Precision tests of fundamental physics

The original core activity of the Walsworth Group was the development of precision measurement tools, such as atomic clocks, and the application of these tools to precision tests of fundamental physics. In particular, we have used noble gas and hydrogen masers to perform some of the most sensitive tests to date of Lorentz symmetry for the neutron and proton.

Lorentz symmetry - i.e., symmetry under spatial rotations and boosts - is a fundamental feature of modern descriptions of nature, including both the Standard Model of particle physics and general relativity. However, these realistic theories are believed to be the low-energy limit of a single fundamental theory at the Planck scale. Even if the underlying theory is Lorentz invariant, spontaneous symmetry breaking at very high energies, such as the Planck scale, might result in small apparent violations of Lorentz symmetry and hence of CPT (symmetry under simultaneous application of Charge conjugation, Parity inversion, and Time reversal) at an observable level. Experimental investigations of the validity of Lorentz symmetry therefore provide valuable tests of the framework of modern theoretical physics.

Clock-comparison experiments serve as sensitive probes of rotation and boost invariance and hence of Lorentz symmetry, essentially by bounding the frequency variation of a clock as its orientation and velocity changes. In practice, some of the most precise limits are obtained by comparing the frequencies of two different co-located clocks as they rotate with the Earth and revolve around the Sun. Typically, the clocks are electromagnetic signals emitted or absorbed by atoms on hyperfine or Zeeman transitions.

We used a two-species 129Xe/3He Zeeman maser to perform sensitive searches for violations of rotation and boost symmetry for the neutron. We search for specific experimental signatures: variations of the maser frequency (i.e., the nuclear Zeeman splitting) with periodicities of a sidereal day and year for violations of rotation and boost symmetry, respectively. Such Zeeman splitting modulation could arise from Lorentz- and CPT-violating couplings of the 3He and 129Xe nuclear spins (each largely determined by a valence neutron) which depend on the instantaneous orientation and velocity of the laboratory. The appeal of the noble-gas maser experiment is the excellent absolute frequency stability, and thus the sensitivity to small, slow variations in spin couplings. To date, we have found no rotation-symmetry violation at a level < 10-31 GeV and no boost-symmetry violation at the level of 10-27 GeV. With ongoing improvements to our noble gas masers, we expect one to two orders of magnitude improvement in sensitivity to violations of CPT and Lorentz symmetry.

We used a hydrogen maser double resonance technique in a sensitive test of rotation symmetry for the proton. The hydrogen maser is an established tool in precision tests of fundamental physics. H masers operate on the ΔF = 1,  ΔmF = 0 hyperfine transition - the "clock" transition - in the atomic hydrogen electronic ground state. H masers built at the Center for Astrophysics over the last 30 years provide fractional frequency stability on the clock transition of better than 10-14 over averaging intervals of minutes to days, and can operate undisturbed for several years before requiring routine maintenance. We utilize a H maser double resonance technique to probe the ΔmF = 1 Zeeman transition with precision ~1 mHz. Unlike the clock transition, the Zeeman transition has leading-order sensitivity to Lorentz and CPT violation, if it exists, because there is a change in the longitudinal orientation of the hydrogen atom's electron and proton spins. We search for rotation-symmetry violation by monitoring the Zeeman frequency as the laboratory reference frame rotates sidereally. To date, we have found no rotation-symmetry violation of the H Zeeman transition at the level of 10-27 GeV. This result can interpreted as a limit for the proton, because of the work at the Univ. of Washington on a spin-polarized torsion pendulum, which has set a bound of 10-29 GeV on rotation-symmetry violation of the electron. Improvements to the environmental control of our experiment may allow one to two orders of magnitude improvement in sensitivity to violations of rotation-symmetry (and hence CPT and Lorentz symmetry).

A general theoretical framework known as the Standard-Model Extension has been developed in recent years to allow a comprehensive and systematic study of the implications of Lorentz violation at observable energies. Information about the Standard-Model Extension can be found at