Hadronic particles composed of a quark–antiquark pair
Mesons are a class of hadrons composed of one quark and one antiquark bound together by the strong interaction. Unlike baryons, mesons are bosons and therefore obey Bose–Einstein statistics.
Mesons play a crucial role in nuclear physics. The exchange of virtual mesons between nucleons explains the strong nuclear force that binds protons and neutrons inside atomic nuclei.
The total spin of a meson is always an integer (0 or 1 in most cases), which makes mesons bosonic particles. Their intrinsic parity depends on the orbital angular momentum between the quark and antiquark.
| Property | Description |
|---|---|
| Composition | One quark + one antiquark |
| Spin | Integer (0 or 1) |
| Statistics | Bose–Einstein statistics |
| Interaction | Strong, electromagnetic, weak |
| Lifetime | Very short (except π⁰, K⁰ varieties) |
Mesons are classified according to their spin (J), parity (P), and charge conjugation (C). The most important categories are pseudoscalar and vector mesons.
| Type | Spin (J) | Examples | Remarks |
|---|---|---|---|
| Pseudoscalar Mesons | 0 | π, K, η | Lowest mass mesons; important in nuclear forces |
| Vector Mesons | 1 | ρ, ω, φ | Higher mass; short-lived |
According to Yukawa’s theory, the strong nuclear force arises due to the exchange of mesons between nucleons. The range of the force is given approximately by
where \( m_\pi \) is the mass of the pion. This relation correctly explains why the nuclear force is short-ranged.
| Meson | Symbol | Charge | Significance |
|---|---|---|---|
| Pion | π⁺, π⁰, π⁻ | +1, 0, −1 | Primary carrier of nuclear force |
| Kaon | K⁺, K⁰ | +1, 0 | Shows strangeness |
| Eta Meson | η | 0 | Neutral pseudoscalar meson |
The pseudoscalar meson octet arises from SU(3) flavor symmetry and contains eight mesons with spin 0.
| Meson | Quark Content |
|---|---|
| π⁺ | u d̄ |
| π⁰ | (u ū − d d̄)/√2 |
| π⁻ | d ū |
| K⁺ | u s̄ |
| K⁰ | d s̄ |
| K⁻ | s ū |
| ṼK⁰ | s d̄ |
| η | (u ū + d d̄ − 2s s̄)/√6 |
Mesons are fundamental to our understanding of strong interactions. They validate the quark model, explain nuclear binding, and provide experimental confirmation of quantum chromodynamics (QCD).