Comparisons of the orbital cyclotron frequencies of antiprotons and protons test the PCT invariance theorem, as do comparisons of the structure of antihydrogen and hydrogen.
Historically, P, which stands for parity, was examined first. To understand the concept, imagine conducting an experiment in which the outcome is watched in a mirror. Now suppose a second experiment is constructed that is the mirror image of the first. If parity is conserved, the outcome of the second experiment should be identical to the outcome observed as the mirror image of the first experiment performed.
Until 1956, it was believed that reality was invariant under such a parity transformation. Early that year, however, Lee and Yang, then at Columbia University and the Institute for Advanced Study in Princeton, N.J., respectively, realized that the invariance of parity in weak interactions, which are responsible for radioactive decay, had not yet been tested. Later that year Wu and her colleagues at Columbia showed that mirror-image experiments did not produce mirror-image results when weak interactions were involved. The widespread belief in parity conservation was shattered.
Faith in a new invariance, PC, rapidly replaced the discredited notion. C stands for charge conjugation, a "thought experiment" process that turns particles into their corresponding antiparticles. To test whether PC is conserved, a mirror-image experiment is constructed, and all the particles in the experiment are replaced with their corresponding antiparticles. In 1964 Cronin and Fitch, then at Princeton University, used particles called kaons to demonstrate, explicitly and unexpectedly, that PC is not conserved.
Today most physicists believe that PCT is invariant (the T stands for time reversal). Thus far theorists have yet to construct a reasonable theory in which PCT is not conserved. To test the invariance of PCT, imagine making a movie of an experiment's mirror image in which all the particles have been replaced by their corresponding antiparticles. Then a second experiment is performed to mimic what one sees in the film when it is run backward - when "time is reversed".
One consequence of PCT invariance is that the circular cyclotron frequencies of the antiproton and proton in a magnetic field should be identical. TRAP looked for differences in the measured cyclotron frequencies and found no evidence of violations of CPT invariance at the level of 9 parts in 1011. The TRAP experiment is currently one of the most accurate tests of PCT invariance, certainly the most stringent test carried out with baryons and antibaryons.
Another consequence of PCT invariance is that the structure of antihydrogen and hydrogen should be identical. This is the prediction that ATRAP intends to test at even higher accuracy. We shall see whether this invariance under PCT continues to hold.