What is everything made of?
Matter is made of atoms. An atom has a nucleus (protons and neutrons) and electrons around it.
An elementary particle is a particle that is not made of anything smaller (as far as we know). The electron is elementary. The proton and neutron are not: they are made of quarks.
Particle physics is the study of these smallest pieces and the forces between them.
Quarks, hadrons, baryons and mesons
There are 6 kinds (flavours) of quark: up, down, charm, strange, top, bottom. Ordinary matter needs only up (charge +2/3 e) and down (charge −1/3 e).
- Proton = u u d → +2/3 + 2/3 − 1/3 = +1
- Neutron = u d d → +2/3 − 1/3 − 1/3 = 0
Particles made of quarks are called hadrons. Hadrons with 3 quarks are baryons (proton, neutron). Hadrons with 1 quark and 1 antiquark are mesons (for example the pion π⁺ = u + anti-d).
Quarks are held by the strong force and are never found alone. This is called confinement.
Leptons and antimatter
Leptons are elementary particles that do not feel the strong force. There are 6: electron, muon, tau, and a neutrino for each. Neutrinos have almost no mass and no charge; billions pass through you every second.
Every particle has an antiparticle with the same mass but opposite charge. The antiparticle of the electron is the positron (e⁺).
Annihilation: particle + antiparticle → energy (usually two gamma photons). Pair production: a high-energy photon near a nucleus can turn into a particle and its antiparticle. Both follow E = mc².
The four forces and exchange particles
| Force | Acts on | Carrier (boson) | Range |
|---|---|---|---|
| Strong | quarks, hadrons | gluon | about 10⁻¹⁵ m |
| Electromagnetic | charged particles | photon | infinite |
| Weak | all quarks and leptons | W⁺, W⁻, Z⁰ | about 10⁻¹⁸ m |
| Gravity | everything with mass | graviton (not yet found) | infinite |
In beta-minus decay a down quark turns into an up quark by the weak force: n → p + e⁻ + anti-neutrino. The Higgs boson, found at CERN in 2012, is linked to how particles get mass.
Accelerators, detectors and conservation laws
Accelerators use electric fields to speed up charged particles and magnetic fields to bend them round a ring. Linear accelerators push in a straight line; cyclotrons and synchrotrons use circles. Detectors track the new particles by the paths they leave (curving in a magnetic field shows their charge).
In every reaction these stay the same: charge, baryon number (quark = +1/3, antiquark = −1/3), lepton number (lepton = +1, antilepton = −1), and energy and momentum. If any one changes, the reaction cannot happen.
The Standard Model = 6 quarks + 6 leptons + force carriers + the Higgs boson. It explains almost everything except gravity and dark matter.
Try it: check a reaction
Open the free-play step in the 3D. Build u + anti-d. Predict the charge first (+2/3 + 1/3). Then build u u d and u d d. On paper, check whether p → n + e⁺ + ν keeps charge, baryon number and lepton number. (It does: this is beta-plus decay, which happens inside some nuclei.)
Key formulas and definitions
- Up quark charge = +2/3 e; down quark charge = −1/3 e
- Proton = uud (charge +1); neutron = udd (charge 0)
- Baryon number: quark +1/3, antiquark −1/3; baryon = +1, meson = 0
- Lepton number: lepton +1, antilepton −1, all other particles 0
- E = mc² (mass–energy in annihilation and pair production)
- Minimum photon energy for pair production = 2mc²
Worked examples
1. Find the charge of a particle made of u, u, u.
+2/3 + 2/3 + 2/3 = 6/3 = +2. This is the Δ⁺⁺ baryon, with charge +2e.
2. Find the baryon number of a pion π⁺ = u + anti-d.
Quark u: +1/3. Antiquark anti-d: −1/3. Total = 0. Mesons always have baryon number 0.
3. Is p + p → p + p + n possible?
Charge: before +2, after +1 +1 + 0 = +2 (kept). Baryon number: before 2, after 3 (not kept). So the reaction is impossible.
4. Show that beta-minus decay n → p + e⁻ + anti-ν keeps lepton number.
Before: 0 (neutron is not a lepton). After: proton 0, electron +1, antineutrino −1. Total 0. Lepton number is kept. Charge: 0 → +1 − 1 + 0 = 0, also kept.
5. An electron and positron (each mass 9.11 × 10⁻³¹ kg) annihilate at rest. Find the total energy released.
Total mass = 2 × 9.11 × 10⁻³¹ = 1.822 × 10⁻³⁰ kg. E = mc² = 1.822 × 10⁻³⁰ × (3 × 10⁸)² = 1.64 × 10⁻¹³ J (about 1.02 MeV). Each of the two gamma photons carries 0.51 MeV.
6. What is the minimum photon energy needed to make an electron–positron pair?
It must make two electron masses: E = 2mc² = 1.64 × 10⁻¹³ J = 1.02 MeV.
Common mistakes
- Thinking the proton and neutron are elementary. They are made of quarks; the electron is elementary.
- Mixing up baryons and mesons. Baryon = 3 quarks; meson = 1 quark + 1 antiquark.
- Thinking an antiparticle has negative mass. It has the same mass, only opposite charge (and opposite quantum numbers).
- Checking only charge in a reaction. Baryon number and lepton number must also be conserved.