Catalog · EDM / neutrino · E004

E004 $d_n$

Neutron electric dipole moment

Status SUBTLETY-ADDED VERIFIED High Code: NO Priority High

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
Neutron electric dipole moment	1.8e-26 e cm	2026	PDG Live 2026 S017EDM neutron electric dipole moment datablock	source ↑
Neutron electric dipole moment	0 10^-26 e cm	2020	Abel et al., Phys. Rev. Lett. 124, 081803 (2020); arXiv:2001.11966	source ↑
Neutron electric dipole moment	3e-26 e cm	2015	PDG Live 2026 S017EDM neutron EDM datablock, Pendlebury 2015 row	source ↑

Why this constrains the RS scan

In anarchic warped or composite-Higgs completions, new complex Yukawa spurions and KK fermion/gauge loops can generate flavor-diagonal CP-odd dipoles even when tree-level $\Delta F=2$ observables are controlled. The neutron EDM is therefore a standard hadronic CP benchmark for quark EDMs and chromo-EDMs, and for threshold-induced Weinberg-operator contributions. It does not map onto the existing neutral-meson mixing basis; it would test a different CP-odd operator lane.

What's changed since the original paper

The 2008 CFW reference point (CsakiFalkowskiWeiler:RSFlavor2008) focused on warped-flavor bounds such as KK-gluon constraints. Since then, the direct neutron EDM experimental limit improved from the PDG-listed Pendlebury-era $3.0\times10^{-26}\ e\,\mathrm{cm}$ bound to the Abel et al. 2020 $1.8\times10^{-26}\ e\,\mathrm{cm}$ bound (PDG2026:NeutronEDM; Abel:NeutronEDM2020). On the model side, Koenig, Neubert, and Straub 2014 (KoenigNeubertStraub:DipoleComposite2014) made quark electromagnetic and chromomagnetic dipoles explicit in composite-Higgs and warped extra-dimensional language, including the role of ``wrong-chirality'' Yukawas and the neutron EDM. On the hadronic side, lattice work such as Bhattacharya et al. 2022 (Bhattacharya:NeutronEDMLattice2022) now directly targets the neutron EDM induced by the QCD topological term, the Weinberg three-gluon operator, and the isovector quark chromo-EDM.

Validity and model dependence

The experimental limit is a robust null bound on $d_n$, but translating it into RS parameters is hadronic- and basis-dependent. A live interpretation must specify the UV matching to quark EDM, quark chromo-EDM, $\theta$, and Weinberg coefficients; their QCD running and threshold matching; and the low-energy neutron matrix elements. This is an EDM-adjacent loop constraint, not a direct $\Delta F=2$ or lepton-universality observable. For anarchic CP phases, EDM constraints should be reviewed jointly with the flavor branching rows that share the same Wilson coefficients (relevant cross-refs: K001, K003, B011, B033, B034), not as an isolated appendix.

Code coverage in this repo

NO. The required catalog greps and a focused search for neutron[ -]?EDM, nEDM, d\_n, chromo-EDM, CEDM, Weinberg, and three-gluon over quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no neutron EDM implementation. The only nearby dipole code is the off-diagonal $\mu\to e\gamma$ helper in flavorConstraints/muToEGamma.py:3, :21, and :81, which is not a hadronic EDM calculation.

Implementation difficulty

HIGH. A production constraint needs new CP-odd dipole and gluonic operator matching, QCD running with thresholds, and hadronic neutron matrix elements. The existing SLL/SLR/VLL/VRR/LR $\Delta F=2$ infrastructure does not cover quark EDMs, chromo-EDMs, or the Weinberg operator.

Reason: Requires new CP-odd quark EDM, chromo-EDM, and Weinberg-operator matching, QCD running and threshold treatment, and neutron hadronic matrix elements. Existing $\Delta F = 2$ and $\mu \to e \gamma$ code paths do not provide this observable.

Key references

Process-local source keys before bibliography consolidation: PDG2026:NeutronEDM, Abel:NeutronEDM2020, CsakiFalkowskiWeiler:RSFlavor2008, KoenigNeubertStraub:DipoleComposite2014, and Bhattacharya:NeutronEDMLattice2022.

value_summary |d_n| < 1.8 x 10^-26 e cm at 90% CL

RESOLVED

Match snapshot line 20

L17: Canonical direct result:
L18:   <0.18, CL = 90%, ABEL 2020, method MRS UCN.
L19: Converted value:
L20:   |d_n| < 1.8 x 10^-26 e cm at 90% CL.
L21: 
L22: PDG measurement note for ABEL 2020:
L23:   d = (0.0 +/- 1.1 +/- 0.2) x 10^-26 e cm, corresponding to the listed limit.

measurement_summary d_n = (0.0 +/- 1.1_stat +/- 0.2_sys) x 10^-26 e cm; |d_n| < 1.8 x 10^-26 e cm at 90% CL in PDG

RESOLVED

Match snapshot line 23

L20:   improved magnetic-field systematic control.
L21: 
L22: Numerical result:
L23:   d_n = (0.0 +/- 1.1_stat +/- 0.2_sys) x 10^-26 e cm.
L24: 
L25: This source is the primary experimental input for the PDG 2026 direct neutron
L26: EDM bound |d_n| < 1.8 x 10^-26 e cm at 90% CL.

value_summary |d_n| < 3.0 x 10^-26 e cm at 90% CL, superseded by Abel 2020

RESOLVED

Match snapshot line 28

L25: Superseded direct result used for post-2008 context:
L26:   <0.30, CL = 90%, PENDLEBURY 2015, method MRS, supersedes BAKER 2006.
L27: Converted value:
L28:   |d_n| < 3.0 x 10^-26 e cm at 90% CL.
L29: PDG measurement note for PENDLEBURY 2015:
L30:   d = (-0.21 +/- 1.82) x 10^-26 e cm, corresponding to the listed limit.
L31: