neutral pion

Deep inside a mountain, where cosmic-ray muons are muffled by a thick layer of rock, stands a detector that catches not light but its absence. Positronium is a tiny atom in which an electron and its antiparticle, the positron, circle each other like a pair of dancers. Usually they come together and vanish in a flash of two or three gamma quanta.

A courtroom interpreter must relay every word without being distracted by noise in the room. In the quantum world, a superconducting resonator plays that role, losing photons on schedule without straying into spurious effects.

The neutron is a particle that usually sits in the atomic nucleus and doesn't disappear. But some theories suggest that occasionally it might transform into its invisible twin — a particle that ordinary detectors can't catch.

In a quiet room, you can hear the faintest whisper. Likewise, in the hydrogen atom, the lightest of all, an uninvited guest is easier to spot. That guest could be the Z' boson, a mediator particle of a new force predicted by many unification theories. The Standard Model already has its own mediator — the Z boson.

The 21-kilometer ring near Protvino was once planned to accelerate protons. Today, the same tunnel could host a very different machine — a muon collider. The muon is a heavy cousin of the electron, and that's its strength. On a curve, an electron radiates like a searchlight, losing precious energy. A muon radiates a billion times less.

Imagine a green beam that, passing through an ingenious optical setup, twists into a three-dimensional funnel of light — an optical vortex. At its heart, like in the eye of a hurricane, the intensity drops to zero. It is there that photons disappear.

In 2021, the LHAASO observatory registered gamma quanta with energies above a petaelectronvolt from sources like the Crab Nebula — a supernova remnant with a pulsating neutron star at its heart.

Since Saul Perlmutter and his colleagues discovered the accelerated expansion of the universe, physicists have been scratching their heads over the nature of dark energy.

In the microscopic world of the strong interaction, nature's most powerful glue, quarks never wander alone. Over large distances, they stick together into the protons and neutrons we know. But under extreme compression, like in the depths of a neutron star, this glue creates far more exotic forms of matter.

Imagine a perfectly tuned orchestra, where every instrument is adjusted with flawless precision, trying to play in a noisy hall. That noise is quantum gravity — the microscopic ripples of spacetime. It constantly threatens to throw the musicians off.

Electrons and muons are tiny magnets. Their magnetism is slightly more than two (in special units), and this 'excess fraction' (anomalous magnetic moment) arises from constant ripples in the vacuum: particle pairs momentarily pop into existence and vanish.

A regular flashlight shines equally forward and backward. But in magnetic crystals, a axion appears — a ghostly excitation, like a wave going around a stadium: atoms pass a magnetic impulse to each other. When light meets an axion, a hybrid forms, whose fate depends on electric and magnetic fields.

In a dark room, a glass bead the size of a dust grain hangs on a laser beam. Beneath it are two mirrors: one aluminum, one silver. If axions (light, invisible particles that could make up the mysterious dark matter) whiz by, they exchange with the mirror atoms, and the bead shifts ever so slightly.

Fusion, the merging of a proton with a boron-11 nucleus, promises a near-perfect energy source: no dangerous neutron flux, just a stream of charged particles that can be directly converted into electric current. But there's a fundamental problem: the Coulomb barrier.

Inside every atom lives a proton. For a long time, it was considered as eternal as matter itself. But theories that unify all forces of nature predict that, sooner or later, the proton will decay. The only question is how long we have to wait. In the Japanese detector Super-Kamiokande — a giant tank of ultra-pure water — physicists searched for signs of such a decay.

A silicon wafer as thin as half a human hair catches a passing particle the way a palm catches a raindrop. But if that drop is a proton accelerated to enormous energy, the palm can get burned. Inside the detector, an avalanche of charge is born, and sometimes it's enough to burn a microscopic hole. This is called single-event burnout.

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 collisions of heavy nuclei, a fireball of thousands of particles is born. Among them are pions and kaons, two kinds of light particles that look very much alike. To understand what happens in this fireball, physicists need to know exactly how many of each particle flew out.

A dark matter detector is like a room where you're trying to hear a whisper while static electricity crackles nearby. This crackle is produced by radon atoms — a radioactive gas that seeps out from materials and air.

The antiproton is the proton's twin, only with a negative charge. When such a twin approaches a nucleus, it doesn't just bounce off: a whole dance of attraction and repulsion unfolds between them.

Inside a piece of magnetic material, electron spins, tiny magnetic arrows, usually arrange into simple patterns. Sometimes they swirl into vortices, like water down a drain, and these vortices can tie themselves into knots.

Igniting a miniature sun is a long-held dream of humanity. Typically, scientists try to confine scorching plasma (a gas of charged particles) at temperatures of hundreds of millions of degrees. But plasma tends to escape magnetic traps, and confinement comes at enormous cost.

In the first moments after the Big Bang, space boiled. A superheated substance permeating the world gave rise to giant bubbles of a new phase – much like a liquid produces vapor. Collisions of these bubbles accelerated particles to energies unimaginable in Earthly accelerators.

All bodies in vacuum fall with the same acceleration: this principle, formulated by Einstein in general relativity, underpins our understanding of gravity. Yet so far it has been experimentally confirmed only for particles of ordinary matter.