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Physicists have for the first time broken down a rare particle decay into its components, like a musical score into notes.
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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. It's like a magician dissecting their trick into individual hand movements to figure out exactly when the rabbit appears.
The D0 meson is a particle made of a charm quark and an up antiquark. It lives for less than a billionth of a second.
The most common decay pathway turns out to be this: first, the D0 transforms into a pair of short-lived K*(892) particles: these are called resonances because they live even shorter and immediately fall apart into a kaon and a pion. The physicists measured that this pathway accounts for about 0.279% of all D0 decays. But the key discovery is how these K*(892) particles behave. It turns out their internal rotation (spin) most often aligns as if two tops are spinning in the same plane but in opposite directions. This is called the S-wave state, and it dominates.
Polarization here isn't about light from glasses; it's about the direction of particle spin. A longitudinal polarization fraction of 0.468 means that in nearly half the cases, the tops spin along the line of flight.
Such analyses help test the calculations of the Standard Model — the theory describing all known particles and their interactions. If measurements don't match predictions, it would be a sign that something new is involved in the decay, like an unknown particle. For now, everything matches, and that's good news: we are understanding the microcosm with ever greater precision.
An amplitude analysis of the singly Cabibbo-suppressed decay $$D^0 \to K^+ K^- \pi^0 \pi^0$$ is performed, for the first time, to determine the relative magnitudes and phases of different intermediate processes. The analysis uses $$e^+e^-$$ collision data collected with the BESIII detector at the center-of-mass energy 3.773~GeV corresponding to an integrated luminosity of 20.3 $$\rm fb^{-1}$$. The absolute branching fraction of $$D^0 \to K^+ K^- \pi^0 \pi^0$$ is measured to be \BF. The dominant intermediate process is $$D^0 \to K^{*}(892)^+K^{*}(892)^-$$, with a branching fraction of $$(2.79 \pm 0.13_{\rm{stat.}} \pm 0.11_{\rm{syst.}}) \times 10^{-3}$$. Amplitude analysis reveals that the $$D^0 \to K^{*}(892)^+K^{*}(892)^-$$ decay is S-wave dominant. The longitudinal polarization fraction of $$D^0 \to K^{*}(892)^+ K^{*}(892)^-$$ is measured to be $$0.468\pm0.046_{\rm{stat.}}\pm0.011_{\rm{syst.}}$$.
The D0 particle was discovered in 1976, and since then its decays have been studied to search for differences between matter and antimatter.
