Strong nuclear interaction

Explanatory table of the 4 fundamental forces.

Nucleus of a helium atom. The two protons have the same burden, but they still remain together due to the residual nuclear force.

The strong nuclear force is one of the four fundamental forces that the Standard Model of particle physics establishes to explain the forces between known particles. This force is responsible for keeping together the nucleons (protons and neutrons) that coexist in the atomic nucleus, defeating the electromagnetic repulsion between the protons that have an electrical charge of the same sign (positive) and making the neutrons, which do not have electric charge, remain bound to each other and also to the protons.

It is the greatest existing force in the entire universe, there is no force comparable to the strong nuclear interaction; for this is what gives the existence of the entire universe as a whole, in addition to the weak nuclear interaction, electromagnetism and gravity.

The effects of this force are only seen at very small distances, the size of atomic nuclei, and are not perceived at distances greater than 1 fm. This feature is known as short-range, as opposed to long-range features such as gravity or electromagnetic interaction, which are strictly of infinite range.

The strong interaction is observable at two ranges and is mediated by two force carriers. On a larger scale (about 1 to 3 femtometers), it is the force (carried by mesons) that binds protons and neutrons (nucleons) to form the nucleus of an atom. On the smallest scale (less than about 0.8 fm, the radius of a nucleon), it is the force (carried by gluons that holds quarks together to form protons, neutrons, and other hadronic particles. In the latter context, is often known as the color force. The strong force inherently has such a high force that the hadrons bound together by the strong force can produce new massive particles. Thus, if the hadrons are struck by particles of high energy, they give rise to new hadrons instead of emitting freely moving (gluon) radiation.This property of the strong force is called color confinement, and it prevents the free "emission" of the strong force: instead, in practice, massive particle jets are produced.

In the context of atomic nuclei, the same strong interaction force (that binds quarks within a nucleon) also binds protons and neutrons to form a nucleus. In this sense, it is called the nuclear force (or residual strong force). So the residue of the strong interaction within the protons and neutrons also binds the nuclei together. As such, the residual strong interaction obeys distance-dependent behavior between nucleons that is quite different from when it acts to bind nucleons. the quarks inside the nucleons. In addition, there are distinctions in the binding energies of the nuclear force of nuclear fusion versus nuclear fission. Nuclear fusion accounts for most of the energy production in the Sun and other stars. Nuclear fission allows the decay of radioactive elements and isotopes, although it is usually mediated by the weak interaction. Artificially, the energy associated with the nuclear force is partially released in nuclear power and nuclear weapons, both fission weapons based on uranium or plutonium and fusion weapons, such as the hydrogen bomb.

The strong interaction is mediated by the exchange of massless particles called gluons that act between quarks, antiquarks, and other gluons. Gluons are thought to interact with quarks and other gluons through a type of charge called a color charge. Color charge is analogous to electromagnetic charge, but it comes in three types (±red, ±green, ±blue) instead of one, resulting in a different type of force, with different rules of behavior. These rules are detailed in the theory of quantum chromodynamics (QCD), which is the theory of quark-gluon interactions.

History

Before the 1970s, physicists weren't sure how the atomic nucleus came together. It was known that the nucleus was made up of protons and neutrons, and that protons had a positive electrical charge, while neutrons were electrically neutral. According to the physics knowledge of the time, positive charges repel each other and positively charged protons should cause the nucleus to be ejected. However, this was never observed. A new physics was needed to explain this phenomenon.

A stronger attractive force was postulated to explain how the atomic nucleus was held together despite the mutual electromagnetic repulsion of the protons. This hypothetical force was called the strong force, which was believed to be a fundamental force acting on the protons and neutrons that make up the nucleus.

Later it was discovered that protons and neutrons were not fundamental particles, but were made up of constituent particles called quarks. The strong attraction between the nucleons was the side effect of a more fundamental force that bound the quarks into protons and neutrons. The theory of quantum chromodynamics explains that quarks carry what is called a color charge, although it has no relation to the visible color. Quarks with different color charges attract each other as a result of the strong interaction, and the The particle that mediates this is called a gluon.

