The metastable a(1)[(3)Sigma(+)] state of PbO has been suggested as a suitable system in which to search for the electric dipole moment of the electron. We report here the development of experimental techniques allowing high-sensitivity measurements of Zeeman and Stark effects in this system, similar to those required for such a search. We observe Zeeman quantum beats in fluorescence from a vapor cell of PbO, with shot-noise limited extraction of the quantum beat frequencies, high counting rates, and long coherence times. We argue that improvement in sensitivity to the electron electric dipole moment by at least 2 orders of magnitude appears possible using these techniques.

We are pursuing an experiment to measure the electric dipole moment of the electron using the molecule PbO. This measurement requires the ability to prepare quantum states with orientation of the molecular axis and, simultaneously, alignment of the electron spin perpendicular to this axis.It also requires efficient detection of the evolution of the spin alignment direction within such a state. We describe a series of experiments that have achieved these goals, and the features and limitations of the techniques. We also discuss possible new approaches for improved efficiency in this and similar systems.A permanent electric dipole moment (EDM) of the electron, d e , would violate both parity and time-reversal invariance [1], since d e = 2d e S (where S is the electron spin).There is substantial interest in measurements of d e with sensitivity beyond the current limit d e < 1.6 × 10 −27 e·cm [2]. A non-zero value of d e within the next few orders of magnitude would be a clear indication of physics beyond the Standard Model (SM). Moreover, a nonzero EDM within this range is predicted to occur in a wide range of theories that extend the SM [3]. Heavy polar molecules have a significant advantage over atoms in the search for d e [4]. As a result of the strong hybridization of the atomic orbitals in such molecules, there is a large internal electric field E int = E intn , wheren is the direction of the molecule's internuclear axis. If d e = 0, unpaired electrons interact with this internal field giving rise to a linear Stark shift described by the Hamiltonian H EDM = −d e · E int . This leads to an observable energy shift in molecules that can be 2-3 orders of magnitude larger than what is achievable in atoms under typical laboratory conditions [5, 6].Despite this advantage, molecules pose significant experimental challenges. For example, the thermal Boltzmann distribution limits the population in any particular rovibrational state. This reduces the counting rate in the experiment, and hence the overall sensitivity to d e . In addition, molecules suitable for measuring d e must have unpaired electron spins; such species are chemical free radicals and generally are not thermodynamically stable. This leads to substantial experimental difficulties and typically reduces counting rates even further.Measurements using the molecule PbO hold considerable promise for improved sensitivity to d e , as described in Refs. [7,8,9] and summarized here. PbO is not a free radical; rather, its ground state X(0) 1 Σ + has closed shells [10]. The EDM measurement is conducted in the metastable a(1) [ 3 Σ + ] state, which has two unpaired electron spins and hence is sensitive to d e . The a(1) state can be populated by laser excitation and has a lifetime of τ a ∼ = 82 µs. The Ω-doublet substructure of this |Ω| = 1 state allows it to be easily polarized with a modest external electric field E 15 V/cm. Together these properties allow an EDM measurement using PbO contained in a closed cell, operating at substantial vapor density. These propertie...

We consider an axisymmetric microwave cavity for an accelerator structure whose eigenfrequency for its second lowest TM-like axisymmetric mode is twice that of the lowest such mode, and for which the fields are asymmetric along its axis. In this cavity, the peak amplitude of the rf electric field that points into either longitudinal face can be smaller than the peak field which points out. Computations show that a structure using such cavities might support an accelerating gradient about 47% greater than that for a structure using similar single-mode cavities, without an increase in breakdown probability.

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