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The decay of a single particle revealed what light scalar mesons are really made of.
Inside the proton and neutron, life is boiling: quarks exchange gluons, and for an instant pairs of particles appear and vanish. Some of them, the so-called scalar mesons, live for only 10⁻²³ seconds — less time than light needs to cross an atomic nucleus. Scientists have long debated what they are: ordinary quark-antiquark pairs, tetraquarks (four quarks together), or even "molecules" made of two mesons stuck together like drops of water.
The BESIII accelerator in Beijing collides electrons with positrons to produce Ds mesons — particles that live long enough for their decays to be studied in detail. In the new experiment, physicists for the first time saw a Ds meson decay into three pions and an eta meson. Among the debris, they spotted a pair of a0(980) and f0(500)—two scalar mesons. The frequency of this decay turned out to be about one percent — much higher than simple models predicted. It's as if, when taking apart a four-piece construction set, every hundredth time two pieces turned out to be glued together by an invisible adhesive you never suspected.
The measured branching fraction of Ds+ → a0(980)+ f0(500) is (0.98 ± 0.16 ± 0.22)%. This is tens of times larger than naive estimates that treat a0 and f0 as simple quark pairs.
Such a surprisingly high yield of the a0(980) and f0(500) pair hints that inside these particles there is something more complex than just two quarks. Perhaps they really resemble loose "molecules" made of other mesons, which form easily in the cramped space of the decay. The new data give theorists hard constraints: any model of the internal structure of scalar mesons must now reproduce this number.
The same experiment allowed a more precise determination of how often the Ds meson decays with the emission of an eta meson. Previously, some of these decays escaped detectors, leaving a "dark" remainder of a few percent. Now, after measuring the decay into π+π0π0η, this unaccounted tail has shrunk to (0.1 ± 3.1)%—practically zero. Physicists searching for rare Ds meson decays now have a cleaner picture: almost all decay paths with an eta meson are now known.
The scalar meson f0(500) was formerly called the sigma meson, and its existence was long disputed: it is so broad and fuzzy on plots, like a blurry smudge instead of a sharp line.