It looks like they've found protonium in the decay of a heavy particle! 🎉
Protonium is made of a proton and an antiproton orbiting each other. It lasts a very short time before they annihilate each other.
It's a bit like a hydrogen atom where the electron has been replaced with an antiproton! But unlike a hydrogen atom, which is held together by the electric force, protonium is mainly held together by the strong nuclear force. It's also much smaller than a hydrogen atom.
There are various ways to make protonium. One is to make a bunch of antiprotons and mix them with protons. This was done accidentally in 2002 during the first experiment that created antihydrogen. They only realized this upon carefully analyzing the data 4 years later.
This time, people were studying the decay of the J/psi particle. The J/psi is made of a heavy quark and its antiparticle. It's 3.3 times as heavy as a proton, so it's theoretically able to decay into protonium. And careful study showed that yes, it does this sometimes!
The new paper on this has over 550 authors, so I won't list them all. It also has a rather dry title - not "We found protonium!"
The idea here is that sometimes the J/ψ particle decays into a gamma ray and 3 pion-antipion pairs. When they examined this decay, they found evidence that an intermediate step involved a particle of mass 1880 MeV/c², a bit more than an already known intermediate of mass 1840 MeV/c².
This new particle is a bit lighter than twice the mass of a proton, 938 MeV/c². So, there's a good chance that it's protonium!
