Letter | Published:

Improved measurement of the shape of the electron

Nature volume 473, pages 493496 (26 May 2011) | Download Citation



The electron is predicted to be slightly aspheric1, with a distortion characterized by the electric dipole moment (EDM), de. No experiment has ever detected this deviation. The standard model of particle physics predicts that de is far too small to detect2, being some eleven orders of magnitude smaller than the current experimental sensitivity. However, many extensions to the standard model naturally predict much larger values of de that should be detectable3. This makes the search for the electron EDM a powerful way to search for new physics and constrain the possible extensions. In particular, the popular idea that new supersymmetric particles may exist at masses of a few hundred GeV/c2 (where c is the speed of light) is difficult to reconcile with the absence of an electron EDM at the present limit of sensitivity2,4. The size of the EDM is also intimately related to the question of why the Universe has so little antimatter. If the reason is that some undiscovered particle interaction5 breaks the symmetry between matter and antimatter, this should result in a measurable EDM in most models of particle physics2. Here we use cold polar molecules to measure the electron EDM at the highest level of precision reported so far, providing a constraint on any possible new interactions. We obtain de = (−2.4 ± 5.7stat ± 1.5syst) × 10−28e cm, where e is the charge on the electron, which sets a new upper limit of |de| < 10.5 × 10−28e cm with 90 per cent confidence. This result, consistent with zero, indicates that the electron is spherical at this improved level of precision. Our measurement of atto-electronvolt energy shifts in a molecule probes new physics at the tera-electronvolt energy scale2.

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  1. 1.

    & CP Violation Without Strangeness (Springer, New York, 1997)

  2. 2.

    & Electric dipole moments as probes of new physics. Ann. Phys. 318, 119–169 (2005)

  3. 3.

    Electric dipole moments of leptons. In Advances in Atomic, Molecular, and Optical Physics Vol. 40, 1–56 (eds & ), Academic Press. (1999)

  4. 4.

    & in Lepton Dipole Moments (eds & ) Ch. 14 (World Scientific, Singapore, 2010)

  5. 5.

    Violation of CP invariance, C asymmetry, and baryon asymmetry of the Universe. Pis'ma ZhETF 5, 32–35 (1967); Sov. Phys. JETP Lett. 5, 24–27 (1967)

  6. 6.

    Angular Momentum in Quantum Mechanics 73–77 (Princeton University Press, 1996)

  7. 7.

    , , & New limit on the electron electric dipole moment. Phys. Rev. Lett. 88, 071805 (2002)

  8. 8.

    , , & Measurement of the electron electric dipole moment using YbF molecules. Phys. Rev. Lett. 89, 023003 (2002)

  9. 9.

    Testing time reversal symmetry using molecules. Phys. Scr. T70, 34–41 (1997)

  10. 10.

    & Enhancement of the electric dipole moment of the electron in the YbF molecule. Phys. Rev. A 49, 4502–4507 (1994)

  11. 11.

    Enhancement of the electric dipole moment of the electron in the YbF molecule. J. Phys. B 30, L607–L612 (1997)

  12. 12.

    , & P,T-odd spin-rotational Hamiltonian for YbF molecule. Phys. Rev. Lett. 77, 5346–5349 (1996)

  13. 13.

    , & Hyperfine and PT-odd effects in YbF2Σ. J. Phys. B 31, L85–L95 (1998)

  14. 14.

    Ab initio calculation of the enhancement of the electric dipole moment of an electron in the YbF molecule. J. Phys. B 31, 1409–1430 (1998)

  15. 15.

    , & Electric dipole moment of the electron in the YbF molecule. J. Phys. B 31, L763–L767 (1998)

  16. 16.

    et al. Pulsed beams as field probes for precision measurement. Phys. Rev. A 76, 033410 (2007)

  17. 17.

    et al. A jet beam source of cold YbF radicals. J. Phys. B 35, 5013–5022 (2002)

  18. 18.

    , , , & Optical Zeeman spectroscopy of ytterbium monofluoride, YbF. J. Phys. Chem. A 113, 8038–8044 (2009)

  19. 19.

    , & Robust Statistics; Theory and Methods 31–32 (Wiley, 2006)

  20. 20.

    & Bootstrap methods for standard errors, confidence intervals, and other measures of statistical accuracy. Stat. Sci. 1, 54–75 (1986)

  21. 21.

    , , & Prospects for measuring the electric dipole moment of the electron using electrically trapped polar molecules. Faraday Discuss. 142, 37–56 (2009)

  22. 22.

    , , , & A robust floating nanoammeter. Rev. Sci. Instrum. 79, 126102 (2008)

  23. 23.

    Better bootstrap confidence intervals. J. Am. Stat. Assoc. 82, 171–185 (1987)

  24. 24.

    et al. Diffusion, thermalization and optical pumping of YbF molecules in a cold buffer gas cell. Phys. Rev. A 83, 023418 (2011)

  25. 25.

    , & A bright, slow cryogenic molecular beam source for free radicals. Preprint at 〈〉 (2011)

  26. 26.

    et al. A cryogenic beam of refractory, chemically reactive molecules with expansion cooling. Preprint at 〈〉 (2011)

  27. 27.

    , & in Cold Molecules: Theory, Experiment, Applications (eds , & ) Ch. 14 (CRC Press, 2009)

  28. 28.

    , & A multichannel phase-sensitive detection method using orthogonal square waveforms. J. Phys. E 4, 750–754 (1971)

  29. 29.

    , , & in Cold Molecules: Theory, Experiment, Applications (eds , & ) Ch. 15 (CRC Press, 2009)

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We acknowledge the contributions of P. Condylis and H. Ashworth. We are grateful for technical assistance from J. Dyne and V. Gerulis. This work was supported by the UK research councils STFC and EPSRC, and by the Royal Society. J.J.H. is supported by an STFC Advanced Fellowship.

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  1. Centre for Cold Matter, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, UK

    • J. J. Hudson
    • , D. M. Kara
    • , I. J. Smallman
    • , B. E. Sauer
    • , M. R. Tarbutt
    •  & E. A. Hinds


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J.J.H. was involved in all aspects of the measurement, led the analysis, and drafted the manuscript. D.M.K. developed many of the systematic tests, worked on taking the data set, and contributed to the analysis. I.J.S. had primary responsibility for taking the data set, and contributed to the development of the data acquisition techniques. B.E.S. was involved in all aspects of the measurement, and designed much of the hardware. M.R.T. built the molecular beam source, contributed to the analysis, and drafted the manuscript. E.A.H. contributed to the analysis, drafted the manuscript and led the team. All authors discussed the results, improved the manuscript and were equally involved in setting the direction of the work.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to E. A. Hinds.

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