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RESEARCH PRODUCT

First Antiprotons in an Ion Trap

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Measurements of the antiproton mass[2,3,4,5] are represented in Fig. 1. All of these are deduced from measurements of the energy of x-rays radiated from highly excited exotic atoms. For example, if an antiproton is captured in a Pb atom, it can make radiative transitions from its n = 20 to n = 19 state. The antiproton is still well outside the nucleus in this case, so that nuclear effects can be neglected. The measured transition energy is essentially proportional to the reduced mass of the nucleus and hence the antiproton mass can be deduced by comparing the measured values with theoretical values, corrected for QED effects. The most accurate quoted uncertainty is 5 × 10-5 and is consistent with the much more accurately know proton mass, indicated by the dashed line. It looks like it would be difficult to extend the accuracy realized with the exotic atom method. It might be possible, however, that proton and antiproton masses could be compared directly in a storage ring, from the spatial separation of counter propagating beams of protons and antiprotons at comparable or somewhat improved accuracies.[6] Based upon precisions obtained with trapped electrons, positrons and protons, it seems very likely that the measurement uncertainty in the ratio of antiproton to proton masses could be reduced by more than 4 orders of magnitude, to order 10-9 or better.