neutrinoless double beta decay

Picture a room where hundreds of people are talking at once while you strain to hear a pin drop. That is essentially how physicists with the PandaX-4T collaboration listened for silence inside 3.7 tonnes of liquid xenon.

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.

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.

Atomic batteries powered by plutonium decay have been running pacemakers and space probes for over sixty years. But the raw material is scarce: worldwide, only a few kilograms of plutonium-238 are produced each year, barely enough for the most critical missions.

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.

A massive star exhausts its fuel, and its core collapses in a fraction of a second. A supernova is born, along with a brief but powerful shower of neutrinos. These particles barely interact with matter, so they fly away freely, carrying information about the catastrophe.

Imagine an hourglass where the sand grains transform into other grains as they fall: that's what unstable atomic nuclei are like. They live for an instant and then decay, ejecting an electron (this is beta decay). In September 2025, theorists and experimentalists gathered under the FRIB Theory Alliance to discuss how to use such decays to probe the structure of matter.

In 1946, proposed a method that revolutionized archaeology: determining age by the decay of radiocarbon. The clock ticks: every 5730 years, half of the atoms vanish. But why so slowly? For a light nucleus, this is an anomaly — typical lifetimes are millions of times shorter. The mystery held for seven decades.

In the first seconds after the Big Bang, the Universe left us not only light but also something far more elusive — the relic neutrino background. This echo of creation permeates every cubic centimeter, yet we barely notice it.

When a massive star runs out of fuel, its core collapses, giving birth to a supernova and unleashing an all-penetrating shower of neutrinos.

In nuclear physics, the belief long reigned that two neutrons cannot form a bound state — they attract too weakly and immediately fly apart. However, on the surface of neutron-rich nuclei, where the matter density drops to a thousandth of normal nuclear density, the situation changes dramatically.

The collision of two neutron stars is a cosmic laboratory where matter is compressed to unimaginable densities. In the first moments after the merger, a hot cloud forms and cools by emitting neutrinos. These ghostly particles carry away energy and dictate how much matter is ejected into space and which heavy elements are synthesized.

On one side of the scale lies all the matter in the Universe; on the other, all the antimatter. According to the laws of physics, at the moment of the Universe's birth, the pans should have been balanced. But we see one pan tipping: around us are stars, planets, people, and almost no antimatter. This imbalance is one of the greatest mysteries.

Deep underground in Japan, a huge tank of ultrapure water waits for rare visitors — neutrinos arriving from distant stellar explosions. Such guests are few and far between: over years of observation, Super-Kamiokande registers only a handful of events.

A translator strains to hear a single word in years of noise. That's the task physicists face when hunting for neutrinoless double beta decay neutrino. This hypothetical process is like an atomic nucleus that emits two electrons but "forgets" to release a pair of neutrinos.

In the first moments after the Big Bang, matter and antimatter should have been created in equal amounts. But today we see only matter — stars, planets, ourselves. Where did the antimatter go? One clue hides in nearly elusive particles: right-handed neutrinos. The usual story is that their decays create a slight bias in favor of matter.

A giant tank of ultrapure water sits deep underground in Japan. Its walls are lined with thousands of light-sensitive sensors that catch faint flashes — the traces of ghostly particles passing through the water.

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 translator halfway across the hall suddenly starts rendering the message backwards. That's roughly how a neutrino, a ghost particle that barely notices matter, might behave if it encounters a cloud of ultralight dark matter.

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.

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?

Neutrinos are tiny particles that pass through the planet's bulk like light through a windowpane. Every second, billions of them fly through your pinky fingernail, but catching even one is no easy feat. To do it, scientists dug a two-kilometer-deep mine in Canada and installed the SNO+ detector.

Colliding two lead nuclei at the Large Hadron Collider is like throwing two hefty bags of marbles at each other. Usually, the marbles scatter in all directions, but sometimes one bag passes so close that the marbles don't collide: they just exchange glances.

In the first minutes of the Big Bang, the universe whipped up a cocktail of hydrogen, helium, and a pinch of lithium.

A neutron star is the dead core of a star that once exploded, compressed to the size of a city. If a normal star is nearby, the neutron star siphons matter from it like a vacuum cleaner. This matter falls onto the surface and forms a crust — a solid layer of neutron-rich atomic nuclei.

In a dark mine under the Italian Alps, a tank holds three tons of liquid xenon. When a particle hits it, it knocks faint flashes of light and electron clouds out of the atoms. Following these tracks, physicists hope to see an ultra-rare decay: a xenon-136 nucleus turning into barium-136, emitting two electrons and no neutrinos.

The tau lepton is a heavy cousin of the electron, living less than a trillionth of a second. It usually decays into lighter particles, but never directly into three muons: such a transition is forbidden by known laws of particle physics. If it did happen, it would mean unknown forces are at play.

Inside an atom's nucleus, protons and neutrons stack in layers, like spectators in a packed hall. When a layer is full, the nucleus becomes especially stable — physicists call these 'full rows' magic numbers. One of them, 32, has long been elusive: in nuclei with 32 neutrons, its effect is weak, and in some neighbors it seems absent altogether.

An atomic nucleus usually follows its own rules: its decay doesn't care about temperature, pressure, or molecular neighbors. But the uranium-235m isotope is a striking exception. This nucleus sits in an excited state, like a tightly coiled spring, and sooner or later it releases the extra energy.

In the first moments after the Big Bang, the universe was a hot soup of particles. Among them could have been neutrinos — very light and nearly elusive particles. Some of them might have been 'sterile': they barely interact with the rest of matter, like a quiet guest at a loud party.

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.

Inside the accelerator, particles collide, and for a fleeting moment a B meson is born — a heavy particle that lives for less than a billionth of a second. Usually, it decays into a shower of debris, but sometimes it transforms into just two particles: a muon and a neutrino. This rare path is like a lottery where the winning ticket appears once in every two million draws.

In the first minutes after the Big Bang, the Universe was like a blazing hot stove. On it, according to the 'recipe' of the standard theory, hydrogen, helium, and a bit of lithium were supposed to cook. But telescopes see only a third of the lithium that this recipe calls for.

A pebble drops into a pond, and ripples spread across the water. But sometimes the pebble doesn't sink right away: it skims beneath the surface, travels a few meters, and only then surfaces, creating ripples in an unexpected spot.

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.

Ultracold atoms can act as one. When a neutrino is born inside such a cloud, it usually slips away unnoticed.

Measured dipole moment of tantalum-181: 3.39 × 10⁻³² e·cm — like spotting a one-atom shift across the entire Solar System. An ordinary atomic nucleus is perfectly balanced, like a spinning top. In tantalum, that balance is broken: the positive and negative charges are slightly offset from each other.

An electron flies out of a tritium nucleus with an energy of 18.57 keV, leaving behind an invisible partner: a neutrino. If that neutrino has access to a fifth dimension, its mass splits into a whole tower of states, and each one takes its own bite out of the spectrum.