Test of CPT and Lorentz Symmetry for the Proton

Using a hydrogen maser double resonance technique, we have performed the best test to date of rotation symmetry for the 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 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. For example, this might occur in string theory. 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 invariance and hence of Lorentz symmetry, essentially by bounding the frequency variation of a clock as its orientation 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.
The hydrogen maser is an established tool in precision tests of fundamental physics. H masers operate on the ΔF = 1, Δm_F = 0 hyperfine transition - the "clock " transition - in the atomic hydrogen electronic ground state. H masers built in our laboratory 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 Δm_F = 1 Zeeman transition with precision of ~ 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. With ongoing improvements to the environmental control of our experiment, we expect one to two orders of magnitude improvement in sensitivity to violations of rotation-symmetry (and hence CPT and Lorentz symmetry).

(We have also used noble gas Zeeman masers to perform the best tests to date of both rotation and boost symmetry for the neutron, as discussed here.)

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 http://www.physics.indiana.edu/~kostelec/faq.html

Recent Posters (click to download)

References:

Testing Lorentz and CPT symmetry with hydrogen masers.

Limit on Lorentz and CPT violation of the proton using a hydrogen maser.

Double resonance frequency shift in a hydrogen maser.

Precision Measurements with Atomic Hydrogen Masers