PolyEDM

We aim to laser cool and optically trap heavy, polyatomic molecules in order to search for the electric dipole moment (EDM) of the electron.

Motivation

Despite its many successes, the Standard Model of particle physics has several problems which must be addressed. For example, the Standard Model, as it stands, does not predict a universe dominated by matter, with scarcely any anti-matter. Yet such is the universe in which we find ourselves.Image of a galaxy and a rendition of an anti-galaxy, which is not observed in our universe.

It is known that any theory explaining this matter/anti-matter asymmetry requires the presence of new time-reversal-violating interactions [1], and EDMs of fundamental particles are one direct signature of such symmetry violation [2]. Progress over many decades has demonstrated that heavy atoms and molecules possess very high sensitivity to the electron EDM [3-6].

PolyEDM

At its core, an electron EDM experiment simply searches for a shift in the electron’s energy due to the presence of an electric field. Performing such a measurement precisely requires large electric fields, long interaction times, and many particles. All of these can be achieved using laser-cooled and trapped, neutral, heavy polyatomic molecules. Furthermore, polyatomic molecules (as opposed to diatomics) promise robust systematic error rejection via internal co-magnetometer states. While some diatomic species offer a subset of these advantages, polyatomics are the only identified platform which simultaneously offer all of them [7].

As one example, an experiment probing 106 such molecules in an optical trap with a 10 s coherence time would probe T-violating physics at the PeV scale after about 1 week of averaging. This is well beyond the energy reach of existing or conceivable accelerators.

Yb-Containing Polyatomics

Yb-containing polyatomic molecules are ideally suited for such an experiment. The high mass of the Yb nucleus yields large relativistic enhancements for the electron EDM [8]. Yb’s alkaline-earth-like structure means that molecules which YbOH and YbOCH3 will be isoelectronic to CaF, a species for which laser cooling and optical trapping have already been demonstrated [9-12], and to SrOH, for which one-dimensional laser cooling has been observed [13].

Figure showing an excited molecule decaying and emitting a photon

A particularly important aspect of the structure of YbOH and YbOCH3 is the presence of ‘parity doublets’ arising from nuclear orbital angular momentum [7]. The bending mode of YbOH consists of a pair of states separated by about 40 MHz. Small laboratory electric fields, ~100 V/cm, will fully mix these states are orient the molecule either along or against the laboratory field. The electron near the Yb nucleus, on the other hand, interacts with an internal electric field of order 25 GV/cm [8]. By selectively populating one or the other orientation and performing a spin precession measurement, the electron EDM interaction can be reversed, thereby suppressing potential systematic errors. YbOCH3 has a similar pair of states due to rigid body rotation, however these states are split by <1 MHz and therefore fully polarized in fields <1 V/cm. The parity doublets in both species live for >10 s, promising long coherence times.

Experimental Work

We have recently produced our first samples of buffer-gas cooled YbOH. We are now working to characterize a molecular beam of YbOH and demonstrate further slowing to the capture velocity of a magneto-optical trap.

In parallel, we have made the first observations of YbOCH3 molecules, in collaboration with the Steimle group at ASU. Our initial measurements demonstrate that YbOCH3 has relatively diagonal Franck-Condon factors, and is another candidate molecule for laser cooling and trapping.

 

 

References

[1] A. D. Sakharov, "Violation of CP Invariance, C asymmetry, and baryon asymmetry of the universe." JETP Lett. 5, 24-27 (1967).

[2] E. D. Commins and D. DeMille, "The Electric Dipole Moment of the Electron," in Lepton Dipole Moments. Advanced Series on Directions in High Energy Physics: Vol. 20. Robers, B. Lee, Ed. https://www.worldscientific.com/worldscibooks/10.1142/7273 (2009).

[3] J. J. Hudson, D. M. Kara, I. J. Smallman, B. E. Sauer, M. R. Tarbutt, and E. A. Hinds, "Improved measurement of the shape of the electron." Nature 473, 493-496 (2011).

[4] ACME Collaboration, "Order of magnitude smaller limit on the electric dipole moment of the electron." Science 17, Vol. 343 Issue 6168, 269-272 (2014).

[5] W. B. Cairncross, D. N. Gresh, M. Grau, K. C. Cossel, T. S. Roussy, Y. Ni, Y. Zhou, J. Ye, and E. A. Cornell, "Precision measurement of the electron's electric dipole moment using trapped molecular ions." Phys. Rev. Lett. 119, 153001 (2017).

[6] ACME Collaboration, "Improved limit on the electric dipole moment of the electron." Nature 562, 355-360 (2018). 

[7] I. Kozyryev and N. R. Hutzler, "Precision measurement of time-reversal symmetry violation with laser-cooled polyatomic molecules." Phys. Rev. Lett. 119, 133002 (2017).

[8] M. Denis, P. A. B. Haase, R. G. E. Timmermans, E. Eliav, N. R. Hutzler, and A. Borschevsky, "Enhancement factor for the electric dipole moment of the electron in the BaOH and YbOH molecules." arXiv:1901.02265v1 (2019).

[9] S. Truppe, H. J. Williams, M. Hambach, L. Caldwell, N. J. Fitch, E. A. Hinds, B. E. Sauer, and M. R. Tarbutt, "Molecules cooled below the Doppler limit." Nat. Phys. 13, 1173-1176 (2017).

[10] L. Anderegg, B. L. Augenbraun, E. Chae, B. Hemmerling, N. R. Hutzler, A. Ravi, A. Collopy, J. Ye, W. Ketterle, and J. M. Doyle, "Radio Frequency Magneto-Optical Trapping of CaF with High Density." Phys. Rev. Lett. 119, 103201 (2017).

[11] L. W. Cheuk, L. Anderegg, B. L. Augenbraun, Y. Bao, S. Burchesky, W. Ketterle, and J. M. Doyle, "Λ-Enhanced Imaging of Molecules in an Optical Trap." Phys. Rev. Lett. 121, 083201 (2018).

[12] L. Anderegg, B. L. Augenbraun, Y. Bao, S. Burchesky, L. W. Cheuk, W. Ketterle, and J. M. Doyle, "Laser cooling of optically trapped molecules." Nat. Phys. 14, 890-893 (2018).

[13] I. Kozyryev, L. Baum, K. Matsuda, B. L. Augenbraun, L. Anderegg, A. P. Sedlack, and J. M. Doyle, "Sisyphus Laser Cooling of a Polyatomic Molecule." Phys. Rev. Lett. 118, 173201 (2017).