger's hypothesis also interested the Pakistani theorist Abdus Salam, who, together with John Ward, built a model similar to Glashow's model. He also encountered the masslessness of gauge bosons and even came up with a way to eliminate it. Salam knew that their masses could not be entered “by hand”, since the theory was becoming non-normalizable, but he hoped to get around this difficulty by using spontaneous symmetry breaking, so that the solutions to the equations of boson motion would not have the gauge symmetry inherent in the equations themselves. This task interested the American Steven Weinberg.

But in 1961, the English physicist Geoffrey Goldstone showed that in relativistic quantum field theories, spontaneous symmetry breaking seems to inevitably generate massless particles. Salam and Weinberg tried to refute Goldstone's theorem, but only strengthened it in their own work. The mystery looked insurmountable, and they moved on to other areas of physics.

Higgs and others

Help came from experts in condensed matter physics. In 1961, Yoichiro Nambu noted that when a normal metal transitions to a superconducting state, the previous symmetry is spontaneously broken, but no massless particles appear. Two years later, Philip Anderson, using the same example, noted that if the electromagnetic field does not obey Goldstone’s theorem, then the same can be expected from other gauge fields with local symmetry. He even predicted that Goldstone bosons and Yang and Mills field bosons could somehow cancel each other out, leaving behind massive particles.

This forecast turned out to be prophetic. In 1964, he was acquitted by physicists from the Free University of Brussels Francois Englert and Roger Braut, Peter Higgs and employees of the Imperial College London Jerry Guralnik, Robert Hagen and Thomas Kibble. They not only showed that the conditions for the applicability of the Goldstone theorem are not met in Yang–Mills fields, but also found a way to provide the excitations of these fields with non-zero mass, which is now called the Higgs mechanism.

These wonderful works were not immediately noticed and appreciated. It was not until 1967 that Weinberg constructed a unified model of the electroweak interaction, in which a trio of vector bosons gain mass based on the Higgs mechanism, and a year later Salam did the same. In 1971, the Dutchmen Martinus Veltman and Gerard 't Hooft proved that this theory is renormalizable and therefore has a clear physical meaning. It firmly stood on its feet after 1973, when in a bubble chamber Gargamelle(CERN, Switzerland), experimenters recorded so-called weak neutral currents, indicating the existence of an uncharged intermediate boson (direct registration of all three vector bosons was carried out at CERN only in 1982–1983). Glashow, Weinberg and Salam received Nobel Prizes for it in 1979, Veltman and 't Hooft - in 1999. This theory (and with it the Higgs boson) has long become an integral part of the Standard Model of elementary particles.

Higgs mechanism

The Higgs mechanism is based on scalar fields with spinless quanta - Higgs bosons. They are believed to have arisen moments after the Big Bang and now fill the entire Universe. Such fields have the lowest energy at a non-zero value - this is their stable state.

It is often written that elementary particles acquire mass as a result of braking by the Higgs field, but this is too mechanistic an analogy. The theory of electroweak interaction involves four Higgs fields (each with its own quanta) and four vector bosons - two neutral and two charged, which themselves have no mass. Three bosons, both charged and one neutral, absorb one Higgs each and as a result acquire mass and the ability to transfer short-range forces (they are denoted by the symbols W +, W – and Z 0). The last boson does not absorb anything and remains massless - it is a photon. The “eaten” Higgs are unobservable (physicists call them “ghosts”), while their fourth brother should be observed at energies sufficient for its birth. In general, these are exactly the processes that Anderson managed to predict.

Elusive particle

The first serious attempts to catch the Higgs boson were made at the turn of the 20th and 21st centuries at the Large Electron-Positron Collider ( Large Electron-Positron Collider, LEP) at CERN. These experiments truly became the swan song of the remarkable installation, in which the masses and lifetimes of heavy vector bosons were determined with unprecedented accuracy.

The standard model makes it possible to predict the channels of production and decay of the Higgs boson, but does not make it possible to calculate its mass (which, by the way, arises from its ability to self-interact). According to the most general estimates, it should not be less than 8–10 GeV and more than 1000 GeV. By the start of the LEP sessions, most physicists believed that the most likely range was 100–250 GeV. The LEP experiments raised the lower threshold to 114.4 GeV. Many experts believed and still believe that if this accelerator had worked longer and increased the energy of colliding beams by ten percent (which was technically possible), the Higgs boson would have been detected. However, CERN management did not want to delay the launch of the Large Hadron Collider, which was to be built in the same tunnel, and at the end of 2000 the LEP was closed.

Boson corral

Numerous experiments, one after another, ruled out possible mass ranges for the Higgs boson. At the LEP accelerator, the lower threshold was set at 114.4 GeV. At the Tevatron, masses exceeding 150 GeV were excluded. Later, the mass ranges were refined to the interval 115–135 GeV, and at CERN at the Large Hadron Collider the upper limit was shifted to 130 GeV. So the Standard Model Higgs boson, if it exists, is confined to fairly narrow mass boundaries.

