Mesons – Hadronic Particles

1. What are Mesons?

Mesons are a class of hadrons composed of a quark–antiquark pair. They are strongly interacting particles and play a crucial role in mediating nuclear forces.

Definition: Mesons are hadrons having baryon number B = 0.

2. Quark Structure of Mesons

Each meson consists of:

One quark + one antiquark (q q̄)

Examples:

Meson Quark Composition Charge
π⁺ u d̄ +1
π⁻ d ū −1
π⁰ (u ū − d d̄)/√2 0
K⁺ u s̄ +1
K⁰ d s̄ 0

3. Properties of Mesons

(a) Spin and Statistics

Mesons have integer spin (0 or 1) and hence obey Bose–Einstein statistics.

(b) Mass

Meson masses are generally greater than leptons but less than baryons.

(c) Lifetime

Mesons are generally unstable and decay via strong or weak interactions.

4. Types of Mesons

Type Examples Spin
Pseudoscalar Mesons π, K, η 0
Vector Mesons ρ, ω, φ 1

5. Interactions of Mesons

Interaction Participate?
Gravitational Yes
Electromagnetic Charged mesons
Weak Yes
Strong Yes

6. Role of Mesons in Nuclear Force

Yukawa proposed that nuclear force between nucleons is mediated by pions (π-mesons).

Exchange of π-mesons explains the short-range nature of nuclear force.

7. Mesons vs Leptons

Property Mesons Leptons
Constituents Quark + Antiquark Elementary
Strong Interaction Yes No
Spin 0 or 1 1/2

8. Summary

✔ Mesons are hadrons made of quark–antiquark pairs ✔ They have zero baryon number ✔ Obey Bose–Einstein statistics ✔ Play a vital role in nuclear forces ✔ Important constituents of particle physics