Matter-antimatter: balancing the scales

Using its innovative experimental set-up, the Japanese-European ASACUSA collaboration recently succeeded in measuring the mass of the antiprotons with an unprecedented accuracy. This has been made possible by applying extremely high-precision laser techniques.


ASACUSA physicist, Masaki Hori, adjusts the optical system of laser beams.

The antiproton is not something you could weigh by putting it on a pair of scales. Besides, it is not its “weight” (i.e. the Earth’s gravitational force on it) that scientists aim to measure but rather its “mass”. In addition, the yardstick against which the antiproton mass was measured is not the familiar kilogram, but the electron’s mass. Technically speaking, this is no easy task, especially when an unprecedented precision is requested.

In the ASACUSA experiment, two counter-propagating ultra-sharp laser beams simultaneously hit an antiprotonic helium atom, where an antiproton orbits around the nucleus in place of one of its two electrons. The researchers tuned the laser frequencies to values at which the antiproton jumped between two of its energy levels. By precisely measuring these two energies, ASACUSA researchers were able to calculate the ratio between the mass of the antiproton and that of the electron. “It's the first time that the double-laser technique has been applied to antiprotonic atoms. Its importance resides in the fact that the first photon excites the antiproton to an intermediate virtual energy state (i.e. one that is not allowed by quantum mechanics) while the second photon completes the transition to the closest real energy state. Taking advantage of this principle, relatively low-power lasers can be used to achieve a very high precision in the proton mass measurement,” explains Masaki Hori, an ASACUSA physicist who also conceived the two-laser set-up.

The antiproton turns out to have the same mass as the proton (of course, with respect to the electron mass), at least to the precision achieved by ASACUSA. The margin of error of the ASACUSA result is only 1.3 parts per billion, about the same as that for the proton-electron mass ratio. Since the Collaboration hopes to do even better in the future, we may soon ‘know’ the antiproton better than we know the proton. In this case, one of the world’s fundamental constants may have to be redefined by measurement from the (so-far unobserved) anti-world.

To perform its measurements, the ASACUSA experiment uses antiprotons from the Antiproton Decelerator, CERN’s antimatter factory, which slows down antiprotons to a speed at which they can be brought to rest in the experiment’s helium target and spontaneously replace electrons in the helium atoms. Such antiprotonic atoms can survive for microseconds in the target, plenty of time for laser spectroscopic studies to be made. “ASACUSA lasers are unique pieces in that they are extremely phase-coherent and highly reliable,” explains Masaki Hori. “We were able to develop the two lasers thanks to collaboration with the Munich group led by Theodor Hänsch, who was awarded the 2005 Nobel Prize in Physics for inventing the optical frequency comb, which is a device used to measure the frequencies of light very accurately. A copy of this instrument is now installed at CERN and allowed us to achieve the high precision on the antiproton mass.”

ASACUSA’s recent result comes after a long period of testing of the experiment’s performance and marks the beginning of a wide-ranging list of physics studies that the Collaboration is planning for the coming years. Together with the other AD experiments, ASACUSA will certainly help to shed light on antimatter properties.

by CERN Bulletin