Measurement of the ground state hyperfine splitting of antihydrogen Antihydrogen is the
antimatter mirror of hydrogen in which the
electron is replaced by a positron and the
proton by an antiproton. The ASACUSA
Collaboration intends to measure the ground
state hyperfine splitting of antihydrogen (the
energy difference between the two states with
parallel and antiparallel electron and
proton spins) at the CERN Antiproton
Decelerator, with an initial precision of one
part per million, to test the symmetry between
matter and antimatter. The fundamental symmetry
of CPT (combination of charge and parity
conjugation and time reversal) predicts that
matter and antimatter have equal or
sign-opposite properties. However, the puzzling
dominance of matter over antimatter in the
universe warrants precise measurements of
antimatter properties - such as the transition
frequencies in antihydrogen atoms - to compare
with their matter counterparts. In fact,
theories beyond the so far well established
Standard Model (such as string theory) predict a
violation of the CPT symmetry at some level.
Antihydrogen is the simplest stable atom composed solely of antimatter, and hydrogen is one of the most precisely studied atomic systems. The hydrogen ground state hyperfine splitting of about 1.42 GHz had been measured earlier very accurately by a maser experiment with a relative precision of about 1 part in 1012. It was also determined recently by ASACUSA, albeit with a more modest precision of a few parts per billion (ref.1) in a hydrogen beam by using the Rabi resonance method, which we will also apply to determine the hyperfine transition frequency of antihydrogen. Figure 1 shows the behaviour of the two hyperfine states of antihydrogen which split to four when switching an external magnetic field (Breit-Rabi diagram). The total angular momentum F and its pro- jection M on the quantisation axis are given. With ASACUSA’s setup two transitions, σ1 and π1, are accessible. The hyperfine transition frequency can then be determined by measuring one of the transitions at several field strengths and extrapolating to zero field. |
Figure 1: Breit-Rabi diagram showing the magnetic field dependence of the four hyperfine states of antihydrogen. |