Electric dipole moments as probes of new physics

TL;DR

This review analyzes how electric dipole moments constrain CP-violating sources beyond the Standard Model, tracing the connection from high-energy CP-odd operators to observable EDMs via a sequence of effective theories. It surveys three broad calculational frameworks—naive dimensional analysis, chiral techniques, and QCD sum rules—for linking quark-gluon level CP violation to nucleon and nuclear EDMs, and discusses EDM predictions in the SM and in supersymmetric models, highlighting the SUSY CP problem and flavor-violating effects. The work emphasizes both the theoretical uncertainties in hadronic and nuclear matrix elements and the experimental progress in paramagnetic, diamagnetic, and nucleon EDM measurements, arguing that EDMs probe scales up to and beyond the TeV regime. It concludes that continued EDM improvements, alongside advances in lattice QCD and nuclear many-body theory, will remain essential for uncovering or constraining new sources of CP violation relevant to cosmology and electroweak symmetry breaking.

Abstract

We review several aspects of flavour-diagonal CP violation, focussing on the role played by the electric dipole moments (EDMs) of leptons, nucleons, atoms and molecules, which consitute the source of several stringent constraints on new CP-violating physics. We dwell specifically on the calculational aspects of applying the hadronic EDM constraints, reviewing in detail the application of QCD sum-rules to the calculation of nucleon EDMs and CP-odd pion-nucleon couplings. We also consider the current status of EDMs in the Standard Model, and on the ensuing constraints on the underlying sources of CP-violation in physics beyond the Standard Model, focussing on weak-scale supersymmetry.

Figures (10)

• Figure 1: A schematic plot of the hierarchy of scales between the CP-odd sources and three generic classes of observable EDMs. The dashed lines indicate generically weaker dependencies.
• Figure 2: Chirally enhanced contribution to the neutron EDM..
• Figure 3: Two classes of diagrams contributing to the CP-odd pion-nucleon coupling constant.
• Figure 4: A leading contribution to the neutron EDM within QCD sum rules. Sensitivity to the $CP$-violating source enters through the two soft quark lines which lead to a dependence on the chiral condensate.
• Figure 5: A particular 3-loop contribution CK to the $d$-quark EDM induced by the KM phase in the standard model. The box vertex denotes a contacted $W$-boson line connected to the light quarks, while it is implicit that the external photon line is to be attached as appropriate to any charged lines.