Measuring Electron's Electric Dipole MomentJanuary 10, 2014
A collimated pulse of ThO molecules enters a magnetically shielded region (not to scale). An aligned spin state (smallest red arrows), prepared via optical pumping, precesses in parallel electric and magnetic fields. The final spin alignment is read out by a laser with rapidly alternating linear polarizations, X and Y, with the resulting fluorescence collected and detected with photomultiplier tubes (PMTs). [from The ACME Collaboration: J. Baron, W.C. Campbell, D. DeMille, J.M. Doyle1, G. Gabrielse..., et al., "Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron," Science 343: no. 6168 | doi: 10.1126/science.1248213. Reprinted with permission from AAAS.]
The Standard Model (SM) of particle physics fails to explain dark matter and why matter survived annihilation with antimatter following the Big Bang. Extensions to the SM, such as weak-scale Supersymmetry, may explain one or both of these phenomena by positing the existence of new particles and interactions that are asymmetric under time-reversal (T). These theories nearly always predict a small, yet potentially measurable (10−27-10−30 e cm) electron electric dipole moment (EDM, de), which is an asymmetric charge distribution along the spin. The EDM is also asymmetric under T. Using the polar molecule thorium monoxide (ThO), the ACME Collaboration, led by Harvard Professors John Doyle and Gerald Gabrielse and Professor David DeMille of Yale University, produced the following measurements: de=(−2.1±3.7stat±2.5sys)×10−29 e cm. This corresponds to an upper limit of |de| < 8.7×10−29 e cm with 90 percent confidence, an order of magnitude improvement in sensitivity compared to the previous best limits. The result constrains T-violating physics at the TeV energy scale.
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