Perspectives Of course, more exciting than limiting the electron electric dipole moment would be to measure it. Why does theoretical physics tell us to expect it, and what do we expect the magnitude to be? These are both challenging questions to Kenneth R. Brown explain simply. An electric dipole moment requires an interaction that vioSchools of Chemistry and Biochemistry; Computational Science and Engineering; and Physics, Georgia lates both parity and time symmetry. Institute of Technology, Atlanta, GA 30332–0400, USA. E-mail: [email protected] Everyday laws of physics are invariant if we relabel forward and backward in Can tabletop experiments using polar molecules to reveal the internal structure of space and time. However, the laws that the electron point to physics beyond the Standard Model? govern subatomic particles are not. They satisfy a combined symmetry of charge conjugation, parity, and time. Ordinary matter is made of fast-moving electrons and slow-moving The Standard Model of particle physics allows for a violation of nuclei. The dynamics of electrons accounts for most material properties time and parity symmetry, and from the known interactions we can esfrom color to reactivity. Our success in predicting the physics of semitimate that the electron electric dipole moment will be smaller than 10−38 conductors and the chemistry of molecules depends on our ability to e⋅cm (5). Extensions to the Standard Model, for example, supersymmetcharacterize the electron. The electron is vanishingly small, and its propric or left-right symmetric models, can produce electron electric dipole erties can be well described by three parameters: mass, charge, and moments near the current limits of detection. As a result, tabletop expermagnetic dipole. All of these parameters have been measured to extreme iments are beginning to limit which theoretical models are compatible precision. Theoretical models of physics predict that the electron will with the observed world. also have a very small electric dipole moment. A strong electric field is Normally, when considering questions about particle physics, one required to detect a small electric dipole, and one proposed method is to thinks of large particle accelerator experiments. The excitement of the observe the properties of the electron in the internal electric field of a discovery of the Higgs boson at the Large Hadron Collider (LHC) (6, 7) polar molecule. Loh et al. (1) recently demonstrated that molecular ions and the award of this year’s Nobel prize for its prediction are testaments can also be used for this task. In this week’s Science Express, Baron et to this method of inquiry. A challenge for the tabletop experiments is al. (2) report on measurements that lower the bound for the electron’s that at present the results at the LHC are consistent with the Standard electric dipole moment. Model. The lack of detection of other new particles also limits possible To observe the strong internal fields, the molecule or molecular ion theoretical models of our world and, as a result, the value of the electron must be aligned. Otherwise, the rotation of the molecule will wash out electric dipole moment. It is unclear at the moment whether investigaany possible signal. For a polar molecule, this alignment can be pertion at high precision or investigation at high energy will be first to reformed by applying an external electric field (2, 3). Baron et al. use a veal additional physics beyond the Standard Model. cryogenic buffer gas beam of ThO molecules between two electric plates. An electric field of approximately 100 V/cm orients the molecuReferences lar dipole, and lasers then interrogate the molecule for a possible signa1. H. Loh et al., Science 342, 1220 (2013). doi:10.1126/science.1243683 ture of the electron electric dipole moment. The measurements of Baron Medline et al. place a bound on the electron electric dipole moment to be below 2. J. Baron et al., Science. 10.1126/science.1248213 −29 8.7 × 10 e⋅cm, an order of magnitude improvement over the previous 3. J. J. Hudson et al., Nature 473, 493 (2011). doi:10.1038/nature10104 bound measured in YbF (3). Medline An alternative approach to neutral beams is to use trapped molecular 4. A. E. Leanhardt et al., J. Mol. Spectrosc. 270, 1 (2011). doi:10.1016/j.jms.2011.06.007 ions. For an ion held in a radio-frequency trap, a static electric field of 5. E. D. Commins, in Advances in Atomic, Molecular, and Optical sufficient strength to polarize the molecule will also push it out of the Physics, B. Bederson, H. Walther, Eds. (Academic Press, 1999), vol. trap. The proposed solution to the problem was applying a strong rotat40, pp. 1–55. ing field (4). In the frame of that field, one gains the benefits of both 6. ATLAS Collaboration, Phys. Lett. B 716, 1 (2012). holding trapped ions for a long time and polarization of the molecule doi:10.1016/j.physletb.2012.08.020 (see the figure). 7. CMS Collaboration, Phys. Lett. B 716, 30 (2012). Loh et al. use the rotating field method to limit the electron electric doi:10.1016/j.physletb.2012.08.021 dipole moment to be less than 1.5 × 10−25 e⋅cm. This initial experiment is three orders of magnitude above the ThO result of Baron et al.. The Published online 19 December 2013 10.1126/science.1246820 experiment of Loh et al. shows that precision measurements can be performed with molecular ions, and future experiments will be competitive with the neutral molecule and atom measurements.

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Probing the Electron

Spin it up and peek inside. Schematic of the experiment of Loh et al. from the perspective of the molecular ion HfF+. The red and blue shapes approximate the orbitals of the two highest energy electrons. The white arrow represents the motion of the molecule as it follows the field.

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Physics Probing the electron.

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