1. The Forbidden Transition in Ytterbium ● Atomic selection rules forbid E1 transitions between states of the same parity. However, the parity-violating weak interaction between thenucleons and electrons can mix states of opposite parity, resulting in asmall parity-violating E1 transition amplitude. ● An external electric field also mixes states of opposite parity. This results in a Stark-induced transition amplitude. This much larger transitionamplitude can be interfered with the small parity-violating transitionamplitude allowing observation of the small parity-violating effects.Why ytterbium? ● In ytterbium, the odd parity state 6s6p 1 P 1 state is near in energy to the even parity 5d6s 3 D 1 state (see energy diagram). In perturbation theory, the mixing of these states is enhanced by the small energy denominator. ● The high charge of the ytterbium nucleus (Z = 70) is important since the parity-violating effects scale as Z 3. The parity violating effect is expected to be ~10 and 100 times larger than those previously studied in thalliumand cesium, respectively. ● Ytterbium has seven stable isotopes (A=168, 170, 171, 172, 173, 174, 176) and the parity-violating effects are expected to be different for eachisotope. This limits the dependence of the measurement upon atomicstructure calculations, which are currently less precise than experimentalmeasurements.

2. Parity Nonconservation In Atoms ● Within an atom there is an interaction due to the weak force. This interaction occurs via the exchange of virtual Z-bosons between theelectrons and nucleons within an atom. Because this interaction does notconserve parity the parity of atomic states, as defined by theelectromagnetic interaction, is not completely preserved. ● The presence of a parity-violating interaction mixes states of opposite parity. This mixing is manifested in the optical properties of the atom. ● Because the Standard Model predicts the size of these parity-violating effects, precision measurements of atomic parity violation provide a low-energy test for the Standard Model and may be sensitive to physics beyondthe Standard Model.

3. Work In Progress Stark-induced E1 Amplitude ●In order to determine the parity-violating effects on an absolute scale we are currently working on measuring the Stark-induced transition amplitude. We use a c.w. laser to excite the 6s 2 1 S 0 → 5d6s 3 D 1 transition (408nm) in an effusive atomic beam (see diagram) within the presence of an electric field and measure the absorption. ●To calibrate the density of the atomic beam we measure the absorption of 556nm light on the 6s 2 1 S 0 → 6s6p 3 P 1 transition. This absorption coefficient is known from the lifetime of the 6s6p 3 P 1 state. ●Given the branching ratios of the decay of the 5d6s 3 D 1 state, we can also use fluorescence to measure the Stark-induced amplitude. The atoms in the excited 5d6s 3 D 1 state decay through the 6s6p 3 P 2, 1, 0 states to the 6s 2 1 S 0 ground state. Comparing the fluorescence from the 6s6p 3 P 1 → 6s 2 1 S 0 transition (556nm), after exciting with 408nm light, with the fluorescence from the 6s6p 3 P 1 → 6s 2 1 S 0 transition (556nm), after exciting with 556nm light, allows for a second method of measurement of the Stark-induced amplitude. M1 Transition Amplitude ●Determining the parity-violating amplitude by observing the interference with the Stark-induced amplitude requires a small M1 amplitude so that the parity-nonconserving amplitude is not masked by the M1 amplitude. ●The M1 amplitude for the 6s 2 1 S 0 → 5d6s 3 D 1 transition is estimated to be highly suppressed, but a direct measurement of the M1 amplitude is necessary do determine any effect its presence may have on the parity nonconservation measurement.

4. Current Experimental Apparatus

5. Observation of the Forbidden Transition This plot shows the fluorescence from the 6s6p 3 P 1 → 6s 2 1 S 0 transition after exciting the 3 D 1 state. The fluorescence is observed with a photomultplier tube as the excitation-laser frequency is scanned.

6. 6s5d 3 D 3 6s5d 3 D 2 6s5d 3 D 1 408 nm 556 nm Low-Lying Energy Levels of Ytterbium 6s6p 1 P 1 6s 2 1 S 0 6s6p 3 P 0 6s6p 3 P 1 6s6p 3 P 2 PNC and Stark Mixing Odd Even

7. Investigation of the 6s 2 1 S 0 → 5d6s 3 D 1 Transition in Atomic Ytterbium C.J. Bowers, D. Budker, E. D. Commins, D. DeMille, S.J. Freedman, G. Gwinner, J.E. Stalnaker

8. Stark Shift Measurement This plot shows the effect of the electric field on both the amplitude and position of the transition for the case of Yb 171 1/2 → 1/2. The electric field is switched between 40kV and 25kV throughout the scan. Points are connected to show time sequence. Each point corresponds to a ~2 second time period.

9. Results Lifetime Measurements ●In order to determine the branching ratios the lifetimes of 21 excited states in atomic ytterbium were measured using time-resolved fluorescence detection after pulsed laser excitation (C.J. Bowers et.al. Phys. Rev. A 53, 3103(1995)). Stark Shifts ●We have measured the Stark shifts of the 6s 2 1 S 0 → 5d6s 3 D 1 transition (408nm). This is done by exciting with laser light at 408nm and observing the cascade fluorescence at 556nm while varying the electric field. ●In order to minimize the effects of temperature drifts of the laser frequency, we switch the electric field between two values as we scan over the resonance (see Stark shift plot). ●The size of the shifts are ~20 MHz for the values of the electric field used (20-50kV). Isotope Shifts and Hyperfine Structure ●Our experimental setup allows us to measure the isotope shifts and hyperfine structure for the 5d6s 3 D 1 states. This is done by exciting the 6s 2 1 S 0 →5d6s 3 D 1 transition and observing the fluorescence of the 6s6p 3 P 1 → 6s 2 1 S 0 transition with the photomultiplier tube.

10. Atomic Oven E Atomic Beam Chopper Wheel Fluorescence Detection PMT Holes in Field Plates to See Fluorescence 556 nm 408 nm Normalization Photodiode Detector 556 nm Normalization Photodiode Detector 408 nm Transmission Photodiode Detector 556 nm Transmission Photodiode Detector Dichroic Mirror Dichroic Mirror Beamsplitter 556nm Laser Beam (~2nW) 408nm Laser Beam (~20  W) Atomic Beam Mirror Chopping of the atomic beam allows lock-in detection of both fluorescence and absorption signals Normalization of laser power reduces noise in absorption signals due to laser power fluctuations. Low laser power avoids optical pumping and saturation effects. Fluorescence detection during a calibrated laser frequency scan (with increased 408nm laser power) is used for measurement of hyperfine structure, isotope shifts, and Stark shifts. Electric Field Plates (~45kV/cm)