Precision laser spectroscopy of antiprotonic helium atoms

Antiprotonic helium atoms are three-body Coulomb systems composed of a helium nucleus with an antiproton and electron orbiting around it. The atoms can be synthesized by stopping antiprotons in a target filled with cryogenic helium gas. By measuring the characteristic transition frequencies of this atom by laser spectroscopy, and comparing the results with precise QED calculations and the antiproton cyclotron frequency, we can determine the charge and mass of the antiproton. 

The (anti)proton-to-electron mass ratio is one of the dimensionless fundamental constants of nature that can be experimentally determined to particularly high precision. Its exact value is an important parameter in the international system of units. The atom also allows us to study the unique QED of a bound baryon-antibaryon system. We succeeded to excite nonlinear two-photon transitions of the antiproton by irradiating the atom with two counter-propagating laser beams. Sharp, sub-Doppler resonances in the deep ultraviolet regions were detected.

More recently we cooled samples of atoms to a temperature of 1.5-1.7 Kelvin by a method called gas buffer cooling. This latest improvement allowed us to determine the antiproton-to-electron mass ratio as 1836.1526734(15) . This agrees with the proton-to-electron value known to a similar precision. A video made by CERN people can be seen here.

Using the high quality antiproton beam provided by the new Extra Low Energy Antiproton (ELENA) facility, it should in principle be possible to determine the transition frequencies of antiprotonic helium to much higher precision; indeed, rapid advances in the last 5 years have made the theoretical calculations many orders of magnitude more precise than the experiment. Precision spectroscopy is however challenging due to a variety of reasons. The atoms can only be synthesized at a low rate. Laser beams of megawatt-scale intensities are needed to excite nonlinear transitions of the antiproton in the atom. The laser resonance signal must be resolved on a background of pi mesons emerging from the target. ASACUSA is currently developing the instruments and techniques to try and improve the experimental precision using the new ELENA machine. 

The energy level diagram of antiprotonic helium is shown below for the two isotopes antiprotonic 4He and 3He. When the atom is first formed, the antiproton occupies a highly-excited Rydberg state with large principle and angular momentum quantum numbers (like n=38, l=37). These antiprotons can be deexcited to a lower-lying state (for example n=37, l=36) by shooting a laser beam onto the atom, its wavelength tuned to one of the atomic transitions shown in the figure (like 529.7 nm).

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