The Standard Model of particle physics is known to be incomplete. Extensions to the Standard Model, such as weak-scale supersymmetry, posit the existence of new particles and interactions that are asymmetric under time reversal (T) and nearly always predict a small yet potentially measurable electron electric dipole moment (EDM), d(e), in the range of 10(-27) to 10(-30) e·cm. The EDM is an asymmetric charge distribution along the electron spin (S(→)) that is also asymmetric under T. Using the polar molecule thorium monoxide, we measured d(e) = (-2.1 ± 3.7stat ± 2.5syst) × 10(-29) e·cm. This corresponds to an upper limit of |d(e)| < 8.7 × 10(-29) e·cm with 90% confidence, an order of magnitude improvement in sensitivity relative to the previous best limit. Our result constrains T-violating physics at the TeV energy scale.

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We recently set a new limit on the electric dipole moment of the electron (eEDM) (J Baron et al and ACME collaboration 2014 Science 343 269-272), which represented an order-of-magnitude improvement on the previous limit and placed more stringent constraints on many charge-parityviolating extensions to the standard model. In this paper we discuss the measurement in detail. The experimental method and associated apparatus are described, together with the techniques used to isolate the eEDM signal. In particular, we detail the way experimental switches were used to suppress effects that can mimic the signal of interest. The methods used to search for systematic errors, and models explaining observed systematic errors, are also described. We briefly discuss possible improvements to the experiment. 9 Note that the limit we report here uses an updated value for  = 78 eff GV cm −1 which is obtained by averaging the results from [29, 30]. 10 A detailed discussion of the sign convention for this Hamiltonian term is provided in section appendix A.3 1 states have very small magnetic moments [58] since the d 3 2 orbital valence electron serves to nearly cancel the magnetic moment of the s 1 2 orbital. The actual magnetic moment of H deviates from zero primarily because of mixing with other states [59]. We express ThO molecule states using the basis Wñ |Y J M , , , , where Y is the electronic state, J is the total angular momentum, M is the projection of J onto the laboratoryẑ-axis, and Ω 1 (V cm −1 ) −1 [67]; this permits full (>99%) polarisation of the state in small applied electric fields,   10 V cm −1 , allowing us to take full advantage of the huge  eff in ThO. The Ω-doublet structure is also useful in rejecting systematic errors since it allows for spectroscopic reversal of  µ -n eff by addressing different  states without reversing the applied electric field [68]. This is discussed in greater detail in section 5.4.The H state in ThO is metastable with a lifetime »1.8 ms [69], limiting our measurement time to t » 1 ms. We note that this is comparable to previous beam-based eEDM measurements where the atomic/molecular states used had significantly longer lifetimes [20,69,70]. 1 (V cm −1 ) −1 (black arrow/lines) is the expectation value of the molecular electric dipole moment in these states [60]. Additionally, a magnetic field   causes a Zeeman shift  m »-Mg z 1 B , with m p » -ǵ 2 6kHz 1 B G −1 (red arrow/lines) [59, 64]. A nonzero eEDM would result in an additional energy shift   »-M d e eff (blue arrow/lines) where  = -1 (+1) when the applied  field is (is not) reversed. The orientation of   eff (green arrows), the spin of the electron in the σ orbital (black arrow next to molecule), the external electric field  , and the external magnetic field   are shown relative to the laboratoryẑ direction which is oriented upwards on the page. Diagram not to scale. 11 Throughout the paper, we give numerical values of energies (with  = 1) in terms of angular frequencies by using the notation p´f 2 , where f ...

Heavy diatomic molecules have been identified as good candidates for use in electron electric dipole moment (eEDM) searches. Suitable molecular species can be produced in pulsed beams, but with a total flux and/or temporal evolution that varies significantly from pulse to pulse. These variations can degrade the experimental sensitivity to changes in the spin precession phase of an electrically polarized state, which is the observable of interest for an eEDM measurement. We present two methods for measurement of the phase that provide immunity to beam temporal variations, and make it possible to reach shot-noise-limited sensitivity. Each method employs rapid projection of the spin state onto both components of an orthonormal basis. We demonstrate both methods using the eEDM-sensitive H 3 1 state of thorium monoxide, and use one of them to measure the magnetic moment of this state with increased accuracy relative to previous determinations.

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