Antihydrogen hyperfine splitting measured to 4 ppm, tightening CPT tests and antimatter structure probes
A CERN ALPHA Collaboration study reports a four-parts-per-million measurement of the antihydrogen ground-state hyperfine splitting (a1S/h) using microwave spectroscopy on ~24,000 trapped anti-atoms in a flattened 1 T magnetic trap. By comparing two spin-flip transitions at magnetic minimums in 1.03 T and 1.07 T, the team determines a1S/h = 1,420,404.8 ± 1.1 (stat) ± 5.6 (sys) kHz, consistent with hydrogen and CPT invariance. The result probes the antiproton’s internal structure (Zemach corrections) at ~40 ppm and complements ongoing 1S–2S measurements, with future plans to push precision further (e.g., Sternheim interval, 0.65 T transitions). Achieving this precision relies on improved magnetic-field control, spin-state manipulation, and antihydrogen accumulation, as well as careful handling of magnetic-field drift and lineshape modeling.