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Tests of the electric charges carried by the positron and antiproton are derived from recent measurements of the cyclotron frequencies of these particles, and from the spectroscopy of exotic atoms in which they are constituents.There has recently been considerable interest in highprecision tests of the equality of particle and antiparticle masses that is required by CPT symmetry [1,2]. A method that has been applied to electrons and positrons [3] and protons and antiprotons [l] utilizes the cyclotron frequency, (o-qB/m, of a particle of mass m and charge q in a magnetic field B. Indeed, comparisons of the cyclotron frequencies of electrons and protons (or ions that contain these particles as constituents) in the same magnetic field are regarded as tests of the particles' mass ratios, because there is independent information that the magnitudes of the electron and proton charges are equal. However, there are no analogous tests for the charges of electrons and positrons or protons and antiprotons. Therefore, cyclotron frequency comparisons for these particle-antiparticle pairs should strictly be regarded as comparisons of the particles' charge-to-mass ratios [4,5], unless the additional assumptions of either charge quantization or CPT symmetry for particle-antiparticle charges are made. These assumptions involve concepts that are as fundamental as the equality of particle and antiparticle masses and hence deserve their own independent tests. In this Letter we shall derive tests of the positron and antiproton charges by combining precision measurements of their cyclotron frequencies with spectroscopic measurements on positronium and antiprotonic atoms. Then we shall discuss how these tests could be improved with experiments on antihydrogen. We start by reviewing the existing experimental tests of charge quantization.

Hydrogen formation has been observed following proton impact on positronium. This is the first observation of charge exchange involving a positronium target. The cross section for hydrogen formation has been experimentally determined at proton energies of 11.3, 13.3, and 15.8 keV. Values of s H 26͑69͒, 7.8͑62.3͒, and 7.6͑64.4͒ 3 10 216 cm 2 were obtained, in reasonable agreement with recent calculations. [S0031-9007(97)02936-0]

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The ratio of the double to single ionisation cross section, R'", has been measured over a wide range of energies for positron and electron impact on He. At high velocities (>1-2 MeV amu-') the electron results, in accord with previous studies, merge with those for antiproton impact. Using a beam of low-energy positrons, a similar effect is found with proton data, further supporting the hypothesis that the observed difference in the electron and proton double ionisation cross sections is a result of their opposite charge.