The ALPHA collaboration at CERN’s Antiproton Decelerator (AD) has reported the most precise direct measurement of antimatter ever made. The team has determined the spectral structure of the antihydrogen 1S–2S transition with a precision of 2 × 10–12, heralding a new era of high-precision tests between matter and antimatter and marking a milestone in the AD’s scientific programme (CERN Courier March 2018 p30).

Measurements of the hydrogen atom’s spectral structure agree with theoretical predictions at the level of a few parts in 1015. Researchers have long sought to match this stunning level of precision for antihydrogen, offering unprecedented tests of CPT invariance and searches for physics beyond the Standard Model. Until recently, the difficulty in producing and trapping sufficient numbers of delicate antihydrogen atoms, and acquiring the necessary optical laser technology to interrogate their spectral characteristics, has kept serious antihydrogen spectroscopy out of reach. Following a major programme by the low-energy-antimatter community at CERN during the past two decades and more, these obstacles have now been overcome.

“This is real laser spectroscopy with antimatter, and the matter community will take notice,” says ALPHA spokesperson Jeffrey Hangst. “We are realising the whole promise of CERN’s AD facility; it’s a paradigm change.”

ALPHA confines antihydrogen atoms in a magnetic trap and then measures their response to a laser with a frequency corresponding to a specific spectral transition. In late 2016, the collaboration used this approach to measure the frequency of the 1S–2S transition (between the lowest-energy state and the first excited state) of antihydrogen with a precision of 2 × 10–10, finding good agreement with the equivalent transition in hydrogen (CERN Courier January/February 2017 p8).

The latest result from ALPHA takes antihydrogen spectroscopy to the next level, using not just one but several detuned laser frequencies with slightly lower and higher frequencies than the 1S–2S transition frequency in hydrogen. This allowed the team to measure the spectral shape, or spread in colours, of the 1S–2S antihydrogen transition and get a more precise measurement of its frequency (see figure). The shape of the spectral line agrees very well with that expected for hydrogen, while the 1S–2S resonance frequency agrees at the level of 5 kHz out of 2.5 × 1015 Hz. This is consistent with CPT invariance at a relative precision of 2 × 10−12 and corresponds to an absolute energy sensitivity of 2 × 10−20 GeV.

Although the precision still falls short of that for ordinary hydrogen, the rapid progress made by ALPHA suggests hydrogen-like precision in antihydrogen is now within reach. The collaboration has also used its unique setup at the AD to tackle the hyperfine and other key transitions in the antihydrogen spectrum, with further seminal results expected this year. “When you look at the lineshape, you feel you have to pinch yourself – we are doing real spectroscopy with antimatter!” says Hangst.

Further reading

M Ahmadi et al. 2018 Nature doi:10.1038/s41586-018-0017-2.