Published by Patrick Mutisya · 14 days ago
In the Standard Model of particle physics, all matter is built from two families of elementary particles: quarks and leptons. Quarks combine to form composite particles called hadrons, while leptons (such as the electron) remain elementary.
There are six flavours of quark: up (u), down (d), charm (c), strange (s), top (t) and bottom (b). Each quark carries a fractional electric charge of either \$+\frac{2}{3}e\$ or \$-\frac{1}{3}e\$, where \$e\$ is the elementary charge.
A hadron is any particle that experiences the strong nuclear force. Hadrons are categorised into two families:
Because quarks have fractional charges, the total charge of a baryon is always an integer multiple of \$e\$. For example, the proton consists of two up quarks and one down quark:
\$\$
Q_{\text{proton}} = 2\left(+\frac{2}{3}e\right) + \left(-\frac{1}{3}e\right) = +e.
\$\$
Similarly, the neutron is made of one up quark and two down quarks:
\$\$
Q_{\text{neutron}} = \left(+\frac{2}{3}e\right) + 2\left(-\frac{1}{3}e\right) = 0.
\$\$
| Baryon | Quark Content | Charge | Spin (ℏ) |
|---|---|---|---|
| Proton (p) | uud | +\$e\$ | ½ |
| Neutron (n) | udd | 0 | ½ |
| Lambda (Λ⁰) | uds | 0 | ½ |
| Delta⁺⁺ (Δ⁺⁺) | uuu | +\$2e\$ | 3/2 |
Mesons are bosons (integer spin) formed from a quark and its corresponding antiquark. Their charges also sum to integer multiples of \$e\$.
Example: the positively charged pion consists of an up quark and an anti‑down quark:
\$\$
Q_{\pi^{+}} = \left(+\frac{2}{3}e\right) + \left(+\frac{1}{3}e\right) = +e.
\$\$
The neutral pion is a mixture of up–anti‑up and down–anti‑down pairs, giving a net charge of zero.
| Meson | Quark Content | Charge | Spin (ℏ) |
|---|---|---|---|
| Pion⁺ (π⁺) | u\$\bar{\text{d}}\$ | +\$e\$ | 0 |
| Pion⁰ (π⁰) | (u\$\bar{\text{u}}\$ – d\$\bar{\text{d}}\$)/√2 | 0 | 0 |
| Kaon⁺ (K⁺) | u\$\bar{\text{s}}\$ | +\$e\$ | 0 |
| Eta (η) | mix of u\$\bar{\text{u}}\$, d\$\bar{\text{d}}\$, s\$\bar{\text{s}}\$ | 0 | 0 |
| Property | Baryon (qqq) | Meson (q\$\bar{\text{q}}\$) |
|---|---|---|
| Number of constituent quarks | 3 | 2 (quark + antiquark) |
| Spin | ½, 3/2, … (fermions) | 0, 1, … (bosons) |
| Typical mass range | ≈ 938 MeV/c² (proton) to a few GeV/c² | ≈ 135 MeV/c² (π⁰) to a few GeV/c² |
| Examples | Proton, neutron, Λ⁰, Δ⁺⁺ | π⁺, π⁰, K⁺, η |