The first spectroscopic measurement of an atom of antimatter - a longstanding goal in antimatter physics - is reported online in Nature this week. The findings represent a significant step towards the development of highly precise tests of whether matter behaves differently from antimatter.
A major puzzle in physics is why today’s Universe seems to consist almost entirely of ordinary matter when the Standard Model of particle physics predicts that there should have been equal amounts of matter and antimatter after the Big Bang. Atoms can be excited by firing light at them, and when they return to their ground state they emit light the frequency distribution of which forms a spectrum that can be precisely measured (spectroscopy). However, antimatter is difficult to produce and to trap because it annihilates on contact with matter, which makes measuring its properties challenging.
Recent advances at CERN’s Antiproton Decelerator have allowed researchers to trap and measure antiprotons and antihydrogen. Now, Jeffrey Hangst and colleagues from CERN’s ALPHA collaboration have magnetically trapped atoms of antihydrogen in a cylindrical vacuum chamber that is 280 millimetres long and has a diameter of 44 millimetres. They shone a laser light through windows in the chamber to measure the 1S-2S transition of the anti-atoms (the transition from the ground state to an excited state). The authors report that the transition frequency of antihydrogen is consistent with that of hydrogen. The spectrum of hydrogen has been characterized to high precision, so improvements in antihydrogen spectroscopy should yield highly sensitive tests of matter-antimatter symmetry.