A measurement of a permanent electric dipole moment on xenon atoms has yielded the null result d( 129 Xe) = ( -0.3 ± 1.1) x 10~2 6 e • cm, achieved by use of spin exchange with optically pumped rubidium atoms to determine the precession frequency of the xenon nuclear spins as a function of applied electric field. This measurement improves the limit on an atomic dipole moment by over four orders of magnitude. It is sensitive to timereversal-asymmetric forces within the atom, setting a new upper limit of 10" 6 Gp on a shortrange tensor-pseudotensor electron-nucleon coupling.PACS numbers: 32.80. Bx, 11.30.Er, 35.10.Di The discovery 1 in 1964 of CP nonconservation in the K 0 meson system provided the first and still the only evidence of time-reversal (T) symmetry violation. In attempting to find other examples of T nonconservation accurate searches have been undertaken for a permanent electric-dipole moment (edm) on an elementary particle or atomic system. Measurements on the neutron 2 ' 3 have yielded the important null value for the dipole moment, d(n) < 4xl0~2 5 e -cm. Searches for an edm on atoms and molecules have set upper limits on Tnonconserving electron-nucleon forces, and on the edm of the electron and the proton. 4 " 8 .We report here the first result in a new search 9 for an atomic edm. We observe precession of the nuclear spins of xenon atoms in a magnetic field, and measure the change in precession frequency when an applied electric field is reversed relative to the magnetic field. Thus far we have obtained the null result:where the quoted error is statistical and one standard deviation. This result is sensitive to several plausible T-nonconserving forces within the atom. For example, it improves the limit on a tensorpseudotensor interaction 5,8 between electrons and nucleons by over a factor of 10. The ground state of 129 Xe is doubly degenerate, with the states distinguished by the axial nuclearspin quantum number, mj= ± \. In external electric and magnetic^fieldsj? and B, the energy operator is H = -d • E -jZ • B, where d^d

Parity-nonconserving optical rotation has been observed and measured on the 8757-A magnetic-dipole absorption line in atomic bismuth vapor. The result, R = Im(E i /M i ) = (-10.4± 1.7) x 10" 8 , is of the approximate size calculated with use of the Weinberg-Salam theory of the weak neutral-current interaction with sin 2 0 w = 0.23.

We have made a very precise test of time-reversal invariance using nuclear-spin resonance in a thallium fluoride molecular beam. We searched for a shift of the 120-kHz Tl resonance when an external electric field was reversed relative to the nuclear spin and found it to be ( -2.2 ± 2.1 )x 10 ~3 Hz, less than 0.02 ppm and a fivefold improvement over any previous measurement. Our result imposes new constraints on the proton electric dipole moment and on CP nonconservation in strong and weak interactions.PACS numbers: 35.20.My, ll.30.Er.33.25.FsThe KQ system provides strong evidence that time reversal (T) is not an exact symmetry of nature, but the origin of the asymmetry remains obscure even today, twenty-three years after its discovery. l Perhaps the best hopes for enlightenment lie in the ongoing searches for other examples of T asymmetry. These include several very sensitive attempts to detect a permanent electric dipole moment (edm) in a nondegenerate system: If it is not zero, both T and P symmetry must be violated. No edm has been found, but the null results to date have nevertheless been important because of the strong limits that they impose on theory. 2 At present, the best limits on possible P-and T-asymmetric interactions come from measurements on neutrons, 3 atoms, 4 and molecules. 5 In this Letter we report the first results from a new apparatus designed to look for T asymmetry in the thallium fluoride (TIF) molecule. As was first pointed out by Sandars, 6 this heavy polar molecule is a sensitive system in which to search because it is much more easily polarized than an atom by a supposed T-odd, P-odd interaction. Many different mechanisms are able in principle to induce an effect of the kind we have tried to detect. These include P-and T-odd nucleon-nucleon or electron-nucleon interactions, as well as the "intrinsic" nucleon edm measured by the neutron experiments. The results described here, which constitute a fivefold improvement over any previous measurement in a molecule, are both complementary to and competitive with the limits derived from the neutron and from atoms. We are able to place new limits on several fundamental quantities.Our experiment is most simply viewed as the search for a .P-odd, T-odd hyperfine interaction between the spin-y Tl nucleus (spin direction

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