Test of CPT and Lorentz symmetry for the neutron

Using a two-species ¹²⁹Xe/³He Zeeman maser, we have performed the best tests to date of Lorentz symmetry (both rotations and boosts) for the neutron.

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 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 have used a two-species ¹²⁹Xe/³He Zeeman maser to perform the most sensitive searches to date 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 the level of 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 have also used hydrogen masers to perform a sensitive rotation symmetry test for the proton, 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:

Bound on Lorentz- and CPT-Violating Boost Effects for the Neutron.

Measurement of the ²¹Ne Zeeman frequency shift due to Rb-²¹Ne collisions.

Limit on Lorentz and CPT violation of the neutron using a two species noble gas maser.

Improved frequency stability of the dual noble gas maser.

Demonstration of a two species noble gas maser.

Fundamental Symmetry Tests using a ¹²⁹Xe/³He Dual Noble Gas Maser