Antiprotons pass latest symmetry test

The Antiprotonic Decelerator at CERN (Courtesy: CERN)

By Hamish Johnston

For something that is rare in the universe, antimatter has certainly been in the news a lot lately.

The latest breakthrough involves antiprotonic helium and is published in Nature today. This exotic “atom” is formed when one electron in a helium atom is replaced with an antiproton, which is negatively charged.

For two decades physicists have known that antiprotonic helium is formed in a metastable state that sticks around for a few milliseconds before decaying. This should make it to possible to study its energy levels and measure the ratio of the antiproton mass to the electron mass. This could then be compared with the well-known proton-to-electron mass ratio to see if the proton and antiproton have different masses. Such an asymmetry goes against the Standard Model of particle physics and its discovery could help physicists understand why the universe is dominated by matter.

Now, physicists working on the Antiprotonic Decelerator at CERN have done just that. Masaki Hori of the Max Planck Institute of Quantum Optics in Garching, Germany, and an international team made laser-spectroscopy measurements and worked out the mass ratio to a remarkable degree of precision.

The experiment begins with pulses of antiprotons being injected into helium gas to create the exotic atoms. The team then fires laser pulses at the atoms to knock the antiproton from its metastable state to an unstable state, causing it to annihilate with the helium nucleus. This produces pions, which are easily detected. By varying the wavelength of the lasers to find the maximum rate of pion production, the team found the exact energy of the transition.

The big challenge for the researchers was that the atoms are moving about, which causes a Doppler broadening of the transition wavelength. Scientists get around this with normal atoms by firing two identical lasers in opposite directions at the target. The atom absorbs one photon from each beam – which is only likely to occur if the atom has no relative motion in the direction of the lasers, eliminating the Doppler broadening.

This is trickier to do with antiprotonic helium, and Hori and colleagues instead used lasers at two different frequencies to eliminate much of the Doppler broadening.

So after all that hard work, did they discover any new physics? I’m afraid not. The antiproton-to-electron mass ratio is the same as the proton-to-electron ratio to an impressive nine significant figures.

The work is described in Nature 475 484.