A guided walk down to the smallest things that exist — and the rules they obey. Scroll down; an electron meets its antimatter twin and vanishes in a flash, quarks snap together into a proton, particle tracks curl in a magnetic field, and two beams smash together inside the LHC.
1 — Particles & antiparticles
Every particle has an antiparticle: same mass, opposite charge. The electron's twin is the positron (e⁺). When a particle meets its antiparticle they annihilate — matter turns entirely into energy.
- Annihilation — e⁻ + e⁺ → 2γ. The mass disappears and reappears as two gamma-ray photons flying apart (momentum is conserved).
- Pair production — the reverse: a high-energy photon near a nucleus can become an e⁻e⁺ pair, if it carries enough energy.
Mass ⇄ energyE = mc² · electron rest energy = 0.511 MeV
e⁻ + e⁺ → 2γ (each γ ≈ 0.511 MeV)
Real use: a PET scanner detects exactly these back-to-back annihilation photons to map the inside of your body.
2 — The four fundamental forces
Everything that happens is one of just four interactions. Each is carried by an exchange particle and has its own strength and range.
| Force | Relative strength | Range | Carrier |
|---|
| Strong | 1 | ~10⁻¹⁵ m | gluon |
| Electromagnetic | ~10⁻² | infinite | photon |
| Weak | ~10⁻⁶ | ~10⁻¹⁸ m | W, Z bosons |
| Gravity | ~10⁻³⁸ | infinite | graviton (?) |
The strong force binds quarks into protons and holds the nucleus together against electric repulsion. Gravity is overwhelmingly the weakest — yet it shapes galaxies because it never cancels out.
3 — Quarks & leptons
Matter is built from two families of fundamental (point-like, indivisible) particles, arranged in three generations.
- Quarks (6) — up, down, charm, strange, top, bottom. They carry fractional charge (+⅔e or −⅓e) and feel the strong force. They never appear alone.
- Leptons (6) — electron, muon, tau and their three neutrinos. The electron carries −1e; neutrinos are neutral and almost massless.
Fractional chargesup u = +⅔ e · down d = −⅓ e
ordinary matter = up & down quarks + electrons
Key idea: everyday matter needs just three of these — the up quark, the down quark and the electron.
4 — Hadrons: baryons & mesons
Particles built from quarks are called hadrons. They feel the strong force. There are two kinds:
- Baryons — three quarks (qqq). The proton (uud) and neutron (udd) are baryons. Baryon number B = +1.
- Mesons — a quark + antiquark (q q̄). Example: the pion π⁺ = u d̄. Mesons have baryon number B = 0.
Hadron recipesbaryon = q q q (e.g. proton uud) → B = +1
meson = q q̄ (e.g. pion u d̄) → B = 0
Confinement: pull two quarks apart and the strong force makes a new pair rather than letting one escape — quarks are never seen free.
5 — Building a proton & a neutron
Add the fractional quark charges and the whole-number charges of the proton and neutron fall out exactly.
Add the quark chargesproton uud: (+⅔) + (+⅔) + (−⅓) = +1
neutron udd: (+⅔) + (−⅓) + (−⅓) = 0
- Proton = u u d → charge +1e, baryon number +1. Stable.
- Neutron = u d d → charge 0, baryon number +1. A free neutron decays in ~15 min.
Beta decay: inside the nucleus a down quark turns into an up (n → p), emitting an electron and an antineutrino — the weak force at work.
6 — Conservation laws
A reaction is only allowed if certain quantities are equal before and after. These rules decide which decays can happen at all.
- Charge (Q) — total electric charge is always conserved.
- Baryon number (B) — quarks count +⅓ each, so a baryon is +1; total B is conserved.
- Lepton number (L) — electrons, muons, taus and their neutrinos count +1; antileptons count −1.
Worked check — neutron β decayn → p + e⁻ + ν̄ₑ
charge: 0 = (+1) + (−1) + 0 ✓
baryon: 1 = 1 + 0 + 0 ✓ · lepton: 0 = 0 + (+1) + (−1) ✓
worked — is it allowed?
Does the proton decay p → e⁺ + γ?
charge ✓ but baryon number 1 → 0 ✗ — forbidden, so the proton is stable.
7 — Detecting particles
We can't see a particle, only the track it leaves. A magnetic field bends a charged particle into a circle, and the curve tells us its story.
Radius of the trackr = p / (qB) → r ∝ momentum (at fixed q, B)
charge sign → which way it curls · neutral particle → no track
- Tight curl — small radius → low momentum (slow particle).
- Gentle curve — large radius → high momentum (fast particle).
- Spiral — the particle loses energy, momentum drops, the radius shrinks as it spirals in.
Reading a photo: opposite charges bend opposite ways — that is how a positron is told apart from an electron.
8 — Accelerators & the LHC
To make heavy particles we must supply the energy E = mc². Accelerators whirl beams to nearly light speed and collide them; the collision energy materialises as new particles.
- The ring — the Large Hadron Collider is a 27 km loop under the French–Swiss border; magnets steer the beams, electric fields speed them up each lap.
- Head-on collision — two proton beams meet, and the huge kinetic energy converts into a shower of brand-new particles for the detectors to record.
Why so much energy?collision energy → mass: E = mc²
bigger energy → heavier particles can be created
Triumph: the LHC discovered the Higgs boson in 2012 — the particle that gives others their mass.
9 — Recap: the Standard Model & the Higgs
Everything in this walkthrough fits into one chart — the Standard Model: 6 quarks, 6 leptons, the force-carrier bosons (gluon, photon, W, Z) and the Higgs boson that gives mass.
- Every particle has an antiparticle; matter + antimatter → annihilate → energy (E = mc²).
- Four forces: strong > electromagnetic > weak > gravity; carried by gluon, photon, W/Z, (graviton).
- Fundamental matter = 6 quarks + 6 leptons; quarks carry fractional charge.
- Hadrons: baryon = qqq (proton uud, B = +1); meson = q q̄ (B = 0).
- Quark charges add up: uud = +1 (proton), udd = 0 (neutron).
- Conserve charge, baryon number & lepton number — they decide what is allowed.
- Tracks: r = p/qB, so radius ∝ momentum; charge sign sets the curl direction.
- Accelerators (LHC) turn collision energy into new mass — the Higgs was found in 2012.