Catalog · Top / Higgs / EW · T004

T004 $\mathcal{B}(t\to u\gamma)$

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

Status REVIEWED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
B(t $ \to \gamma$ q), q = u,c	< 9.5e-6 branching fraction, 95% CL upper limit	2026	Particle Data Group pdgLive t-quark listing	source ↑
B(t $ \to $ u $\gamma$), left-handed tqgamma benchmark	< 0.85e-5 branching fraction, 95% CL upper limit	2023	ATLAS arXiv:2205.02537v3	source ↑
B(t $ \to $ u $\gamma$), right-handed tqgamma benchmark	< 1.2e-5 branching fraction, 95% CL upper limit	2023	ATLAS arXiv:2205.02537v3	source ↑
B(t $ \to $ u $\gamma$), single nonzero coupling at a time	< 0.95e-5 branching fraction, 95% CL upper limit	2024	CMS-TOP-21-013 public result page	source ↑

Why this constrains the RS scan

In warped or partially composite flavor models, non-universal fermion profiles and mass-basis rotations can induce top FCNC couplings. The photon channel is a dipole-like probe, complementary to $t\to uZ$, $t\to ug$, and $t\to uh$. Compared with charm final states, T004 isolates the top-up flavor rotation and therefore probes a different corner of the anarchic up-sector structure.

What's changed since the original paper

The comparison baseline is arXiv:0804.1954 (CFW2008:T004:rs\_flavor\_context). Since that reference epoch, the important change is experimental: ATLAS and CMS set direct mode-resolved limits (ATLAS2023:T004:dataset; CMS2024:T004:dataset). The high-energy single-production strategy also became a standard way to discuss future sensitivity for $t\to uZ/u\gamma$-equivalent interactions.

Validity and model dependence

Experimentally this is a clean null-search upper limit; the SM rate is far below current reach and is lower than the corresponding $t\to c\gamma$ rate because of CKM suppression (AguilarSaavedra2017:T004:SM\_and\_projection\_context). The interpretation is not model-independent: ATLAS quotes separate chiral $tq\gamma$ tensor benchmarks, and an RS backend must choose an operator normalization before comparing scan points to the branching-fraction limit.

Code coverage in this repo

Coverage is NO. The author reran the required catalog greps across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/. The targeted $t\to u\gamma$, $tu\gamma$, $tq\gamma$, and top-photon FCNC search returned no observable, Wilson coefficient, or branching-ratio implementation.

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

Implementation difficulty

Implementation difficulty is HIGH. The repo has no top-FCNC photon dipole basis, no RS-to-$tu\gamma$ matching convention, and no top radiative decay-width or production-recast module. A production backend would need those ingredients before this limit could become a scan cut.

Reason: Cataloging the limit is straightforward, but production integration needs a new electroweak dipole/top-FCNC operator convention, RS-to-top-flavor matching for t-u rotations, and a $t \to u \gamma$ decay-width or production/decay recast path not present in the current $\Delta F = 2$ and $\mu \to e \gamma$ code.

Key references

PDG2026TopTGammaQ; ATLAS2023TopFCNCTQGamma; CMS2024TopFCNCTQGamma; CsakiFalkowskiWeiler2008WarpedFlavor; AguilarSaavedra2017UltraBoostedTopFCN.

value < 9.5e-6

RESOLVED

Match snapshot line 9

L6: 
L7: Relevant catalog value:
L8: - t-quark decay modes include Gamma_7: t -> gamma q (q = u,c).
L9: - Listed fraction Gamma_7/Gamma: < 9.5 x 10^-6, CL = 95%.
L10: 
L11: Notes for T004:
L12: - PDG presents this row as the combined photon-plus-light-quark mode q = u,c,

value < 0.85e-5

RESOLVED

Match snapshot line 16

L13:   FCNC top decay are considered.
L14: - No significant excess over the background prediction was observed.
L15: - 95% CL observed bound, left-handed tqgamma coupling:
L16:   B(t -> u gamma) < 0.85 x 10^-5.
L17: - 95% CL observed bound, right-handed tqgamma coupling:
L18:   B(t -> u gamma) < 1.2 x 10^-5.
L19:

value < 1.2e-5

RESOLVED

Match snapshot line 18

L15: - 95% CL observed bound, left-handed tqgamma coupling:
L16:   B(t -> u gamma) < 0.85 x 10^-5.
L17: - 95% CL observed bound, right-handed tqgamma coupling:
L18:   B(t -> u gamma) < 1.2 x 10^-5.
L19: 
L20: Notes for T004:
L21: - This is the tightest flavor-separated t -> u gamma limit found for this PKA

value < 0.95e-5

RESOLVED

Match snapshot line 15

L12: - Search mode: single top plus photon and ttbar events with one FCNC top decay.
L13: - No significant deviation from the standard-model background prediction was observed.
L14: - 95% CL observed expected upper limits:
L15:   B(t -> u gamma) < 0.95 x 10^-5 observed, 1.20 x 10^-5 expected.
L16: - CMS notes that the t -> u gamma bound is similar to the current best limit.
L17: 
L18: Notes for T004: