First of all, there are really six types or “flavors” of quarks, as they are normally called: up, down, top, bottom, strange, and charm quarks. They are a part of the fundamental or elementary particles in the Standard Model, a table of 16 elements which includes their mass, spin, electromagnetic force, and name.
The only force that quarks have which is not represented by the Standard Model is color charge, which will be explained subsequently. Quarks are classified in the top left corner of the table, and they belong to a family of particles called fermions.
They are actually classified into three generations or pairs, which are the first generation up and down quarks, the second generation strange and charm quarks, and the third generation top and bottom quarks. Following the discovery of these, there have been efforts to find a fourth generation, but at this time, all efforts have failed. One of the qualities that puts the quark in the fermions category is its spin because all quarks are spin-1/2 particles, which classifies them as fermions according to the spin statistics theorem.
Quarks are the only particles that have all of the fundamental interactions or forces, which include the aforementioned electromagnetism, gravity, strong interaction, and weak interaction. Above all, however, the main characteristic of quarks is that they are the only particles which do not have whole number, that is to say, whole-number multiples, of electric charges.
Each of the quarks is also able to change into the other quarks in its generation by a process called particle decay, which is where the mass of one of the quarks changes, thus transforming it into its partnering quark. Of course, everything must have an opposite, and the same is true with quarks because there are also anti-quarks which are almost identical to regular quarks but have the opposite electric charge.
The combinations that quarks and anti-quarks produce are either called hadrons, mesons or heavy particles depending on their constituents. Protons and neutrons, the most stable hadrons, are formed of 2 up quarks and 1 down quark, or 1 up quark and 2 down quarks, respectively.
Baryons are made up of 3 quarks, therefore the anti-baryon is made up of 3 anti-quarks, and mesons are made of one regular quark and one anti-quark, normally of the same flavor as the regular one. There have also been experiments to determine if there might be other “exotic” hadrons which would have up to four or five valence quarks, but to this day, there has been no evidence of their existence.
History of the Quark:
In the beginning, no one knew that quarks really existed. They merely came into being as a term for explaining something they did not know of. At that time, the atom menagerie, that is to say the known atoms at the time, consisted mostly of what we now call hadrons, so they would have been atoms similar to the proton and neutron.
Then came along Murray Gell-Mann and his colleague George Zweig, who proposed that the atoms they knew of were not the smallest that existed, but instead, each of these was made up of smaller ones. The term they used to explain these smaller atoms were “up,” “down,” and “strange” quarks, although they did not have any evidence of their existence.
The correct term for what they used quarks for would be an “abstract representation.” The year after this proposal, Sheldon Lee Glashow and James Bjorken suggested a fourth type of quark called “charm” to better explain the weak interaction between the atom, causing atom decay.
The discovery of this new quark made it so that the number of quarks they knew of equaled the number of leptons they knew existed, and it led to a formula for calculating mass, which accurately reproduced the same masses as those of the known mesons.
Then in 1961, Murray Gell-Mann and George Zweig came back with an atom classification system for quarks called the Eightfold Way, which demonstrated symmetry, and once again, in 1964, to demonstrate their new quark model. But up to this point, there was no real evidence that quarks existed.
It was only four years later, in a deep inelastic scattering experiment at SLAC, that the discovery was made that the proton was made up of minuscule point-like objects, therefore making the proton disappear from the list of simple atoms. At the time of the discovery, and still used from time to time today, these were called “partons.”
Subsequently, they realized that they were actually the fabled up and down quarks. The rest of the quarks were discovered one at a time until the last quark, the top quark, was discovered in 1995. The team that discovered this atom was hugely surprised at its mass because it was so large. It is said to have a mass “almost as great as a gold atom.”
Standard Model: The Standard Model is composed of (in order from left to right in rows of four): up quark (u), charm quark (c), top quark (t), photon (γ), down quark (d), strange quark (s), bottom quark (b), gluon (g), electron neutrino (νe), muon neutrino (νμ), tau neutrino (ντ), Z boson (Z0), electron (e), muon (μ), tau (τ), W boson (W±). Plus the Higgs boson (H0), which was recently discovered but is not included in the model. There are also the antiparticles, which, though they are not on this table, are the same elements, indicated by the symbol with a bar on top of it.
Quark properties: Electric charge: This comes in positive and negative and is present in every sort of matter because it is one of the constituents that holds matter together. Positively charged atoms are attracted to negatively charged atoms and vice versa, but they are repelled by other positively charged atoms.
The electric charge of a substance is normally referred to or measured in coulombs (C), but when working with atoms, the usual electric measuring is in simple charge designated by the symbol e. One e is equal to 1.602 x 10^-19. One proton has an electric charge of e, and an electron has an electric charge of -e. However, if we delve into quarks, the electric charge is measured in 1/3e.
The up, charm, and top quarks have a charge of +2/3e, while down, strange, and bottom quarks have a charge of -1/3e. Their corresponding anti-quarks have the same amount of electric charge but the opposite charge. In hadrons, the total charge will always be a whole number charge, such as in protons and neutrons, which have +1 and 0 respectively, and are normally made up of 3 quarks (heavy particles), 3 anti-quarks (antibaryon), or a quark and an anti-quark, which always have a whole number charge (mesons).
