A new type of exotic atom, a pionic helium atom was created and examined by members of the ASACUSA Program running at the European Organization for Nuclear Research (CERN). The results of the measurements were first published in the scientific journal Nature. Two Hungarians are among the authors of the article: Dániel Barna, Wigner Research Centre for Physics, Budapest and Anna Sótér, ETH, Zürich. This achievement is seen as a breakthrough, as although the existence of this atom had already been predicted, its existence had not been proven till now. The creation of the new atom was identified with the help of a laser device at the Paul Scherrer Institute (PSI).

A measurement system known as the Antimatter Factory has been operated by CERN since 1999. This is where antimatter was first created and exact spectroscopy measurements were carried out on it. One of the projects pursued here is the ASACUSA Program, headed by Japanese researchers. Besides Austrian, German and Italian research groups, a team of Hungarian researchers from the Wigner Institute, Budapest and Atomki, Debrecen were the founders of the Program. “There are several first discoveries and experiments among the successes of the project, including proof that apart from the sign of their electric charge, all the properties of a proton and its corresponding antimatter particle, the antiproton, are identical to a very high degree,” writes Dezső Horváth, a researcher at the Wigner Institute, discussing the new results. Dezső Horváth was among the founding fathers of the Program, working in the project for several years.

Measurements have been performed for two decades on this long-lived exotic atom, which is made up of an atomic nucleus of helium, an electron and an antiproton, i.e., one of the electrons of the helium is replaced by a negatively charged antiproton. In the cold atom, the antiproton was moved between relatively stable and unstable states with a fine tuned laser beam. A recent idea research workers had was to test whether a novel exotic atom can be created using negative pions. One of the leaders of the ASACUSA experiment, Masaki Hori arranged for the construction of a special measurement device at CERN. The apparatus was later transported to the Paul Scherrer Institute (PSI) near Zürich, where the largest capacity pion source in the world is operated. The aim of the experiment was to determine the weight of pions more precisely than before, in a similar manner to the weighing of antiprotons. The biggest challenge was, however, the short lifespan of pions. In contrast to stable antiprotons, a pion decays in 26 nanoseconds. In the course of the experiment, negative pions were fed by a magnetic field into the target containing superfluid helium cooled down to almost absolute zero.

During the experiment, a pion was inserted into the helium atom, which replaced one of the two electrons. In the next step, the helium atom containing the pion was excited by laser, which caused the atomic nucleus to absorb the pion, and in turn the nucleus decomposed into its particles. The creation of the helium atom containing the pion was identified by a finely tuned laser: at a certain frequency the pion was absorbed through resonance by the helium’s atomic nucleus, which led to an explosion, i.e., the nucleus decomposed into smaller particles in a similar manner to atomic fission. Researchers identified the following particles in the experiment after the decomposition: proton, neutron and deuteron (i.e., a stable particle composed of a proton and a neutron).

Now the researchers aim to make the measurement of the laser transition even more precise in order to determine the weight of a pion. According to their preliminary calculations, this method can increase accuracy a hundred times, which could effectively test the Standard Model of particle physics.