ALPHA measures the optical spectrum of antihydrogen

Dec. 19, 2016
First results show that the antimatter atom has the same spectrum as hydrogen.
"Using a laser to observe a transition in antihydrogen and comparing it to hydrogen to see if they obey the same laws of physics has always been a key goal of antimatter research," says Jeffrey Hangst, spokesperson of the ALPHA collaboration, who can be seen here among the ALPHA hardware. (Image: Maximilien Brice/ CERN)

In the ALPHA collaboration at CERN, the European Organization for Nuclear Research (Geneva, Switzerland), antihydogen atoms are created from antiprotons and positrons and then levitated in a magnetic field so that their properties can be measured and compared with those of ordinary hydrogen. The properties, if identical, simply confirm the standard physical theory -- the charge-conjugation, parity-reversal, time-reversal (CPT) portion of the Standard Model); if different, however, the properties would point the way toward new physics and possibly help us to understand the puzzle of the matter-antimatter imbalance in the universe.

Now, ALPHA has completed the the first-ever measurement on the optical spectrum of an antimatter atom -- and the results, within experimental limits, are identical to those for a hydrogen atom. However, while the Standard Model is safe for the moment, the ALPHA collaboration expects to improve the precision of its measurements in the future, once again opening up the possibility of new physics.

The antihydrogen is made by mixing plasmas of about 90,000 antiprotons from the Antiproton Decelerator with positrons, resulting in the production of about 25,000 antihydrogen atoms per attempt. Antihydrogen atoms can be trapped if they are moving slowly enough when they are created. Using a new technique in which the collaboration stacks anti-atoms resulting from two successive mixing cycles, it is possible to trap on average 14 anti-atoms per trial, compared to just 1.2 with earlier methods.

By illuminating the trapped atoms with a laser beam at a precisely tuned 243 nm wavelength, scientists can observe a two-photon interaction of the beam with the internal states of antihydrogen. The measurement was done by observing the 1S-2S transition excited by the laser. The 2S state in atomic hydrogen is long-lived, leading to a narrow natural line width, so it is particularly suitable for precision measurement. The results were with CPT invariance to a level of 2 × 10-10.



1. M. Ahmadi et al., Nature (2016); doi: 10.1038/nature21040.

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