Number 218 (Story #1), March 17, 1995 by Phillip F. Schewe and Ben Stein CPT CONSERVATION, SPECIAL RELATIVITY AND THE SINGLE ANTIPROTON: Gerald Gabrielse (617-495-4381) of Harvard and his colleagues can adjust the electrical voltages of their tabletop antiproton trap at CERN to remove antiprotons until only a single one remains. Measuring a single antiproton, without the perturbing influences of many particles, not only improves precision in measurements of its charge-to-mass ratio but also allows the effects of special relativity to become manifest. The rate at which the circulating antiproton completes an orbit around the trap, known as the cyclotron frequency, is equal to the product of magnetic field strength and its electrical charge divided by its mass. In separate measurements of a single proton in their trap, they confirmed that the cyclotron frequency of antiprotons and protons is identical to one part in a billion--a factor of 40 improvement over their previous measurements involving many trapped particles and 45,000 times more precise than earlier measurements with hot antiprotons. Not only does this measurement confirm that proton and antiproton masses are equal to a new level of precision (assuming that they have the same magnitude of charge), but it stringently tests the CPT theory, which holds that the outcome of one experiment involving a set of particles is identical to a time-reversed mirror image of the experiment in which all of the particles are replaced by their antiparticles. This is the first exact test of CPT theory for baryonic matter (precision tests of the PCT theorem have already been performed for leptons and pions). In separate measurements, the researchers then applied a brief radio pulse to add energy to the antiproton, increasing its velocity and consequently its mass according to special relativity. As the antiproton dissipated this energy in the trap, the researchers observed an increase in cyclotron frequency, signalling a decrease in the mass. (G. Gabrielse et al., 1 May in Physical Review Letters.)