The following search cycles were carried out on the Tevatron (on the CDF and DZero detectors) and on the LHC. As Dmitry Denisov, one of the leaders of the DZero collaboration, told PM, Tevatron began collecting statistics on Higgs in 2007: “Although there was enough energy, there were many difficulties. The collision of electrons and positrons is the “cleanest” way to catch Higgs, because these particles do not have an internal structure. For example, during the annihilation of a high-energy electron-positron pair, a Z 0 boson is born, which emits a Higgs without any background (however, in this case, even dirtier reactions are possible). We collided protons and antiprotons, loose particles consisting of quarks and gluons. So the main task is to distinguish the birth of the Higgs from the background of many similar reactions. The LHC teams have a similar problem.”

Traces of unseen beasts

There are four main ways (as physicists say, channels) of the birth of the Higgs boson.

The main channel is the fusion of gluons (gg) in the collision of protons and antiprotons, which interact through loops of heavy top quarks.
The second channel is the fusion of virtual vector bosons WW or ZZ (WZ), emitted and absorbed by quarks.
The third channel of Higgs boson production is the so-called associative production (together with the W- or Z-boson). This process is sometimes called Higgstrahlung(by analogy with the German term bremstrahlung- bremsstrahlung).
And finally, the fourth is the fusion of a top quark and an antiquark (associative creation together with top quarks, tt) from two top quark-antiquark pairs generated by gluons.

“In December 2011, new messages arrived from the LHC,” continues Dmitry Denisov. - They looked for Higgs decays either by top-quark and its antiquark, which annihilate and turn into a pair of gamma quanta, or into two Z 0 bosons, each of which decays into an electron and a positron or a muon and an antimuon. The data obtained suggest that the Higgs boson is pulling at about 124–126 GeV, but this is not enough to draw definitive conclusions. Now both our collaborations and physicists at CERN continue to analyze the results of the experiments. It is possible that we and they will soon come to new conclusions, which will be presented on March 4 at an international conference in the Italian Alps, and I have a feeling that we won’t be bored there.”

The Higgs boson and the end of the world

So, this year we can expect either the discovery of the Higgs boson of the Standard Model, or its, so to speak, cancellation. Of course, the second option will create the need for new physical models, but this can also happen in the first case! In any case, this is what one of the most authoritative experts in this field, professor at King's College London John Ellis, thinks. In his opinion, the discovery of a “light” (not more massive than 130 GeV) Higgs boson will create an unpleasant problem for cosmology. It will mean that our Universe is unstable and will someday (perhaps even at any moment) transition to a new state with less energy. Then the end of the world will happen - in the fullest meaning of the word. We can only hope that either the Higgs boson will not be found, or Ellis is mistaken, or the Universe will delay suicide a little.

From approximately 1000 seconds (for a free neutron) to a negligible fraction of a second (from 10 −24 to 10 −22 s for resonances).

The structure and behavior of elementary particles is studied by particle physics.

All elementary particles are subject to the principle of identity (all elementary particles of the same type in the Universe are completely identical in all their properties) and the principle of particle-wave dualism (each elementary particle corresponds to a de Broglie wave).

All elementary particles have the property of interconvertibility, which is a consequence of their interactions: strong, electromagnetic, weak, gravitational. Particle interactions cause transformations of particles and their collections into other particles and their collections, if such transformations are not prohibited by the laws of conservation of energy, momentum, angular momentum, electric charge, baryon charge, etc.

Main characteristics of elementary particles: lifetime, mass, spin, electric charge, magnetic moment, baryon charge, lepton charge, strangeness, isotopic spin, parity, charge parity, G-parity, CP-parity.

Classification

By lifetime

• Stable elementary particles are particles that have an infinitely long lifetime in a free state (proton, electron, neutrino, photon and their antiparticles).
• Unstable elementary particles are particles that decay into other particles in a free state in a finite time (all other particles).

By weight

All elementary particles are divided into two classes:

• Massless particles are particles with zero mass (photon, gluon).
• Particles with non-zero mass (all other particles).

By largest back

All elementary particles are divided into two classes:

By type of interaction

Elementary particles are divided into the following groups:

Compound particles

• Hadrons are particles that participate in all types of fundamental interactions. They consist of quarks and are divided, in turn, into:
  • mesons are hadrons with integer spin, that is, they are bosons;
  • baryons are hadrons with half-integer spin, that is, fermions. These, in particular, include the particles that make up the nucleus of an atom - proton and neutron.

Fundamental (structureless) particles

• Leptons are fermions that have the form of point particles (that is, not consisting of anything) up to scales of the order of 10 −18 m. They do not participate in strong interactions. Participation in electromagnetic interactions was experimentally observed only for charged leptons (electrons, muons, tau leptons) and was not observed for neutrinos. There are 6 known types of leptons.
• Quarks are fractionally charged particles that are part of hadrons. They were not observed in the free state (a confinement mechanism has been proposed to explain the absence of such observations). Like leptons, they are divided into 6 types and are considered structureless, however, unlike leptons, they participate in strong interactions.
• Gauge bosons are particles through the exchange of which interactions are carried out:
  • photon is a particle that carries electromagnetic interaction;
  • eight gluons - particles that carry the strong force;
  • three intermediate vector bosons W + , W− and Z 0, which tolerate weak interaction;
  • graviton is a hypothetical particle that carries the gravitational force. The existence of gravitons, although not yet experimentally proven due to the weakness of gravitational interaction, is considered quite probable; however, the graviton is not included in the Standard Model of elementary particles.

