Catalog · Top / Higgs / EW · T008

T008 $\mathcal{B}(t\to H u)$

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

Status REVIEWED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
B(t $ \to $ H u), PDG headline limit from CMS H $ \to \gamma\gamma$	< 1.9e-4	2026	PDG2026:T008:tHu_headline	source ↑
CMS H $ \to \gamma\gamma$ dataset	137 fb^-1 at sqrt(s) = 13 TeV	2022	CMS2022:T008:dataset	source ↑
B(t $ \to $ H u), CMS H $ \to \gamma\gamma$	< 0.019%	2022	CMS2022:T008:tHu_diphoton	source ↑
ATLAS multilepton tHq dataset	140 fb^-1 at sqrt(s) = 13 TeV	2024	ATLAS2024:T008:dataset	source ↑
B(t $ \to $ H u), ATLAS multilepton-only result	< 2.8e-4	2024	ATLAS2024:T008:tHu_multilepton	source ↑
B(t $ \to $ H u), ATLAS combination with H $ \to $ bb, $\gamma\gamma$, and $\tau \tau$ searches	< 2.6e-4	2024	ATLAS2024:T008:tHu_combined	source ↑
CMS same-sign dilepton tHq dataset	138 fb^-1 at sqrt(s) = 13 TeV, collected in 2016-2018	2025	CMS2025:T008:dataset	source ↑
B(t $ \to $ H u), CMS same-sign dilepton search	< 0.072%	2025	CMS2025:T008:tHu_samesign	source ↑
B(t $ \to $ H u), CMS combination with H $ \to \gamma\gamma$ and H $ \to $ bb searches	< 0.019%	2025	CMS2025:T008:tHu_combined	source ↑

Why this constrains the RS scan

Warped anarchic flavor and composite-Higgs variants can generate off-diagonal scalar Yukawa couplings after rotating to the quark mass basis. The $t\to H u$ channel probes up-top scalar flavor, complementary to the charm channel in T007 and to light-meson $\Delta F=2$ gates. Associated $tH$ production is especially sensitive to an up-quark initial state, so the experimental interpretation is not identical to simply rescaling $t\to Hc$.

What's changed since the original paper

The requested baseline, arXiv:0804.1954 (CsakiFalkowskiWeiler2008WarpedFlavor), predates direct LHC top-Higgs FCNC searches. Since then, ATLAS and CMS moved the constraint from model estimates to direct searches in diphoton, multilepton, $b\bar b$, and $\tau\tau$ Higgs final states. The PDG headline currently comes from the CMS diphoton search, while ATLAS and CMS now also provide multi-channel combinations that treat $t\bar t$ FCNC decay and associated $tH$ production together.

Validity and model dependence

The experimental statement is a direct 95\% CL upper limit under a single anomalous $Hut$ coupling assumption (PDG2026:T008:tHu\_headline). Mapping it into this repo would require choosing a scalar-Yukawa or SMEFT convention, deciding whether $Hut$ and $Htu$ are independent, and preserving the analysis-specific combination of $t\bar t$ decay and associated-production topologies. Correlations with $t\to Hc$ are not universal, so T008 keeps only the up-channel bound.

Code coverage in this repo

Coverage is NO. The required plan greps and a targeted search for $t\to Hu$, $tHu$, $tuH$, $Hut$, $Htu$, and $tqH$ were rerun across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/. The only targeted hits outside the catalog were generic FCNC prose/comments in quarkConstraints/PAPER\_0710\_1869.md:35 and tests/test\_paper\_couplings.py:258; there is no $t\to Hu$ observable, likelihood, matching layer, or limit check.

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

Implementation difficulty

Production integration is HIGH. Limit cataloging is straightforward, but a live scan needs a new top-Higgs FCNC matching layer, scalar-coupling normalization, and an analysis prescription for the $tH$ plus $t\bar t$ interpretation.

Reason: A live constraint needs a new top-Higgs FCNC matching layer, scalar Htu/Hut coupling convention, and an analysis-level prescription for combining associated tH production with ttbar FCNC decay interpretations.

Key references

PDG2026TopHu; CMS2022TopHuDiphoton; ATLAS2024TopHuMultilepton; CMS2025TopHuHiggsMediated; CsakiFalkowskiWeiler2008WarpedFlavor.

pdg_units 1.9

RESOLVED

Match snapshot line 18

L15: 
L16: VALUE (units 10^-4), CL%, DOCUMENT ID, TECHNIQUE, COMMENT
L17: 
L18: < 1.9, 95, TUMASYAN 2022A, CMS, t -> H u (H -> gamma gamma)
L19: 
L20: The following data are listed below the PDG "do not use for averages, fits,
L21: limits, etc." separator:

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

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Match snapshot line 17

L14: 
L15: Relevant abstract excerpt:
L16: The analysis uses pp collision data collected at sqrt(s) = 13 TeV with the CMS
L17: detector, corresponding to 137 fb^-1. Final states with two photons are used
L18: to search for top-Higgs FCNC signatures.
L19: 
L20: No significant excess is observed. The observed (expected) 95% CL upper limits

value < 0.019%

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Match snapshot line 21

L18: to search for top-Higgs FCNC signatures.
L19: 
L20: No significant excess is observed. The observed (expected) 95% CL upper limits
L21: are B(t -> H u) < 0.019% (0.031%) and
L22: B(t -> H c) < 0.073% (0.051%). The abstract describes these as the strictest
L23: limits yet determined at the time of publication.

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

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Match snapshot line 16

L13: Related DOI: 10.1140/epjc/s10052-024-12994-1.
L14: 
L15: Relevant abstract excerpt:
L16: The analysis uses 140 fb^-1 of pp data at sqrt(s) = 13 TeV. It considers ttbar
L17: production with one top decaying via t -> H q and associated Ht production,
L18: where q = u,c.
L19:

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

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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. Same-sign lepton-pair events with at least one jet
L17: are considered.
L18: 
L19: No significant excess is observed. The same-sign search observed (expected)

value < 0.072%

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Match snapshot line 20

L17: are considered.
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

value < 0.019%

RESOLVED

Match snapshot line 24

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
L25: B(t -> H c) < 0.037% (0.035%).