Spin: Spin is an intrinsic property of elementary particles, meaning that it is independent of the amount of matter there is or any other obstructions. So spin is wholly free of any obstructions, but its direction gives the particle an important degree of freedom, which determines the substance’s state.
However, the spin of a particle is not like the Earth spinning around the Sun but rather like the Earth’s wobbling on its own axis at 23.5°. Spin is, therefore, a vector quantity, which has velocity and direction, and it determines the amount of electromagnetism the particle will have depending on how fast it is spinning.
Quarks have the lowest degree of spin (for it is not measured in distance per time but rather degree per time) which is equivalent to + or – h/2 where h is the reduced Planck constant, the smallest amount possible. This is why quarks, protons, neutrons, and other fermions are referred to as spin-1/2 particles.
These types of particles must also obey the Pauli Exclusion Principle, which dictates that no two fermions, primarily electrons, can occupy the same state or space at the same time.
Weak interaction: It is true that quarks can change from an up-type quark (up, charm, or top) to any down-type quark (down, strange, or bottom), but this can only happen because of one of the four fundamental interactions, weak interaction.
Weak interaction is where a particle, like the quarks, absorb a W-boson to move from down to up or releasing a W-boson to move from up to down. This is, in turn, possible because the quarks are named after their mass and the way they react, so by adding a W-boson, the mass goes up, and the particle adheres to the particle, making it behave slightly differently.
The emission or response of a W-boson is a procedure called beta decay, which is a radioactive procedure that typically “splits” a neutron into a proton, an electron, and an electron neutrino.
This happens because one of the down-type quarks in the neutron emits a virtual W-boson, which means an atom that only exists for a limited amount of time and space, and then turns the down quark into an up-type, transforming the neutron (udd) into a proton (udu).
The W-boson then veers off and becomes an electron (e-) by absorbing an electron antineutrino (Ve). Now, while any of the up-type quarks can turn into any down-type quarks, they do prefer to change into their corresponding quark, which is the other quark in their generation.
There is also another weak interaction graph/matrix made for the leptons (right side of the table), which can explain all the connection between the quark spirits, but the links between the two graphs are not clear yet.
Strong interaction and color charge: Quarks are said to each have a color charge, and there are three types of color charges labeled red, green, and blue. The corresponding anti-quarks have colors of their own, which are the anti-colors or antigreen, antired, and antiblue (antigreen = magenta, antired = baby blue, antiblue = yellow).
These atoms are able to move in this manner because of a mediator atom called gluons, which are force-transporting atoms carrying the color force. Therefore, when a quark and an anti-quark combine, the resulting color charge has a value of 0. The color force or strong interaction is explained better with the special unitary groups, but these are too complicated for my understanding.
Mass: When calculating the mass of quarks, there are two different masses to take into account. There is the “current quark mass,” which is the mass of the quark by itself, and then there is the constituent quark mass, which is the combined mass of the quark and the gluon field that surrounds it.
It is interesting to know that, even though gluons are practically massless, they are the main body of the quark’s mass. This is because of the amount of energy they carry. So while the mass of the three quarks of a proton is only 11 MeV/c², the mass of the hadron is about 938 MeV/c².
The name for the type of energy the gluons possess is called quantum chromo kinetic binding energy or QCBE. The Standard Model derives the masses of these atoms with aid from the Higgs mechanism, which is in relation to the Higgs boson.
Other properties:
Total angular momentum: Total angular momentum is a parameter that includes both the orbital angular momentum of a particle combined with its intrinsic angular momentum, otherwise known as its spin. The total angular momentum is measured by a quantum number (J), which is used very frequently in the field of physics.
Baryon number: The baryon number of a particle is the remaining spin quantum number left when the quarks and anti-quarks in the hadron are counted. For example, a meson will have a baryon number of 0, a baryon will have +1, and an antibaryon will have -1. Other more exotic hadrons could also be considered to be baryons depending on their baryon number.
Isospin: Interestingly, the isospin is a number that does not have either spin or angular momentum. Isospin is named so because of the resemblance of its mathematical expression to that of the expression for the spin of particles. It is a number that enables individuals to understand the different possibilities of spin for that type of particle. For example, the isospin number for a proton or a neutron would be I3=+1/2 or -1/2 because protons and neutrons are almost exactly alike in every aspect except their charge.
Charm: The charm number is simply a number that represents the difference between the number of charm quarks and the number of anti-charm quarks.
Strangeness: Strangeness is a number that indicates the difference between the number of strange quarks versus the number of anti-strange quarks.
Topness: Topness is a number that indicates the difference between the number of top quarks and anti-top quarks.
Bottomness: Bottomness is a number that indicates the difference between the number of bottom quarks and anti-bottom quarks. *Note that for charm, topness, bottomness, and strangeness numbers are seldom used because the regular strange, bottom, charm or top quarks have a +1 value and their corresponding anti-quarks have a -1 value.
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