Antiprotonic helium

An antiprotonic helium atom is a three-body object 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 have a charge of -1, whereas helium nuclei have a charge of +2.

Production

These atoms can be produced by simply mixing antiprotons with ordinary helium gas; in this reaction the antiproton first removes one of the two electrons contained in a normal helium atom, and then begins to orbit the helium nucleus in the electron's place. 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 many microseconds, before finally falling to its surface and annihilating. This remarkably long lifetime is seen only in the case of antiprotons mixed in helium; in all other substances, antiprotons annihilate a million times faster, in less than a picosecond.

Laser spectroscopy

Antiprotonic helium is 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. By measuring the particular wavelength of this laser light needed to resonate the atom, the mass and electric charge of the antiproton can be precisely measured. This determination also relies on separate measurements of the antiproton's cyclotron frequency carried out by the ATRAP collaboration at CERN.

Measurement of antiproton mass and charge

Scientists believe that nature has a fundamental symmetry called CPT (short for Charge, Parity, and Time reversal). This law says that all physical laws would appear the same if you simultaneously reversed the charge, parity, and the flow of time (or equivalently, the direction of motion) in the universe. One of the important predictions of this theory is that particles and their antiparticles should have exactly the same mass. Any difference between the masses of antiprotons and protons, however small, would show that this fundamental symmetry is broken. The most recent measurements using the antiprotonic helium atom show that the antiproton's mass (and the absolute value of the charge) is the same as the proton's to a precision of 2 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 for the first time in 2005.

References

• Laser spectroscopy experiments on antiprotonic helium atoms