Absolutely bonkers experiment measures antiproton orbiting helium ion

In Wednesday's issue of Nature, a new paper describes a potentially useful way of measuring the interactions between normal matter and exotic particles, like antiprotons and unstable items like kaons or elements containing a strange quark. The work is likely to be useful, as we still don't understand the asymmetry that has allowed matter to be the dominant form in our Universe.

But the study is probably most notable for the surprising way that it collected measurements. A small research team managed to put an antiproton in orbit around the nucleus of a helium atom that was part of some liquid helium chilled down to where it acted as a superfluid. The researchers then measured the light emitted by the antiproton's orbital transitions.

Why would anyone want to do this?

There are many reasons you'd want to get precise measurements of this sort of thing. For one, the measurements will be sensitive to the properties of antimatter and strange quarks, which are short-lived and are often created in environments that make precision measurements challenging. In addition, this system involves interactions between antimatter and regular matter, which can be difficult to capture due to their violent ends. Finally, the specific interactions here—between an atomic nucleus and an object in the orbitals that surround it—are sensitive to properties that are fundamental to the Universe.

In this case, the antimatter was an antiproton. As it's the opposite of a proton, it has a negative charge. From the perspective of the nucleus, the antiproton looks a lot like a morbidly obese electron: it will occupy orbitals with precise energies around the nucleus but with a different shape from those occupied by the electron. And just like an electron, the antiproton can shift between orbitals by absorbing or emitting a photon.

The energy of the emitted photons provides information about the interactions between the antiproton and the atomic nucleus. That information is what the researchers were after.

Doing these measurements presented a significant challenge, however, and not just because of the tendency of matter and antimatter to annihilate each other. Any motion by the atoms being studied will typically cause the photon to be red- or blue-shifted relative to its actual value. In a high-energy environment, this process will turn what should be a sharp peak at a specific wavelength into an imprecise blur that doesn't give us useful answers.

Trying something that hasn’t worked

The simplest way to avoid this problem is to slow the atoms down, which means cooling them. In the case of helium, however, sufficient cooling will create a superfluid, at which point its atoms will flow without losing energy to viscosity. This transition has the potential to make things worse. In the past, researchers targeted temperatures right at the transition, where the liquid helium is at its most dense (and where it's notably denser than hydrogen, which might otherwise be an option for this type of experiment).

But those experiments haven't worked out, as the measurements produced the broad peaks typical of imaging samples that are moving around a lot. Researchers have speculated about why the experiments haven't worked, but the new work favors one explanation, which we'll come back to.

In any case, the experimental setup involves a bit of recycling, using antiprotons that might otherwise be thrown away. CERN is producing antiprotons to use in the creation of antihydrogen atoms, but this process only works with antiprotons that are below a certain energy. Once the low-energy particles are shuttled off to that experiment, CERN is left with a beam of moderate-energy antiprotons. This beam was what was used in the experiments.