Marie-Anne Bouchiat

Parity violation in atoms

Optical experiments have demonstrated cases in which mirror symmetry in stable atoms is broken during the absorption or emission of light. Such results, which are in conflict with quantum electrodynamics, support the theory of unification of the electromagnetic and weak interactions. The interpretation of the experimental results is based on exchanges of weak neutral bosons between the electrons and the nucleus of the atom. A concise review of these phenomena in atomic physics is presented. The role of precise caesium parity-violation experiments, as a source of valuable information about electroweak physics, is illustrated by examples pertaining to experimental conditions which, in some cases, are not accessible to accelerator experiments. We give the basic principles of experiments, some under way and others completed, where a quantitative determination of the nuclear weak charge, , which plays for the exchange the same role as the electric charge for the Coulomb interaction is to be, or has been achieved. In the most recent and most precise experiment the accuracy on is limited to 1 % by the uncertainty due to atomic physics calculations. Such a result challenges specialists in atomic theory and nuclear structure, since a more accurate determination of would mean more stringent constraints upon possible extensions of the standard model. Moreover, clear evidence has recently been obtained for the existence of the nuclear anapole moment, which describes the valence electron interaction with a chiral nuclear-magnetization component induced by the parity-violating nuclear forces. In writing this review, our hope was to make clear that any improvement in atomic parity-violation measurements will allow the exploration of new areas of electroweak physics.

New manifestation of atomic parity violation in cesium: a chiral optical gain induced by linearly polarized 6S-7S excitation.

We have detected, by using stimulated emission, an atomic parity violation (APV) in the form of a chiral optical gain of a cesium vapor on the 7S-6P(3/2) transition, consecutive to linearly polarized 6S-7S excitation. We demonstrate the validity of this detection method of APV, by presenting a 9% accurate measurement of expected sign and magnitude. We stress several advantages of this new approach which fully exploits the cylindrical symmetry of the setup. Future measurements at the percent level will provide an important cross-check of an existing more precise result obtained by a different method.

Parity violation in forbidden transitions: Detection of the electroweak alignment or polarization in the upper state by stimulated emission

Abstract We present a new method for detecting the parity violation induced by weak neutral currents in a forbidden atomic transition such as the 6 S-7 S Cs transition. The 7 S atoms are produced by pulsed excitation and detected through the subsequent transient gain which appears at the 7 S-6 P frequency. A right-left asymmetry in this gain is expected as a result of parity violation, with possible amplification of this asymmetry when the vapor becomes optically thick. With respect to previous observation conditions, substantial improvement in signal/noise ratio is expected.

Measuring the Fr Weak Nuclear Charge by Observing a Linear Stark Shift with Small Atomic Samples

We study the chirality of ground-state alkali atoms in E and B fields, dressed with a circularlypolarized laser beam detuned from an E-field-assisted forbidden transition, such as 7S − 8S in Fr. We predict a parity violating energy shift of their sublevels, linear in E, and the weak nuclear charge QW . A dressing beam of 10 kW/cm 2 at 506 nm produces a shift of ∼ 100 μHz at E=100 V/cm, B ∼30 mG. It should be observable with ∼ 10 4 Fr atoms confined in an optical dipole trap. We discuss optimal conditions, parameter reversals and a calibration procedure to measure QW .

Sensitive pulsed pump-probe atomic polarimetry for parity-violation measurements in caesium

We describe an ongoing experiment to measure parity violation in atomic caesium, based on detection by stimulated emission. Our goal is to measure to 1 a left-right asymmetry of to test electroweak theory and look for new physics beyond the Standard Model. The Cs highly forbidden transition, , is excited in a vapour (5-10 mtorr) by a pump laser pulse in a longitudinal electric field . The PV asymmetry resulting from the weak interaction during optical excitation is converted into an anisotropy in the gain of a probe laser pulse which stimulates the allowed transition , and manifests itself as a tiny -odd rotation of the probes linear polarization. Differential polarimetry allows dark-field detection of the rotation angle with a baseline defined to better than and discrimination between true and pseudo-rotation. Lineshape-independent angle calibration is performed using a parity-conserving -even anisotropy. To isolate the parity-violating effect, we exploit the symmetry of revolution of the experiment by (i) rotating pump and probe linear polarizations around the beam axis and (ii) reversing in a cylindrically symmetric cell. After describing the apparatus and data acquisition procedure, we summarize the current experimental status and short-term prospects.

