Absolutely Bonkers Experiment Measures Antiproton Orbiting Helium Ion

An anonymous reader quotes a report from Ars Technica: 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.[...]At temperatures above the point at which liquid helium becomes a superfluid, the transition created a broad peak instead of a sharp one. The peak narrowed as the temperature dropped, and it eventually separated into two distinct peaks at the transition temperature. This separation -- called the hyperfine split -- is caused by interactions between the antiproton and the helium nucleus. The fact that it can be detected with this level of precision indicates that an experimental system can be used to tell us about both the antimatter and the fundamental physics behind these interactions. Why did this experiment work when previous attempts to measure the properties of molecules in liquid helium failed? The researchers suggest their success is mostly due to the fact that they were essentially measuring an odd form of helium in a pool of helium. In the other cases, researchers measured a molecule that was dissolved in the helium, producing very different behavior. (One suggestion is that the helium forms a cage around any molecules dissolved in it, and the cage is large enough to allow the molecule to move around freely.) The researchers are excited about the idea that this process could be used more generally to get these sorts of measurements. Technically, any moderately sized, negatively charged particle could be put in orbit around a helium nucleus, provided it can be slowed down enough -- the researchers specifically mention "negatively charged mesons and hyperons that include strange quarks." The authors suggest that helium with an unusual nuclear composition would also work.

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