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Physicists have for the first time observed the J/psi particle transforming into three others while emitting light.
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The J/psi particle lives for an instant and then decays into lighter particles, much like a snapped string scattering into pieces. Physicists from the BESIII collaboration caught over ten billion such decays to figure out exactly what 'pieces' are produced.
It turns out that most often J/psi first transforms into a neutral pion and a b1 meson, or a pion and a rho(1450) meson, or an eta meson and an h1 meson. These intermediate particles live even shorter and immediately decay into a photon, an eta meson, and a pion. This cascade resembles a relay race: the runner passes the baton, but does so differently depending on the race.
For the first time, three rare decays have been observed where J/psi emits a photon and produces scalar mesons a0(980), a2(1320), and a2(1700). Previously, such transitions were unseen: they are very weak.
The total probability of J/psi decaying into a photon, an eta meson, and a neutral pion has been measured with twice the previous precision. This is important because J/psi serves as a kind of laboratory for studying the strong interaction that glues quarks together inside particles. The more precisely we know how J/psi decays, the better we understand this interaction.
J/psi consists of a charm quark and its antiquark. It's like a pair of dancers spinning together until they decide to part ways, and then other particles are born.
Using $$(10\,087 \pm 44) \times 10^6$$ $$J/\psi$$ events collected with the BESIII detector, we perform the first amplitude analysis of the process $$J/\psi\to\gamma\eta\pi^0$$. The decay is dominated by the intermediate processes $$J/\psi\to\pi^0 b_1(1235)^0 \to\gamma\eta\pi^0$$, $$J/\psi\to\pi^{0}\rho(1450)^0 \to\gamma\eta \pi^0$$ and $$J/\psi\to\eta h_1(1170) \to\gamma\eta\pi^0$$. Contributions from $$J/\psi\to\gamma a_0(980)^0\to\gamma\eta\pi^0$$, $$J/\psi\to\gamma a_2(1320)^0\to\gamma\eta\pi^0$$ and $$J/\psi\to\gamma a_2(1700)^0\to\gamma\eta\pi^0$$ are observed with a statistical significance exceeding $$5\sigma$$, constituting the first observation of radiative transitions of $$J/\psi$$ to isospin-triplet scalar mesons. The total branching fraction of $$J/\psi\to\gamma \eta \pi^0$$ is measured to be $$(25.7\pm0.3\pm1.5)\times 10^{-6}$$, where the first uncertainty is statistical and the second systematic. This result is consistent with the previous measurement, with the precision improved by more than a factor of two.
The J/psi particle was discovered simultaneously in 1974 by two groups — one in the USA and one in China. In China it was called the 'J particle,' and in the USA the 'psi particle,' hence the double name used today.
In the TV series 'The Big Bang Theory,' the characters sometimes discuss particle decays, and J/psi is mentioned as an example of a particle composed of charm quarks.