Leptons vs Quarks
It has been our understanding for over three hundred years that matter consists of atoms. Atoms are thought to be indivisible until the 20th century, but the 20th century physicist discovered that the atom can be broken into smaller pieces, and all the atoms are made of different compositions of these particles. These are known as subatomic particles and namely, the proton, neutron and electron.
Further investigation reveals that these particles (subatomic particles also have internal structure, and made of smaller things). These particles are known as elementary particles, and Leptons and Quarks are their two main categories. Quarks are bound together to form a larger particle structure known as Hadrons.
Particles known as electrons, muons (µ), tau (Ƭ) and their corresponding neutrinos are known as the family of leptons. Electron, muon, and tau have a charge of -1, and they differ from each other only from the mass. The muon is three times more massive than the electron, and tau is 3500 times more massive than the electron. Their corresponding neutrinos are neutral and relatively massless. Each particle and where to find them are summarized in the following table.
a) In atoms
b) Produced in beta radioactivity
a) Large numbers produced in the upper atmosphere by cosmic radiation
Observed only in laboratories
Electron neutrino (νe)
Muon neutrino (νµ)
Tau neutrino (νƬ)
a) Beta radioactivity
b) Nuclear reactors
c) In nuclear reactions in the stars
a) Produced in nuclear reactors
b) Upper atmospheric cosmic radiation
Only generated in laboratories
The stability of these heavier particles is directly related to their masses. Massive particles have a shorter half-life than the less massive ones. The electron is the lightest particle; that is why the universe is abundant with electrons, but the other particles are rare. To generate muons and tau particles, a high level of energy is needed and in present day can only be seen in instances where there is a high energy density. These particles can be produced in particle accelerators. Leptons interact with each other by the electromagnetic interaction and weak nuclear interaction.
For each lepton particle, there are anti-particles known as antileptons. Anti-leptons have similar mass and opposite charge. The anti-particle of the electron is known as positrons.
The other major category of the elementary particles is known as quarks. Since scientist grew tired of giving difficult foreign names for the particles they found, they were given common names as up, down, strange, and charm. Each particle’s properties can be summarized as follows. (The mass of each particle is shown below the name itself. The accuracy of the of these numbers is highly debatable)
Quarks interact strongly with each other by strong nuclear interaction to form combinations of quarks. These combinations are known as Hadrons. In fact, isolated quarks do not exist in our universe at present. It is reasonable to say that all the quarks in this universe are in some form of hadrons.
Quarks have an internal property, which is the only one, known as the baryon number. All quarks have a baryon number of 1/3, and the anti-quarks have baryon numbers -1/3. In a reaction involving elementary particles, this property known as the baryon number is conserved.
There are other properties, which cannot be explicitly categorized as internal properties. Quarks have another property called the flavour. A number is assigned to denote the flavour of the particle known as the flavour number. The flavours are referred to as, Upness (U), Downness (D), Strangeness (S) and so on. The up quark has an upness of +1 and 0 strangeness and Downness.
Most common and known types of the hadrons are protons and neutrons.
What is the difference between Leptons and Quarks?
• Quarks and leptons are two categories of the elementary particles and when taken together known as fermions.
• Leptons are less interactive in strong interaction, but interact through electromagnetic and weak interaction. Quarks are interacting through strong interaction.
• Leptons can exist as individual particles in nature, but quarks have very strong interaction; therefore, form hadrons.
• Lepton particles, the electron, muon and tau, have a negative one charge, which is the charge of the electrons. Relatively they have very small mass. Compared to hadrons, neutrinos are considered to be massless, and they have no charge.
• Quarks have fractional charges, such as -1/3 and 2/3, and they are very much heavier than the leptons. Most of the visible matter is in the form of hadrons.