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Physicists have spotted a rare wave in the decay of a charmed particle for the first time.
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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. In the BESIII experiment, a D0 meson, a particle with a charmed quark, decayed this way. It transformed into a kaon, a pion, an electron, and a neutrino.
Among the fragments, scientists looked for different types of motion: they call them waves, like ripples on water. The main wave is the P-wave, where the kaon and pion fly apart like two swimmers pushing off the pool edge. But for the first time, they also managed to spot the D-wave — a more complex motion, as if the swimmers also rotate around a common center. Its contribution is tiny: just 0.16% of all events, but the signal is so clear that there is no doubt.
The precision is such that if you were counting grains of rice, you would be off by one grain in a thousand.
These data allowed precise measurement of how the D0 meson transforms into the K*(892) particle. It turned out that electrons and muons behave identically — the difference in decay probabilities does not exceed a couple of percent. This confirms a rule of nature: all leptons are equal before the forces. They also tested the symmetry between different kaon charges, and everything matched.
The decay of the D0 meson is a window into the world of the strong interaction, which holds quarks together inside particles. Each new measurement refines the picture, like adding pixels to a photograph. And although the D-wave is just a small fragment, without it the snapshot would remain incomplete.
The D0 meson contains a charmed quark: it was discovered in 1974 simultaneously in two laboratories, an event dubbed the 'November Revolution' in physics.