Catalog · Top / Higgs / EW · T006

T006 $\mathcal{B}(t\to u g)$

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

Status SUBTLETY-ADDED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
ATLAS dataset for u g $ \to $ t single-top FCNC search	139 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2022	PDG2025:T006:ATLAS2022:dataset	source ↑
$\sigma(u$ g $ \to $ t) * B(t $ \to $ b W) * B(W $ \to $ l $\nu$)	< 3.0 pb, 95% CL upper limit	2022	PDG2025:T006:ATLAS2022:ug_to_t_cross_section	source ↑
B(W $ \to $ l $\nu$) sum over e, $\mu$, $\tau$ modes used by ATLAS	0.325 branching fraction	2022	PDG2025:T006:ATLAS2022:w_leptonic_sum	source ↑
$\|C_{uG}^{ut}\|$ / $\Lambda^2$	< 0.057 TeV^-2, 95% CL upper limit	2022	PDG2025:T006:ATLAS2022:CuGut	source ↑
B(t $ \to $ u g), ATLAS AAD 2022T EFT interpretation	< 6.1e-5 branching fraction, 95% CL upper limit	2022	PDG2025:T006:ATLAS2022:t_ug	source ↑
CMS dataset for t-channel single top FCNC search	5.0 fb^-1 at sqrt(s) = 7 TeV and 19.7 fb^-1 at sqrt(s) = 8 TeV integrated luminosity and collision energy	2017	CMS2017:T006:dataset	source ↑
$\|\kappa_{tug}\|$ / $\Lambda$	< 4.1e-3 TeV^-1, 95% CL upper limit	2017	CMS2017:T006:kappa_tug	source ↑
B(t $ \to $ u g), CMS t-channel single-top interpretation	< 2.0e-5 branching fraction, 95% CL upper limit	2017	CMS2017:T006:t_ug	source ↑
Standard Model estimate for B(t $ \to $ u g) inferred from source scaling	approximately 3.6e-14 branching fraction	2004	AguilarSaavedra2004:T006:t_ug_SM	source ↑

Why this constrains the RS scan

This channel probes color-carrying first--third generation up-sector flavor violation rather than the neutral-meson $\Delta F=2$ four-quark operators already audited in the quark scan. In anarchic warped flavor, the heavy color sector and the IR-localized top can generate non-universal mass-basis couplings and loop-level chromomagnetic dipoles. A $t\to u g$ limit is therefore a direct top-flavor diagnostic, complementary to $D^0$ mixing and to electroweak channels such as $t\to uZ$ or $t\to uH$.

What's changed since the original paper

The paper-era CFW reference point, arXiv:0804.1954, predates the LHC single-top FCNC program and is used here only as RS-flavor context. Since then, CMS and ATLAS have interpreted single-top data in top-gluon FCNC operators, with the ATLAS Run-2 dataset supplying the current PDG/pdgLive entry. The resulting bounds remain far above the SM prediction, so they are new-physics limits rather than SM precision tests.

Validity and model dependence

The experimental upper bound is clean because the SM signal is negligible at collider sensitivity. Its use as an RS scan cut is model dependent: ATLAS quotes a left-handed SMEFT $C_{uG}^{ut}$ interpretation, while CMS uses an older $\kappa_{tug}$ convention. The conversion from single-top production to ${\cal B}(t\to u g)$ assumes one FCNC operator at a time, standard top decays, and a specific normalization of the chromomagnetic interaction. A model producing both chiralities, contact terms, or correlated heavy-gluon resonance effects needs a dedicated reinterpretation. In down-aligned or kaon-protected RS variants, top FCNCs become leading rather than secondary diagnostics.

Code coverage in this repo

NO. The required plan greps were rerun on 2026-05-16, followed by a targeted search for $t\to u g$, $tug$, $utg$, $tqg$, $C_{uG}$, and $ug\to t$ across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/. The only implementation-directory hits were generic FCNC prose/comments at quarkConstraints/PAPER\_0710\_1869.md:35 and tests/test\_paper\_couplings.py:258. The modern phenomenology surface enumerates only $\epsilon_K$, $K$, $B_d$, $B_s$, and $D^0$ at quarkConstraints/modern/phenomenology.py:23, and states that it is policy-only at quarkConstraints/modern/phenomenology.py:167.

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

Implementation difficulty

HIGH. Cataloging the external limit is straightforward, but live integration would need a top chromomagnetic FCNC operator convention, RS-to-SMEFT matching, decay-width and $ug\to t$ production normalization, PDF/collider acceptance assumptions or a validated recast, and QCD-running choices not present in the existing $\Delta F=2$ code.

Reason: Limit cataloging is MEDIUM, but production-grade integration is HIGH because the repo lacks a top chromomagnetic FCNC operator convention, RS-to-SMEFT matching, $t \to u$ g width and u $g \to t$ production normalization, collider/PDF reinterpretation machinery, and QCD-running choices for this operator.

