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February 26, 2017

ASACUSA experiment produces first beam of antihydrogen atoms at CERN

The ASACUSA experiment at CERN has succeeded for the first time in producing a beam of antihydrogen atoms. In a paper published in Nature Communications, the ASACUSA collaboration reports the unambiguous detection of 80 antihydrogen atoms 2.7 meters downstream of their production, where the influence of the magnetic fields used initially to produce the antiatoms is small. This result is a significant step towards precise hyperfine spectroscopy of antihydrogen atoms. The feat will help them test whether these atoms differ from their matter counterparts, which could give scientists clues about how our universe formed.

Primordial antimatter has so far never been observed in the universe, and its absence remains a major scientific challenge. Nevertheless, it is possible to produce significant amounts of antihydrogen in experiments at CERN by mixing antielectrons and low energy antiprotons produced by the Antiproton Decelerator. Hydrogen is the simplest atom in existence, consisting of one electron and one proton. Antihydrogen, made of an antiproton and an antielectron (also called a positron), is hydrogen’s antiparticle.Scientists on ASACUSA will use a technique called spectroscopy to study beams of atoms, measuring the energies of the electrical emissions coming from matter atoms versus those coming from antimatter atoms. The process will be somewhat like examining how white light splits into a rainbow after traveling through different types of prisms.

The spectra of hydrogen and antihydrogen are predicted to be identical, so any tiny difference between them would immediately open a window to new physics, and could help in solving the antimatter mystery. With its single proton accompanied by just one electron, hydrogen is the simplest existing atom, and one of the most precisely investigated and best understood systems in modern physics. Thus comparisons of hydrogen and antihydrogen atoms constitute one of the best ways to perform highly precise tests of matter and antimatter symmetry.

Theoretically, the spectroscopic measurements of hydrogen and antihydrogen should be identical. If there are any differences, it could be an indication of something not explained by our current understanding of physics, and it could help solve one of the biggest mysteries in the cosmos. It is believed that the same amount of matter and antimatter was formed during the big bang. Yet when matter and antimatter meet, their particles transform into energy. If they existed in equal amounts in the early universe, nothing should remain today except free-floating energy, or there should be a separate antimatter universe in addition to our matter universe. But, “these days, in nature, primarily only matter exists,” Yamazaki says.

Matter and antimatter annihilate immediately when they meet, so aside from creating antihydrogen, one of the key challenges for physicists is to keep antiatoms away from ordinary matter. To do so, experiments take advantage of antihydrogen’s magnetic properties (which are similar to hydrogen’s) and use very strong non-uniform magnetic fields to trap antiatoms long enough to study them. However, the strong magnetic field gradients degrade the spectroscopic properties of the (anti)atoms. To allow for clean high-resolution spectroscopy, the ASACUSA collaboration developed an innovative set-up to transfer antihydrogen atoms to a region where they can be studied in flight, far from the strong magnetic field.

The team created a new tool that combines magnetic fields of different strengths to move the antiatoms into a controlled beam about 3 meters from the origin point. In their first run, ASACUSA scientists counted 80 antihydrogen atoms downstream from where they were initially produced. Now that the scientists know the beam works, they’ll be able to analyze the antihydrogen atoms in detail in the next run.

“Antihydrogen atoms having no charge, it was a big challenge to transport them from their trap. Our results are very promising for high-precision studies of antihydrogen atoms, particularly the hyperfine structure, one of the two best known spectroscopic properties of hydrogen. Its measurement in antihydrogen will allow the most sensitive test of matter/antimatter symmetry. We are looking forward to restarting this summer with an even more improved set-up,” says Yasunori Yamazaki of RIKEN, Japan, a team leader of the ASACUSA collaboration. The next step for the ASACUSA experiment will be to optimize the intensity and kinetic energy of antihydrogen beams, and to understand better their quantum state.

“Now that we know we can make the beam, we’re going to study the properties of antihydrogen, and we’re going to figure out this matter-antimatter discrepancy,” Yamazaki says.The ASACUSA collaboration consists of about 40 people from institutions and universities in Japan, Austria, Denmark, Hungary, Italy, the United Kingdom and Germany. The group will reconvene to collect more data when the CERN accelerator complex restarts this summer after a period of planned maintenance and upgrades.