[Paper Review] Electric dipole moments and the search for new physics

Ricardo Alarcón, Alexander, Jim+141 more|arXiv (Cornell University)|Mar 15, 2022

Atomic and Subatomic Physics Research28 citations

TL;DR

This white paper reviews how static electric dipole moments (EDMs) across particles, atoms, molecules, and storage rings probe beyond-Standard-Model CP violation, outlining theory, current limits, and future experimental paths.

ABSTRACT

Static electric dipole moments of nondegenerate systems probe mass scales for physics beyond the Standard Model well beyond those reached directly at high energy colliders. Discrimination between different physics models, however, requires complementary searches in atomic-molecular-and-optical, nuclear and particle physics. In this report, we discuss the current status and prospects in the near future for a compelling suite of such experiments, along with developments needed in the encompassing theoretical framework.

Motivation & Objective

• Motivate EDM searches as probes of new CP-violating physics beyond the Standard Model.
• Present the theoretical framework connecting high-energy CP violation to low-energy EDM observables via effective field theory and lattice QCD.
• Summarize current experimental status (neutron, atomic, molecular EDMs) and outline near-future sensitivity goals.
• Describe storage-ring EDM concepts and their potential reach for proton, deuteron, and muon EDMs.
• Advocate a coordinated, multi-system EDM program to diagnose the origin of CP violation and its cosmological implications.

Proposed method

• Use Standard Model and Beyond-Standard-Model CP-violating frameworks to relate high-scale operators to low-energy EDMs via SM effective field theory.
• Employ chiral EFT and lattice QCD inputs to connect quark/gluon level CPV operators to hadronic, nuclear, and atomic EDMs.
• Assess experimental capabilities and systematics across neutron, atomic, molecular, and storage-ring EDM experiments.
• Estimate mass-scale reach for new physics given EDM bounds using order-of-magnitude scaling relations and loop counting.
• Propose a multi-pronged experimental strategy to disentangle contributions from θ̄, quark/gluon EDMs, and four-fermion operators.

Experimental results

Research questions

• RQ1 What high-scale CP-violating sources can generate observable EDMs in current or near-future experiments?
• RQ2 How can EDM measurements in multiple systems be used to disentangle contributions from θ̄ versus beyond-Standard-Model operators?
• RQ3 What is the estimated mass-scale reach for new CP-violating physics given current and projected EDM sensitivities (single vs multiple systems)?
• RQ4 How can lattice QCD and chiral EFT reduce hadronic uncertainties to sharpen constraints from EDM measurements?
• RQ5 What experimental pathways (neutron, atoms/molecules, storage rings) offer the most complementary and robust discovery potential for CP violation?

Key findings

• Static EDMs probe BSM mass scales well beyond direct collider reach under maximal CP violation.
• Current neutron EDM limit is dn < 1.8×10^-26 e·cm (90% C.L.) as of 2020, with goals aiming at 10^-27 to 10^-28 e·cm in the next 10–15 years.
• Atomic and molecular EDMs provide strong constraints on electron EDMs and quark chromo-EDMs, and offer complementary access to CP-violating hadronic interactions.
• Storage-ring EDM experiments (proton, deuteron, muon) promise exceptional sensitivity (e.g., 10^-29 e·cm for proton in ~10 years) and potential reach for dark sector or axion-related CP violation.
• A single EDM measurement cannot uniquely diagnose the underlying CP-violating source; a coordinated suite across systems is essential to identify the responsible operators.
• Reducing theoretical uncertainties in hadronic matrix elements via lattice QCD and chiral EFT is crucial to maximizing the physics reach of EDM experiments.