Catalog · Top / Higgs / EW · T019

T019 $\mathcal{B}(h\to e\tau)$

Lepton-flavor-violating Higgs decay h $ \to $ e $\tau$

Status REVIEWED VERIFIED High Code: NO Priority Medium

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
B(h $ \to $ e $\tau$), ATLAS full Run-2 direct search	< 0.20% branching fraction	2025	PDG2025:T019:atlas_run2	source ↑
B(h $ \to $ e $\tau$), CMS full Run-2 direct search	< 0.22% branching fraction	2025	PDG2025:T019:cms_run2	source ↑
B(H $ \to $ e $\tau$), ATLAS full Run-2 direct search	< 0.20% branching fraction	2023	ATLAS2023:T019:etau_limit	source ↑
ATLAS dataset for h $ \to $ e $\tau$ direct search	138 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2023	ATLAS2023:T019:dataset	source ↑
B(H $ \to $ e $\tau$), CMS full Run-2 direct search	< 0.22% branching fraction	2021	CMS2021:T019:etau_limit	source ↑
CMS dataset for h $ \to $ e $\tau$ direct search	137 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2021	CMS2021:T019:dataset	source ↑
B(H $ \to $ e $\tau$), ATLAS partial Run-2 direct search	< 0.47% branching fraction	2019	ATLAS2019:T019:etau_limit	source ↑
ATLAS partial Run-2 dataset for h $ \to $ e $\tau$ direct search	36.1 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2019	ATLAS2019:T019:dataset	source ↑
Generic LFV-Higgs EFT allowance for h $ \to \tau$ e, theory context	order 10% branching fraction, theory-context allowance	2012	HarnikKoppZupan2012:T019:LFV_Higgs_EFT_allowance	source ↑

Why this constrains the RS scan

In warped or partially composite lepton sectors, flavor-dependent localization can explain charged-lepton masses while leaving the physical Higgs Yukawa matrix misaligned with the mass matrix. The $h\to e\tau$ limit directly tests the $e$-$\tau$ off-diagonal charged-lepton Yukawa entries. It is complementary to dipole constraints such as $\mu\to e\gamma$, but it is not part of the current quark $\Delta F=2$ scan lane.

What's changed since the original paper

The baseline arXiv:0804.1954 predates direct LHC Higgs LFV searches (CsakiFalkowskiWeiler2008WarpedFlavor). Harnik, Kopp, and Zupan later emphasized that $h\to\tau e$ could be a discovery channel and that branching fractions of order $10\%$ were still possible in generic LFV-Higgs EFT settings after low-energy constraints (HarnikKoppZupan2012FlavorViolatingHiggs). ATLAS performed an early $13~\mathrm{TeV}$ search with $36.1~\mathrm{fb}^{-1}$, setting $\mathcal{B}(H\to e\tau)<0.47\%$ (ATLAS2019HiggsLFVETau). The full Run-2 CMS and ATLAS analyses then pushed the direct e-tau limit to the few-$10^{-3}$ level, with no significant excess (CMS2021HiggsLFVETau; ATLAS2023HiggsLFVETau).

Validity and model dependence

Experimentally this is a direct upper limit on a Standard-Model-forbidden decay. A literal branching-ratio cut assumes the Higgs production mixture and total width used by the collaborations. A model with altered Higgs production rates, new invisible width, or correlated LFV tau decays would need a recast. For the catalog, the limit is robust as an experimental observable but model-dependent as an RS acceptance criterion.

Code coverage in this repo

Coverage is NO. The required greps and a targeted search for $h \to e \tau$ , $H \to e \tau$ , etau, Y\_e\_tau, and Y\_tau\_e across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no observable implementation. The related LFV backend is specifically $\mu\to e\gamma$ (flavorConstraints/muToEGamma.py:1; flavorConstraints/muToEGamma.py:75), and the scan applies that filter at scanParams/scan.py:523--scanParams/scan.py:524. The charged-lepton Yukawa helper is diagonal (yukawa/charged\_lepton.py:1; yukawa/charged\_lepton.py:36).

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

Implementation difficulty

Production integration is HIGH. The external limit is simple to catalog, but a faithful RS constraint needs a new charged-lepton Higgs-Yukawa misalignment calculation, a convention for $Y_{e\tau}$ and $Y_{\tau e}$, and a Higgs-rate/width interpretation before scan points can be accepted or rejected.

