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An accelerator in Beijing has spotted a charmed particle turning into a neutron with two light companions.
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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.
A neutron is a familiar building block of atomic nuclei, while pions and eta mesons are light particles that live for mere fractions of a second.
To fish this rare decay out of billions of collisions, the researchers used a neural network based on the Transformer architecture — the same idea behind large language models, but trained on detector signals. It filtered out background noise like an experienced fisherman tossing small fry back into the sea. The signal was so clean that its significance reached 9.5 sigma — far beyond the threshold of chance.
They also measured the fraction of such decays: about 3 cases per thousand. However, an intermediate step involving the mysterious particle a0(980) was not confirmed — only an upper limit was set. This means the decay proceeds directly, without extra intermediaries.
Such measurements are like marking a map of the invisible world. Each new arrow on the decay scheme refines how strongly the charmed quark interacts with other particles. It's a step toward understanding how matter is structured at the deepest level.
By analysing 6.1 $${\rm fb}^{-1}$$ of data collected at center-of-mass energies between $$\sqrt{s}=4.600$$ and 4.843 $$\rm GeV$$ with the BESIII detector at the BEPCII collider, we observe the decay $$\Lambda_c^+\to n\pi^+\eta$$ for the first time with a statistical significance of $$9.5\sigma$$. The ratio of branching fractions $$\mathcal{B}(\Lambda_c^+\to n\pi^+\eta)/\mathcal{B}(\Lambda_c^+\to \Lambda\pi^+\eta)$$ is measured to be $$0.155\pm0.031_{\rm stat.}\pm0.012_{\rm syst.}$$ Taking the world average of $$\mathcal{B}(\Lambda_c^+\to \Lambda\pi^+\eta)$$ as reference, the absolute branching fraction is calculated to be $$\mathcal{B}(\Lambda_c^+\to n\pi^+\eta)=(2.94\pm0.59_{\rm stat.}\pm0.23_{\rm syst.}\pm0.13_{\rm ref.})\times10^{-3}$$. The intermediate process $$\Lambda_c^+\to na_0(980)^+$$ is also searched for in the $$\pi^+\eta$$ invariant mass spectrum. Since no significant signal is found, the upper limit on $$\mathcal{B}(\Lambda_c^+\to na_0(980)^+)\times\mathcal{B}(a_0(980)^+\to\pi^+\eta)$$ is set to $$8.4\times10^{-4}$$ at 90\% confidence level. A sophisticated deep learning approach using a Transformer-based architecture is employed to distinguish signals from prevalent hadronic backgrounds, complemented by thorough validation and systematic uncertainty quantification.
