# Direct measurement of the intrinsic electric dipole moment in pear-shaped thorium-228

## Abstract

Atomic nuclei with certain combinations of proton and neutron numbers can adopt reflection-asymmetric or octupole-deformed shapes at low excitation energy. These nuclei present a promising avenue in the search for a permanent atomic electric dipole moment—the existence of which has implications for physics beyond the Standard Model of particle physics. Theoretical studies have suggested that certain thorium isotopes may have large octupole deformation. However, due to experimental challenges, the extent of the octupole collectivity in the low-energy states in these thorium nuclei has not yet been demonstrated. Here, we report measurements of the lifetimes of low-energy states in 228Th (Z = 90) with a direct electronic fast-timing technique, the mirror symmetric centroid difference method. From lifetime measurements of the low-lying Jπ = 1 and Jπ = 3 states, the E1 transition probability rates and the intrinsic dipole moment are determined. The results are in agreement with those of previous theoretical calculations, allowing us to estimate the extent of the octupole deformation of 228Th. This study indicates that the nuclei 229Th and 229Pa (Z = 91) may be good candidates for the search for a permanent atomic electric dipole moment.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

## Code availability

The code used to process the data presented in this paper is available from the corresponding author upon reasonable request.

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## Acknowledgements

We acknowledge useful discussions with L. Robledo, who was the recipient of a SUPA Distinguished Visitor grant awarded to the University of the West of Scotland. Financial support for this work has been provided by the Scottish Funding Council (SFC) and the UK Science and Technology Facilities Council (STFC). We acknowledge the support of the UK EPSRC (grant nos EP/J018171/1, EP/J500094/1 and EP/N028694/1), the EC’s LASERLAB-EUROPE (grant no. 654148), EuCARD-2 (grant no. 312453), EuPRAXIA (grant no. 653782), ARIES (grant no. 730871) and the Extreme Light Infrastructure (ELI) European Project.

## Author information

Authors

### Contributions

D.O’D. conceived the investigation; M.M.R.C., D.O’D. and G.B. set up the instrumentation; M.M.R.C., D.O’D., G.B. and P.S. performed the data analysis; M.M.R.C., D.O’D. and M.S. interpreted the results; D.O’D., M.B., D.A.J., B.S.N.S., M.S., P.S. and J.F.S. contributed to writing the manuscript.

### Corresponding author

Correspondence to D. O’Donnell.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Extended data

### Extended Data Fig. 1 Prompt response function.

The prompt response differences (PRD) of the fast-timing apparatus. The top panel shows the $$\gamma-\gamma$$ time-walk dependence with $$\gamma$$ -ray energy of the fast-timing apparatus. Data points correspond to transitions depopulating prompt states in $${}^{152}$$ Sm and $${}^{152}$$ Gd following the $$\beta$$ -decay of $$^{152}$$ Eu. Error bars, representing two standard deviations, are smaller than the symbols for the data points. The function fitted to the data has the form $$\text{PRD}(E_\gamma) = \frac{a}{\sqrt{E_\gamma+b}}+c{E_\gamma}^2+dE_\gamma+e$$ where $$E_\gamma$$ is the $$\gamma$$ -ray energy and a,b,c,d and e are parameters to be determined. The bottom panel shows the differences between the data and the fitted prompt response function with the dashed lines showing the mean uncertainty of the fit corresponding to two standard deviations.

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