Introduction

Forces in the atomic nucleus

Before the 1970s, the proton and neutron were assumed to be fundamental particles. Then the expression strong force or strong nuclear force referred to what today is called nuclear force or residual strong force. This residual strong force is responsible for the cohesion of the nucleus and today it is interpreted as the force field associated with pions emitted by protons, neutrons and other hadrons (whether baryons or mesons). According to quantum chromodynamics, the existence of that pion field that holds the atomic nucleus together is only a residual effect of the true strong force acting on the inner components of hadrons, the quarks. The forces that hold quarks together are much stronger than those that hold neutrons and protons together. In fact, the forces between quarks are due to gluons and are so strong that they produce the so-called confinement of color that makes it impossible to observe naked quarks at ordinary temperatures, while in heavy nuclei it is possible to separate some protons or neutrons by nuclear fission or bombardment with fast particles of the atomic nucleus.

Historically, the strong nuclear force was theoretically postulated to compensate for the repulsive electromagnetic forces that were known to exist inside the nucleus when it was discovered that it was made up of protons with a positive electric charge and neutrons with zero electric charge. It was also postulated that its range could not be greater than the radius of the nucleus itself so that other nearby nuclei would not feel it, since if it had a greater range, all the nuclei in the universe would have collapsed to form a large conglomerate of nuclear mass. For that reason it was called at that time strong force. Yukawa's (1935) model satisfactorily explained many aspects of the strong nuclear force or residual strong force.

Quark model

Quarks structure of a proton.

After the discovery of a large number of hadrons that did not seem to play any fundamental role in the constitution of atomic nuclei, the term particle zoo was coined, given the wild profusion of different types of particles whose existence was not well understood.

Many of these particles appeared to interact through a type of interaction similar to the strong force, so schemes were sought to understand such diversity of particles. One model postulated to explain the existence of all the great variety of baryons and mesons was the quark model of 1963. This model postulated that the experimentally found hadrons and mesons were in fact combinations of more elementary quarks. Subsequent experiments at higher energies showed that the baryons themselves did not appear to be elemental and seemed to be made up of parts held together by some kind of misunderstood interaction. These discoveries could finally be interpreted naturally in terms of quarks.

The acceptance of quarks as constituents of hadrons made it possible to reduce the variety contained in the zoo of particles to a much smaller number of elementary constituents, but it opened up the problem of how those more elementary constituents joined together to form neutrons, protons and other hadrons. Since that force had to be very strong and the term "strong force" or "strong interaction" began to be used instead of "strong nuclear force" since the strong interaction appeared in contexts other than the atomic nucleus. Theoretical attempts to understand the interactions between quarks led to quantum chromodynamics, a theory of the strong force that describes the interaction of quarks with a field of gluons, which is what actually forms protons and neutrons (which definitely ceased to be considered as elementary particles). For some time thereafter, what had previously been called strong force was called residual strong force, calling the new strong interaction color force.

Nucleon-pion interaction

Pion-nucleon interaction and its simplest quark model

The need to introduce the concept of strong interactions arose in the 1930s, when it became clear that neither the gravitational phenomenon nor the electromagnetic interaction phenomenon could answer the question of what binds nucleons in nuclei. In 1935, the Japanese physicist H. Yukawa constructed the first quantitative theory of the interaction that causes the exchange of nucleons for new particles that are now known as pi mesons (or pions). Pyonies were later discovered experimentally in 1947.

In this "pion-nucleon theory", the attraction or repulsion of two nucleons was described as the emission of a pion by one nucleon and its subsequent absorption by another nucleon (by analogy with electromagnetic interaction, which is described as the exchange of a virtual photon). This theory has successfully described a wide range of phenomena in nucleon-nucleon collisions and bound states, as well as pion-nucleon collisions. The numerical coefficient, which determines the "efficiency" of the pion emission, turned out to be very large (compared to the analogous coefficient for the electromagnetic interaction), which determines the "strength" of the strong interaction.

