Lepton, Quark and Standard Model


Leptons are the light weight elementary particles. They do not have strong interaction. There are six types of leptons. They are electron, muon, tau particle, electron neutrino, muon neutrino and tau neutrino. All the members of lepton family have different anti-particles.

Hadrons: Hadrons are the strongly interacting particles. Each hadrons has anti particles. Hadrons are divided into two types:

  1. Mesons: Mesons are the hadrons made up of two quarks. They have integer spin and are called bosons. For mesons, baryon number is zero. Examples: Kaon (Κ), pion (π) , eta (n) etc.
  2. Baryons: Baryons are the hadrons made up of two quarks.  They have 1/2 integer spin and are called fermions. The baryon number for each baryon is +1 and for anti-baryon is -1. Example: omega(Ω), sigma(σ), Psi(Ψ) etc.

Note: A fermion is any particle that has an odd half-integer. Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions. Bosons are those particles which have an integer spin (0, 1, 2…). All force carrying particles are bosons.


Scientist at the beginning thought that atoms  are the smallest indivisible building blocks of matter. But later they found not only atoms,  its constituents proton, neutron are further made up  of smaller particle which they called as quark. Quarks are the most tiny particles that cannot be found isolated. But combination of multiple quarks can form proton and neutrons.  There are six types of quark namely: Up, down, strange, anti up, anti down and anti strange.

The charge of up quark is 2e/3 and that of down quark and strange quark is -1/3.

Quark composition of  some of the Baryons:

Proton = uud = 2/3 + (-1/3) + (-1/3) = +1, So proton has charge +1.

Neutron: udd = -1/3 + 2/3 + 2/3 = 0, So neutron has no charge.

In this way quark composition determines the charge of the baryons as well as mesons.

Standard Model

We know particle physics deals with all the basic structure and laws of particle governing the interaction of these particles.  After the discovery of proton, neutron and electrons great simplification were made to study the nature of the elements. The more protons were added,  the different elements were made. With the growing research,  many sub atomic particles were discovered. So, a new fundamental particle called quark was predicted and the whole zoo of particles could be described by the combination of quarks. This gave birth to  the standard model.

The standard model

The theory that describes the properties, structure of the fundamental particles on the basis of their charge, spin, mass and lifetime of the is called as standard model. Standard model is a extremely successful and explains all the phenomena that we see in microscopic scale.   According to this theory, fundamental particles are categorized into three types: Leptons., quarks and mediator particles.  These are the building blocks of all the matter in this universe. There are six quarks, six leptons and six mediator particles in the current standard model. More particles will be added soon as the research is going on. 




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