Protonium

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 it’s much smaller than a hydrogen atom. And unlike a hydrogen atom, which is held together by the electric force, protonium is mainly held together by the strong nuclear force.

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. 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 a rather dry title—not “We found protonium!” But it has over 550 authors, which hints that it’s a big deal. I won’t list them.

• Observation of the anomalous shape of X(1840) in J/ψ→γ3(π+π−), Phys. Rev. Lett. 132 (2024), 151901.

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 GeV/c², a bit more than an already known intermediate of mass 1840 GeV/c².

This new particle is a bit lighter than twice the mass of a proton, 938 GeV/c². So, there’s a good chance that it’s protonium!

But how did physicists made protonium by accident in 2002? They were trying to make antihydrogen, which is a positron orbiting an antiproton. To do this, they used the Antiproton Decelerator at CERN. This is just one of the many cool gadgets they keep near the Swiss-French border.

You see, to create antiprotons you need to smash particles at each other at almost the speed of light—so the antiprotons usually shoot out really fast. It takes serious cleverness to slow them down and catch them without letting them bump into matter and annihilate.

That’s what the Antiproton Decelerator does. So they created a bunch of antiprotons and slowed them down. Once they managed to do this, they caught the antiprotons in a Penning trap. This holds charged particles using magnetic and electric fields. Then they cooled the antiprotons—slowed them even more—by letting them interact with a cold gas of electrons. Then they mixed in some positrons. And they got antihydrogen!

But apparently some protons got in there too, so they also made some protonium, by accident. They only realized this when they carefully analyzed the data 4 years later, in a paper with only a few authors:

• N. Zurlo, M. Amoretti, C. Amsler, G. Bonomi, C. Carraro, C. L. Cesar, M. Charlton, M. Doser, A. Fontana, R. Funakoshi, P. Genova, R. S. Hayano, L. V. Jorgensen, A. Kellerbauer, V. Lagomarsino, R. Landua, E. Lodi Rizzini, M. Macri, N. Madsen, G. Manuzio, D. Mitchard, P. Montagna, L. G. Posada, H. Pruys, C. Regenfus, A. Rotondi, G. Testera, D. P. Van der Werf, A. Variola, L. Venturelli and Y. Yamazaki, Production of slow protonium in vacuum, Hyperfine Interactions 172 (2006), 97–105.

Protonium is sometimes called an ‘exotic atom’—though personally I’d consider it an exotic nucleus. The child in me thinks it’s really cool that there’s an abbreviation for protonium, Pn, just like a normal element.