Antiprotonic helium
Antiprotonic helium is a three-body atom composed of an antiproton and an electron orbiting around a helium nucleus. It is thus made partly of matter, and partly of antimatter. The atom is electrically neutral, since both electrons and antiprotons each have a charge of −1, whereas helium nuclei have a charge of +2. Production. These exotic atoms can be produced by simply mixing antiprotons with ordinary helium gas; the antiproton spontaneously removes one of the two electrons contained in a normal helium atom in a chemical reaction, and then begins to orbit the helium nucleus in the  electron's place. This will happen in the case of approximately 3% of the antiprotons introduced to the helium gas. The antiproton's orbit, which has a large principal quantum number and angular momentum quantum number of around 38, lies far away from the surface of the helium nucleus. The antiproton can thus orbit the nucleus for tens of microseconds, before finally falling to its surface and annihilating. Laser spectroscopy. Antiprotonic helium atoms are under study by the ASACUSA experiment at CERN. In these experiments, the atoms are first produced by stopping a beam of antiprotons in helium gas. The  atoms are then irradiated by powerful laser beams, which cause the antiprotons in them to resonate and jump from one atomic orbit to another. In 2022 ASACUSA found unexpected narrowing of antiprotonic helium spectral lines. Measurement of the mass ratio between the antiproton and electron. By measuring the particular frequency of the laser light needed to resonate the atom, the ASACUSA experiment determined the mass of the antiproton, which they measured at times more massive than an electron. This is the same as the mass of a "regular" proton, within the level of certainty of the experiment. This is a  confirmation of a fundamental symmetry of nature called CPT (short for charge, parity, and time reversal). This law says that all physical laws would remain unchanged under simultaneous reversal of the charge axis, parity of the space axes, and the orientation of the time axis. One important prediction of this theory is that particles and their antiparticles should have exactly the same mass. Comparison of antiproton and proton masses and charges. By comparing the above results on laser spectroscopy of antiprotonic helium with separate high-precision measurements of the antiproton's cyclotron frequency carried out by the ATRAP and BASE  collaborations at CERN, the mass and electric charge of the antiproton can be precisely compared with the proton values. 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. Antiprotonic helium ions. An antiprotonic helium ion is a two-body object composed of a helium nucleus and orbiting antiproton. It has an electric charge of +1. Cold ions with lifetimes of up to 100 ns were produced by the ASACUSA experiment in 2005. Pionic helium. 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 in a atom containing a lepton. Its existence had been predicted in 1964 by George Condo at University of Tennessee to explain some anomalies from bubble chamber tracks but no definite proof of its existence had ever been obtained. 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 deuterium. The experiment proved highly technical to perform and took 8 years, including the design and construction of the experiment.