There are two types of stable particles present in an atom’s nucleus. They are called protons and neutrons. The fundamental and elementary particles which constitute protons are known as quarks. They are never found in isolation. Quarks are unstable in their elementary form. They combine to form hadrons. The hadrons are stable. The other elementary particles inside the protons and neutrons include the baryons and mesons which are found in the quark-gluon plasma.
Quarks are invariably found in pairs with their antiparticles. The antiparticles are known as antiquarks. Antiquarks combine with each other to form hadrons which in turn, combine to form subatomic particles such as mesons and baryons. So far, quarks are the elementary particles which are not broken down to other smaller particles. Quarks don’t have a structure. In particle physics, they are the basic elementary particles which exist in nature.
The quark model came into existence in 1964. The credit goes to two physicists from the USA, Murray Gell-Man and George Zweig. George named the particles as ‘aces’. As per the Gell-Mann’s model, mesons were made of quark and its antiquark. On the other hand, baryons were made of three quarks. So according to the model, there are six types of quarks which are found in three pairs. Rather, there are six ‘flavors’ of quarks.
Up and down, top and bottom and charm and strange are the six flavors. The up and down quarks are the lightest particles. Whereas, the four other heavier particles tend to convert rapidly to up and down quarks. There are plenty of up and down quarks in the universe. They are stable. Other elementary particles including charm, strange, top and bottom quarks are produced as a result of high energy collisions such as cosmic rays and with the help of particle accelerators like cyclotrons.
In quarks, It is very rare that electric charges on the particles are not an integer. While the electric charge on up, charm, and top quarks are equal to ⅔ e (positive fractional charge), the electrical charge on down, strange and bottom quarks is equal to – ⅓ e (negative fractional charge). Letter ‘e’ denotes the value of charge on a single electron without the sign. All quarks have antiparticles. It should be noted that the electrical charges on the antiparticles are contrary to their corresponding quarks.
The up antiquark coexists with its up quark. It has an electric charge of – ⅔ e. Protons and neutrons in an atom’s nucleus are made by these particles. A proton contains two up quarks and one down quark. Meanwhile, a neutron has one up quark and two down quarks. As a result, protons have +1 e charge and neutrons have 0 e charge on it.
Quarks are trivial in size. It is impossible to measure and see them. According to quantum chromodynamics (QCD), the particles are point-like structures. They are very small. 10−19 meters. The size was discovered in 1968 via deep inelastic scattering experiments at the Stanford Linear Accelerator Center.
Mass of the quark is variable. It depends on its mass and the mass of adjacent fluid containing gluons. The significant portion of the mass of hadrons, which are made by these quarks, comes from the gluons. The binding constituent keeps the quarks together rather than the quarks themselves.
From up quark to top quark, their mass varies. While the top quark is the heaviest quarks, the up quark is the lightest one. Weight of the up quark is around 2to8MeV/c2. The weight of the top quark is 170to180 ×103MeV/c2. The unit of mass of a quark is evaluated in terms of MeV/c2, here MeV is the energy of quark in terms of mega-electron volts and c indicates the velocity of light in vacuum.
There are three types of quark’s colors in nature as per QCD. These colors are red, blue and green. This property of quarks is known as color charge. On the other hand, antiquarks have similar colors but are anti red, anti blue and anti green. Gluons also have two color charges. One from red, blue and green and the other from anti red, anti blue and anti green. In a proton, it is compulsory to show all the three colors of the quarks.
Every particle in the universe has certain properties. These include intrinsic properties like mass, charge or angular momentum. Every elementary particle has angular momentum, it is termed as spin.
A proton consists of three quarks, two up and one down quark. They bind together with massless, color charged particles, and gluons. Every quark has 12 spin and gluons also possess spin. They contribute to the proton’s spin. The contribution to the proton’s spin is only 30 percent.
Up and down quark flavors are stable. Meanwhile, top, bottom, charm and strange, are unstable. In the 4 unstable quark flavors, the interactive forces are weak, they do not hold for much time and eventually get converted into stable quark flavors. In the particle decay process, the unstable flavors get transformed to stable flavors.
Check your knowledge
Answer) Quarks are the smallest elementary particles in the universe. They are constituent matter of protons and neutrons in the atomic nuclei.
Answer) Hadrons, baryons, mesons and gluons.
Answer) There are six types of flavors, namely, up, down charm, strange, top and bottom.