Quark

In an atom’s nuclei, there are two types of stable particles present. They are protons and neutrons. These protons are made up of other fundamental and elementary particles known as Quarks or Quark. These subatomic particles, quarks, are unstable in its elementary form and never found in isolation. They combine to form hadrons which are stable. Inside the protons and neutrons, other elementary particles like baryons and mesons are found in the quark-gluon plasma.

Quarks are always found in pairs with their antiparticles. These antiparticles are antiquarks. They combine with each other to form hadrons which in turn, combine to form other subatomic particles like mesons and baryons. Till date, quarks are supposed to be the elementary particles which are not broken down to other smaller particles as they have no structure apparently. In particle physics, only quarks are the basic elementary particles which exist in nature.

History and Types of Quarks

In 1964, the quark model came to the existence and two physicists, Murray Gell-Mann and George Zweig (both from the United States of America) have proposed it, though George called the particles as “aces”. In the Gell-Mann’s model, it was shown that all mesons were made of a quark and its antiquark whereas all the baryons were made of three quarks. According to this model, there are six types of quarks and are found in three pairs. The type of quarks is also termed as “flavour” so that there are six flavours of quarks.

These six flavours are up and down, top and bottom and charm and strange. Out of these flavours, up and down quarks are lightest particles and four other heavier particles which tend to convert rapidly to up and down quarks. It means that up and down quarks are most common in the universe as they are stable whereas other elementary particles charm, strange, top and bottom quarks can only be produced when high energy collisions occur such as cosmic rays and particle accelerators like cyclotrons.

Properties of the Quarks

Electric Charge:

In quarks, it is very strange that the electric charges on these particles are not an integer. The electric charge on up, charm and top quarks are equal to \(\frac{2}{3}e\), which is a positive fractional charge and on the other hand, the electric charge on down, strange and bottom quarks is equal to \(-\frac{1}{3}e\) which is a negative fractional charge. In this denotation, the letter ‘e’ represents the value of charge on a single electron without the sign. Here, all flavours of quarks have antiparticles (or antiquarks) to each and the electrical charges on these antiparticles are opposite to their corresponding quarks.

It means up antiquark coexists with its up quark and has the electric charge of \(-\frac{2}{3}e\). In the atom’s nuclei, protons and neutrons are made by these particles. A proton consists of two up quarks and one down quark whereas a neutron consists of one up quark and two down quarks. In this way, a proton has +1 e charge and neutron has 0 e charge on it.

Size:

Quarks are negligible in size and they are impossible to measure and see. In the quantum chromodynamics (QCD), these particles are point-like structures and they are very small to the range of \(10^{-19}\) meters. In 1968, the size was discovered in deep inelastic scattering experiments at the Stanford Linear Accelerator Center (SLAC).

Mass:

Mass of the quark is variable and depends on its mass and the mass of surrounding fluid consisting gluons. The major portion of the mass of hadrons, made by these quarks, comes from the gluons the binding constituent is keeping the quarks together rather than the quarks themselves.

The mass of quarks varies from up quark to top quark. The top quark is the heaviest quarks whereas up quark is the lightest one. Weight of up quark is in the range of \(2 to 8 \frac{MeV}{c^{2}}\) and the weight of top quark is \(170 to 180 × 10^{3} \frac{MeV}{c^{2}}\). The unit of mass of quark is measured in terms of \(\frac{MeV}{c^{2}}\), where MeV is the energy of quark in terms of mega-electron volts and c is the velocity of light in vacuum.

Colour Charge:

According to the quantum chromodynamics (QCD), there are three types of quark’s colours in nature. These colours are red, blue and green. This property of quarks is a colour charge. Antiquarks have the same colours but are represented as antired, antiblue and antigreen. Gluons are also having two colour charges one from red, blue and green and another from antired, antiblue and antigreen. In a proton, it is mandatory to show all the three colours of the quarks.

Spin:

Every particle in the universe has some property which cannot be separated from them. These properties are intrinsic properties like mass, charge or angular momentum. The intrinsic property, angular momentum is termed as spin and every elementary particle has this property.

A proton has three quarks, two up and one down quark and these are binding together with massless, colour charged particles, gluons. Each quark has \(\frac{1}{2}\) spin and gluons also have spin. The spin of these quarks and gluons contribute to proton’s spin. Their contribution to proton’s spin is only 30%.

Weak Interaction:

As discussed in the above, only two quark flavours, up and down, are stable and other 4 quark flavours, top, bottom, charm and strange, are unstable. In these 4 flavours, the interactive forces are so weak that they do not hold for much time and convert into stable quark flavours, i.e., up and down quarks. This transformation of flavours from unstable to stable flavours is through the particle decay process.

FAQs on the Oscillatory Motion

Q.1: What is the quark?

Answer: In the universe, quarks are the smallest elementary particles which are the constituent matter of proton and neutrons of the atomic nuclei. They are non-separable from the protons and neutrons as they cannot stable in the elementary form. Hence they are not found isolated in nature but inside the protons and neutrons.

Q.2: What are other particles inside a proton?

Answer: There are other particles found inside a proton which are hadrons, baryons, mesons and gluons. All these particles except gluons are made by quarks. Gluons are plasma-like elements which bind the quarks and held them inside the proton.

Q.3: Describe the types of quarks and establish relationships among them?

Answer: There are six types of quarks. The type of quarks has another term, flavour. Thus, there are six types of flavours, namely, up, down, charm, strange, top and bottom. These six flavours are in three pairs. These pairs are

1. Up and down flavours
2. Charm and strange flavours and
3. Top and bottom flavours

Out of these six flavours, two flavours up and down are stable and lightest in weight. Other 4 flavours are heavier and unstable in nature and tend to convert rapidly to up and down quarks through the particle decay process.

Q.4: What are the properties of quarks? Explain one of them.

Answer: The properties of the particles which can’t be separated from them are known as intrinsic properties. Some of them are mass, electric charge and angular momentum. Other properties which may change are size, spin, interaction forces and colour charge. According to QCD, there are three colour charges in quarks. These basic colours are red, blue and green. Antiquarks are also in the same colour charges but they are termed as antired, antiblue and antigreen respectively.