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Physics

Antimatter illuminated

Chukman So Measurements by CERN’S ALPHA collaboration indicate that the antihydrogen positron (illustration) shifts orbit and absorbs light like a hydrogen electronChukman So

Hydrogen, the simplest and most abundant element in the universe, is made up of a single electron, or particle of negatively charged electricity, orbiting around a nucleus that consists of a single proton, or positively charged particle. Conversely, antihydrogen atoms are extremely rare in the universe. They are composed of an antielectron – more commonly known as a positron – which is a particle precisely like an electron but with its electrical charge switched. The positron spins around an antiproton, which is a particle identical to a proton except that its electrical charge is reversed. Understanding why practically all matter in the universe is apparently made up only of atoms like hydrogen and not of antiatoms, like antihydrogen, is one of the great unanswered questions in physics. For the first time, an international team of physicists has succeeded in measuring with precision how antimatter atoms interact with light. The new measurements show that the antihydrogen positron shifts orbit and absorbs light of the same wavelength as the hydrogen electron (Nature, December 19, 2016). One of the challenges in conducting this type of measurement is that it is hard to create and store antiatoms in the laboratory because antimatter always annihilates and generates energy whenever it comes in contact with matter. The study was conducted by the ALPHA collaboration, an international team headquartered at the European Organization for Nuclear Research (CERN). Since 2010, ALPHA has been working to produce and confine antihydrogen atoms in a magnetic trap, using an antiproton decelerator. “ALPHA is one of the collaborations that uses the only equipment in the world capable of obtaining antiprotons at low energies,” explains Claudio Lenz Cesar, physicist at the Federal University of Rio de Janeiro (UFRJ) and project member since ALPHA’s inception in 2006. The team succeeded in confining thousands of antihydrogen atoms for some minutes, long enough to shine laser beams on them. Their goal was to find out whether the antiatoms would interact with light in the same way atoms do. Lenz Cesar says that this year the ALPHA team plans to work on arriving at more precise results and exploring other properties of antihydrogen, like gravitational mass.

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