Antiprotonic helium
[2] The ASACUSA Collaboration has studied p3He+ and p4He+ atoms with the p occupying a high Rydberg state with large principal and orbital quantum numbers,
[2] Counterpropagating Ti:Sapphire lasers with pulses of duration 30−100 ns excited nonlinear 2-photon transitions in the deep UV, including spectral lines of wavelengths,
However, the probability is increased by a factor of 105 when the laser frequencies sum to within 10 GHz of an intermediate state
States were selected pairwise such that Auger emission to pHe2+ and rapid annihilation produced a detectable Čerenkov signal.
Comparison of the results with three-body quantum electrodynamics calculations made possible a determination of the antiproton to electron mass ratio of 1836.1526736(23).
This is a confirmation of a fundamental symmetry of nature called CPT (short for charge, parity, and time reversal).
The most recent such measurements show that the antiproton's mass (and the absolute value of the charge) is the same as the proton's to a precision of 0.5 parts in a billion.
In 2020 ASACUSA in collaboration with the Paul Scherrer Institut (PSI) reported the experimental verification of long lived pionic helium by spectroscopic measurements, the first time on an exotic atom containing a meson.
In the experiment negatively charged pions from a ring cyclotron were magnetically focused into a tank filled with superfluid helium so that they would expel an electron from the atom and take its place.
Later, to confirm the production, laser light was fired at various frequencies until they found a specific one at 1631 nm where the pion would resonate undergoing a quantum jump from its orbit into an inner one and eventually into the nucleus which would break down into a proton, a neutron and a deuteron.
