Catalog · Charged lepton · L003

L003 $R_{\mu e}^{\rm Al}$

Coherent muon-to-electron conversion in aluminum

Status REVIEWED VERIFIED High Code: NO Priority High

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
$\Gamma(\mu^-$ Au $ \to $ e- Au(g.s.)) / $\Gamma_{capture}(\mu^-$ Au)	7e-13	2006	SINDRUM II Collaboration, JuSER metadata for EPJ C 47, 337-346 (2006)	source ↑

Why this constrains the RS scan

In a warped lepton-flavor extension, coherent conversion probes dipole, scalar, and vector lepton-quark operators. KK gauge exchange, $Z$-like flavor violation, Higgs-mediated scalar currents, and lepton-sector dipoles can feed the same nuclear final state. The observable is complementary to $\mu\to e\gamma$ because it can be contact-operator dominated, so it tests lepton-extension structure not covered by the current dipole checker.

What's changed since the original paper

Relative to the arXiv:0804.1954 RS-flavor baseline (L003.yaml:pdg\_or\_equivalent.post\_2008\_baseline), the main change is experimental. Mu2e published staged aluminum sensitivity projections, and Mu2e-II records a proposed PIP-II upgrade aiming for at least one additional order of magnitude beyond Mu2e (L003.yaml:pdg\_or\_equivalent.aluminum\_projections[4]). COMET published Phase-I and Phase-II aluminum strategies (L003.yaml:pdg\_or\_equivalent.aluminum\_projections[2] and L003.yaml:pdg\_or\_equivalent.aluminum\_projections[3]). The process therefore moved from SINDRUM-II context to a near-term dedicated aluminum search program.

Validity and model dependence

The experimental signature is robust: a monoenergetic conversion electron from a stopped negative muon with no neutrinos. The mapping to RS parameters is model-dependent. It requires a lepton-quark operator basis, target-dependent overlap integrals and capture rates, and a convention for combining dipole, scalar, and vector contributions. A gold limit cannot be treated as an aluminum limit without a nuclear/operator translation.

Code coverage in this repo

NO. The required greps over quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no Mu2e, COMET, SINDRUM, $R_{\mu e}$, or $\mu-e$-conversion implementation. The only adjacent LFV code is the $\mu\to e\gamma$ dipole checker at flavorConstraints/muToEGamma.py:75 and its scan call at scanParams/scan.py:524.

Linked evidence (opens GitHub blob at flavor-catalog-website/2026q2):

Implementation difficulty

HIGH. A production implementation needs new lepton-quark Wilson coefficients, target-dependent conversion formulae for aluminum, nuclear overlap/capture inputs, and likely EFT running or matching shared with $\mu\to e\gamma$ and $\mu\to3e$. This is beyond adding a new numerical limit to the existing dipole-only LFV path.

Reason: Missing implementation needs a new mu-e conversion observable, lepton-quark Wilson/operator convention, aluminum nuclear overlap and capture inputs, and likely EFT running or matching shared with $\mu \to e \gamma$ and $\mu \to 3e$. This exceeds a new-operator-only catalog update.

Key references

Process-local source keys before bibliography consolidation: SINDRUMII2006\_GoldMuE, Mu2e2016\_FullProjection, Mu2eRunI2023, COMETPhaseITDR2020, COMETStrategy2018, Mu2eII2022, and CFW2008.

value 7e-13

RESOLVED

Match snapshot line 18

L15: 
L16: Minimal extracted value:
L17: The record abstract reports a search for mu-e conversion in muonic gold with
L18: Gamma(mu- Au -> e- Au(g.s.)) / Gamma_capture(mu- Au) < 7 x 10^-13 at 90% C.L.
L19: 
L20: Use in L003:
L21: This is the current published all-target benchmark limit cited by Mu2e and