"Determining the antiproton-to-electron mass ratio by laser spectroscopy of antiprotonic helium atoms", Dr. Masaki Hori (MPQ)

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An antiprotonic helium atom is a 3-body Coulomb system composed of a helium nucleus, antiproton, and electron that retains microsecond-scale lifetimes against antiproton annihilation in the helium nucleus. The ASACUSA collaboration at CERN carries out precise laser spectroscopy of this object to measure the characteristic frequencies of transitions of the antiproton within this atom. By comparing the results with three-body QED calculations, the antiproton-to-electron mass ratio was recently determined to a fractional precision of 800 parts per trillion. The results agreed with the known proton-to-electron value. In this experiment we used the technique of buffer gas cooling, i.e., to lower the temperature of our samples of antiprotonic helium atoms to T=1.5 - 1.7 Kelvin by simply allowing the antiprotonic atoms to collide with normal helium atoms at cryogenic temperatures.
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