Antimatter is notoriously volatile, but physicists have learned to control it so well that they are now starting to harness it as a tool for the first time. In a project that began last month, researchers will transport antimatter by truck and then use it to study the strange behaviour of rare radioactive nuclei. The work aims to provide a better understanding of fundamental processes inside atomic nuclei and to help astrophysicists learn about the interiors of neutron stars, which contain the densest form of matter in the Universe.
“Antimatter has long been studied for itself, but now it is mastered well enough that people can start to use it as a probe for matter,” says Alexandre Obertelli, a physicist at the Technical University of Darmstadt in Germany, who leads the project, known as PUMA (antiProton Unstable Matter Annihilation), which will take place at CERN, Europe’s particle physics laboratory near Geneva, Switzerland.
CERN’s antimatter factory makes antiprotons — the rare mirror image of protons — by slamming a proton beam into a metal target, then dramatically slowing the emerging antiparticles so they can be used in experiments. Obertelli and his colleagues plan to use magnetic and electric fields to trap a cloud of antiprotons in a vacuum (see ‘Antimatter to go’). Then they will load the trap into a van and drive it a few hundred metres to the site of a neighbouring experiment, known as ISOLDE, that produces rare, radioactive atomic nuclei that decay too quickly to be transported anywhere themselves. “It’s almost science fiction to be driving around antimatter in a truck,” says Charles Horowitz, a theoretical nuclear physicist at Indiana University Bloomington. “It’s a wonderful idea.”
Because antiprotons annihilate so readily, both with protons and with neutrons, they present a unique way to study the unusual configurations of radioactive nuclei. Whereas everyday atomic hearts host protons and neutrons in roughly equal measure, radioactive isotopes are stuffed with extra neutrons. This imbalance can give rise to exotic characteristics, including a surface ‘skin’ that is richer in neutrons than protons, or an extended halo in which neutrons orbit alone, as in lithium-11 (see ‘Probing a halo’). By observing how often antiprotons annihilate with a proton versus a neutron, the team will be able to understand the relative densities of these particles at the very edge of the nucleus. And because annihilation happens so rapidly, the test will be fast enough to probe even short-lived nuclei. “It’s a kind of test we haven’t been able to do before on these new, more exotic nuclei, which may have very interesting structures,” says Horowitz.
Radioactive nuclei act as microcosms for learning about neutron stars, objects that squash more mass than is contained in the Sun into the size of a city, and which are key to understanding how the Universe’s heavy elements form. The cores of these super-dense stars remain a mystery, but their structure is dictated by the same little-understood interaction that creates bizarre phenomena in neutron-rich nuclei. “One of the reasons understanding neutron skins and halos is so important is to make the most of astrophysical observations,” says Panagiota Papakonstantinou, a nuclear physicist at the Institute for Basic Science in Daejeon, South Korea.
Obertelli and his collaborators hope to create a trap that can store a record 1 billion antiprotons — more than 100 times as many as can be stored inany existing experiment. Another difficulty will be keeping them for weeks at a time, something so far achieved for no more than a few dozen antiparticles at once. This will mean storing them at 4 degrees above absolute zero and in a vacuum comparable with that of intergalactic space. “It’s a challenging project,” says Chloé Malbrunot, an antimatter physicist at CERN. “But I do think it is feasible.”
Developing and testing technology for the portable trap will take around four years, with the first measurements scheduled for 2022. If the method works, physicists could transport antimatter much farther afield, allowing other scientists who aren't involved in the six experiments huddled at CERN’s antiproton source to study and use the elusive matter.
“As soon as they can demonstrate one billion protons and keep them for several weeks, then many more experiments will join, and people with new ideas will come,” says Malbrunot. “I think it will really open up the field.”
Nature 554, 412-413 (2018)