Bubble chambers. A bubble chamber holds a liquid, often liquid hydrogen, just above its boiling point. A charged particle ionises the liquid along its path, and tiny bubbles grow on the ions. A cloud chamber works the same way with droplets in a vapour. Liquid hydrogen is much denser than air, so more interactions happen inside the chamber, and every hydrogen nucleus is a proton for the beam to hit.

No track means no charge. Only charged particles ionise the liquid. Photons, neutrons, K⁰ and Λ leave no track at all. You find them from gaps: a track that ends with nothing visible coming out, or a V that starts from nowhere.

The curve. The magnetic force on a moving charge is at right angles to its velocity, so the track curves. Positive and negative particles curve opposite ways. The radius is r = p/BQ, so a particle with more momentum curves less. A particle that loses energy curves more and more tightly and spirals.

Track thickness. A slow, heavy particle ionises more strongly than a fast, light one, so it leaves a thicker, denser trail of bubbles. A proton of a few hundred MeV/c is much slower than a pion with the same momentum, so its track is thicker.

Strangeness. When strange particles are formed, by the strong interaction, strangeness is conserved, so they are made in pairs: one with S = +1 and one with S = −1. When they decay, by the weak interaction, strangeness is not conserved: it can change by 0, +1 or −1. Weak decays are slow, about 10⁻¹⁰ s, which is long enough for a fast neutral particle to travel several centimetres: that is the gap before each V.

The model. The momenta are typical values and the tracks are drawn to scale in a chamber 40 cm wide, with a field of 1.5 T into the page. Each event is shown on its own; a real photograph contains many overlapping tracks.