Observation of Hyperfine Transitions in Trapped Ground-State Antihydrogen

This paper discusses the first observation of stimulated magnetic resonance transitions between the hyperfine levels of trapped ground state atomic antihydrogen, confirming its presence in the ALPHA apparatus. Our observations show that these transitions are consistent with the values in hydrogen to within 4~parts~in~${10}^3$. Simulations of the trapped antiatoms in a microwave field are consistent with our measurements.