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    August 2018
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●Ytterbium has seven stable isotopes (A=168, 170, 171, 172, 173, 174, 176) and the parity-violating effects are expected to be different for each isotope. This limits the dependence of the measurement upon atomic structure calculations, which are currently less precise than experimental measurements. ●The high charge of the ytterbium nucleus (Z = 70) is important since the parity-violating effects scale as Z3. The parity violating effect is expected to be ~10 and 100 times larger than those previously studied in thallium and cesium, respectively. ●In ytterbium, the odd parity state 6s6p 1P1 state is near in energy to the even parity 5d6s 3D1 state (see energy diagram). In perturbation theory, the mixing of these states is enhanced by the small energy denominator. Why ytterbium? ●An external electric field also mixes states of opposite parity. This results in a Stark-induced transition amplitude. This much larger transition amplitude can be interfered with the small parity-violating transition amplitude allowing observation of the small parity-violating effects. ●Atomic selection rules forbid E1 transitions between states of the same parity. However, the parity-violating weak interaction between the nucleons and electrons can mix states of opposite parity, resulting in a small parity-violating E1 transition amplitude. The Forbidden Transition in Ytterbium 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 the electrons and nucleons within an atom. Because this interaction does not conserve parity the parity of atomic states, as defined by the electromagnetic 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 lowenergy test for the Standard Model and may be sensitive to physics beyond the Standard Model. ●The M1 amplitude for the 6s2 1S0 → 5d6s 3D1 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. ●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. M1 Transition Amplitude ●Given the branching ratios of the decay of the 5d6s 3D1 state, we can also use fluorescence to measure the Stark-induced amplitude. The atoms in the excited 5d6s 3D1 state decay through the 6s6p 3P2, 1, 0 states to the 6s2 1S0 ground state. Comparing the fluorescence from the 6s6p 3P1→ 6s2 1S transition (556nm), after exciting with 408nm light, with the 0 fluorescence from the 6s6p 3P1→ 6s2 1S0 transition (556nm), after exciting with 556nm light, allows for a second method of measurement of the Stark-induced amplitude. ●To calibrate the density of the atomic beam we measure the absorption of 556nm light on the 6s2 1S0 → 6s6p 3P1 transition. This absorption coefficient is known from the lifetime of the 6s6p 3P1 state. ●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 6s2 1S0 → 5d6s 3D1 transition (408nm) in an effusive atomic beam (see diagram) within the presence of an electric field and measure the absorption. Stark-induced E1 Amplitude Work In Progress Current Experimental Apparatus Observation of the Forbidden Transition 0.00 171 1/2 -> 1/2 170 173 5/2 -> 5/2 173 5/2 -> 7/2 Fluorescence -0.05 173 5/2 -> 3/2 176 171 1/2 -> 3/2 -0.10 172 -0.15 174 8000 6000 4000 2000 0 Relative Frequency (in MHz) This plot shows the fluorescence from the 6s6p 3P1 → 6s2 1S0 transition after exciting the 3D1 state. The fluorescence is observed with a photomultplier tube as the excitation-laser frequency is scanned. Low-Lying Energy Levels of Ytterbium 6s5d 3D3 6s5d 3D2 6s5d 3D1 6s6p 1P1 6s6p 3P2 6s6p 3P1 6s6p 3P0 408 nm 556 nm 6s2 1S0 Odd Even Investigation of the 6s2 1S0 → 5d6s 3D1 Transition in Atomic Ytterbium C.J. Bowers, D. Budker, E. D. Commins, D. DeMille, S.J. Freedman, G. Gwinner, J.E. Stalnaker Stark Shift Measurement Fluorescence -5 -10 -15 -20 -25x10 -3 400 300 200 Realative Frequency (in MHz) 100 0 This plot shows the effect of the electric field on both the amplitude and position of the transition for the case of Yb171 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. 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 Phys. Rev. A 53, 3103(1995)). Stark Shifts ●We have measured the Stark shifts of the 6s2 1S0 → 5d6s 3D1 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 3D1 states. This is done by exciting the 6s2 1S0 →5d6s 3D1 transition and observing the fluorescence of the 6s6p 3P1→ 6s2 1S0 transition with the photomultiplier tube. Chopping of the atomic beam allows lock-in detection of both fluorescence and absorption signals 408 nm Transmission Photodiode Detector 556 nm Transmission Photodiode Detector Normalization of laser power reduces noise in absorption signals due to laser power fluctuations. Low laser power avoids optical pumping and saturation effects. Mirror 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. Atomic Holes in Field Fluorescence Beam Plates to See Detection Chopper Fluorescence PMT Wheel Dichroic Mirror Electric Field Plates (~45kV/cm) Atomic Beam 556 nm Atomic Oven E 408 nm Normalization Photodiode Detector 408nm Laser Beam (~20mW) Beamsplitter Dichroic Mirror 556 nm Normalization Photodiode Detector 556nm Laser Beam (~2nW) Mirror Beamsplitter