D. Franco

A bullet that doesn't just pierce one target but leaves marks in several layers of armor. That's roughly how superheavy dark matter behaves, and it's what the DarkSide-50 detector was built to catch for the first time. The usual suspects for dark matter (weakly interacting massive particles, or WIMPs) pass through matter almost unnoticed, occasionally bumping into a nucleus.

The MicroBooNE detector is a large tank of liquid argon that catches particles flying through it. When a particle passes, it leaves a trail of charge and a flash of light. These trails combine into a three-dimensional picture of the event, as if the particle were drawing its path in a transparent cube.

In the dark chamber of the detector, argon waits. When an invisible particle (say, a dark matter candidate) flies through, it gives the argon nucleus a gentle shove. The nucleus is knocked out of place, and charges are born around it. Exactly how many charges is no trivial question: physicists use that number to tell what kind of visitor dropped by.

Neutrinos are tiny particles that zip through Earth, barely noticing it. In the MicroBooNE experiment, they are caught in a tank of liquid argon. When a neutrino strikes an argon nucleus, it produces an electron and protons, like splashes from a stone thrown into water.

In a carbon atom's nucleus, particles occupy their own 'apartments'—like residents who cannot share the same room. Physicists call this rule the Pauli exclusion principle: two identical inhabitants of a nucleus cannot be in the same state. But what if the rule is sometimes broken?

A chamber the size of a small truck is filled with liquid argon at minus 186 degrees Celsius. When a particle flies in, it knocks electrons off argon atoms, and those electrons drift toward the walls, tracing a three-dimensional picture of the event.