Physicists from CERN’s ALPHA experiment today report the first ever measurement on the optical spectrum of an antimatter atom.
Artist’s impression of a cloud of trapped antihydrogen atoms. Image credit: Chukman So.
“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,” said Dr. Jeffrey Hangst, spokesperson of the ALPHA collaboration.
The ALPHA result is the first observation of a spectral line in an antihydrogen atom, allowing the light spectrum of matter and antimatter to be compared for the first time.
Within experimental limits, the result shows no difference compared to the equivalent spectral line in hydrogen.
This is consistent with the Standard Model of particle physics, the theory that best describes particles and the forces at work between them, which predicts that hydrogen and antihydrogen should have identical spectroscopic characteristics.
Details of the research were published today in the journal Nature.
ALPHA is an experiment at CERN’s Antiproton Decelerator facility, able to produce antihydrogen atoms and hold them in a specially-designed magnetic trap, manipulating antiatoms a few at a time. Trapping antihydrogen atoms allows them to be studied using lasers or other radiation sources.
“Moving and trapping antiprotons or positrons is easy because they are charged particles,” Dr. Hangst said.
“But when you combine the two you get neutral antihydrogen, which is far more difficult to trap, so we have designed a very special magnetic trap that relies on the fact that antihydrogen is a little bit magnetic.”
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 novel technique in which the researchers stack 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 frequency, they can observe the interaction of the beam with the internal states of antihydrogen.
The measurement was done by observing the so-called 1S-2S transition.
The 2S state in atomic hydrogen is long-lived, leading to a narrow natural line width, so it is particularly suitable for precision measurement.
“In our experiment, we trapped antihydrogen atoms in our magnetic trap and illuminated them with laser light with a wavelength close to 243 nm,” the physicists explained.
“In one series of runs, we tuned the light so that it is in resonance with the 1S-2S transition in hydrogen, and in a second series, so that it was detuned by 200 kHz. Interactions between the laser and the trapped atoms should cause atoms to be lost from the trap.”
“In each run, after 600s of illumination, we counted the number of atoms left in the trap using our annihilation imaging detector.”
“When the laser was tuned to resonance, we observed 67 atoms in 11 runs; when the laser was detuned, we counted 159 atoms in the same number of runs.”
“We also searched for signs of the atoms annihilating as they left the trap while the laser illuminated. When the laser was on-resonance, we observed 79 events that pass our criteria for inclusion, and 27 when off-resonance. Both of these comparisons help us conclude that the on-resonance laser light is interacting with the antihydrogen atoms via their 1S-2S transition.”
“This first result implies that the 1S-2S transition in hydrogen and antihydrogen are not too different, and the next steps are to measure the transition’s lineshape and increase the precision of the measurement.”
The ALPHA collaboration expects to improve the precision of its measurements in the future.
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M. Ahmadi et al. Observation of the 1S–2S transition in trapped antihydrogen. Nature, published online December 19, 2016; doi: 10.1038/nature21040
