Determination of the antiproton-to-electron mass ratio

High-precision laser spectroscopy experiments were carried out on antiprotonic helium atoms^a (pHe+) in 2004 using a femtosecond optical frequency comb and a continuous-wave pulse-amplified laser. The transition frequencies between different antiprotonic states have been measured with an accuracy of ~10-8. Comparing these results with three-body QED calculations, we could deduce an antiproton-to-electron mass ratio of M`p / me = 1836.152674(5)^b. This is consistent with the value for the proton (see Fig. 1). We also concluded that the mass and charge of the antiproton agree with those of the proton to a precision of 2 × 10–9. A press release on this article was published through the press office of the Austrian Academy of Sciences.

Collisions between antiprotonic helium atoms and hydrogen/deuterium molecules

Simple hydrogenic exchange reactions like D + H2 → H + DH at low temperatures (10-50 K) play an important role in the early development of interstellar and protostellar clouds, including our own solar system. However, no data on the reaction rate of these exchange reaction exist at such low temperatures due to various technical difficulties.
The antiprotonic helium atom,`pHe⁺ consists of a helium nucleus, an antiproton, and an electron. From the electron’s point of view, the helium nucleus and the antiproton together appear as a single 'nucleus' with an effective charge of +1.7 e. Thus an antiprotonic helium atom behaves as an exotic, 'heavy' hydrogen atom in collisions with other molecules including H₂ or D₂. In these reactions, the H₂/D₂ molecules are most likely dissociated, and one of the H/D atoms is attached to the pHe⁺ molecule to form a compound molecule. This compound molecule is short-lived, and its destruction ('quenching') is immediately followed by the annihilation of the antiproton. Since these annihilations are easy to detect, we can measure the cross section (i.e. the reaction rate) of the collisions, which can provide an insight into the behaviour of the above-mentioned hydrogenic exchange reactions as well.

The ASACUSA collaboration at CERN used laser spectroscopy to measure the quenching cross sections of several different states of antiprotonic helium at various temperatures in 2003. After a careful analysis of the data, the results were published in 2006c. They revealed that all but one state behaved as expected from a simple activation barrier model i.e. their cross sections decreased with decreasing temperature. However, the cross section of one state increased with decreasing temperature, showing a 1/v dependence on the thermal velocity (see Fig. 2). This suggests that there is no activation barrier for this state, and is consistent with the Wigner threshold law of exothermic reactions involving neutral particles. An article on this result and the observation of protonium formation through a chemical reaction in the Penning trap of the ATHENA collaboration appeared in the December 2006 issue of the CERN Courier under the title "Serendipity at the Antiproton Decelerator opens the way to new antiproton chemistry".

Two-photon laser spectroscopy

During the beam time of 2006, the ASACUSA collaboration successfully carried out preliminary sub-Doppler laser spectroscopy experiments of antiprotonic helium atoms. The resonance profile of the transition (n,l) = (36,34) → (35,33) at wavelength l = 417 nm has a width of ~1 GHz, mainly due to the Doppler broadening. A two-photon technique was used to eliminate this Doppler broadening to the first order.
Two laser beams arranged in a counter propagating geometry, had non-equal frequencies adjusted such that their combined frequencies were tuned to the two-photon transition (n,l) = (36,34) → (34,32) where the virtual intermediate state was tuned to within a few GHz of a real state (n,l) = (35,33). Fig. 3 shows that, in comparison to the single photon in 2004, with two-photon spectroscopy the hyperfine structure is now resolvable.

Outlook

In 2007, detailed and systematic measurements of the two-photon transitions in both`p<sup>3</sup>He<sup>+</sup> and`p⁴He⁺ are planned. These studies will immediately lead to an improvement of the known value of the antiproton-to-electron mass ratio of a factor of 2-4. The laser system is being improved so that with a higher laser intensity and resolution the signal-to-noise ratio of the two-photon signal should be improved by a factor of 5-10 compared to the 2006 results.

Antiprotonic helium ion

During the 2006 beam time, the first laser spectroscopy measurements of`pHe<sup>2+</sup> were attempted. These antiprotonic helium ions are two body systems which are much more easily calculated, and therefore less dependent on theory than neutral three-body`pHe⁺ systems. The disadvantage of measuring such systems is their short lifetimes (5-6 ns), and the fact that the difference between the lifetimes of the parent and daughter states is too small compared to our experimental resolution. Despite taking a lot of statistics, the results were inconclusive, and therefore no further measurements are intended for the ion.

^a T. Yamazaki et al., Phys. Rep. 366, 183 (2002).

^bM. Hori et al., Phys. Rev. Lett. 96, 233401 (2006).

^c B. Juhász et al., Chem. Phys. Lett. 427, 246 (2006).