Key references

PDG2025TopFCNCTUG; ATLAS2022TopFCNCTUG; CMS2017TopFCNCTUG; AguilarSaavedra2004TopFCNC; CsakiFalkowskiWeiler2008WarpedFlavor.

value 139 fb^-1 at sqrt(s) = 13 TeV

RESOLVED

Match snapshot line 19

L16: and kappa^{ctg}/Lambda" and lists values in TeV^{-1} with 95% CL.
L17: 
L18: PDG footnote 1, AAD 2022T / ATLAS:
L19: - Based on 139 fb^{-1} of pp data at sqrt(s) = 13 TeV.
L20: - Single top-quark production limits:
L21:   sigma(u g -> t) * B(t -> b W) * B(W -> l nu) < 3.0 pb.
L22:   sigma(c g -> t) * B(t -> b W) * B(W -> l nu) < 4.7 pb.

value < 3.0

RESOLVED

Match snapshot line 21

L18: PDG footnote 1, AAD 2022T / ATLAS:
L19: - Based on 139 fb^{-1} of pp data at sqrt(s) = 13 TeV.
L20: - Single top-quark production limits:
L21:   sigma(u g -> t) * B(t -> b W) * B(W -> l nu) < 3.0 pb.
L22:   sigma(c g -> t) * B(t -> b W) * B(W -> l nu) < 4.7 pb.
L23: - EFT coupling limits:
L24:   |C_uG^{ut}| / Lambda^2 < 0.057 TeV^{-2}.

value 0.325

RESOLVED

Match snapshot line 28

L25:   sigma(u+g -> t) * B(t -> Wb) * B(W -> l nu) < 3.0 pb.
L26:   sigma(c+g -> t) * B(t -> Wb) * B(W -> l nu) < 4.7 pb.
L27: - The W leptonic branching fraction used for the sum over e, mu, tau modes is
L28:   B(W -> l nu) = 0.325.
L29: - EFT interpretation:
L30:   |C_uG^{ut}| / Lambda^2 < 0.057 TeV^{-2}.
L31:   |C_uG^{ct}| / Lambda^2 < 0.14 TeV^{-2}.

value < 0.057

RESOLVED

Match snapshot line 24

L21:   sigma(u g -> t) * B(t -> b W) * B(W -> l nu) < 3.0 pb.
L22:   sigma(c g -> t) * B(t -> b W) * B(W -> l nu) < 4.7 pb.
L23: - EFT coupling limits:
L24:   |C_uG^{ut}| / Lambda^2 < 0.057 TeV^{-2}.
L25:   |C_uG^{ct}| / Lambda^2 < 0.14 TeV^{-2}.
L26: - Equivalent FCNC top-decay branching-fraction limits:
L27:   B(t -> u g) < 6.1e-5.

value < 6.1e-5

RESOLVED

Match snapshot line 27

L24:   |C_uG^{ut}| / Lambda^2 < 0.057 TeV^{-2}.
L25:   |C_uG^{ct}| / Lambda^2 < 0.14 TeV^{-2}.
L26: - Equivalent FCNC top-decay branching-fraction limits:
L27:   B(t -> u g) < 6.1e-5.
L28:   B(t -> c g) < 3.7e-4.
L29: 
L30: PDG footnote 2, KHACHATRYAN 2017G / CMS:

value 5.0 fb^-1 at sqrt(s) = 7 TeV and 19.7 fb^-1 at sqrt(s) = 8 TeV

RESOLVED

Match snapshot line 21

L18: - CMS used t-channel single top-quark events to search for FCNC top-gluon
L19:   interactions.
L20: - Dataset: pp collisions at sqrt(s)=7 TeV with 5.0 fb^{-1} and sqrt(s)=8 TeV
L21:   with 19.7 fb^{-1}.
L22: - Event topology: one muon and two or three jets.
L23: - Observed event distributions are consistent with the Standard Model
L24:   prediction.

value < 4.1e-3

RESOLVED

Match snapshot line 26

L23: - Observed event distributions are consistent with the Standard Model
L24:   prediction.
L25: - 95% CL top-gluon FCNC coupling limits:
L26:   |kappa_tug| / Lambda < 4.1e-3 TeV^{-1}.
L27:   |kappa_tcg| / Lambda < 1.8e-2 TeV^{-1}.
L28: - Equivalent branching-fraction limits:
L29:   B(t -> u g) < 2.0e-5.

value < 2.0e-5

RESOLVED

Match snapshot line 29

L26:   |kappa_tug| / Lambda < 4.1e-3 TeV^{-1}.
L27:   |kappa_tcg| / Lambda < 1.8e-2 TeV^{-1}.
L28: - Equivalent branching-fraction limits:
L29:   B(t -> u g) < 2.0e-5.
L30:   B(t -> c g) < 4.1e-4.

value approximately 3.6e-14

RESOLVED

Match snapshot line 25

L22: |V_ub / V_cb|^2 approximately 0.0079.
L23: 
L24: Using the review's stated scaling, the corresponding Standard Model estimate
L25: for t -> u g is approximately 3.6e-14. The source emphasizes that top FCNC
L26: interactions are negligible in the Standard Model but can be enhanced in
L27: extensions, so an observed t -> u g signal would indicate new physics.