With the current global pandemic and dominance of negative news in the media, it is easy to look more optimistic stories that show how the world continues to function. In particular, it is easy to overlook stories of scientific advancements than are happening in spite of isolation and world health issues.
   
And while there are many rapid advances in the field of virology and immunology, the news story that I want to cover today is in a far more esoteric branch of science - the study of exotic atoms.

This story begins with a team of researchers from the ASACUSA collaboration, who have used obsolete equipment from CERN to produce a particle that was predicted to exist, but until now has never been observed in the lab. The researchers involved were able to take an ordinary helium atom, and replace the bound electron with a negatively charged pion. They then allowed this exotic atom to absorb and re-emit photons of various wavelengths, creating the first spectroscopic measurements of such a meson-nuclei bound state.

When the electron in an atom is replaced with a heavy, negatively charged particle, the result is an exotic atom. Such atoms tend to have relatively short lifetimes, but they can provide a very useful method for studying the properties of the negatively charged particle and to search for physics phenomena not currently predicted or explained by the Standard Model.

(Just for review, and for those who are not familiar with the quark model of particles, are best theories right now state that protons and neutrons, the building blocks of atomic nuclei, are formed from smaller particles known as quarks. Each nucleon has three quarks, and the type of quarks determine if it will be a proton or a neutron, or something more exotic. A meson is a similar particle to a nucleon, but formed from a single quark and an anti-quark bound together. The pions are the lightest of the mesons, and are formed from the same types of quarks as the proton and the neutron.)

To produce these exotic atoms, the ASACUSA group produced negatively charged pions using a 590 MeV ring cyclotron facility at the Paul Scherrer Institut (PSI) near Zurich – the world’s most intense source of such pions – and focused them using a magnet into superfluid helium.

In order to confirm that the atoms had indeed been created and to study how they absorb and resonate with light, the researchers applied laser light of various frequencies to the target and searched for instances in which the pions made a quantum jump between different energy levels of their host atoms, analogous to how electron transitions in ordinary matter can be used to detect the presence of specific atoms.

At present the results are preliminary, but they do prove the existence of the exotic meson-nucleus bound state that researchers were seeking. The next stages of the experiment are expected to increase the precision at which the pion mass can be measured, and will also produce stronger limits on a number of particle physics theories that extend the Standard Model - or ideally they will detect the effects of such theories and provide theoretical physicists with a new mystery to solve. Only time will tell what the ultimate results of this experiment will be.

As we all continue to adapt to the new standards of life in isolation, it is uplifting to know that scientists are continuing to explore the limits of our understanding of nature.