The ALPHA collaboration has reported the most precise direct measurement of antimatter ever made, revealing the spectral structure of the antihydrogen atom in unprecedented detail. The result, published on 4 April in Nature, is the culmination of three decades of research and development at CERN, and opens a completely new era of high-precision tests between matter and antimatter.
Measurements of the hydrogen atom’s spectral structure agree with theoretical predictions at the level of a few parts in a quadrillion (1015). Researchers have long sought to match this level of precision for antihydrogen, with a view to comparing the hydrogen measurements with those of antihydrogen. Such a comparison would allow testing charge-parity-time (CPT) invariance and searching for physics beyond the Standard Model. Until now, however, it has been all but impossible to produce and trap sufficient numbers of antihydrogen atoms, and to acquire the necessary optical interrogation technology, to make serious antihydrogen spectroscopy possible.
The ALPHA team makes antihydrogen atoms and confines them in a magnetic trap. Laser light is then shone onto the atoms, their response measured and finally compared with that of hydrogen. In 2016, the team used this approach to measure the frequency of the transition between the lowest-energy state and the first excited state (1S to 2S transition) of antihydrogen with a precision of a couple of parts in ten billion, finding good agreement with the equivalent transition in hydrogen. The measurement involved using two laser frequencies — one matching the frequency of the 1S–2S transition in hydrogen and another “detuned” from it — and counting the number of atoms that dropped out of the trap as a result of interactions between the laser and the atoms.
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 of the 1S–2S antihydrogen transition and get a more precise measurement of its frequency. The shape matches that expected for hydrogen extremely well, and ALPHA was able to determine the 1S–2S antihydrogen transition frequency to a precision of a couple of parts in a trillion — a factor of 100 better than the 2016 measurement.
Although the precision still falls short of that for ordinary hydrogen, the rapid progress made by ALPHA suggests hydrogen-like precision in antihydrogen — and thus unprecedented tests of CPT symmetry — are now within reach. “This is real laser spectroscopy with antimatter, and the matter community will take notice,” explains Jeffrey Hangst, spokesperson for the ALPHA experiment. “We are realising the whole promise of CERN’s AD facility; it’s a paradigm change.”
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