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For the first time, scientists caught the Σ0 particle turning into a Λ and an electron-positron pair.
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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.
At the BEPCII collider in Beijing, the BESIII detector accumulated 10 billion J/ψ production events — imagine a huge bag filled with billions of glass marbles, and you need to find a few special ones. The scientists used a double-tag method: first, they identified decays of J/ψ into a Σ0 and an anti-Σ0 pair, and then they recorded the desired signature from one of them. This allowed them to measure for the first time how often Σ0 transforms into a Λ and an electron-positron pair: about 6 cases out of 1000.
The obtained number slightly disagrees with calculations — a discrepancy of two standard deviations. It's like predicting that out of 1000 coin flips you'd get 500 heads, but you get 506: the difference is small, but it makes you think.
They also measured the magnetic moment of Σ0—a quantity that shows how the particle responds to a magnetic field. It turned out to be 1.74 in units of the nuclear magneton. For comparison, the proton's value is about 2.79. Such numbers help understand how charges and currents are distributed inside Σ0, what it is 'made of'.
Now theorists have a new clue: perhaps the decay involves not only simple particle exchanges but also more subtle effects. And experimentalists have a refined double-tag method that will be useful for other rare decays.
Based on a data sample of 10 billion $$J/\psi$$ events collected with the BESIII detector operating at the BEPCII collider, the Dalitz decay $$\Sigma^0 \rightarrow \Lambda e^+e^-$$ is studied experimentally for the first time. The $$\Sigma^0$$ hyperons are produced through the process $$J/\psi \rightarrow \Sigma^0\bar{\Sigma}^0$$ and analyzed using a double-tag method. The absolute branching fraction is measured to be $$\mathcal{B}(\Sigma^0 \rightarrow \Lambda e^+e^-) = (6.34 \pm 0.25_{\rm stat.} \pm 0.23_{\rm syst.}) \times 10^{-3}$$. This result shows a $$2\sigma$$ discrepancy from the theoretical calculation quoted in the PDG, where the uncertainties are statistical and systematic, respectively. In addition to the branching fraction, the transition magnetic moment $$\mu$$ is determined to be $$(1.74 \pm 0.03_{\rm stat.} \pm 0.09_{\rm syst.})\,\mu_N$$, where $$\mu_N=e/(2m_p)$$ represents the nucleon magnetic moment, providing valuable insight into the intrinsic structure of the $$\Sigma^0$$ hyperon.
