Leptons vs Hadrons
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 are made of smaller things. These particles are known as Elementary particles, and Leptons and Quarks are known to be two main categories of elementary particles. Quarks are bound together to form a larger particle structure known as Hadrons.
Leptons
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.
|
1st Generation |
2nd Generation |
3rd Generation |
|
Electron (e) |
Muon (µ) |
Tau (Ƭ) |
|
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.
Hardrons
The other major category of the elementary particles is known as quarks. They are up, down, strange, top, and bottom quarks. These quarks have fractional charges. Quarks also have anti-particles known as anti-quarks. They have the same mass but opposite charge.
|
Charge |
1st Generation |
2nd Generation |
3rd Generation |
|
+2/3
|
Up 0.33 |
Charm 1.58 |
Top 180 |
|
-1/2 |
Down 0.33 |
Strange 0.47 |
Bottom 4.58 |
N.B. particle masses shown in the bottom are in GeV/c2.
These particles interact through strong force to form larger particles known as hadrons and hadrons have integer number charge.
Basically, quarks combine with quarks itself or with anti-quarks, to form stable hadrons. Three main categories of hadrons are baryons, antibaryons, and mesons. Baryons consists of three quarks (qqq) bound with strong force, and antibaryons are three anti-quarks ([latex]\bar{q}\bar{q}\bar{q}[/latex]) bound. Mesons are quark and antiquark ([latex]q\bar{q}[/latex]) paired together.
What is the difference between Hadrons and Leptons?
• Quarks and leptons are two categories of the elementary particles and taken together, known as fermions.
• The quarks combine through strong nuclear interaction to form hadrons; until now, no internal structures of leptons are discovered, but Hadrons have internal structure. Leptons exist as individual particles.
• Hadrons are more massive particles compared to leptons.
• Leptons interact through electromagnetic and weak force, while quarks interact through strong interactions.