Catalog · Charged lepton · L008

L008 $\mathcal{B}(\tau^-\to e^-\gamma)$

Tau to e $\gamma$ charged-lepton-flavor-violating dipole decay

Status REVIEWED VERIFIED Medium Code: NO Priority Medium

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
Branching fraction upper limit for $\tau^- \to $ e- $\gamma$	3.3e-8	2025	Particle Data Group 2025 PDGLive tau decay summary, PDGID S035.32	source ↑
Branching fraction upper limit for $\tau \to $ e $\gamma$	3.3e-8	2010	BaBar Collaboration full-data tau -> l gamma search	source ↑
Branching fraction upper limit for $\tau^+- \to $ e+- $\gamma$	5.6e-8	2021	Belle Collaboration full-data tau -> l gamma search	source ↑
Projected branching-fraction sensitivity for $\tau \to $ e $\gamma$	?	2019	Belle II Physics Book	source ↑

Why this constrains the RS scan

In warped lepton-flavor extensions, this mode constrains brane-localized or loop-induced charged-lepton dipoles with a $\tau e$ flavor spurion. It is not a constraint on the current quark $\Delta F=2$ scan, but it is a useful catalog entry for any future charged-lepton implementation because alignment that suppresses $\mu\to e\gamma$ need not suppress the tau-to-electron transition in the same way. The Perez--Randall warped-neutrino source gives the lepton-MFV/GIM-like assumptions under which charged-lepton flavor violation is controlled.

What's changed since the original paper

Relative to the arXiv:0804.1954 RS flavor baseline, the main experimental development is the BaBar 2010 full-data result, which remains the canonical PDG value for $\tau\to e\gamma$ (L008.yaml:pdg\_or\_equivalent.primary\_experiment; L008.yaml:pdg\_or\_equivalent.rs\_context). Belle later repeated the search with its full $988\,{\rm fb}^{-1}$ data set and found no significant excess, setting the weaker electron-mode limit $5.6\times10^{-8}$ at $90\%$ C.L. (L008.yaml:pdg\_or\_equivalent.recent\_belle\_cross\_check). The author pass found no Belle II measurement superseding the BaBar/PDG bound; the sidecar records only the $50\,{\rm ab}^{-1}$, $1.2\times10^{-8}$ Belle II prospect (L008.yaml:pdg\_or\_equivalent.belle\_ii\_projection).

Validity and model dependence

Experimentally this is a clean null search for charged-lepton flavor violation, with a negligible Standard Model rate. Its RS interpretation is model-dependent: the bound constrains a dipole coefficient only after choosing the lepton localization, flavor symmetry, Yukawa normalization, and KK-mass convention. The catalog should therefore classify it as robust experimentally, but lepton-extension-only and dipole-model-dependent for this repo.

Code coverage in this repo

NO. The required catalog grep over quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no $\tau\to e\gamma$ implementation. The adjacent code surface is only $\mu\to e\gamma$: flavorConstraints/muToEGamma.py:1 documents that process, flavorConstraints/muToEGamma.py:75 defines check\_mu\_to\_e\_gamma, and scanParams/scan.py:523 and scanParams/scan.py:524 call that checker in scans. Those lines are reusable design evidence, not L008 coverage.

Implementation difficulty

MEDIUM. A future implementation can reuse the existing LFV dipole pattern, but it must generalize the flavor indices and rate normalization from $\mu\to e\gamma$ to tau decays, add a tau-sector branching-ratio limit, and decide whether the same RS lepton-spurion assumptions apply. No new hadronic matrix elements, lattice inputs, or QCD running are needed.

Reason: The repo has an adjacent $\mu \to e \gamma$ dipole pattern, but L008 needs a generalized lepton-dipole observable with tau-e flavor indices, tau lifetime/normalization, and a process-specific branching-ratio limit. No new hadronic matrix elements, lattice inputs, or QCD running are needed.

Key references

Process-local source keys before bibliography consolidation: PDG2025\_TauEGamma, BaBar2010\_TauLGamma, Belle2021\_TauLGamma, BelleIIPhysicsBook2019\_TauLFV, CFW2008\_RSWarpedFlavor, and PerezRandall2008\_WarpedNeutrinos.

value 3.3e-08

RESOLVED

Match snapshot line 17

L14: PDGID: S035.32
L15: Decay mode summary: tau- -> e- gamma
L16: Gamma_186 / Gamma_total:
L17:   tau- -> e- gamma  < 3.3 x 10^-8
L18:   confidence level: 90%
L19: 
L20: PDG API related-data entry:

value 3.3e-08

RESOLVED

Match snapshot line 23

L20: Upsilon(3S), and Upsilon(2S) resonances. No evidence of signals was found.
L21: The 90% confidence-level upper limits were:
L22: 
L23:   BR(tau -> e gamma)  < 3.3 x 10^-8
L24:   BR(tau -> mu gamma) < 4.4 x 10^-8
L25: 
L26: Extraction note:

value 5.6e-08

RESOLVED

Match snapshot line 23

L20: the branching fractions are set at 90% confidence level:
L21: 
L22:   B(tau+- -> mu+- gamma) <= 4.2 x 10^-8
L23:   B(tau+- -> e+- gamma)  <= 5.6 x 10^-8
L24: 
L25: Extraction note:
L26: This is the main post-2008 Belle full-data update for tau -> l gamma. For