Catalog · Charged lepton · L006

L006 $P_{M\bar M},\;G_C/G_F$

Muonium-antimuonium conversion

Status REVIEWED VERIFIED Medium Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
MACE prospective probability-sensitivity improvement target	2 orders of magnitude improvement	2022	Bai2022:L006:mace_sensitivity_improvement	source ↑
MACE target muonium-to-antimuonium conversion-probability reach	1e-13 conversion probability	2024	Bai2024:L006:mace_probability_reach	source ↑

Why this constrains the RS scan

Muonium conversion probes a four-lepton contact structure $(\bar\mu\Gamma e)(\bar\mu\Gamma e)$ that violates muon and electron family numbers by two units. In an RS lepton extension, such operators could arise from flavor-misaligned heavy neutral or doubly charged states, KK-sector lepton-current misalignment, or other non-dipole lepton interactions. It is therefore complementary to L001: the present repo checks only $\mu\to e\gamma$, while L006 would require a separate $\Delta L=2$ four-lepton observable.

What's changed since the original paper

Relative to the CFW2008 era, the experimental bound itself has not changed: PDG still uses the 1999 MACS/PSI result. The main post-2008 movement is prospective. The sidecar prospects entries record the Snowmass 2021 statement that MACE is intended to improve probability sensitivity by more than two orders of magnitude and the 2024 MACE conceptual-design target beyond the $10^{-13}$ level. On the theory side, post\_2008\_theory\_context records recent EFT work separating $\Delta L_\mu=-\Delta L_e=2$ operators from ordinary one-unit LFV and noting that muonium conversion probes only part of that operator class.

Validity and model dependence

The experimental signature is clean and lepton-sector-only: a confirmed signal would be physics beyond the Standard Model. The quoted $G_C/G_F$ limit is not operator-universal, however; it is tied to the PDG $(V-A)\times(V-A)$ normalization. Any catalog-to-code translation must choose the Lorentz basis, magnetic-field treatment, and spin-state normalization before comparing a model Wilson coefficient to the MACS probability limit.

Code coverage in this repo

NO. A targeted grep for muonium|antimuonium|Muonium|Antimuonium|MACE|MACS over quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ returned no hits. The adjacent charged-lepton LFV implementation is the $\mu\to e\gamma$ dipole checker at flavorConstraints/muToEGamma.py:75, called from scanParams/scan.py:524; it does not cover muonium conversion.

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

Implementation difficulty

MEDIUM. L006 needs a new $\Delta L_\mu=-\Delta L_e=2$ four-lepton operator convention and a bound-state conversion-probability wrapper. It does not require lattice inputs or hadronic long-distance calculations for a first catalog-to-code implementation, but a production version should handle operator-dependent magnetic-field and spin factors explicitly.

Reason: Missing implementation needs a new $\Delta L$_mu = - $\Delta L$_e = 2 four-lepton/contact operator convention and a bound-state conversion-probability observable wrapper. It does not require lattice or hadronic long-distance inputs for a first implementation.

Key references

Process-local keys before bibliography consolidation: PDG2026\_MuoniumAntimuoniumS004MC, Willmann1999\_MACS, Bai2022\_SnowmassMuonium, Bai2024\_MACECDR, HeeckSokhashvili2024\_DeltaLTwo, AgasheBlechmanPetriello2006\_RSLFV, and CFW2008.

value 2

AMBIGUOUS

Match 1 of 9 snapshot line 1

L1: Source snapshot: Snowmass2021 whitepaper on muonium to antimuonium conversion
L2: Process: L006, muonium-antimuonium conversion
L3: Access date: 2026-05-16
L4: URL: https://arxiv.org/abs/2203.11406

Match 2 of 9 snapshot line 3

L1: Source snapshot: Snowmass2021 whitepaper on muonium to antimuonium conversion
L2: Process: L006, muonium-antimuonium conversion
L3: Access date: 2026-05-16
L4: URL: https://arxiv.org/abs/2203.11406
L5: arXiv id: 2203.11406
L6: DOI: 10.48550/arXiv.2203.11406

Match 3 of 9 snapshot line 4

L1: Source snapshot: Snowmass2021 whitepaper on muonium to antimuonium conversion
L2: Process: L006, muonium-antimuonium conversion
L3: Access date: 2026-05-16
L4: URL: https://arxiv.org/abs/2203.11406
L5: arXiv id: 2203.11406
L6: DOI: 10.48550/arXiv.2203.11406
L7:

Match 4 of 9 snapshot line 5

L2: Process: L006, muonium-antimuonium conversion
L3: Access date: 2026-05-16
L4: URL: https://arxiv.org/abs/2203.11406
L5: arXiv id: 2203.11406
L6: DOI: 10.48550/arXiv.2203.11406
L7: 
L8: Title:

Match 5 of 9 snapshot line 6

L3: Access date: 2026-05-16
L4: URL: https://arxiv.org/abs/2203.11406
L5: arXiv id: 2203.11406
L6: DOI: 10.48550/arXiv.2203.11406
L7: 
L8: Title:
L9: Snowmass2021 Whitepaper: Muonium to antimuonium conversion

Match 6 of 9 snapshot line 9

L6: DOI: 10.48550/arXiv.2203.11406
L7: 
L8: Title:
L9: Snowmass2021 Whitepaper: Muonium to antimuonium conversion
L10: 
L11: Authors:
L12: Ai-Yu Bai, Yu Chen, Yukai Chen, Rui-Rui Fan, Zhilong Hou, Han-Tao Jing,

Match 7 of 9 snapshot line 19

L16: and Luping Zhou.
L17: 
L18: Date:
L19: Submitted 2022-03-22.
L20: 
L21: Minimal extracted use note:
L22: The whitepaper identifies spontaneous muonium-to-antimuonium conversion as a

Match 8 of 9 snapshot line 23

L20: 
L21: Minimal extracted use note:
L22: The whitepaper identifies spontaneous muonium-to-antimuonium conversion as a
L23: charged-lepton-flavor-violation process and summarizes post-2008 theoretical
L24: and experimental developments. It states that MACE is proposed to improve the
L25: sensitivity to the conversion probability by more than two orders of magnitude
L26: relative to the current PSI/MACS upper constraint.

Match 9 of 9 snapshot line 29

L26: relative to the current PSI/MACS upper constraint.
L27: 
L28: Catalog use:
L29: Post-2008 developments and prospects only. This is not the source for the
L30: current experimental limit.

value 1e-13

RESOLVED

Match snapshot line 27

L24: 
L25: Minimal extracted value:
L26: The abstract states that MACE aims to discover or constrain the rare process at
L27: conversion probability beyond the level of 10^-13.
L28: 
L29: Catalog use:
L30: Post-2008 experimental development and future sensitivity context only. This is