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Original: "Measurement of Born Cross Sections for $$e^+e^-\to\Sigma^-\bar\Sigma^+$$ at $$\sqrt{s}=3.51-4.95$$ GeV and Observation of $$\psi(3770)\to\Sigma^-\bar\Sigma^+$$"
Physicists have for the first time observed the decay of a particle that lives less than a billionth of a second.
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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.
Among them is a pair of sigma-minus and anti-sigma-plus. These are unusual particles: they consist of three quarks, like protons, but live only about 10⁻¹⁰ seconds. Physicists have for the first time measured how often such a pair is produced at energies from 3.51 to 4.95 GeV. It turned out that at an energy of 3.77 GeV, the production sharply increases: this is the signature of the ψ(3770) particle, which decays into the sigma pair. This decay was observed for the first time, and its significance reached 5.5 sigma — meaning chance is virtually ruled out.
The ψ(3770) particle is a bound state of a charm quark and its antiquark. It is 3.77 times heavier than the proton and lives about 10⁻²³ seconds.
The measured production rates of the sigma particles will help test the vector dominance model, which describes how a photon transforms into particles via intermediate vector mesons. If the model is correct, the ratios of probabilities for different charge states of the sigma particles should follow simple rules. The new data will allow these rules to be refined and deepen our understanding of the strong interaction that binds quarks inside particles.
Sigma particles were discovered in the 1950s, but their exact properties are still being refined. Each new measurement is like adding a new detail to the overall picture of the microworld.