Optical orientation, via spin-orbit coupling, in the final state of the stark induced 6S–7S cesium transition

Abstract In the excitation process of the forbidden 6S 1 2 –7S 1 2 Cs transition by a circularly polarized laser beam in a static electric field, spin-orbit coupling causes the creation of an electronic polarization in the upper state with a component not only along the beam, but also transverse to it. This effect is studied experimentally by observing the Hanle effect in the 7S 1 2 state on the 7S 1 2 →6P 1 2 fluorescence light, in various field configurations. This provides a unique way of determining the relative sign of the scalar part and spin-dependent vector part of the polarizability which connects the transition electric dipole with the electric field.

Two-beam linear magneto-optical spectroscopy of atomic transitions between short lived states

Two-beam, linear magneto-optical spectroscopy is a powerful tool for studying short-lived states. We present both measurements and a quantitative theoretical analysis of magneto-rotation observed in the forward scattering of a linearly polarised laser beam passing through an amplifying atomic medium placed in a longitudinal magnetic field. The probed transition connects two short-lived, excited atomic levels, the upper state (here the 7S1/2 level of cesium) being prepared initially via another transition from ground state, excited by a linearly polarised pump beam. The probe polarisation undergoes three different magneto-optical processes: optical rotation, with separate contributions from the two transitions, and linear dichroism due to Hanle precession of the upper state alignment. Complete resolution of the hyperfine structures and ninety degree switching of the probe polarisation enable us to isolate all of these processes. To lowest order in optical thickness the relative intensities and lineshapes are well interpreted.

New measurement of the Cs 6S → 7S scalar to vector polarizability ratio

Abstract We have measured the ratio |α/β| of the scalar to vector polarizabilities of the forbidden 6S 1 2 → 7S 1 2 transition of cesium by a method free of background difficulties. Our result, | α / β | = 9.90 ± 0.10, agrees with that of Hoffnagle et al. The slight disagreement with our earlier result is examined.

Demonstration of an optical polarization magnifier with low birefringence

In any polarimetric measurement technique, enhancing the laser polarization change of a laser beam before it reaches the analyzer can help in improving the sensitivity. Enhancement of a small polarization rotation can be performed using an optical component having a large linear dichroism, the enhancement factor being equal to the square root of the ratio of the two transmission factors. A pile of parallel plates at Brewster incidence seems appropriate for realizing such a polarization magnifier. In this article, we address the problem raised by the interference in the plates and between the plates, which affects the measurement by giving rise to birefringence. We demonstrate that wedged plates provide a convenient and efficient way to avoid this interference. We have implemented and characterized devices with four and six wedged plates at Brewster incidence, which have led to a decisive improvement of the signal-to-noise ratio in our ongoing parity violation measurement.

Linear Stark shift in dressed atoms as a signal to measure a nuclear anapole moment with a cold-atom fountain or interferometer.

We demonstrate theoretically the existence of a linear dc Stark shift of the individual substates of an alkali atom in its ground state, dressed by a circularly polarized laser field. It arises from the electroweak nuclear anapole moment violating P but not T. It is characterized by the pseudoscalar xi k wedge E x B involving the photon angular momentum and static electric and magnetic fields. We derive the relevant left-right asymmetry with its complete signature in a field configuration selected for a precision measurement with cold-atom beams. The 3, 3 --> 4, 3 Cs transition frequency shift amounts to 7 microHz for a laser power of approximately 1 kW at 877 nm, E=100 kV/cm and B approximately > 0.5 G.