Magnetic and electric dipole moments of theH3Δ1state in ThO

Abstract

The metastable H 3 1 state in the thorium monoxide (ThO) molecule is highly sensitive to the presence of a CP-violating permanent electric dipole moment of the electron (eEDM) [E. R. Meyer and J. L. Bohn, Phys. Rev. A 78, 010502 (2008)]. The magnetic dipole moment μ H and the molecule-fixed electric dipole moment D H of this state are measured in preparation for a search for the eEDM. The small magnetic moment μ H = 8.5(5) × 10 −3 μ B displays the predicted cancellation of spin and orbital contributions in a 3 1 paramagnetic molecular state, providing a significant advantage for the suppression of magnetic field noise and related systematic effects in the eEDM search. In addition, the induced electric dipole moment is shown to be fully saturated in very modest electric fields (<10 V/cm). This feature is favorable for the suppression of many other potential systematic errors in the ThO eEDM search experiment. Measurable CP violation is predicted in many proposed extensions to the standard model, and could provide a clue to the observed dominance of matter over antimatter in the universe [1]. The permanent electric dipole moment of the electron (eEDM) is a sensitive probe for flavor-diagonal CP violation in the lepton sector [2]. A number of experimental efforts are currently focused on searching for this elusive quantity [3]. Many of these experiments take advantage of the large internal electric field E mol experienced by valence electrons in a polar molecule [4]. Following the suggestion of Meyer et al. [5], states with a 3 1 character in heavy molecules are being used in several new eEDM experiments [6][7][8]. In addition to a large intrinsic eEDM sensitivity, there are two key attractive features of this kind of molecular state for eEDM searches: closely spaced opposite parity doublets ( -doublets) [5,9] and small magnetic moments [5,10]. The -doublets enable the molecule to be completely polarized in small electric fields. The fully polarized molecule accesses the full eEDM sensitivity of the molecule, while suppressing E-field-induced systematic errors such as those due to leakage currents and geometric phases [7,11,12]. The extremely small magnetic moment of the H 3 1 state makes the molecule less sensitive to effects arising from fluctuating B fields and motional ( v × E/c 2 ) magnetic fields [7]. Here we report measurements of both of these key parameters in the H state of ThO. I. EXPERIMENTAL SETUPThe measurements were carried out using a molecular beam of ThO, produced in an apparatus similar to one described elsewhere [7]. The apparatus uses helium buffer gas at 4 K to cool a pulse of ThO molecules (produced by pulsed laser ablation of ThO 2 ), which are extracted into a molecular beam and probed 30 cm downstream. The lowest rovibrational level (v = 0,J = 1) in the H state was populated by optical pumping from the ground electronic X 1 + (v = 0,J = 1) state via the higher-lying, short-lived A 3 0 + (v = 0,J = 0) state. It * amar.vutha@gmail.com was subsequently probed a few millimeters downstream by e...

Communication: Theoretical study of ThO for the electron electric dipole moment search

“…One of the most promising experiments towards the measurement of eEDM is proposed and now prepared on the metastable 3 1 state of the thorium monoxide (ThO) molecule by ACME Collaboration (groups of DeMille, Gabrielse, and Doyle). 7,8 A very high sensitivity to eEDM is expected in the nearest future, up to an order of magnitude and more than that attained in the YbF and Tl experiments, due to some unique combination of experimental advantages of the molecule. Even the value for eEDM compatible with zero will lead to serious consequences for the modern theory of fundamental symmetries.…”

Zeeman interaction in ThOH3Δ1for the electron electric-dipole-moment search

“…This means that the experimental signature of d e can be detected either by performing the measurement in the other -doublet state or by reversing the external electric field E. On the other hand, the internal field of an atom or molecule without doublets can be reversed only by reversing E, which makes the measurement susceptible to systematic errors associated * alexsandernp@gmail.com with changing leakage currents, field gradients, and motional fields [1,4]. Molecules with doublets are very robust against these effects, since the -doublet structure acts as an "internal comagnetometer" [19]; the spin-precession frequencies in the two -doublet states can be subtracted from each other, which heavily suppresses many effects related to magnetic fields [19] or geometeric phases [20] but doubles the eEDM signature. The advantages of doublets for suppression of systematic effects were first proposed [19] and realized [14,21] in the PbO eEDM search.…”

“…The molecule-fixed dipole moment, D , is taken from experiment [31]. The positive value for D means that the unit vector n along the molecular axis is directed from O to Th.…”

Shot-noise-limited spin measurements in a pulsed molecular beam

“…Here u ∼ = 180 m/s is the forward velocity of molecules in the beam [4], and μ H ∼ = 0.008μ B is the H -state magnetic moment (where μ B is the Bohr magneton) [17]. At the end of the interaction region, the final state is…”

“…Our measurements are performed on the H 3 1 state of thorium monoxide (ThO). This state has been identified as a promising system for detecting the eEDM [11,13] due to its -doublet energy level structure (which provides both high eEDM sensitivity and powerful means to reject systematic errors [14,15]), its small magnetic moment [16,17] (which suppresses sensitivity to magnetic noise and systematics [13,18]), and its long lifetime [11,18,19] (which enables high sensitivity to the phase φ). We show two separate methods that allow projective measurements onto both states of the measurement basis for each particle, making it possible to form the N -independent asymmetry A in both cases.…”