It is relevant that atom is made up of nuclei and electron and it is bound by electromagnetic force and it is also known that mass of the electron is very small when compared to mass of nuclei( it consists of neutron and proton) and they ( electrons) are negatively charged and protons are present in point like quarks feature which are “up” and “down” in the nuclei.
According to the particle physics , it is stated that leptons are elementary particles with half integral spin(½) and it does not go under any strong interaction and it is divided into two branches that are:
Electrically charged lepton ( also called electron like lepton)
Neutrally charged lepton ( also called neutrinos )
The charged leptons can combine with other particles to form composite particles such as atoms and positronium ( it is the reverse of electron it has the same mass but with a positive charge )while the neutrino rarely reacts with any other particle and they are rarely taken into observation and the best known lepton is electrons( electrons also have least mass of all charged leptons).
According to the above two classes it is divided into six types and grouped into three generation that are:
The first generation leptones are called electronic leptons which comprises of electron and electron neutrino
The second generation leptons are called muonic leptones which comprises of muon and muon neutrino
The third generation leptons are called tauonic leptones which comprises of tau and tau neutrino
The electric charge on the charged lepton is -1 e, 0 e , +1 e
History of Leptons
J.J Thomason with his team of british physicist in 1897 for the first time discovered electrons ( which was the first lepton to be discovered).
To preserve the conservation of energy, conservation of momentum and conservation of angular momentum in beta decay Wolf in 1930 gave its postulate for electron neutrino( in the starting it was only called neutrino as it was known yet that neutrino can have different flavours or be present in different generation ) and muon was discovered by Carl D Anderon in 1936 ( nearly after 40 years the discovery of the electrons ) and due to its mass it was in the beginning considered as a meson later it was observed that muon is more similar to electron than to meson.
Martin Lewis Perl with his colleagues SLAC LBL group discovery tau during a series of experiments between 1974 and 1977 ( while tau neutrino was discovered by the missing energy and momentum in the decay ).
And all the present data is consistent with all the three generations of leptons but particle physicists are still trying to find the fourth generation of the leptons.
Properties of Leptons
The spin statistics theorem tells us that leptons are fermions and thus Pauli’s exclusion law implies on them and thus no two leptons having the same species cannot have the same state at the same time.
Leptons closely related property chirality can be compared to a more easily visual properties known as helicity ( it is the direction of the spin of an particle related to its relative momentum).
The particles having the same spin with its momentum is known as right handed and otherwise they are called left handed and the Standard Model’s weak interaction treats left handed and right handed fermions differently and it is also states that the left handed fermions only participate in the weak interaction which is an example parity violation explicitly written in the records.
It is stated that right hand neutrino and left hand anti neutrino cannot have interaction with any particle.
It is also found that right handed fermions which are electrically charged and they don't interact or engage in weak interaction specifically but they can interact electrically and thus are combined in the weak electro force although with different strengths.
Another of the important property of leptons is their electric charge and the electric charge determines the strength of their electromagnetic interaction and it also determines its electric field and how will it react to other electric and magnetic field and leptons possess intrinsic rotation in the form of their spin and the charged leptons generate magnetic field.
Thus each generation contains one electrically charged lepton and one non-electrically lepton with zero electric charge and the electromagnetic interaction of the charged leptons is expressed by the fact that the particles interact with the quantum of the electromagnetic field the photons.
The size of the magnetic dipole moment is given by a formula( it is also referred to as anomalous magnetic dipole moments are very sensitive to the details of the quantum field theory model thus it gives or provides the opportunity for a precise model and structure.) and the given electron anomalous magnetic dipole moment theoretical and measured value is within the eight significant figures.
The members of each generation of the weak isospin doublets are assigned with the leptonic numbers which used under the standard model it is also known that neutrino oscillation violates the individual leptonic numbers and these violation are indicates that there is also a physics beyond the Standard Model and thus it gives a more stronger conservation law the conservation of the total number of leptons and neutrino oscillation also obeys these law but this law is violated by a tiny amount of chiral anomaly.