Composite Particles of the Strong Interaction
Hadrons are composite subatomic particles that participate in the strong nuclear interaction. They are made up of elementary particles called quarks, which are bound together by gluons according to the principles of Quantum Chromodynamics (QCD).
The word hadron is derived from the Greek word hadrós, meaning “thick” or “heavy”, indicating particles that are subject to strong interactions and are generally heavier than leptons.
Based on their quark composition and spin, hadrons are broadly classified into two categories:
Baryons consist of three quarks and have half-integer spin, making them fermions. The most familiar baryons are the proton and neutron, which form atomic nuclei.
\[ B = \frac{1}{3}(n_q - n_{\bar{q}}) \]For baryons, \(B = 1\).
| Baryon | Symbol | Quark Content | Spin |
|---|---|---|---|
| Proton | p | uud | 1/2 |
| Neutron | n | udd | 1/2 |
| Lambda | Λ⁰ | uds | 1/2 |
Mesons are hadrons composed of a quark and an antiquark. They have integer spin and are therefore classified as bosons.
\[ B = 0 \]| Meson | Symbol | Quark Content | Spin |
|---|---|---|---|
| Pion | π⁺ | u\(\bar{d}\) | 0 |
| Kaon | K⁰ | d\(\bar{s}\) | 0 |
| Rho | ρ | u\(\bar{u}\) | 1 |
Determined by the charges of constituent quarks:
\[ Q = \sum_i q_i \]The total spin of a hadron arises from the intrinsic spins of quarks and their orbital angular momentum.
Some hadrons contain strange quarks and exhibit non-zero strangeness, especially hyperons and kaons.
Most hadrons are unstable and decay via the strong or weak interaction. Only protons are considered stable in isolation, while neutrons decay outside the nucleus.
\[ n \rightarrow p + e^- + \bar{\nu}_e \]