Light spectrum of antimatter observed for first time

Scientists at CERN have measured the optical spectrum of an antimatter atom for the first time, opening up a completely new era in high-precision research of the material composed of antiparticles. Antiparticles have the same mass as particles of ordinary matter, but opposite charges, lepton and baryon numbers. 

The work is a result of over 20 years of research by the antimatter community at CERN (European Organisation for Nuclear Research) in Switzerland. "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 Jeffrey Hangst, Spokesperson of the ALPHA collaboration, a unique experiment at CERN's Antiproton Decelerator facility. 

ALPHA is 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. With its single proton and single electron, hydrogen is the most abundant and well-understood atom in the universe. Its spectrum has been measured to very high precision.

Antihydrogen atoms, on the other hand are poorly understood. Since the universe appears to consist entirely of matter, the constituents of antihydrogen atoms - antiprotons and positrons - have to be produced and assembled into atoms before the antihydrogen spectrum can be measured. 

Any measurable difference between the spectra of hydrogen and antihydrogen would break basic principles of physics and possibly help understand the puzzle of the matter-antimatter imbalance in the universe. 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, which best describes particles and the forces at work between them and predicts that hydrogen and antihydrogen should have identical spectroscopic characteristics. Measuring the antihydrogen spectrum with high-precision offers an extraordinary new tool to test whether matter behaves differently from antimatter and thus to further test the robustness of the Standard Model.

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. They 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 frequency, scientists can observe interaction of the beam with the internal states of antihydrogen. The study was published in the journal Nature.