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Hyperfine structure of antihydrogen

Final Report Summary - HBAR-HFS (Hyperfine structure of antihydrogen)

Antihydrogen is the simplest atom consisting entirely of antimatter. Since its hydrogen counterpart is one of the most precisely measured atoms in physics, a comparison of antihydrogen and hydrogen offers one of the most sensitive tests of CPT symmetry. The CPT symmetry is one of the cornerstones of the Standard Model of particle physics, based on the mathematical properties of the quantum field theories used in its description. Furthermore, the perfect matter-antimatter symmetry required by the Standard Model at the microscopic scale is not found on cosmological scales, where the apparent absence of antimatter in the Universe is one of the most important open questions. A search for violation of CPT is thus a search for new physics beyond the Standard Model of particle physics.

In this project, the techniques to perform in-beam spectroscopy of the ground-state hyperfine splitting (GS-HFS) of antihydrogen were developed and verified by performing the best ever measurement of this quantity for hydrogen in a beam yielding a precision of 2.7 ppb. A measurement of GS-HFS for antihydrogen and its comparison to hydrogen will allow one of the most sensitive tests of CPT symmetry.

The method requires a spin-polarized beam of cold antihydrogen atoms, a microwave cavity to flip the spin, a superconducting sextupole magnet to analyze the spin direction, and an antihydrogen detector to record the antihydrogen atoms. Within the project the spectroscopy beam line (cavity, sextupole, and antihydrogen detector) were successfully constructed. By analyzing data taken in 2012 it could be shown that the formation method in a so-called CUSP trap by the ASACUSA collaboration is able to produce a beam of antihydrogen atoms 2.7 m down streams of the formation region, in a field-free region suited for precision spectroscopy. The formation of antihydrogen atoms was studied and first measurements of the quantum number distribution of the emerging antihydrogen beam were performed, but the flux created was too low to attempt hyperfine spectroscopy.

In parallel a source of mono-atomic polarized cold hydrogen atoms was built and measurements on hydrogen were done using the same cavity and sextupole as will be used for antihydrogen. A precision of 2.7 ppb was reached, proving that the method is able to perform high-precision hyperfine spectroscopy of antihydrogen.

A second way of producing a cold beam of antihydrogen atoms is pursued by the AEgIS collaboration. Here we participate in several aspects, primarily in the production of positrons and positronium needed for the formation of antihydrogen in collision with cold antiprotons. Major progress was achieved during the project by producing Rydberg positronium atoms by two-step laser excitation. The main goal of AEgIS is to measure the gravitational interaction of antimatter by measuring the cold antihydrogen beam fall in the Earth’s gravitational field. A proof-of principle measurement using the planned technique of a moiré deflectometer but with antiprotons was published that showed that the absolute spatial resolution of 1 micrometer needed could be reached.