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The hadrons we know all fall into multiplets that reflect underlying internal symmetries. To express this fact in a simple and concrete way, it was hypothesized that hadrons are composed of more elementary constitutes with certain basic symmetries called quarks.

The existence of quarks has been confirmed indirectly in experiments that probe hadronic structure by means of electromagnetic and weak interactions and with the discovery of heavy quark atoms. The available evidence is consistent with the picture that hadrons participate in these interactions not as elementary entities but through quarks. 

What is Quark?

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Quark is defined as an elementary particle, every atom are made up of this particle. The significance of quark is that it can not exist independently. The different combination of quark is responsible for the different behaviour of matter.
There are six types of quarks. The details are given in the following section.

Antiquarks :
These particles are similar to the quarks but the spin and the sign of the charge is opposite to that of quarks. But it has same mass and charge according to their corresponding quarks.

Quark Theory

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The central role played by the property of colour screening becomes more and more crucial. But is the argument that quarks are confined within hadrons any more than an excuse for our failure to observe free quarks? The answer is that this picture makes unique and dramatic predictions; as the virtual photon is produced at rest, the quark and antiquark should emerge in opposite directions to conserve momentum.

Therefore, the two sprays of hadrons should be observed on opposite sides of the annihilation point where the photon is produced. The verification of this prediction was a real boost for the quark theory; any pre-quark theory of elementary particles had led us to expect a uniform, isotropic distribution of the emerging hadrons.

Types of Quarks

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The classification of quark is described below :
  • Up
  • Down
  • Strange
  • Charm
  • Top
  • Bottom

Types of Quarks

Up: It is a lightest quark due to its less mass and high stability. The mass is given as 1.7 - 3.1 $\frac{MeV}{c^{2}}$ and the charge is $\frac{2}{3}$e. Protons and neutrons are made up of Up quarks.

Down: This type is also high stable and light weight quark. The mass is given as 4.1 - 5.7 $\frac{MeV}{c^{2}}$ and charge is -$\frac{1}{3}$e.

Strange: It is the third lightest quark. The mass and charge is 100 $\frac{MeV}{c^{2}}$ and -$\frac{1}{3}$e respectively.

Charm:
The mass is given as 2900 times that of the electron and the electric charge is $\frac{2}{3}$e.

Top: The mass and charge of this quark are 172.9 ± 1.5 $\frac{GeV}{c^{2}}$ and $\frac{2}{3}$e respectively.

Bottom: The mass of this quark is 4.1 $\frac{GeV}{c^{2}}$. And the electric charge is given as -$\frac{1}{3}$e.

Quark Particle

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Some of the properties of quark particle is listed below :

  • Quark particle is considered as the elementary particle in physics.
  • It does not situated separately.
  • Quark particles have anti particle also.
  • The interaction between quark and antiquark particle is explained by using basic forces.
  • One can not measure mass, spin and pairing etc directly.