Elementary particles are the fundamental constituents of matter and radiation. They are classified based on intrinsic properties such as spin, statistics, and interaction type.
Spin is an intrinsic angular momentum of particles, measured in units of \(\hbar\). Based on spin, particles are classified into two major groups.
| Spin Type | Spin Value | Particle Class | Examples |
|---|---|---|---|
| Half-integer | \(\frac{1}{2}, \frac{3}{2}, \dots\) | Fermions | Electron, Proton, Neutron |
| Integer | \(0, 1, 2, \dots\) | Bosons | Photon, Pion, Gluon |
Fermions are particles with half-integer spin and obey Fermi–Dirac statistics. They follow the Pauli Exclusion Principle.
\[ n_i = \frac{1}{e^{(E_i - \mu)/kT} + 1} \]Fermions are further classified into:
Bosons are particles with integer spin and obey Bose–Einstein statistics. They do not follow the Pauli exclusion principle.
\[ n_i = \frac{1}{e^{(E_i - \mu)/kT} - 1} \]Bosons act as force carriers and include both fundamental and composite particles.
These particles have no known internal structure.
These particles are made up of quarks bound by strong interaction.
Hadrons are strongly interacting composite particles. They are divided into baryons and mesons based on quark structure.
| Hadron Type | Quark Structure | Spin | Statistics |
|---|---|---|---|
| Baryons | qqq | Half-integer | Fermions |
| Mesons | q\(\bar{q}\) | Integer | Bosons |
Particles may also be classified according to the fundamental interactions they participate in.
| Interaction | Mediator | Particles Involved |
|---|---|---|
| Strong | Gluons | Quarks, Hadrons |
| Electromagnetic | Photon | Charged particles |
| Weak | W⁺, W⁻, Z⁰ | All fermions |
| Gravitational | Graviton (hypothetical) | All particles |
The classification of elementary particles provides a systematic framework to understand their properties, interactions, and roles in nature. Spin-based classification forms the foundation of modern particle physics.