W. D. Li

At a particle accelerator in Beijing, electrons collide with positrons, and for a fleeting moment, a particle called eta-c is born. It lives for a tiny fraction of a second and then decays into other particles. One of the rarest paths is to split into two photons, particles of light. Until now, no one could say exactly how often this happens.

In an accelerator, particles collide, and for a fleeting instant a D meson is born — a particle that lives for less time than it takes light to cross a grain of sand. It promptly shatters into lighter fragments: a kaon, a lepton, and a neutrino.

In a particle accelerator in Beijing, an anti-Lambda hyperon, a rare antimatter particle, slams into an ordinary proton. Instead of simply disappearing, they transform into a whole bouquet of other particles: a positively charged kaon, two pions, and sometimes additional neutral pions. Such transformations for the anti-Lambda had never been observed before.

In a particle accelerator, the anti-lambda particle is a rare guest. It is born on the fly, lives for a tiny fraction of a second, and usually decays on its own. But sometimes it manages to collide with an ordinary proton, and then a whole shower of light particles is born: kaons, pions.

In the BEPCII accelerator, electrons and positrons collide, producing billions of particles. Among them is the J/ψ meson, which lives for less than a billionth of a second. Its decay sometimes yields an eta meson (ηc), an even shorter-lived clump of quarks.

Two twins look identical, but one is left-handed and the other right-handed. In the world of elementary particles, there are similar pairs: Ξ- and its antiparticle Ξ+. They are born together in the decay of the J/ψ particle and, like entangled twins, maintain a connection even at a distance.

In a circular accelerator, electrons are accelerated to nearly the speed of light and smash into positrons: their antiparticles. At the collision point, a burst of pure energy briefly appears, from which new particles are born. The BESIII detector, like a high-speed camera, captures the tracks of these short-lived guests.

When an electron and a positron collide at the BESIII collider, they produce a short-lived particle — the charmed baryon Λc+. Inside it, like in an overstuffed box, another particle quickly emerges: the Λ hyperon. This hyperon has its own rotation, its spin, and physicists have for the first time been able to measure how much that rotation is aligned with the particle's direction of motion.

In the accelerator, an electron and a positron collide, and for an instant, a particle is born that lives for less than a billionth of a second. Its decay resembles the explosion of a tiny firecracker: fragments scatter in all directions, and physicists reconstruct what was inside from their trajectories.

Inside the proton and neutron, life is boiling: quarks exchange gluons, and for an instant pairs of particles appear and vanish. Some of them, the so-called scalar mesons, live for only 10⁻²³ seconds — less time than light needs to cross an atomic nucleus.

The J/psi particle lives for an instant and then decays into lighter particles, much like a snapped string scattering into pieces. Physicists from the BESIII collaboration caught over ten billion such decays to figure out exactly what 'pieces' are produced.

When a particle decays, it doesn't just vanish: it transforms into a whole set of other particles, and each transformation has its own script. Physicists from the BESIII collaboration have, for the first time, broken down the rare decay of the D0 meson step by step: it turns into two kaons and two pions.

In an accelerator near Beijing, electrons collide with positrons, and for an instant, particles are born that you wouldn't find in ordinary matter. One of them is lambda-c-plus: inside it sits a heavy charmed quark, like a stone in a slingshot. Now physicists have for the first time seen this particle break apart into a neutron, a positive pion, and an eta meson.

Every second, billions of decays happen inside our bodies, and nobody notices. But physicists do: they smash particles in accelerators and watch what fragments they fly into. Hidden in those fragments is the answer to why we exist at all. Right after the Universe was born, there were equal amounts of particles and antiparticles, but today all we see around us is particles.

Imagine a balance scale with identical weights, yet one pan always tips lower. In the world of elementary particles, this imbalance is called CP violation: particles and their mirror twins (antiparticles) behave slightly differently.

The charmed particle D+ lives for a tiny fraction of a second before breaking apart into lighter particles, like an overloaded bridge losing its supports. Physicists at the BESIII collider caught millions of such decays and sorted them out. In one decay mode, four pions and an eta meson are produced. The probability of this event turned out to be higher than expected: about 3 chances in 1000.

Quarks are the tiny building blocks that make up protons and neutrons. Usually, they stick together in pairs: a particle and its mirror-image antiparticle. But sometimes quarks form more exotic combinations, which physicists call exotic states.

Among the debris of particle collisions, short-lived 'guests' sometimes appear — hyperons. One of them, Σ0, lives for less than one ten-billionth of a second and decays on the fly. Catching it directly is nearly impossible, but its traces can be spotted: it leaves behind another particle, Λ, and an electron-positron pair.

An archaeologist carefully brushes away layer after layer, and each new stratum at the charmonium dig is a rare decay captured by the detector. Charmonium — a bound state of a charm quark and its antiquark — lives for mere fractions of a second, yet it manages to tell us about the forces that hold quarks together.