The Standard Model of Particle Physics

In the Standard Model a force is an exchange of force-carrying particles, called gauge bosons. Two electrons repel by exchanging photons; quarks are held together by exchanging gluons; the weak force, carried by the heavy W and Z bosons, is responsible for beta decay and lets one type of quark change into another. Gravity is so feeble between individual particles that it plays no role in particle physics: the electric repulsion between two protons is more than $10^{36}$ times their gravitational attraction.

There are twelve matter particles, in three generations. Each generation is a heavier copy of the one before, and the heavier particles quickly decay, so the ordinary matter around you is built almost entirely from the first: up quarks, down quarks and electrons. Quarks feel the strong force and carry fractional charges. They also carry a property called colour charge (nothing to do with visible colour), and they are confined: no one has ever isolated a single quark, because pulling two apart puts so much energy into the gluon field that new quark-antiquark pairs form instead. Leptons do not feel the strong force. Neutrinos are neutral and interact so rarely that most pass straight through Earth; the Sudbury Neutrino Observatory, deep in a mine near Sudbury, Ontario, helped show that neutrinos change type as they travel, which means they have mass, earning Arthur McDonald a share of the 2015 Nobel Prize in Physics. The final piece, the Higgs boson, is associated with the field that gives fundamental particles their mass; it was discovered at CERN in 2012.

Hadrons are all the particles built from quarks held together by the strong force. The two families differ in their recipe. A baryon is three quarks; a meson is a quark bound to an antiquark. To find a hadron's charge, just add the charges of its quarks — an antiquark has the opposite charge of its quark, so an anti-down quark $\bar{\text{d}}$ has charge $+\frac{1}{3}$. Every combination that actually exists adds up to a whole-number charge.

Every particle has an antiparticle with the same mass and the opposite charge. The antielectron, or positron ($e^{+}$), was predicted by Paul Dirac's theory and found by Carl Anderson in cosmic rays in 1932. Relativity's $E=m{c}^2$ lets energy and matter trade places. In pair creation, a high-energy gamma photon passing close to a nucleus turns into an electron and a positron; the nucleus is needed to absorb some momentum. In annihilation, a particle meets its antiparticle and both vanish, their mass-energy leaving as photons — for an electron and a positron at rest, two gamma photons flying off in opposite directions so that total momentum stays zero. PET medical scanners detect exactly these pairs of photons.