A consequence of the pion-nucleon interaction between nucleons is the presence of an exchange component in the nuclear forces, along with the usual forces (Wigner forces, which arise as a result of the exchange of neutral pions). If the state of two interacting nucleons depends on their spatial and spin coordinates, then there are three different forms of such an exchange:

• The nucleons exchange spatial coordinates with constant spinal variables. The forces caused by such exchange are called Majorana forces (exchange of loaded pions while the spin of nucleons is maintained);
• The nucleons exchange spinal variables in constant spatial coordinates. The forces between nucleons arising from this method of exchange are called strength of Bartlett (exchange of neutral pyons);
• The nucleons exchange spatial and spinal coordinates simultaneously. The resulting exchange forces are called strength of Heisenberg (exchange of loaded pions with a change in the spin of nucleons).

In addition, nuclear forces depend on charge coordinates and have a tensor component.

The potential energy operator in the phenomenological description of the nuclear interaction of two nucleons at low energies has the form:

V(r1→ → ,r2→ → ,σ σ 1^ ^ ,σ σ 2^ ^ ,Δ Δ 1^ ^ ,Δ Δ 2^ ^ )=U1(r→ → )+U2(r→ → )(σ σ 1^ ^ σ σ 2^ ^ )+U3(r→ → )S12+(Δ Δ 1^ ^ Δ Δ 2^ ^ ){U4(r→ → )+U5(r→ → )(σ σ 1^ ^ σ σ 2^ ^ )+U6(r→ → )S12!{cHFFFFFF}{cHFFFFFF}{cHFFFFFF}{cHFFFFFF}{cHFFFFFF}{cHFFFFFF}{cHFFFFFF}{cHFFFFFFFF}{cHFFFFFF}{cHFFFF}{cHFFFF}{cHFFFFFFFF}{cH00}{cH00}{cH00}{cH00}{cH00}{cH00}{cH00}{cH00}{cHFFFFFFFFFFFFFFFFFF}{cHFFFFFFFFFFFFFFFFFF}{cH00}{cH00}{cH00}{cH00}{cHFFFFFF}{cHFFFFFFFFFFFFFFFFFF}{cH00}{cH00}{cH00}{cH00}{cH00}{cH00}{,

where r=r1− − r2{displaystyle r=r_{1}-r_{2}}}, r1→ → ,r2→ → {displaystyle {vec {r_{1}}},{vec {r_{2}}}}}} They're space coordinates. σ σ 1^ ^ ,σ σ 2^ ^ {displaystyle {hat {sigma _{1}}}},{hat {sigma _{2}}}}}} are Pauli operators and Δ Δ 1^ ^ ,Δ Δ 2^ ^ {displaystyle {hat {tau}}},{hat {tau _{2}}}}} are operators of isotopian spine.

Majorana forces (interchange of space coordinates) correspond to the term with (σ σ 1^ ^ σ σ 2^ ^ )(Δ Δ 1^ ^ Δ Δ 2^ ^ ){displaystyle ({hat {sigma _{1}}}{hat {sigma _{2}}}})({hat {tau _{1}}}{hat {tau _{2}}}}})} Bartlett forces (exchange of spinal variables) correspond to the term with (σ σ 1^ ^ σ σ 2^ ^ ){displaystyle ({hat {sigma _{1}}}{hat {sigma _{2}}}}}}}Heisenberg forces (interchange of space and spinal variables) correspond to the term with (Δ Δ 1^ ^ Δ Δ 2^ ^ ){displaystyle ({hat {tau _{1}}}{hat {tau _{2}}}}}}}... In addition, the operator S12{displaystyle S_{12}} takes into account the tensorial interaction, (Δ Δ 1^ ^ Δ Δ 2^ ^ )S12{displaystyle ({hat {tau}}{hat {tau _{2}}}}} is the interplay of tensorial exchange.

