Precision measurements with atomic hydrogen masers

We report two experimental results and a theoretical study involving atomic hydrogen masers oscillating on the ΔF = 1; Δ m_F = 0 hyperfine transition. In the first experiment, we placed a new limit on Lorentz and CPT violation of the proton in terms of a recent standard model extension. By placing a bound on sidereal variation of the F = 1, Δm_F = ±1 Zeeman frequency in atomic hydrogen, our search set a limit on violation of Lorentz and CPT symmetry of the proton at the 10−27 GeV level, independent of nuclear model uncertainty, and improved significantly on previous bounds. This test utilized a double resonance technique in which the oscillation frequency of a hydrogen maser is shifted by applied radiation near the F = 1, Δm_F = ±1 Zeeman resonance. We used the dressed atom formalism to calculate this frequency shift and found excellent agreement with a previous calculation made in the bare atom basis. Qualitatively, the dressed atom analysis gave a simpler physical interpretation of the double resonance process. In the second experiment, we investigated low temperature hydrogen-hydrogen spin-exchange collisions using a cryogenic hydrogen maser. Operational details of the apparatus are presented and a description of our measurement of the semi-classical spin-exchange shift cross section $l$₀ at 0.5 K is given. We report a value of $l$₀ = 56.70 Å2 with a statistical error of 15.51 Å2 and a systematic uncertainty between 80.6 and 318.8 Å2. A discussion of this systematic is given and the possibility of an improved measurement is discussed.