Measuring the antiproton-to-electron mass ratio
by precise laser spectroscopy of antiprotonic helium,
and a new laser spectroscopy experiment of pionic helium

Antiprotonic helium

Antiprotonic helium is a three-body atom composed of a helium nucleus with an antiproton and electron orbiting around it. It is the most long-lived (i.e., stable) matter-antimatter atom known. By using lasers to excite the antiproton from one orbit to another and precisely measuring the atom's transition frequencies, and comparing the results with quantum-electrodynamics (QED) calculations, ASACUSA determines the mass of the antiproton relative to the electron mass. The consistency of the fundamental matter-antimatter (CPT) symmetry was thus tested to a precision below 1 part-per-billion.

Antiprotonic helium

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:

Nature 484, 475 (2011)

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.

Science 371, 610 (2016)
A video for the general public made by the 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.

Nature 603, 411 (2022)

When atoms are placed into liquids, their optical spectral lines corresponding to the electronic transitions are greatly broadened compared to those of single, isolated atoms. This linewidth increase can often reach a factor of more than a million, obscuring spectroscopic structures and preventing high-resolution spectroscopy, even when superfluid helium, which is the most transparent, cold and chemically inert liquid, is used as the host material. Here we show that when an exotic helium atom with a constituent antiproton is embedded into superfluid helium, its visible-wavelength spectral line retains a sub-gigahertz linewidth. An abrupt reduction in the linewidth of the antiprotonic laser resonance was observed when the liquid surrounding the atom transitioned into the superfluid phase. This resolved the hyperfine structure arising from the spin–spin interaction between the electron and antiproton with a relative spectral resolution of two parts in 106, even though the antiprotonic helium resided in a dense matrix of normal matter atoms. The electron shell of the antiprotonic atom retains a small radius of approximately 40 picometres during the laser excitation. This implies that other helium atoms containing antinuclei, as well as negatively charged mesons and hyperons that include strange quarks formed in superfluid helium, may be studied by laser spectroscopy with a high spectral resolution, enabling the determination of the particle masses. The sharp spectral lines may enable the detection of cosmic-ray antiprotons or searches for antideuterons that come to rest in liquid helium targets.

Pionic helium

We have also recently succeeded in carrying out laser spectroscopy of metastable pionic helium atoms using the 590 MeV cyclotron facility at the Paul Scherrer Institute near Zurich. This is a three-body atom that consists of a helium nucleus, electron, and orbital pion. In the early 1960’s, experimental data in the form of photographs showing negative pions traversing bubble chambers filled with liquid helium suggested that some fraction of the pions were surviving long enough to decay into muons. In 1964, a theory was presented by a researcher of Tennessee University to explain this anomalous effect; it was assumed to be due to the formation of the metastable pionic helium atom. It was however not possible to verify the theory because the hypothetical atom’s lifetime is so short it does not emit any atomic fluorescence photon which may be used to identify it. Calculations showed that the transition frequencies of the atom lie in the optical region which is subject to intense backgrounds. Several theoretical calculations on the lifetime of this atom (which is far more unstable than the antiprotonic helium atom due to the lower mass of the pion) gave values that did not agree with each other or with experiment.

     Nature 581, 37 (2020) 

We were able to verify this hypothesis, by shooting 800 picosecond long laser pulses of many gigawatts of peak power tuned to exactly the correct wavelength to induce the pionic transition in the hypothetical atom. This caused the pion to execute a resonant transition from one pionic orbital to another, and triggered an electromagnetic cascade process that ended when the pion was absorbed into the helium nucleus. The nucleus broke apart into its constituent protons, neutrons, and deuterons. By detecting these particles, we were able to measure the laser resonance of metastable pionic helium, thus verifying the existence of this atom. We hope to improve the experimental precision of this spectroscopic measurement in the future, thereby determining the negative pion mass far more precisely than before. The measurement, if made very precisely, would also be able to set upper limits on possible forces that may involve the pion that cannot be explained by the Standard Model, in the same way that we have done for antiprotons.

Masaki Hori, 25 May 2022