Catalog · Charged lepton · L005

L005 $R_{\mu e}^{\rm Ti}$

Coherent muon-to-electron conversion in titanium

Status REVIEWED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
Mu2e coherent $\mu^-$ N $ \to $ e- N program improvement context	4 orders of magnitude improvement beyond previous published conversion limits	2015	Mu2eTDR2015:L005:program_improvement_context	source ↑
COMET Phase-I Al coherent $\mu^-$ N $ \to $ e- N single-event sensitivity	3.1e-15 conversion ratio single-event sensitivity	2020	COMETPhaseITDR2020:L005:single_event_sensitivity	source ↑
COMET Phase-I Al coherent $\mu^-$ N $ \to $ e- N expected upper limit	7e-15 conversion ratio upper limit	2020	COMETPhaseITDR2020:L005:expected_upper_limit_90cl	source ↑

Why this constrains the RS scan

In a warped lepton-flavor extension, $\mu-e$ conversion probes lepton-quark contact operators, flavor-changing $Z$-like couplings, scalar currents, and dipoles. It is therefore complementary to $\mu\to e\gamma$, which tests only the dipole lane in the current repository. Titanium is useful historically because it is a real published conversion target, but its interpretation is not target-universal.

What's changed since the original paper

No newer titanium-target measurement was found in the PDG stream used here after the CFW2008 RS-flavor baseline. The main developments are prospective aluminum programs: the sidecar post\_2008\_developments entry for the Mu2e TDR records a coherent $\mu^-N\to e^-N$ program aiming roughly four orders of magnitude beyond previous published conversion limits, and the COMET Phase-I entry records an aluminum sensitivity goal $3.1\times10^{-15}$ with an expected $90\%$ upper limit $7.0\times10^{-15}$. These are not titanium measurements; they explain why the Ti number is now primarily a legacy benchmark and target-comparison input.

Validity and model dependence

The experimental statement is robust as a null search normalized to the ordinary muon-capture rate in titanium. The mapping to RS parameters is model-dependent: dipole, vector, and scalar amplitudes can interfere, and the nuclear overlap integrals and capture normalization are target-specific. Direct comparison with gold or aluminum therefore requires an operator basis and nuclear-convention choice.

Code coverage in this repo

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

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

Implementation difficulty

HIGH. A production constraint needs a new coherent conversion observable, lepton-quark Wilson coefficients, titanium nuclear overlap and capture inputs, and likely EFT matching/running shared with $\mu\to e\gamma$ and $\mu\to3e$. This is a new target-dependent mode calculation, not a new limit for the existing dipole checker.

Reason: Missing implementation needs a new coherent mu-e conversion observable, lepton-quark Wilson/operator convention, titanium target nuclear overlap and capture inputs, and likely EFT matching/running shared with $\mu \to e \gamma$ and $\mu \to 3e$.

Key references

Process-local source keys before bibliography consolidation: PDG2026\_MuonTiConversion, SINDRUMII1993\_TitaniumMuE, Mu2eTDR2015, COMETPhaseITDR2020, and CFW2008.

value 4

AMBIGUOUS

Match 1 of 3 snapshot line 4

L1: Source snapshot: arXiv metadata and abstract for the Mu2e Technical Design Report
L2: Process: L005, mu-e conversion in titanium
L3: Access date: 2026-05-16
L4: URL: https://arxiv.org/abs/1501.05241
L5: arXiv id: 1501.05241
L6: arXiv DOI: 10.48550/arXiv.1501.05241
L7:

Match 2 of 3 snapshot line 5

L2: Process: L005, mu-e conversion in titanium
L3: Access date: 2026-05-16
L4: URL: https://arxiv.org/abs/1501.05241
L5: arXiv id: 1501.05241
L6: arXiv DOI: 10.48550/arXiv.1501.05241
L7: 
L8: Title:

Match 3 of 3 snapshot line 6

L3: Access date: 2026-05-16
L4: URL: https://arxiv.org/abs/1501.05241
L5: arXiv id: 1501.05241
L6: arXiv DOI: 10.48550/arXiv.1501.05241
L7: 
L8: Title:
L9: Mu2e Technical Design Report

value 3.1e-15

RESOLVED

Match snapshot line 20

L17: Minimal extracted values from the arXiv abstract:
L18: COMET Phase-I searches for neutrinoless conversion of muons into electrons in
L19: the field of an aluminium nucleus, mu- N -> e- N. The experimental sensitivity
L20: goal is 3.1 x 10^-15, with a 90% upper limit on the branching ratio of
L21: 7 x 10^-15. The abstract describes this as a factor of 100 improvement over
L22: the existing limit and quotes an expected background of 0.032 events.
L23:

value 7e-15

RESOLVED

Match snapshot line 21

L18: COMET Phase-I searches for neutrinoless conversion of muons into electrons in
L19: the field of an aluminium nucleus, mu- N -> e- N. The experimental sensitivity
L20: goal is 3.1 x 10^-15, with a 90% upper limit on the branching ratio of
L21: 7 x 10^-15. The abstract describes this as a factor of 100 improvement over
L22: the existing limit and quotes an expected background of 0.032 events.
L23: 
L24: Use in L005: