All classes based on their internal composition: mesons
All particles can be classified into two broad categories: leptons and hadrons. The main difference between the two is whether they interact through the strong interaction. Hadrons are particles that interact through all four fundamental interactions of nature, which include, strong, electromagnetic, weak, and gravitational interactions.
Hadrons, the strongly interacting particles, can be further subdivided into two classes based on their internal composition: mesons and baryons. Originally, mesons and baryons were classified according to their masses. Baryons were heavier than mesons, and both were heavier than leptons. Today mesons and baryons are distinguished by their internal structure. Baryons have masses greater than the proton mass.
All hadrons are composed of two or three fundamental particles, which came to be known as quarks. A quark is always combined with one or two other quarks. According to the original model proposed by Murray Gell-Mann and George Zweig in 1963, there were three types of quarks indicated by the symbols u, d, and s. These were given the arbitrary names up, down, and sideways (now referred to as strange). Associated with each quark is an anti-quark, which are the antimatter equivalents of quarks, opposite in electric charge.
Later evidence allowed theorists to propose the existence of several more quarks: charm(c), top (t), and bottom (b). These six quarks species are paired with their flavors: up and down, top and bottom, and charm and strange.A baryon is a “heavy” subatomic particle having strong interactions (a hadron) which either is a nucleon or can transform or decays into a final state of stable particles including a single nucleon plus eventually some additional electrons, photons, neutrinos and/or nucleonanti-nucleon pairs. This definition is only suitable if the quality characterizing a baryon is conserved in all involved reactions or decays and this is made apparent introducing a baryon number, which is +1 for a baryon and -1 for an anti-baryon. Baryons also include hyperons, which are created in particle accelerators.All observed baryons can be described as quark compounds containing three quarks or, for anti-baryons, of three anti-quarks.
Some examples of baryons are protons which contains three quarks (u, u, and d) and neutrons, which also contain three quarks (u, d, and d). From the properties of quarks, it follows that all baryons made in this way have integral electric charge, half integer spin and baryon number +1 (quarks have baryon number +1/3). The possible combinations of higher baryon number are identified with the atomic nuclei, but it is not excluded that they could exist as bound quark states too. The proton has charges of +2/3e, +2/3e, and 1/3e. The total charge of the proton is +e. The neutron has charges of +2/3e, -1/3e, and 1/3e.
The total charge of quarks in a neutron is zero. The rule you need to remember for baryons is that they are always made from 3 quarks. Each quark has to have a different color charge from the others inside the baryon. The three-color charges are labeled in accordance with the three primary colors RED, GREEN, and BLUE.
The demand that a baryon must have three quarks, one RED, one GREEN, and one BLUE, makes the baryon have an overall white color. This is a null color. Hence, the color charge of the baryon is zero. Scientists have discovered four kinds of so-called long-lived hyperons, which last longer than one-thousandth of one-billionth of a second. They named these hyperons lambda, sigma, xi, and omega. The quarks neutron and lambda have the same electric charge and spin. Protons and anti-protons have the same mass and spin.
Scientists have also found shorter-lived hyperons.The term baryon comes from the Greek word for heavy. The lightest baryon, the proton, has a mass 1,836 times that of an electron.
The neutron’s mass is slightly higher, at 1,839 times the electron mass. All baryons, except protons, naturally decay (break down) into two or more other particles. For example, when removed from the nucleus, a neutron decays into a proton, an electron, and a neutrino.
When a baryon decays, it produces another, lighter baryon. Physicists call this principle the law of conservation of baryons. According to this law, a proton cannot decay because there is no baryon lighter than a proton. Protons and neutrons are the main building blocks of atoms. If protons decayed, they could not form lasting atoms. The fact that protons do not decay keeps the objects in our world from collapsing.
An important property of baryons is it mass. In general, only a small part of the mass of a hadron (such as a proton) is due to the rest mass of the quarks in it. Some of the mass of a hadron comes from the kinetic energy of the quarks due to confinement. The volume of a hadron is small. By the Heisenberg Uncertainty Principle, the kinetic energy of the quark is inversely proportional to the radius of its confinement. This energy contributes to the mass of the hadron (baryon).The Standard Model is the combination of two theories of particle physics into a single framework to describe all interactions of subatomic particles, except those due to gravity.
The two components of the standard model are electroweak theory, which describes interactions with the electromagnetic and weak forces, and quantum chromodynamics, the theory of the strong nuclear force. Both these theories are gauge field theories, which describe the interactions between particles in terms of the exchange of intermediary “messenger” particles that have one unit of intrinsic angular momentum, or spin. In addition to these force-carrying particles, the standard model includes two families of subatomic particles that build up matter and that have spins of one-half unit. These particles are the quarks and the leptons, and there are six varieties, or “flavors,” of each, related in pairs in three “generations” of increasing mass. Everyday matter is built from the members of the lightest generation such as the “up” and “down” quarks that make up the protons and neutrons of atomic nuclei. Heavier types of quark and lepton have been discovered in studies of high-energy particle interactions, both at scientific laboratories with particle accelerators and in the natural reactions of high-energy cosmic-ray particles in the atmosphere.
The standard model has proved a highly successful framework for predicting the interactions of quarks and leptons with great accuracy. Yet it has a number of weaknesses that lead physicists to search for a more complete theory of subatomic particles and their interactions. The present standard model, for example, cannot explain why there are three generations of quarks and leptons. It makes no predictions of the masses of the quarks and the leptons neither of the strengths of the various interactions. Physicists hope that, by probing the standard model in detail and making highly accurate measurements, they will discover some way in which the model begins to break down and thereby find a more complete theory.
This may prove to be what is known as a grand unified theory, which uses a single theoretical structure to describe the strong, weak, and electromagnetic forces. We are seeing only the conceptual results of this very mathematical theory, but we should realize that is it based on experimental evidence.Bibliography: