The Antiproton's Mass and Charge

The antiproton's mass and charge have been measured to within 60 parts per billion, affording new tests of quantum mechanics. Paul Dirac's 1930 prediction of a whole shadow family of particles, antiparticle counterparts of the known particles, was quickly borne out. In 1932 the anti-electron, the positron, was discovered and in 1955 antiprotons (p-bar) were made artificially in an accelerator for the first time. Since that time physicists have sought to determine that antimatter plays by the same rules as ordinary matter.

An excellent place for these studies is at the CERN Antiproton Decelerator in Geneva, where antiprotons are created in high energy collisions, then collected, cooled, decelerated, and directed toward a number of experimental setups. One such experiment, staffed by a Japanese-European collaboration, sends the antiprotons into a bottle of cold helium.

About a million of the p-bars at a time ingratiate themselves into helium atoms, essentially taking the place of an electron and, at least in principle, obeying all known laws of atomic physics, including the ability to make quantum jumps between energy states of this exotic "antiprotonic" helium atom.

In fact the p-bar intruder begins in a somewhat circular orbit but after about one microsecond undergoes a transition to a closer orbit. It does this again and again until the antiproton eventually annihilates with a proton or neutron in the helium nucleus.

Before this happens, however, the CERN scientists have more than enough time to perform some crucial atomic physics, including the first-ever measurement of ultraviolet transitions in this kind of exotic atom. Not waiting for the transitions to occur, the researchers actually induce them with a beam of laser light.

Knowing the laser frequency at which the transitions occur allows one to calculate a number proportional to the antiproton charge squared times the antiproton mass. When this number is combined with a separate measurement of the antiproton's motion in an atom trap (see Update 426), which supplies a value for the ratio of the antiproton's charge to its mass (a ratio measured with uncertainties of only 90 parts per trillion), then a separate value for the mass and charge of the antiproton can be determined. In this case the values agree with those of the proton (allowing for the opposite charge) to within 60 parts per billion. (Hori et al., Physical Review Letters, 27 August 2001; contact Masaki Hori, masaki.hori@cern.ch, 41-22-762-8306, or John Eades at CERN, john.eades@cern.ch; also see CERN website.)