LASER SPECTROSCOPY OF ANTIPROTONIC HELIUM ATOMS: WEIGHING THE ANTIPROTON

摘要

Antiprotonic helium atom (pHe~+) is a metastable (τ～ 3μs) neutral three-body Coulomb system consisting of an antiproton, a helium nucleus, and an electron. It was serendipitously discovered that about 3% of antiprotons stopped in a low-temperature helium gas target automatically form such metastable atoms. Precision laser spectroscopy of pHe~+ is possible by inducing a laser-resonant transition from a metastable antiproton orbit to a neighboring short-lived orbit and by detecting antiproton annihilation events. By comparing the experimental results with the state-of-the-art three-body QED calculations, antiproton to proton mass and charge comparison has been done to a precision of 10 parts per billion, which is currently the most precise test of the CPT symmetry (matter-antimatter symmetry) for baryons.