Quantum Chromodynamics

Currently the strong interaction is considered to be well explained by quantum chromodynamics (QCD for Quantum Chromodynamics). Quantum chromodynamics is a theory that is part of the Standard Model of particle physics and mathematically it is a non-Abelian gauge theory based on an internal symmetry group (gauge) based on the SU(3) group. According to this theory, the dynamics of quarks is given by a Lagrangian that is invariant under transformations of the SU(3) group; this invariance by Noether's theorem entails the existence of conserved magnitudes or special conservation laws. Specifically, the invariance of this Lagrangian under SU(3) implies the existence of certain color charges, somewhat analogous to the conservation of electric charge (which is associated with the invariance under the U(1) group). Quantum chromodynamics therefore describes the interaction of objects that have a color charge, and how the existence of these color charges implies the existence of an associated gauge field (gluon field) that defines how these color charged particles interact.

Quantum chromodynamics as gauge theory implies that for there to be local gauge invariance, there must exist a field associated with symmetry, which is the gluon field. The quarks, color charge carriers, interact with each other by exchanging gluons, which is what causes them to be linked to each other. In turn, the gluons themselves have a color charge, so they interact with each other. In addition, quantum chromodynamics explains that there are two types of hadrons: baryons (formed by three quarks each with different color charges) and mesons (formed by two conjugated quarks with opposite color charges). All hadrons, made up of quarks, interact with each other through the strong force (although they can interact weakly, electromagnetically and gravitationally). The intensity of the strong interaction is given by a characteristic coupling constant, much higher than those associated with electromagnetic and gravitational interaction. Therefore, quantum chromodynamics explains both the cohesion of the atomic nucleus and the integrity of hadrons through one of the "color-associated forces" of quarks and antiquarks. Quarks and antiquarks, in addition to the other characteristics attributed to the rest of the particles, are assigned a new characteristic, the "color charge", and the strong interaction between them is transmitted by other particles, called gluons. These gluons are electrically neutral, but they have "color charge" and are therefore also subject to the strong force. The force between colored charged particles is very strong, much stronger than the electromagnetic or gravitational forces, to such an extent that color confinement occurs.

Colour Loading

Quarks, antiquarks, and gluons are the only fundamental particles that contain non-zero color charge, and therefore participate in strong interactions. Gluons, particles that carry the strong nuclear force, which hold quarks together to form other particles, as explained, also have a color charge and therefore can interact with each other. An effect that would derive from this is the theoretical existence of groups of gluons (globules). Quarks can have six types of charge: red, blue, green, antired, antiblue, and antigreen. The anti-red, anti-blue, and anti-green charges are related to the corresponding red, blue, and green charges in a similar way to the negative and positive electrical charges. Gluons for their part have a more complex type of charge, their charge is always the combination of a different color or anticolor (for example, you can have a red-antiblue gluon or a green-antired gluon, etc.)

Strong Nuclear Force as Residual Force

The force that makes the constituents of an atom's nucleus stick together is associated with the strong nuclear interaction. Although today we know that this force that holds protons and neutrons together in the nucleus is a residual force from the interaction between the quarks and gluons that make up these particles (up and down). It would be similar to the effect of the bonding forces that appear between atoms to form molecules, as opposed to the electrical interaction between the electrical charges that form those atoms (protons and electrons), but its nature is totally different.

^{Before quantum chromodynamics, this residual force holding the protons in the nucleus together was considered to be the essence of the strong nuclear interaction, although the force holding the protons together is now assumed to be a secondary effect. of the color force between quarks, so the interactions between quarks are considered a more fundamental reflection of the strong force.}

