Laser spectroscopy of long-lived antiprotonic and pionic helium in superfluid helium

The PiHe collaboration of PSI recently carried out laser spectroscopy of an infrared transition in three-body pionic helium atoms that were synthesized in a superfluid (He-II) helium target. Analogous measurements of antiprotonic helium atoms embedded in liquid helium were carried out by the ASACUSA collaboration of CERN. Spectral lines of unexpectedly narrow, sub-gigahertz linewidth were revealed in the antiproton case. An abrupt reduction in the linewidth was observed when the liquid helium (He-I) surrounding the atom transitioned into the superfluid phase.