Sizes of elementary particles

Despite the wide variety of elementary particles, their sizes fit into two groups. The sizes of hadrons (both baryons and mesons) are about 10 −15 m, which is close to the average distance between the quarks included in them. The sizes of fundamental, structureless particles - gauge bosons, quarks and leptons - within the experimental error are consistent with their point nature (the upper limit of the diameter is about 10 −18 m) ( see explanation). If in further experiments the final sizes of these particles are not discovered, then this may indicate that the sizes of gauge bosons, quarks and leptons are close to the fundamental length (which very likely may turn out to be the Planck length equal to 1.6 10 −35 m) .

It should be noted, however, that the size of an elementary particle is a rather complex concept that is not always consistent with classical concepts. Firstly, the uncertainty principle does not allow one to strictly localize a physical particle. A wave packet, which represents a particle as a superposition of precisely localized quantum states, always has finite dimensions and a certain spatial structure, and the dimensions of the packet can be quite macroscopic - for example, an electron in an experiment with interference on two slits “feels” both slits of the interferometer, separated by a macroscopic distance . Secondly, a physical particle changes the structure of the vacuum around itself, creating a “coat” of short-term virtual particles - fermion-antifermion pairs (see Vacuum polarization) and bosons that carry interactions. The spatial dimensions of this region depend on the gauge charges possessed by the particle and on the masses of the intermediate bosons (the radius of the shell of massive virtual bosons is close to their Compton wavelength, which, in turn, is inversely proportional to their mass). Thus, the radius of an electron from the point of view of neutrinos (only weak interaction is possible between them) is approximately equal to the Compton wavelength of W-bosons, ~3 × 10 −18 m, and the dimensions of the region of strong interaction of the hadron are determined by the Compton wavelength of the lightest of hadrons, the pi-meson (~10 −15 m), acting here as a carrier of interaction.

Story

Initially, the term “elementary particle” meant something absolutely elementary, the first brick of matter. However, when hundreds of hadrons with similar properties were discovered in the 1950s and 1960s, it became clear that hadrons at least have internal degrees of freedom, that is, they are not elementary in the strict sense of the word. This suspicion was later confirmed when it turned out that hadrons consist of quarks.

Thus, physicists have moved a little deeper into the structure of matter: leptons and quarks are now considered the most elementary, point-like parts of matter. For them (together with gauge bosons) the term “ fundamental particles".

In string theory, which has been actively developed since about the mid-1980s, it is assumed that elementary particles and their interactions are consequences of various types of vibrations of especially small “strings”.

Standard model

The Standard Model of elementary particles includes 12 flavors of fermions, their corresponding antiparticles, as well as gauge bosons (photons, gluons, W- And Z-bosons), which carry interactions between particles, and the Higgs boson, discovered in 2012, which is responsible for the presence of inertial mass in particles. However, the Standard Model is largely viewed as a temporary theory rather than a truly fundamental one, since it does not include gravity and contains several dozen free parameters (particle masses, etc.), the values ​​of which do not follow directly from the theory. Perhaps there are elementary particles that are not described by the Standard Model - for example, such as the graviton (a particle that hypothetically carries gravitational forces) or supersymmetric partners of ordinary particles. In total, the model describes 61 particles.

Fermions

The 12 flavors of fermions are divided into 3 families (generations) of 4 particles each. Six of them are quarks. The other six are leptons, three of which are neutrinos, and the remaining three carry a unit negative charge: the electron, muon, and tau lepton.

Generations of particles

First generation	Second generation	Third generation
Electron: e−	Muon: μ −	Tau lepton: τ −
Electron neutrino: ν e	Muon neutrino: ν μ	Tau neutrino: ν τ (\displaystyle \nu _(\tau ))
u-quark (“up”): u	c-quark (“charmed”): c	t-quark (“true”): t
d-quark (“down”): d	s-quark (“strange”): s	b-quark (“lovely”): b

Antiparticles

There are also 12 fermionic antiparticles corresponding to the above twelve particles.

Antiparticles

First generation	Second generation	Third generation
positron: e+	Positive muon: μ +	Positive tau lepton: τ +
Electron antineutrino: ν ¯ e (\displaystyle (\bar (\nu ))_(e))	Muon antineutrino: ν ¯ μ (\displaystyle (\bar (\nu ))_(\mu ))	Tau antineutrino: ν ¯ τ (\displaystyle (\bar (\nu ))_(\tau ))
u-antique: u ¯ (\displaystyle (\bar (u)))	c-antique: c ¯ (\displaystyle (\bar (c)))	t-antique: t ¯ (\displaystyle (\bar (t)))
d-antique: d ¯ (\displaystyle (\bar (d)))	s-antique: s ¯ (\displaystyle (\bar (s)))	b-antique: b ¯ (\displaystyle (\bar (b)))

Quarks

Quarks and antiquarks have never been discovered in a free state - this is explained by the phenomenon