Catalog · Top / Higgs / EW · T007

T007 $\mathcal{B}(t\to Hc)$

Flavor-changing neutral-current top decay t $ \to $ H c

Status SUBTLETY-FIXED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
B(t $ \to $ H c), PDG headline limit from ATLAS five-Higgs-mode combination	< 3.4e-4 branching fraction, 95% CL upper limit	2026	PDG2026:T007:tHc_combined	source ↑
ATLAS dataset for tHq/t $ \to $ Hq multilepton analysis	140 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2024	ATLAS2024:T007:dataset	source ↑
B(t $ \to $ H c), ATLAS multilepton-only result	< 3.3e-4 branching fraction, 95% CL upper limit	2024	ATLAS2024:T007:tHc_multilepton	source ↑
B(t $ \to $ H c), ATLAS combination with H $ \to $ bb, $\gamma\gamma$, and $\tau \tau$ searches	< 3.4e-4 branching fraction, 95% CL upper limit	2024	ATLAS2024:T007:tHc_combined	source ↑
CMS dataset for Higgs-mediated top FCNC search	138 fb^-1 at sqrt(s) = 13 TeV, collected in 2016-2018 integrated luminosity, collision energy, data-taking years	2025	CMS2025:T007:dataset	source ↑
B(t $ \to $ H c), CMS same-sign dilepton search	< 0.043% branching fraction, 95% CL upper limit	2025	CMS2025:T007:tHc_samesign	source ↑
B(t $ \to $ H c), CMS combination with H $ \to \gamma\gamma$ and H $ \to $ bb searches	< 0.037% branching fraction, 95% CL upper limit	2025	CMS2025:T007:tHc_combined	source ↑

Why this constrains the RS scan

Anarchic warped models and composite-Higgs variants can generate off-diagonal scalar Yukawa couplings after rotating to the quark mass basis. The $t\to Hc$ mode probes the same top-sector localization and mixing structure that also controls the large top Yukawa, so it is complementary to light-meson $\Delta F=2$ constraints. For this repo it is best treated as a direct top-sector diagnostic, not as a replacement for the existing kaon, $B$, and $D$-mixing gates.

What's changed since the original paper

The requested baseline is arXiv:0804.1954 (CsakiFalkowskiWeiler2008WarpedFlavor), which predates direct LHC top-Higgs FCNC bounds. Since then, ATLAS and CMS moved the search from indirect model estimates to direct limits using many Higgs final states. ATLAS now combines $H\to WW$, $H\to ZZ$, $H\to\tau\tau$, $H\to\gamma\gamma$, and $H\to b\bar b$ channels in the PDG headline (PDG2026:T007:tHc\_combined; ATLAS2024:T007:tHc\_combined). CMS has an independent 2025 combination with $H\to\gamma\gamma$ and $H\to b\bar b$ inputs in addition to the same-sign lepton analysis (CMS2025:T007:tHc\_combined). The classic top-FCNC review arXiv:hep-ph/0409342 remains useful context for why SM top FCNC Higgs amplitudes are negligible while BSM scalar flavor can be visible (AguilarSaavedra2004TopFCNC).

Validity and model dependence

The experimental statement is a 95\% CL upper limit on an assumed single anomalous top-Higgs flavor coupling. The model interpretation is operator-convention dependent: an RS implementation must choose whether the scan outputs scalar $Htc$, $Hct$, or SMEFT Wilson coefficients, and whether associated $tH$ production is included together with $t\bar t$ FCNC decays. Correlations with $t\to Hu$ are analysis-dependent, so this entry keeps only the charm-channel bound as the T007 catalog value. In down-aligned or kaon-protected RS variants, top FCNCs become leading rather than secondary diagnostics. Note: for the observed Higgs, on-shell $h\to tc$ is kinematically forbidden; $Htc$ coupling constraints enter through $t\to Hc$ decay and associated single-top + $H$ production.

Code coverage in this repo

Coverage is NO. The author reran the required plan greps and a targeted search for $t\to Hc$, $tHc$, $tcH$, $Htc$, and $tqH$ across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/. The only targeted non-doc hits were generic FCNC prose/comments in quarkConstraints/PAPER\_0710\_1869.md:35 and tests/test\_paper\_couplings.py:258; there is no $t\to Hc$ observable, likelihood, or limit check.

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

Implementation difficulty

Production integration is HIGH. Recording the external upper limit is straightforward, but using it in scans needs a new top-Higgs FCNC matching layer, a scalar-coupling convention, and a decision about combining $tH$ production with $t\bar t$ FCNC-decay interpretations.

Reason: Limit cataloging is MEDIUM, but production integration is HIGH because the repo has no top-Higgs FCNC operator basis, no RS-to-SMEFT Htc matching, and no tH/tbar t reinterpretation machinery.

Key references

PDG2026TopHc; ATLAS2024TopHcMultilepton; CMS2025TopHcHiggsMediated; AguilarSaavedra2004TopFCNC; CsakiFalkowskiWeiler2008WarpedFlavor.

pdg_units 3.4

RESOLVED

Match snapshot line 17

L14: 
L15: VALUE (units 10^-4), CL%, DOCUMENT ID, TECHNIQUE, COMMENT
L16: 
L17: < 3.4, 95, AAD 2024AU, ATLAS, pp -> tH or t -> H c
L18: (combined H -> W W, Z Z, tau tau, gamma gamma, b b)
L19: 
L20: The following data are listed below the PDG "do not use for averages, fits,

value 140 fb^-1 at sqrt(s) = 13 TeV

RESOLVED

Match snapshot line 19

L16: A search is presented for FCNC interactions involving the top quark, the Higgs
L17: boson, and an up-type quark q = u,c. The analysis uses leptonic top decays and
L18: Higgs decays to W W, Z Z, or tau+ tau-, focusing on two same-charge leptons or
L19: three leptons. The dataset is 140 fb^-1 at sqrt(s) = 13 TeV.
L20: 
L21: No significant excess is observed. The multilepton-only observed (expected)
L22: 95% CL limits are B(t -> H u) < 2.8 (3.0) x 10^-4 and

value 138 fb^-1 at sqrt(s) = 13 TeV, collected in 2016-2018

RESOLVED

Match snapshot line 16

L13: 
L14: Relevant abstract excerpt:
L15: The search uses pp collision data collected in 2016-2018 at sqrt(s) = 13 TeV,
L16: corresponding to 138 fb^-1. Events with a same-sign lepton pair and at least
L17: one jet are considered.
L18: 
L19: No significant excess is observed. The same-sign search observed (expected)

value < 0.043%

RESOLVED

Match snapshot line 21

L18: 
L19: No significant excess is observed. The same-sign search observed (expected)
L20: 95% CL limits are B(t -> H u) < 0.072% (0.059%) and
L21: B(t -> H c) < 0.043% (0.062%).
L22: 
L23: Combining with CMS searches in H -> gamma gamma and H -> b b final states gives
L24: observed (expected) 95% CL limits B(t -> H u) < 0.019% (0.027%) and

value < 0.037%

RESOLVED

Match snapshot line 25

L22: 
L23: Combining with CMS searches in H -> gamma gamma and H -> b b final states gives
L24: observed (expected) 95% CL limits B(t -> H u) < 0.019% (0.027%) and
L25: B(t -> H c) < 0.037% (0.035%).