University Electric

IOP PUBLISHING JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS

doi:10.1088/0953-4075/43/7/074007

J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 074007 (9pp)

Search for the electric dipole moment of

the electron with thorium monoxide

A C Vutha1, W C Campbell2, Y V Gurevich3, N R Hutzler3, M Parsons3,

D Patterson3, E Petrik3, B Spaun3, J M Doyle3, G Gabrielse3 and

D DeMille1

1

Department of Physics, Yale University, New Haven, CT 06520, USA

2

Joint Quantum Institute, University of Maryland and National Institute of Standards and Technology,

College Park, MD 20742, USA

3

Department of Physics, Harvard University, Cambridge, MA 02138, USA

E-mail: ****.*****@****.***

Received 17 August 2009, in nal form 24 September 2009

Published 19 March 2010

Online at stacks.iop.org/JPhysB/43/074007

Abstract

The electric dipole moment of the electron (eEDM) is a signature of CP-violating physics

beyond the standard model. We describe an ongoing experiment to measure or set improved

limits to the eEDM, using a cold beam of thorium monoxide (ThO) molecules. The metastable

H 3 1 state in ThO has important advantages for such an experiment. We argue that the

statistical uncertainty of an eEDM measurement could be improved by as much as three orders

of magnitude compared to the current experimental limit, in a rst-generation apparatus using

a cold ThO beam. We describe our measurements of the H state lifetime and the production of

ThO molecules in a beam, which provide crucial data for the eEDM sensitivity estimate. ThO

also has ideal properties for the rejection of a number of known systematic errors; these

properties and their implications are described.

(Some gures in this article are in colour only in the electronic version)

of new physics at the electroweak scale, plus order-unity

1. Introduction

T-violating phases, naturally gives rise to values of de that

Electric dipole moments of elementary particles have been are at or near the current experimental limit [7]. Experimental

the focus of experimental searches for over 50 years [1, 2]. limits on the size of de already impose stringent constraints on

The observation of a non-zero electric dipole moment of the supersymmetry and other physics beyond the SM [7, 8].

electron (eEDM) would be evidence of CP-violation in the Experiments to search for the eEDM attempt to measure

lepton sector, with deep implications for our understanding energy shifts arising from the interaction of an internal atomic

of particle physics and cosmology [3, 4]. The existence of or molecular electric eld with a bound electron, a relativistic

an electric dipole moment directed along the spin of a particle effect which can be enormously enhanced in some electronic

requires non-invariance under both parity (P) and time reversal states [9 11]. The current best limit on de comes from

(T). T-violation has been observed and measured in the decays the Berkeley experiments using atomic beams of thallium

of K- and B-mesons, and the results are all consistent with [6]. Compared to atoms however, polar molecules offer the

T-violation arising from a single source: the complex phase possibility of much larger internal electric elds that can be

SM 0.99 rad appearing in the quark mixing matrix in the fully oriented with modest laboratory electric elds [12].

For a single electron, the eEDM vector de = 2de S, where

standard model (SM) of particle physics [5]. This phase gives

S is the electron spin. (We set h = 1 everywhere, for the

rise to a non-zero predicted value of the eEDM de in the SM:

de 10 40 e cm [3], 13 orders of magnitude smaller than the convenience of notation.) In heavy molecules, where the

current limit de

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