ALPHA is an international collaboration based at CERN, and which is working with trapped antihydrogen atoms, the antimatter counterpart of the simplest atom, hydrogen. By precise comparisons of hydrogen and antihydrogen, the experiment hopes to study fundamental symmetries between matter and antimatter.


On February 19, 2020, ALPHA published the first investigation of the fine structure of Antihydrogen. These measurements of transitions in the 1S-2P manifold, in combination we previous detailed measurements of the 1S-2S transition has allowed us to determine the 2S1/2-2P1/2 splitting (the classic Lamb shift) in antihydrogen. The results appeared today in Nature.


In the News.

In 2018 the ALPHA experiment was expanded with the addition of ALPHA-g. To deliver antiparticles to both ALPHA-g and
ALPHA-2 (Spectroscopy trap) a beamline to transport both antiprotons and positrons was also installed. The timelapse
videos below show the massive changes, from installation of the beamline, to the addition of new equipment platforms (in
stainless steel(!)) and the ALPHA-g main solenoid and cryostat as well as control systems etc.






The ALPHA collaboration has for the first time observed single-photon excitation of antihydrogen atoms from the ground (1S) state to the 2P state using 121nm pulsed laser light - the so-called lyman-alpha line of the Lyman series.

The results were published in Nature on August 22nd 2018 and also demonstrate how the pulsed laser-light can be used to measure the temperature of the antihydrogen atoms. The single-photon lyman-alpha transition takes ALPHA one step closer to laser-cooling of antihydrogen, a feat that would dramatically improve the potential for more precise measurements of the also recently detailed 1S-2S transition as well as measurements of the gravitational influence on antihydrogen. The expansion of ALPHA to do the latter in ALPHA is currently underway.



Our latest breakthrough, the first detailed study of one of the hyperfine components of the 1S-2S line in trapped antihydrogen has been published in Nature and is the most precise and most accurate measurement of antimatter to date. This builds on years of work, developing techniques to manipulate super-cold antiprotons and positrons, create trapped antihydrogen and detect the very few atoms that are available to the experiment. It is another crucial step towards precision comparisons of antihydrogen and hydrogen. 

Directly measuring if there are any differences between the antimatter partners may help us understand why our Universe is made almost entirely of matter, even though matter and antimatter should have been produced in equal quanitites in the Big Bang.

In our experiment, we trapped antihydrogen atoms in our magnetic trap and illuminated them with laser light with a wavelength close to 243nm. In four series of runs, we tuned the light so that it was positioned in a number of different places relative to the calculated resonance with the 1S-2S transition in hydrogen. If the light excites the antiatoms to the 2S state it has a good probability for getting photoionised by subsequent photons and thus be lost from the trap. By repeating the experiment for a number of different frequencies we could map out the resonance position of the transition and it's shape.

In each run, after 300s of illumination, we counted both the number of atoms escaping during illumination and remaining in the trap following illumination using our annihilation imaging detector. In total we did four sets of four frequencies each, using about 4000 antiatoms in each set, composed of about 21 individual runs. Each run stacked antihydrogen from 3 mixing cycles of positrons and antiprotons. Combining these four sets allowed us to extract a value for the centre frequency of 2 466 061 103 079.4 (5.4) kHz, to be compared with our calculation for hydrogen in the same magnetic field of 2466 061 103 080.3 (0.6) kHz. We thus found that our measurement of antihydrogen was consistent with the expected value for hydrogen to a precision of about 2 parts in 10 to the power 12. While this is still about 3 orders of magnitude short of the state-of-the-art in hydrogen, it is nontheless the most precise and accurate measurement done on any antimatter system to date.

In the news!

Following 30 years of effort by the low-energy antimatter community at CERN, the ALPHA collaboration has made seminal measurements of antihydrogen’s spectral structure in a bid to test nature’s fundamental symmetries.

Read full article here.