A team of researchers from CERN’s Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration have taken experimental equipment from CERN to the Paul Scherrer Institute (PSI) near Zurich to create a theoretically predicted, but never before verified, exotic atom and made the first measurements of how it absorbs and resonates with light. The results, published May 6 in the journal Nature, mark the first time such spectroscopic measurements have been made on an exotic atom containing a meson, a particle consisting of two fundamental particles called quarks.

Replace an electron in an atom with a heavy, negatively charged particle, and you get a so-called “exotic atom.” Such atoms usually have very short lifetimes, and they provide excellent tools for both studying the properties of the replacement particle and searching for physics phenomena not predicted by the Standard Model.

“Spectroscopic measurements of exotic atoms containing mesons could be used to determine, with high precision, the mass and other properties of the constituent mesons, as well as place limits on possible new forces involving mesons,” says ASACUSA co-spokesperson, Masaki Hori. “For the meson used in this study, one of the lightest mesons, we might eventually be able to determine its mass with a precision of less than about one part in a hundred million. That would be 100 times more precise than today’s achievements and would allow a precise comparison with the Standard Model prediction to be made.”

The new atom verified by the experiment consists of a nucleus from an isotope of helium (helium-4), an electron and a negatively charged pion in a high lying energy state. Its lifetime is more than a thousand times longer than any other atom containing a pion. To make such atoms, the team took negatively charged pions provided by PSI’s 590 MeV ring cyclotron facility—the world’s most intense source of such pions—and focused them using a magnet into a target containing superfluid helium. Both the target and the magnet were made at CERN and brought to PSI for this study.

Next, to confirm that the atoms had indeed been created—and to study how they absorb and resonate with light—the researchers fired laser light of various frequencies at the target and searched for instances in which the pions made a quantum jump between different energy levels of their host atoms.

After some trial-and-error playing with different laser frequencies, the researchers were able to identify a specific jump. This jump was predicted to result in the absorption of the pion by the helium nucleus and the subsequent breaking of the latter into a proton, a neutron and a composite particle made up of a proton and a neutron. The researchers detected these fragments using an array of particle detectors that was also made at CERN and brought to PSI, thereby confirming that the pions had indeed made the jump.

Next on the researchers’ agenda is to improve the precision with which the jump was identified and to search for other jumps, with the aim of using them to measure the mass of the pions and test the Standard Model.