Theoretical Prediction and Impact of Fundamental Electric Dipole Moments

TL;DR

This paper predicts extremely small electric dipole moments (EDMs) for electrons and neutrons within certain string/M-theory frameworks, providing testable bounds that could reveal insights into fundamental Yukawa couplings and high-scale physics.

Contribution

It extends previous work by analyzing the dependence of low-scale EDMs on fundamental Yukawa couplings, predicting upper limits that are smaller than other models but larger than the Standard Model.

Findings

Electron EDM should not exceed 5×10^{-30} e·cm.

Neutron EDM should not exceed 5×10^{-29} e·cm.

Predicted EDMs are below typical effective theory predictions but above Standard Model values.

Abstract

The predicted Standard Model (SM) electric dipole moments (EDMs) of electrons and quarks are tiny, providing an important window to observe new physics. Theories beyond the SM typically allow relatively large EDMs. The EDMs depend on the relative phases of terms in the effective Lagrangian of the extended theory, which are generally unknown. Underlying theories, such as string/M-theories compactified to four dimensions, could predict the phases and thus EDMs in the resulting supersymmetric (SUSY) theory. Earlier one of us, with collaborators, made such a prediction and found, unexpectedly, that the phases were predicted to be zero at tree level in the theory at the unification or string scale $\sim\mathcal{O}(10^{16}$ GeV). Electroweak (EW) scale EDMs still arise via running from the high scale, and depend only on the SM Yukawa couplings that also give the CKM phase. Here we extend the earlier work by studying the dependence of the low scale EDMs on the constrained but not fully known fundamental Yukawa couplings. The dominant contribution is from two loop diagrams and is not sensitive to the choice of Yukawa texture. The electron EDM should not be found to be larger than about $ 5\times 10^{-30} e$ cm, and the neutron EDM should not be larger than about $5\times 10^{-29}e$ cm. These values are quite a bit smaller than the reported predictions from Split SUSY and typical effective theories, but much larger than the Standard Model prediction. Also, since models with random phases typically give much larger EDMs, it is a significant testable prediction of compactified M-theory that the EDMs should not be above these upper limits. The actual EDMs can be below the limits, so once they are measured they could provide new insight into the fundamental Yukawa couplings of leptons and quarks. We comment also on the role of strong CP violation. EDMs probe fundamental physics near the Planck scale.