Catalog · Charged lepton · L001

L001 $\mathcal{B}(\mu^+\to e^+\gamma)$

Muon to electron $\gamma$ charged-lepton-flavor-violating dipole decay

Status REVIEWED VERIFIED Low Code: YES Priority High

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
Branching fraction upper limit for $\mu^+ \to $ e+ $\gamma$	1.5e-13	2025	MEG II Collaboration	source ↑
Branching fraction upper limit for $\mu^+ \to $ e+ $\gamma$	3.1e-13	2025	Particle Data Group 2025 muon listing, NODE=S004R3	source ↑
RS lepton dipole NDA estimate and paper-era $\mu \to $ e $\gamma$ bound	?	-	Perez and Randall 2008 / JHEP 2009	source ↑

Why this constrains the RS scan

In the lepton extension of the warped model, $\mu\to e\gamma$ probes brane-localized or loop-induced dipoles controlled by the off-diagonal combination $(\bar Y_N\bar Y_N^\dagger)_{12}$. The repo implements the Perez--Randall NDA form $\mathcal{B}\simeq 4\times10^{-8}\,|(\bar Y_N\bar Y_N^\dagger)_{12}|^2 (3\,{\rm TeV}/M_{\rm KK})^4$, converts an experimental branching-ratio limit to $C=\sqrt{\mathcal{B}_{\rm lim}/(4\times10^{-8})}$, and uses the updated MEG II limit in the scan default. Those numerical inputs are the L001.yaml:paper\_era\_reference and L001.yaml:repo\_default entries: the live default gives $C\simeq1.936\times10^{-3}$, compared with the paper-era $C=0.02$.

What's changed since the original paper

Relative to the contemporaneous Perez--Randall lepton paper, the experimental story changed substantially. Perez--Randall used the MEGA-era $\mathcal{B}(\mu\to e\gamma)<1.2\times10^{-11}$ input (L001.yaml:paper\_era\_reference). The full MEG 2009--2013 data set then gave $4.2\times10^{-13}$, MEG II's 2021-only result gave $7.5\times10^{-13}$, and the MEG+MEG II combination gave $3.1\times10^{-13}$ (L001.yaml:prior\_experimental\_limits). The MEG II 2021--2022 analysis now sets the $1.5\times10^{-13}$ primary value.

Validity and model dependence

The experimental bound is clean: a signal would be charged-lepton flavor violation with negligible Standard Model background. Its translation into this repo is model-dependent, because it relies on the chosen RS lepton spurion, NDA prefactor, and $M_{\rm KK}$ convention rather than a full loop calculation. The catalog should therefore classify it as a robust experimental limit but a lepton-extension-only, dipole/NDA model interpretation.

Code coverage in this repo

YES. The sidecar code\_coverage.evidence block lists the implementation in flavorConstraints/muToEGamma.py: the module documents the $\mu\to e\gamma$ dipole constraint, records the paper-era $C=0.02$ coefficient, defines $\texttt{PREFAC\_BR}=4.0\times10^{-8}$, converts branching-ratio limits to $C$, and exposes $\texttt{check\_mu\_to\_e\_gamma}$. The scan default is the MEG II 2025 value: scanParams/scan.py:32--33 sets $\texttt{BR\_LIMIT\_MEGII\_2025}=1.5\times10^{-13}$, and line 264 makes it the default $\texttt{ScanConfig.br\_limit}$. Tests at tests/test\_mu\_to\_e\_gamma.py:40 and tests/test\_scan.py:50 exercise the updated limit.

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

Implementation difficulty

LOW for cataloging and present code coverage, because the observable is already implemented and the scan default matches the primary 2025 result. A future production refresh is MEDIUM: the code should audit the paper-era NDA normalization, $M_{\rm KK}$ convention, and whether the final catalog should expose both paper-reproduction and MEG II 2025 modes.

Reason: The process already has a live LFV dipole/NDA implementation and scan default. Cataloging only requires source tracking and documenting the difference between the paper-era coefficient and the MEG II 2025 scan default.

Key references

Process-local source keys before bibliography consolidation: MEGII2025\_MuEGamma, MEGII2024\_MuEGammaFirst, MEG2016\_MuEGammaFull, PDG2025\_MuonListing, and PerezRandall2008\_WarpedNeutrinos.

value 1.5e-13

RESOLVED

Match snapshot line 26

L23: 
L24: Relevant published-article conclusion excerpt:
L25: The MEG II collaboration reports the same 2021-2022 result, with
L26: S_90 = 2.2 x 10^-13 and B(mu+ -> e+ gamma) < 1.5 x 10^-13 at 90% C.L.
L27: The article states that data from 2023-2024 are expected to add a 1.5-fold
L28: larger statistic and that planned 2025-2026 data should add another 2.6-fold,
L29: with a target sensitivity near 6 x 10^-14.

value 3.1e-13

AMBIGUOUS

Match 1 of 2 snapshot line 15

L12: 
L13: Relevant listing excerpt for mu+ -> e+ gamma, NODE=S004R3:
L14: 
L15:   [<3.1 x 10^-13 (CL = 90%) OUR 2025 BEST LIMIT]
L16:   AFANACIEV 25A SPEC +, MEG II at PSI
L17: 
L18: The same listing marks older entries as not used for averages/fits/limits:

Match 2 of 2 snapshot line 20

L17: 
L18: The same listing marks older entries as not used for averages/fits/limits:
L19: 
L20:   AFANACIEV 24 SPEC +, MEG II at PSI: <3.1 x 10^-13 at 90% CL
L21:   BALDINI 16 SPEC +, MEG at PSI: <4.2 x 10^-13 at 90% CL
L22:   ADAM 13B SPEC +, MEG at PSI: <5.7 x 10^-13 at 90% CL
L23:   ADAM 11 SPEC +, MEG at PSI: <2.4 x 10^-12 at 90% CL