The most precisely tested theory ever written
By the mid-twentieth century experiments had uncovered a bewildering zoo of particles. The Standard Model is the triumphant ordering of that zoo: a single quantum field theory that, from a short list of fundamental particles and three of the four forces, predicts an enormous range of measurements — some agreeing with experiment to better than one part in a billion. It is the best-tested theory in the history of science.
Be honest about its limits from the start. The Standard Model does not include gravity; it offers no candidate for the dark matter that dominates galaxies; and neutrino masses had to be grafted on after neutrino oscillation was discovered. It is spectacularly successful and demonstrably incomplete — both things are true, and Guide 5 is about the gap.
Two families: matter and force
Every fundamental particle falls into one of two families set apart by their spin. Fermions carry half-integer spin (\tfrac12) and make up matter. Bosons carry integer spin ($0 or 1$) and carry the forces. This is not an arbitrary label: the spin-statistics theorem ties half-integer spin to the Pauli exclusion principle, which is why matter takes up space at all.
The matter fermions come in two kinds. Quarks feel the strong force and are never found alone. Leptons — the electron and its heavier cousins, plus the ghostly neutrinos — do not feel the strong force. There are six of each: six quarks and six leptons.
The force-carrying bosons are the gauge bosons: the photon for electromagnetism, eight gluons for the strong force, and the W and Z bosons for the weak force. Standing apart is the spin-0 Higgs boson, the quantum of the field that gives the others their mass. That completes the roster: 12 matter fermions, 12 force bosons (with the gluon and W counted by charge), and one Higgs.
Reading the chart: three generations
The matter fermions are laid out in three columns called generations. Each generation repeats the same pattern — an up-type quark, a down-type quark, a charged lepton, a neutrino — but heavier. The first generation (up, down, electron, electron-neutrino) builds every atom in your body. The second and third are heavier copies, unstable and made only in accelerators or cosmic rays.
Speaking the language: mass is energy
Particle physicists measure mass in units of energy, because of the deepest fact in the field: mass and energy are the same thing. A particle's rest mass is quoted as the energy mc^2 locked inside it, in electron-volts (eV), usually mega-eV (MeV) or giga-eV (GeV).
The rest energy of a particle of mass m. An electron's $0.511$ MeV, a proton's $938$ MeV, the Higgs' $125$ GeV — all are masses stated as energies.
A moving particle also carries momentum, and rest mass, energy and momentum are locked together by the relativistic energy-momentum relation. This single equation governs every collision and decay in the rest of this track — memorize it now.
Energy, momentum and mass form a right triangle. For a massless particle like the photon, E = pc; for one at rest, E = mc^2.