Observation of a Large Atomic Parity Violation Effect in Ytterbium

Atomic parity violation has been observed in the $6s^2{}^{1}S_0\to 5d6s{}^{3}D_1$ 408-nm forbidden transition of ytterbium. The parity-violating amplitude is found to be 2 orders of magnitude larger than in cesium, where the most precise experiments to date have been performed. This is in accordance with theoretical predictions and constitutes the largest atomic parity-violating amplitude yet observed. This also opens the way to future measurements of neutron distributions and anapole moments by comparing parity-violating amplitudes for various isotopes and hyperfine components of the transition.

Figure 1

Low-lying energy eigenstates of Yb and transitions relevant to the APV experiment.

Figure 2

Orientation of fields for PV-Stark interference experiment and schematic of the present APV apparatus. Not shown is the vacuum chamber containing all the depicted elements, except the photomultiplier (PMT) and the photodiode (PD). PBC-power buildup cavity. Light is applied collinearly with $\mathbf{x}$.

Figure 3

Discrimination of the PV effect by $E$-field modulation under static magnetic field.

Figure 4

A profile of the $B$-field-split 408-nm spectral line of ${}^{174}\mathrm{Yb}$ recorded at 1st and 2nd harmonic of the modulation. Also a simulated PV contribution is shown for clarity. $\tilde{E}=5\mathrm{kV}/\mathrm{cm}$; dc $\mathrm{\text{offset}}=40\mathrm{V}/\mathrm{cm}$; $\theta =\pi /4$; an effective integration time is 10 s per point.

Figure 5

The PV-interference parameter $\zeta /\beta$. Mean value: $39(4{)}_{\mathrm{stat}}(5{)}_{\mathrm{syst}}\mathrm{mV}/\mathrm{cm}$, $|\zeta |=8.7\pm 1.4\times {10}^{-10}e{a}_0$. The 68% confidence band includes both the statistical and the systematic uncertainties.