Catalog · Top / Higgs / EW · T003

T003 $\mathcal{B}(t\to c\gamma)$

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

Status REVIEWED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
CMS dataset for tqgamma FCNC search	138 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2024	CMS2024:T003:dataset	source ↑
B(t $ \to $ c $\gamma$), CMS single-nonzero-coupling interpretation	< 1.51e-5 branching fraction, 95% CL upper limit	2024	CMS2024:T003:tcgamma	source ↑
B(t $ \to $ u $\gamma$), CMS single-nonzero-coupling interpretation	< 0.95e-5 branching fraction, 95% CL upper limit	2024	CMS2024:T003:tugamma	source ↑
ATLAS dataset for tqgamma FCNC search	139 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2023	ATLAS2023:T003:dataset	source ↑
B(t $ \to $ c $\gamma$), left-handed tqgamma benchmark	< 4.2e-5 branching fraction, 95% CL upper limit	2023	ATLAS2023:T003:tcgamma_left	source ↑
B(t $ \to $ c $\gamma$), right-handed tqgamma benchmark	< 4.5e-5 branching fraction, 95% CL upper limit	2023	ATLAS2023:T003:tcgamma_right	source ↑
$\Gamma(t \to \gamma$ q) / $\Gamma(t \to $ W b), q = u,c generic PDG row	< 9.5e-6 ratio, 95% CL upper limit	2026	PDG2026:T003:gammaq_summary	source ↑

Why this constrains the RS scan

In anarchic warped flavor, the IR-localized top and non-universal fermion profiles can generate mass-basis flavor violation in the up sector. The top-photon channel probes an electromagnetic dipole or heavy-vector matching direction that is absent from the existing neutral-meson $\Delta F=2$ lane. It is therefore complementary to T001 ($t\to cZ$) and T005 ($t\to cg$): the same top-charm rotations can appear with different gauge quantum numbers.

What's changed since the original paper

Relative to the arXiv:0804.1954 RS-flavor baseline (CsakiFalkowskiWeiler2008WarpedFlavor), the main change is experimental. The LHC program moved this channel from indirect expectations to direct full-Run-2 searches with separate $tu\gamma$ and $tc\gamma$ hypotheses. ATLAS reached the $4.2\times10^{-5}$ left-handed charm benchmark in its 2023 result (ATLAS2023:T003:tcgamma\_left). CMS then used $138~\mathrm{fb}^{-1}$ and improved the charm-specific observed limit to $1.51\times10^{-5}$ (CMS2024:T003:dataset; CMS2024:T003:tcgamma). The Standard Model expectation is below current limits by many orders of magnitude, so present bounds are new-physics searches rather than SM precision tests.

Validity and model dependence

The upper limit is experimentally clean because the SM decay rate is negligible. Its interpretation is model dependent: CMS quotes a single nonzero anomalous coupling at a time, while ATLAS separates left- and right-handed $tq\gamma$ benchmarks. A warped-model scan would need to map its top-sector rotations and electromagnetic dipoles into the same operator normalization before using the number as a hard exclusion.

Code coverage in this repo

NO. The required greps across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no $t\to c\gamma$, $tc\gamma$, $tq\gamma$, or top-photon FCNC implementation. Existing coverage is limited to neutral-meson $\Delta F=2$ inputs at quarkConstraints/deltaf2.py:209, the modern policy list $\epsilon_K,K,B_d,B_s,D^0$ at quarkConstraints/modern/phenomenology.py:23, and a lepton-sector $\mu\to e\gamma$ dipole check at flavorConstraints/muToEGamma.py:75.

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

Implementation difficulty

HIGH. Cataloging the external upper limit is straightforward, but a live RS constraint would require a top electromagnetic FCNC operator convention, RS-to-SMEFT or direct dipole matching, a $t\to c\gamma$ width normalization, and a choice about how to reinterpret production-plus-decay LHC limits when more than one chirality or flavor coupling is present.

Reason: MEDIUM to store the external upper limit, but HIGH for production-grade RS integration because the repo lacks a top electromagnetic FCNC operator basis, RS-to-dipole matching, a $t \to c \gamma$ width convention, and a validated production-plus-decay recast.

Key references

CMS2024TopFCNCTQGamma; ATLAS2023TopFCNCTQGamma; PDG2026TopGammaQSummary; AguilarSaavedra2004TopFCNC; CsakiFalkowskiWeiler2008WarpedFlavor.

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

RESOLVED

Match snapshot line 21

L18: Relevant source excerpt, normalized:
L19: 
L20: The analysis uses pp collision data at sqrt(s) = 13 TeV corresponding to
L21: 138 fb^-1. It searches for single top plus photon production and top-pair
L22: events in which one top decays through a tqgamma FCNC vertex, with q = u or c.
L23: No significant excess over the Standard Model prediction is observed.
L24:

value < 1.51e-5

RESOLVED

Match snapshot line 27

L24: 
L25: Observed (expected) 95% CL upper limits on branching fractions:
L26:   B(t -> u gamma) < 0.95 x 10^-5 (1.20 x 10^-5 expected)
L27:   B(t -> c gamma) < 1.51 x 10^-5 (1.54 x 10^-5 expected)
L28: 
L29: CMS also quotes the corresponding anomalous coupling limits:
L30:   kappa_tugamma < 6.2 x 10^-3

value < 0.95e-5

RESOLVED

Match snapshot line 26

L23: No significant excess over the Standard Model prediction is observed.
L24: 
L25: Observed (expected) 95% CL upper limits on branching fractions:
L26:   B(t -> u gamma) < 0.95 x 10^-5 (1.20 x 10^-5 expected)
L27:   B(t -> c gamma) < 1.51 x 10^-5 (1.54 x 10^-5 expected)
L28: 
L29: CMS also quotes the corresponding anomalous coupling limits:

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

RESOLVED

Match snapshot line 21

L18: Relevant source excerpt, normalized:
L19: 
L20: The analysis uses pp collision data at sqrt(s) = 13 TeV collected during
L21: LHC Run 2, corresponding to 139 fb^-1. Both FCNC top-quark production and
L22: top-quark decay are considered. Events contain a charged lepton, missing
L23: transverse momentum, a b-tagged jet, one high-momentum photon, and possibly
L24: additional jets. No significant excess is observed.

value < 4.2e-5

RESOLVED

Match snapshot line 28

L25: 
L26: Observed 95% CL upper limits on branching fractions for left-handed tqgamma:
L27:   B(t -> u gamma) < 0.85 x 10^-5
L28:   B(t -> c gamma) < 4.2 x 10^-5
L29: 
L30: Observed 95% CL upper limits on branching fractions for right-handed tqgamma:
L31:   B(t -> u gamma) < 1.2 x 10^-5

value < 4.5e-5

RESOLVED

Match snapshot line 32

L29: 
L30: Observed 95% CL upper limits on branching fractions for right-handed tqgamma:
L31:   B(t -> u gamma) < 1.2 x 10^-5
L32:   B(t -> c gamma) < 4.5 x 10^-5
L33: 
L34: This is the ATLAS number often quoted before the later CMS-TOP-21-013
L35: charm-specific improvement.

value < 9.5e-6

RESOLVED

Match snapshot line 14

L11: Relevant pdgLive / summary-table content, normalized:
L12: 
L13: The t-quark decay-mode summary includes
L14:   Gamma_7: gamma q (q = u, c) < 9.5 x 10^-6 at 95% CL.
L15: 
L16: The associated note says:
L17:   This limit is for Gamma(t -> gamma q) / Gamma(t -> W b).