Particles and Forces
Learn the particle zoo: quarks, leptons, and gauge bosons. Understand the four forces, the Standard Model table, and what the LHC does.
Particle Physics
The Standard Model, quarks, leptons, gauge bosons, the Higgs field, and the search for physics beyond the Standard Model.
Overview
Particle physics studies the fundamental constituents of matter and the forces between them. The Standard Model — a quantum field theory developed through the 1960s and 70s — describes three of the four fundamental forces and all known elementary particles. It has been tested to extraordinary precision and remains the most successful predictive framework in all of science.
The Standard Model
Matter Particles — Fermions
Matter is built from fermions (spin-½ particles obeying the Pauli exclusion principle). There are six quarks — up, down, charm, strange, top, bottom — that feel the strong force and combine into hadrons (protons, neutrons, pions). There are six leptons — electron, muon, tau, and their three corresponding neutrinos — that do not feel the strong force.
Force Carriers — Gauge Bosons
Forces are mediated by gauge bosons: the photon (electromagnetic force), W⁺, W⁻, Z⁰ bosons (weak nuclear force), and eight gluons (strong force). Gravity has no quantum description in the Standard Model — the hypothetical graviton is not part of it.
The Higgs Mechanism
The Higgs field fills all of space. Particles acquire mass by interacting with it: the W and Z bosons get mass via electroweak symmetry breaking, while quarks and charged leptons get mass through Yukawa couplings. The quantum excitation of the Higgs field — the Higgs boson — was discovered at CERN's LHC in 2012 at a mass of ~125 GeV/c².
Quantum Chromodynamics
QCD is the gauge theory of the strong force, based on the SU(3) colour symmetry. Quarks carry colour charge (red, green, blue); gluons carry colour too, which is why the strong force is self-interacting. Confinement means free quarks are never observed — they always bind into colour-neutral hadrons. Asymptotic freedom means quarks become weakly interacting at very high energies.
Beyond the Standard Model
Despite its success, the Standard Model has known gaps: it does not include gravity, does not explain dark matter or dark energy, does not account for the matter-antimatter asymmetry, and has too many free parameters. Active research areas include supersymmetry, extra dimensions, grand unified theories, and dark matter candidates.
Learning Pathways
Quantum Field Theory Basics
Feynman diagrams, conservation laws, symmetry groups, CP violation, and the Higgs mechanism.
Beyond the Standard Model
Supersymmetry, grand unification, neutrino masses, baryogenesis, and the open problems of particle physics.
Key Articles
Quarks and Leptons
The fundamental building blocks of matter and their quantum numbers.
Read →QCD and Confinement
Why free quarks are never seen and how gluons hold the nucleus together.
Read →Leptogenesis
How CP violation in the early universe may explain the matter-antimatter asymmetry.
Read →The CP Violation Story
Why matter and antimatter behave slightly differently — and why that matters.
Read →Pentaquarks at LHCb
The discovery of five-quark states at the LHC and what they reveal about QCD.
Read →The Muon g-2 Anomaly
A persistent discrepancy between theory and experiment that hints at new physics.
Read →Frequently Asked Questions
What is the Standard Model?
The quantum field theory of three fundamental forces (EM, weak, strong) and all known elementary particles: six quarks, six leptons, gauge bosons, and the Higgs.
What holds the nucleus together?
Gluons bind quarks into protons and neutrons (strong force). A residual strong force (pion exchange) holds the nucleus together.
What is the Higgs boson?
The quantum excitation of the Higgs field, which gives elementary particles their mass. Discovered at the LHC in 2012 at ~125 GeV/c².
What is antimatter?
Every particle has an antiparticle with opposite quantum numbers. Particle-antiparticle annihilation produces photons. The matter-antimatter asymmetry is a major open problem.