Reason: The direct experimental limit is simple to catalog, but production-grade RS integration needs a new charged-lepton Higgs-Yukawa misalignment calculation, a convention for Y_e_tau/Y_tau_e and their relation to the branching ratio, and a Higgs production/total-width interpretation. The current code only has diagonal charged-lepton Yukawas and a $\mu \to e \gamma$ dipole filter.

Key references

PDG2025HiggsLFVReview; ATLAS2023HiggsLFVETau; CMS2021HiggsLFVETau; ATLAS2019HiggsLFVETau; HarnikKoppZupan2012FlavorViolatingHiggs; CsakiFalkowskiWeiler2008WarpedFlavor; PerezRandall2008WarpedLeptonFlavor.

value < 0.20%

RESOLVED

Match snapshot line 22

L19: The full 13 TeV Run-2 ATLAS and CMS datasets set 95% CL upper limits on LFV
L20: Higgs branching ratios. For h -> e tau, the corresponding observed (expected)
L21: limits are:
L22:   B(h -> e tau) < 0.20% (0.12%) from ATLAS [439].
L23:   B(h -> e tau) < 0.22% (0.16%) from CMS [440].
L24: 
L25: The same paragraph gives h -> mu tau limits and notes that direct ATLAS/CMS

value < 0.22%

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Match snapshot line 23

L20: Higgs branching ratios. For h -> e tau, the corresponding observed (expected)
L21: limits are:
L22:   B(h -> e tau) < 0.20% (0.12%) from ATLAS [439].
L23:   B(h -> e tau) < 0.22% (0.16%) from CMS [440].
L24: 
L25: The same paragraph gives h -> mu tau limits and notes that direct ATLAS/CMS
L26: limits on Br(h -> mu tau) and Br(h -> e tau) are more than an order of

value < 0.20%

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Match snapshot line 26

L23: 
L24: The observed (expected) 95% CL upper limits obtained with the MC-template
L25: method from a simultaneous measurement are:
L26:   B(H -> e tau) < 0.20% (0.12%).
L27:   B(H -> mu tau) < 0.18% (0.09%).
L28: 
L29: The best-fit branching-ratio difference measured with the Symmetry method in

value 138 fb^-1 at sqrt(s) = 13 TeV

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Match snapshot line 19

L16: Relevant abstract extract:
L17: 
L18: The searches use pp collision data collected by ATLAS at sqrt(s) = 13 TeV,
L19: corresponding to an integrated luminosity of 138 fb^-1. Leptonic and hadronic
L20: tau decays are considered. No significant excess of events is observed, and
L21: the results are interpreted as upper limits on lepton-flavour-violating Higgs
L22: branching ratios.

value < 0.22%

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L22: 
L23: The observed (expected) upper limits on the branching fractions at 95% CL are:
L24:   B(H -> mu tau) < 0.15% (0.15%).
L25:   B(H -> e tau) < 0.22% (0.16%).

value 137 fb^-1 at sqrt(s) = 13 TeV

RESOLVED

Match snapshot line 19

L16: Relevant abstract extract:
L17: 
L18: The CMS search considers LFV Higgs decays to mu tau and e tau using
L19: 137 fb^-1 of pp collisions at sqrt(s) = 13 TeV. No significant excess is
L20: found, and results are interpreted as upper limits on LFV Higgs branching
L21: fractions.
L22:

value < 0.47%

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L21: 
L22: The observed (median expected) 95% CL upper limits on the LFV branching ratios
L23: are:
L24:   B(H -> e tau) < 0.47% with median expected 0.34% (+0.13%, -0.10%).
L25:   B(H -> mu tau) < 0.28% with median expected 0.37% (+0.14%, -0.10%).

value 36.1 fb^-1 at sqrt(s) = 13 TeV

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Match snapshot line 19

L16: Relevant abstract extract:
L17: 
L18: The ATLAS Letter searches for H -> e tau and H -> mu tau using
L19: 36.1 fb^-1 at sqrt(s) = 13 TeV. No significant excess over Standard Model
L20: background is observed.
L21: 
L22: The observed (median expected) 95% CL upper limits on the LFV branching ratios

value order 10%

RESOLVED

Match snapshot line 18

L15: new-physics frameworks including two-Higgs-doublet models, extra dimensions,
L16: and compositeness. It rederives rare-decay, dipole-moment, and meson-oscillation
L17: constraints and argues that h -> tau mu and h -> tau e branching ratios of
L18: order 10% can be allowed by low-energy constraints. It also motivates
L19: dedicated h -> tau mu and h -> tau e searches.