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Question 12: Write a comprehensive note on hadrons, leptons and quarks.


Initially, it was thought that atom was indivisible. However, the later evidences proved atom consisted of a number of subatomic particles. This showed atom was not an elementary particle. The subatomic particles are protons, neutrons and electrons. Still, during the last century, scientists found that even most of the subatomic consist of smaller particles called quarks. Following is an account of these particles.


Hadrons are particles that interact in strong force. They are further divided in two classes.

  1. Mesons: Mesons have masses between the mass of protons and electrons. All mesons finally decay into electrons, positrons, neutrinos and photons.
  2. Baryons: Baryons have masses equal to or greater than the mass of proton. Proton and neutron are in baryon family. Except proton, all baryons when decay the end product includes proton.


Leptons are those particles which participate in weak force. They are considered to be the elementary particles because they do not have any internal structure. This group of particles includes electrons, muons and neutrinos. They all are less massive than hadrons.


Quarks are the building blocks of hadrons (mesons and baryons). Quark model is based on the following assumptions.

  1. There are six types of quarks; (1) the up quarks (2) the down quarks (3) the strange quarks (4) the charmed quarks (5) the bottom quarks (6) the top quarks. They are also referred to as u, d, s, c, b and t quarks.
  2. For every type of quark, there is an anti-quark.
  3. Quarks combine in threes to form particles like protons and neutrons (hadrons). Anti-quarks also combine in threes to form antiparticles like antiproton and antineutron.
  4. A meson consists of a quark and an anti-quark.

In terms of charge of the electron, the up quark (u), the charm quark (c) and the top quark (t) each carry a charge of + 2/3e. The remaining three quarks (b, s and d quarks) have charges each of – 1/3e. Similarly, each anti-quark has an equal (in magnitude) and opposite (in sign) charge to its corresponding quark. Symbols for anti-quark are the same as that of the corresponding quark but having a bar over the top.

Thus, according to the above-mentioned assumptions;

  • Proton consists of three quarks; two up quarks and one down quark.
  • Neutron consists of three quarks; one up quark and two down quarks.


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