Trails of ghosts

The ancient Greeks thought the fundamental particle was the atom. But when the electron was discovered in 1897, we realised atoms can be split.

Since then, physicists have continued the quest for subatomic particles. All we can see of them is the tracks they leave, like the contrails planes leave in the sky.

This vapour-filled cloud chamber detected the first positron – the antimatter partner to an electron – in 1932. Created by a cosmic ray collision, the positron entered from the bottom. Its path was bent by a magnetic field, displaying its positive charge. The shape of the track revealed its mass.

This first image of antimatter is among the most important photographs in the history of science.

Neutrinos make sparks fly …

To capture the blazing trail of a muon neutrino that travels very close to the speed of light and slips through matter like a ghost, Nobel prize-winning American physicists Melvin Schwartz (shown here), Leon Lederman and Jack Steinberger used a spark chamber. A voltage is applied between neighbouring metal plates that are separated by insulating helium and neon gas. Muon neutrinos, created in a nearby atom smasher, barrelled through a 5,000-tonne steel wall made of old battle ship plates and through the gas, leaving a trail of sparks in their wake.

… and leave a trail of bubbles

These psychedelic swirls made their mark on particle physics in the ’60s and ’70s (perhaps their time had come). The bubble chamber is similar to a cloud chamber but filled with a fluid, typically liquid hydrogen, heated to just below boiling point.

The hydrogen boils as charged particles zoom through, leaving a trail of bubbles. In this image, a muon neutrino generated by an atom smasher has torn in from the left. The neutrino itself left no track but it collided with a neutron which exploded in a “shower” of particles. The more energetic the particle, the longer its track.

Quarks zap delicate wires

The “top quark” was discovered using a drift chamber. This machine moved particle detection into the digital age. It could pick up thousands of particles a second – handy for tracking one of the most fleeting of subatomic particles, the top quark, which decays within a trillionth of a trillionth of a second. The picture shows the Central Outer Tracker. It operated between 1986 and 1996 as part of the Collider Detector at Fermilab near Chicago and discovered the top quark in 1995. It contained tens of thousands of delicate gold-plated tungsten wires bathed in argon gas. As charged particles generated in atom smashers shot through the tracker, they knocked electrons from argon atoms and flung them against the nearest conductive wire. Each “pulse” allowed the physicists to track the particles in 3-D.

Hunting the Higgs boson

Today’s most powerful atom smasher – the Large Hadron Collider – requires high-tech detectors such as the Compact Muon Selenoid detector. Four layers record particles’ momentum, energy, charge, mass and path.

In 2013 it detected the remnants of a Higgs boson – the final unverified particle of the Standard Model of physics. Forged from two colliding protons, the Higgs decayed into a pair of photons – as the Standard Model predicted. They are shown here as yellow dotted lines and green towers.

But the Standard Model only explains around 5% of the Universe. “There’s so much more dark energy and dark matter out there – there’s lots of unanswered questions,” says particle physicist Gavin Hesketh. Will the fundamental particle of dark matter be found next?