nuclear recoil

In a tank holding three tons of liquid xenon, buried deep underground, something stirred one day. Not the xenon itself, but one of its atomic nuclei: it received a kick with an energy of 248 thousand electronvolts and recoiled like a billiard ball struck by an invisible cue ball.

Inside an atomic nucleus, protons and neutrons don't sit still like peas in a pod. Each particle occupies its own orbit — a region of space where it can be found. The larger the orbit, the more 'smeared out' the particle is.

Buried in Antarctic ice, there's a detector the size of a cubic kilometer. It doesn't catch light but ghostly particles — neutrinos. Yet sometimes, a different beast might flash through: a pair of muons born together and flying apart in a wide fan. No known particle of ordinary matter can paint that pattern.

In 1976, physicists predicted that an electron could do more than just fly past a nucleus: it could slam into it, transferring energy and making the nucleus 'shudder.' They called this effect NEEC — nuclear excitation by electron capture. But for nearly fifty years, no one has managed to observe it in an experiment.

Solar neutrinos are tiny particles born in the Sun's core by the trillions every second. They pass through Earth almost without noticing it, and for a long time they were considered absolutely stable. But perhaps they too can "age"—decaying imperceptibly, vanishing on their way to us.

264 kilometres per second — that's how fast the Solar System hurtles through the cloud of dark matter. Its existence was given away by galaxies: they spun too fast to be held by visible mass, and Vera Rubin was first to suspect a hidden scaffolding.

Quantum entanglement — a phenomenon where the states of two or more particles become interconnected regardless of the distance between them — usually links intrinsic properties such as spin or polarization. However, a new experiment has for the first time realized entanglement of the positions of two atoms in space.

A billiard ball slams into another. Impact, and the balls scatter. But if the push is not sharp but very gentle, the balls just roll together without bouncing. An atom behaves much the same way when hit by a dark matter particle.

When a massive star dies, its core collapses, and a supernova explosion occurs. The cataclysm births streams of particles — like ripples from a thrown stone. The first to arrive are swift neutrinos, which pierce the Earth, hardly interacting. Then, hours later, ghost particles with a tiny charge appear.

In the first moments after the Big Bang, the universe was a boiling soup of elementary particles. Even the light neutrinos were actively colliding with everything around. But the rapid expansion cooled this soup, and the neutrinos broke free, becoming almost indifferent to the surrounding world.

A neutron star is the dead core of a massive star, crushed to the size of a city. A teaspoon of its material weighs as much as a mountain. Dark matter particles fly through such a dense medium almost unimpeded, but occasionally they do hit neutrons and get stuck, like bullets bogging down in deep sand.

Conventional dark matter detectors are like a soccer goalkeeper waiting for a powerful shot while the ball barely rolls. The invisible particles filling our galaxy often move too slowly to be noticed by sensitive sensors.

When two heavy bodies collide, they don't always stay put. Merging black holes get a powerful kick, like the recoil of a gun, but instead of gunpowder, it's the curvature of spacetime doing the work. This kick can send the new hole flying out of a galaxy's center or even beyond its outskirts.

A gamma-ray spectrum looks like a picket fence: each line is the precise energy a nucleus released as a photon. But sometimes ghosts appear in that fence — faint lines shifted by the same amount from the main ones. Such a shift could mean that part of the energy went into creating a light particle we cannot yet see directly.

In a dark chamber where the temperature sits just four degrees above absolute zero, a sapphire crystal the size of a grain of rice lies waiting. When a neutrino, a ghostly particle that barely notices matter, flies through it, the crystal shudders and warms by a tiny fraction of a degree.

In the mid-20th century, astronomer Vera Rubin discovered that stars on the outskirts of galaxies were moving too fast — held by an invisible mass. Thus the mystery of dark matter was born. It doesn’t emit light but reveals itself through gravity. The big question: why is there exactly as much as we see?

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.

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.

Dark matter is the invisible glue that keeps galaxies from falling apart. Back in the 1930s, Fritz Zwicky suspected its existence, and in the 1970s Vera Rubin proved that without it, stars would fly apart.

A muon is like an electron, but about 200 times heavier. When it stops in a piece of silicon, it gets pulled into the atomic nucleus like a nail to a magnet. The nucleus gains extra energy and starts to "boil": various particles fly out — protons, deuterons, tritons, and alpha particles.

At the heart of every galaxy slumbers a supermassive black hole. When it devours matter, its surroundings flare up: this is an active nucleus. How can we learn about past outbursts if the black hole is silent? The answer lies in light echoes.

Visible matter is a drop in the sea of cosmic soup. The foundation of the Universe is made of invisible dark matter, whose gravity keeps galaxies from flying apart. The best candidate for this role is sterile neutrinos, ghost particles connected to us only by the thinnest of bridges.

A silicon sphere 166 nanometers in diameter floats in a beam of infrared laser light. The pressure is below 10⁻⁸ mbar, and the motional temperature is about 10 millikelvin. That is colder than the cosmic microwave background.