The strong nuclear force between nucleons is realized by pions, which are mass bosons, and for this reason this force has such a short range. Each neutron or proton can "emit" and "absorb" charged or neutral pions, the emission of charged pions involves the transmutation of a proton into a neutron or vice versa (in fact, in terms of quarks this interaction is due to the creation of a quark pair -antiquark, the charged pion will be nothing more than a bound state of one of the original quarks and one more quark or antiquark of those that have just been created). The emission or absorption of charged pions respond to one of the following two interactions:

p++n0→ → (n0+π π +)+n0→ → n0+(π π ++n0)→ → n0+p+{displaystyle p^{+}+n^{0}to (n^{0}+pi ^{+})+n^{0}to n^{0}+(pi ^{+}+n^{0})to n^{0}

n0+p+→ → (p++π π − − )+p+→ → p++(π π − − +p+)→ → p++n0{displaystyle n^{0}+p^{+}to (p^{+}+pi ^{-})+p^{+}to p^{+}+(pi ^{-}+p^{+})to p^{+}

In the first reaction above a proton initially emits a positive pion becoming a neutron, the positive pion is reabsorbed by a neutron becoming a proton, the net effect of that exchange is an attractive force. In the second, a neutron emits a negative pion and becomes a proton, the negative pion being reabsorbed by another proton gives rise to a neutron. From a semiclassical point of view, the pion field can be approximated by a Yukawa potential:

V(r)=− − gs4π π re− − mrc {displaystyle V(r)=-{frac {g_{s}}{4pi r}e^{-{frac {mrc}{hbar }}}}}}}}

Where:

gs{displaystyle g_{s},}, it is the constant of enforcing that gives the intensity of the effective force.

m{displaystyle m,}It's the pion mass exchanged.

r{displaystyle r,}It's the distance between nucleons.

c, {displaystyle c,hbar ,}, are the speed of light and the constant of rationalized Planck.

So at very small distances the interaction decays approximately according to the inverse of the square, however, at distances of the order of the atomic nucleus, exponential decay predominates, so that at distances greater than atomic distances the effect of pions is practically imperceptible.

Strong interactions in high energy reactions

There are a number of high-energy hadron collision processes where there is no hard scale, so the computation of perturbation theory in the framework of quantum chromodynamics is no longer reliable. Among these reactions are total hadron collision cross sections, elastic scattering of hadrons at small angles, and diffraction processes. From the point of view of kinematics, in such reactions, only the total energy of the colliding particles in their rest frame is large enough, but not the momentum transferred.

Since the 1960s, the main properties of such reactions have been successfully described using a phenomenological approach based on Regge theory. In the framework of this theory, the scattering of high-energy hadrons occurs due to the exchange of some composite objects - reggeons. The most important reggeon in this theory is pomeron, the only reggeon whose contribution to the scattering cross section does not decrease with energy.

In the 1970s, it turned out that many of the properties of reggeons can be derived from quantum chromodynamics. The corresponding approach in quantum chromodynamics is called the Balitsky-Fadin-Kuraev-Lipatov (BFKL) approach.

Current state of the theory of strong interactions

The theoretical description of strong interactions is one of the most developed and at the same time rapidly developing areas of theoretical elementary particle physics. Although the fundamental nature of strong interactions (the color interaction between quarks and gluons, described by quantum chromodynamics) is understood, the mathematical laws that express it are very complex and therefore, in many specific cases, calculations from first principles are still impossible. As a result, an eclectic picture emerges: alongside mathematically rigorous calculations, semi-quantitative approaches based on quantum mechanics, intuitions, which nevertheless perfectly describe the experimental data.

Let's outline the general structure of the modern theory of strong interactions: First, quantum chromodynamics is the basis of the theory of strong interactions. In this theory, the fundamental degrees of freedom are quarks and gluons, the Lagrangian of their interaction is known. Approaches to describe strong interactions essentially depend on the type of object being studied. The following main groups can be distinguished:

• The hard hadronic reactionsin which quarks and gluons play the main role and are well described by the theory of disturbance in QCD;
• The semi-rigid reactionsin which for a reasonable description it is necessary to take into account an infinite number of terms of the series of the theory of disturbances, and in certain cases limits this can be done.
• The low-energy hadronic reactions (suaves)in which the linked states of the quarks (hadrones) become more reasonable degrees of freedom and the laws of interaction are studied.
• The static properties of hadronsin which, depending on the specific case, different approaches can be used.