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arXiv:2108.02809 · New Journal of Physics

Theoretical aspects of radium-containing molecules amenable to assembly from laser-cooled atoms for new physics searches

Timo Fleig, David P. DeMille

Published Nov 1, 2021• 31 citations
arXivHTMLPDF

Abstract

Abstract We explore the possibilities for a next-generation electron-electric-dipole-moment experiment using ultracold heteronuclear diatomic molecules assembled from a combination of radium and another laser-coolable atom. In particular, we calculate their ground state structure and their sensitivity to parity- and time-reversal ( P , T ) violating physics arising from flavor-diagonal charge-parity ( CP ) violation. Among these species, the largest P , T -violating molecular interaction constants—associated for example with the electron electric dipole moment—are obtained for the combination of radium (Ra) and silver (Ag) atoms. A mechanism for explaining this finding is proposed. We go on to discuss the prospects for an electron EDM search using ultracold, assembled, optically trapped RaAg molecules, and argue that this system is particularly promising for rapid future progress in the search for new sources of CP violation.

DOI
10.1088/1367-2630/ac3619

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•Mar 1, 2020•1 citation
Abstract We explore the possibilities for a next-generation electron-electric-dipole-moment experiment using ultracold heteronuclear diatomic molecules assembled from a combination of radium and another laser-coolable atom. In particular, we calculate their ground state structure and their sensitivity to parity- and time-reversal ( P , T ) violating physics arising from flavor-diagonal charge-parity ( CP ) violation. Among these species, the largest P , T -violating molecular interaction constants—associated for example with the electron electric dipole moment—are obtained for the combination of radium (Ra) and silver (Ag) atoms. A mechanism for explaining this finding is proposed. We go on to discuss the prospects for an electron EDM search using ultracold, assembled, optically trapped RaAg molecules, and argue that this system is particularly promising for rapid future progress in the search for new sources of CP violation.
We propose high-spin Σ -state polar molecules assembled from ultracold atoms to probe charge-parity-violating physics beyond the standard model. We identify YbCr as a prime candidate to search for the electric dipole moment of the electron. We show that the combination of relativistic ytterbium and high-spin chromium, amenable to magnetoassociation, leads to molecules with easy-to-polarize parity doublets and large intramolecular electric fields. Based on results for molecular constants, we predict a sensitivity of δ d e = ( 6 × 10 − 31 / n day ) e cm via standard spin-precession measurements, assess the experimental feasibility, and discuss potential extensions to more advanced quantum control as well as searches of the nuclear magnetic quadrupole moment. This work paves the way to next-generation searches for new physics with ultracold molecules in both the leptonic and hadronic sectors.
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First Measurement of the Atomic Electric Dipole Moment of 225^{225}225Ra
The radioactive radium-225 (225^{225}225Ra) atom is a favorable case to search for a permanent electric dipole moment (EDM). Due to its strong nuclear octupole deformation and large atomic mass, 225^{225}225Ra is particularly sensitive to interactions in the nuclear medium that violate both time-reversal symmetry and parity. We have developed a cold-atom technique to study the spin precession of 225^{225}225Ra atoms held in an optical dipole trap, and demonstrated the principle of this method by completing the first measurement of its atomic EDM, reaching an upper limit of ∣|∣ddd(225^{225}225Ra)∣|∣ $
Probing P,TP,TP,T-Symmetry Violation with Optically Trapped Asymmetric Top Molecules
Searches for P,TP,TP,T-violating electromagnetic moments are among the most sensitive probes of physics beyond the Standard Model. Extending beyond current limits will benefit from molecules with fully controllable orientation at low electric fields, long coherence times, and laser coolability---all offered by asymmetric top molecules (ATMs). Exploiting the intrinsic rotational KKK-doubling in ATMs and the associated long-lived (T1≳10T_1 \gtrsim 10T1​≳10 s) parity doublets afforded by C2vC_{2v}C2v​ symmetry and nuclear-spin statistics, these species combine large electric polarizability with long coherence times in modest laboratory fields. We study alkaline-earth(-like) monoamides, M\mathcal{M}M--NH2_22​ (M\mathcal{M}M = Ca, Sr, Ba, Yb, Ra), which possess favorable electronic structure for laser cooling. We perform \textit{ab initio} calculations of fine and hyperfine constants, identifying the importance of relativistic effects in the spin-rotation tensor. An effective Hamiltonian then models the rotational and hyperfine structure of the vibronic ground state, quantifying electron electric dipole moment (EDM) sensitivities and identifying feasible measurement schemes. We compute the effect of external fields and identify engineered clock transitions that suppress sensitivity to external perturbations while retaining strong EDM sensitivity, and characterize the magic trapping conditions that null differential light shifts in an optical trap. Under these conditions we project a statistical electron-EDM sensitivity over an order of magnitude beyond current best experimental limits, with further gains available from increased molecule number and coherence time. Our results establish asymmetric top molecules as a tunable platform for sensitive symmetry-violation measurements with long coherence times.
That the universe contains a disproportionate amount of matter compared to antimatter cannot be explained by the standard model of particle physics; this is because it contains too little charge-parity (CP) symmetry violation. Low energy experiments with atoms and molecules can be extremely sensitive probes for many types of new physics. An active area of research is the search for electric dipole moments (EDMs) of fundamental particles or nuclei; a nonzero EDM would be evidence for new sources of CP violation. Ytterbium monofluoride (YbF) molecules in a supersonic beam are currently being used to measure the electric dipole moment of the electron (eEDM). This thesis presents progress towards a new, more sensitive eEDM experiment that will use a slower YbF beam source and transverse laser cooling to collimate this beam. We have demonstrated laser cooling of YbF in one transverse dimension to less than 100 μK, which is below the Doppler cooling limit. This was done using a Sisyphus-type mechanism called magnetic field induced laser cooling. We also demonstrate polarisation gradient cooling of YbF. After rebuilding and upgrading much of the experiment, we cooled the beam in both transverse dimensions. By scanning experimental parameters, we have gained insight into the cooling mechanisms, and the optimum conditions for laser cooling. We placed an upper limit of a few mK on the transverse temperature of the molecules, although I argue that the true temperature must be very much lower. The number of ultracold molecules was carefully quantified and found to be 2.0(4)x10^5 per pulse. These experiments lay the foundations for an eEDM sensitivity improvement by up to a factor of 100.