Catalog · Top / Higgs / EW · T001

T001 $\mathcal{B}(t\to cZ)$

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

Status SUBTLETY-ADDED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
B(t $ \to $ Z c), left-handed t-c-Z benchmark	< 1.3e-4 branching fraction, 95% CL upper limit	2025	PDG2025:T001:tZc_left	source ↑
B(t $ \to $ Z c), right-handed t-c-Z benchmark	< 1.2e-4 branching fraction, 95% CL upper limit	2025	PDG2025:T001:tZc_right	source ↑
ATLAS2023:T001:dataset	139 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2023	ATLAS2023:T001:dataset	source ↑
$ATLAS2023:T001:tZc_{left}$	< 13e-5 branching fraction, 95% CL upper limit	2023	ATLAS2023:T001:tZc_left	source ↑
$ATLAS2023:T001:tZc_{right}$	< 12e-5 branching fraction, 95% CL upper limit	2023	ATLAS2023:T001:tZc_right	source ↑
CMS2017:T001:dataset	19.7 fb^-1 at sqrt(s) = 8 TeV integrated luminosity and collision energy	2017	CMS2017:T001:dataset	source ↑
CMS2017:T001:tZc	< 0.049% branching fraction, 95% CL upper limit	2017	CMS2017:T001:tZc	source ↑
AguilarSaavedra2004:T001:SM	1e-14 Standard Model branching-fraction estimate	2004	AguilarSaavedra2004:T001:SM	source ↑
PDG2025:T001:SM	order 1e-14 Standard Model branching-fraction estimate	2025	PDG2025:T001:SM	source ↑
$AirenFranceschini2026:T001:HL_{LHC}_{projection}$	approximately 1e-6 $projected equivalent BR(t \to c Z) reach$	2026	AirenFranceschini2026:T001:HL_LHC_projection	source ↑

Why this constrains the RS scan

In warped or partially composite flavor models, non-universal fermion profiles and mass-basis rotations can produce flavor off-diagonal neutral-current couplings. The $t\to cZ$ channel is especially diagnostic of the top sector: it probes the $t$-$c$ rotation and the degree of right-handed top compositeness more directly than light-meson $\Delta F=2$ observables. A direct top FCNC limit is therefore complementary to the existing kaon, $B$, and $D$-mixing lane rather than a substitute for it.

What's changed since the original paper

The CFW-era reference point is arXiv:0804.1954, which framed warped flavor constraints before the full LHC top-FCNC program. Since then, the key experimental change is the move from Tevatron and early-LHC inclusive $t\to Zq$ limits to flavor-separated LHC constraints. CMS used $19.7~\mathrm{fb}^{-1}$ at $\sqrt{s}=8~\mathrm{TeV}$ (CMS2017:T001:dataset) and found $\mathcal{B}(t\to Zc)<0.049\%$ (CMS2017:T001:tZc). ATLAS later used $139~\mathrm{fb}^{-1}$ at $\sqrt{s}=13~\mathrm{TeV}$ (ATLAS2023:T001:dataset), combining single-top $tZ$ production and $t\bar t$ FCNC-decay topologies, and improved the $t\to Zc$ reach to the $10^{-4}$ level (ATLAS2023:T001:tZc\_left, ATLAS2023:T001:tZc\_right). A 2026 RS-specific study, arXiv:2601.14966, recasts post-LHC constraints on $Ztc$ and notes that HL-LHC direct searches may reach an equivalent $\mathcal{B}(t\to cZ)$ level of about $10^{-6}$ (AirenFranceschini2026:T001:HL\_LHC\_projection).

Validity and model dependence

Experimentally this is a clean upper limit; there is no SM-sized background from true $t\to cZ$ decays. The SM branching estimate is about $10^{-14}$, so any observed signal would be unambiguous new physics (AguilarSaavedra2004:T001:SM; PDG2025:T001:SM). The interpretation is not fully model independent: ATLAS quotes separate left- and right-handed tensor-SMEFT benchmarks, while an RS model may generate vector-like $Ztc$ couplings, heavy-vector exchange, or correlated contact interactions. A scan backend would need to map the RS parameters into a chosen operator convention before applying the PDG-number as a hard cut. In down-aligned or kaon-protected RS variants, top FCNCs become leading rather than secondary diagnostics.

Code coverage in this repo

Coverage is NO. The author reran the required plan greps and a targeted search for $t\to cZ$, $tZc$, $tcZ$, and generic FCNC text across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/. The only non-doc hits were generic FCNC prose/comments, not an observable, likelihood, or branching-ratio check.

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

Implementation difficulty

The cataloging difficulty is medium, but production integration is HIGH. The repo has no electroweak top-FCNC operator basis, no $tcZ$ width or single-production reinterpretation, and no top-sector flavor-rotation interface. Implementing it honestly would require a new RS-to-SMEFT or direct $Ztc$ matching convention before this limit can be compared to scan points.

Reason: MEDIUM to store the external upper limit, but HIGH for production-grade RS integration because the repo lacks electroweak top-FCNC matching, a tcZ decay-width/production interpretation, and top-sector flavor-rotation plumbing.

Key references

PDG2025TopFCNCTZQ; ATLAS2023TopFCNCTZQ; CMS2017TZQFCNC; AguilarSaavedra2004TopFCNC; CsakiFalkowskiWeiler2008WarpedFlavor; AirenFranceschini2026RSTopFCNC.

pdg_units 0.13

RESOLVED

Match snapshot line 16

L13: VALUE (units 10^-3), CL%, DOCUMENT ID, TECHNIQUE, COMMENT
L14: 
L15: < 0.062, 95, AAD 23AS, ATLAS, B(t -> Z u), left-handed t u Z coupling
L16: < 0.13, 95, AAD 23AS, ATLAS, B(t -> Z c), left-handed t c Z coupling
L17: < 0.066, 95, AAD 23AS, ATLAS, B(t -> Z u), right-handed t u Z coupling
L18: < 0.12, 95, AAD 23AS, ATLAS, B(t -> Z c), right-handed t c Z coupling
L19: < 0.22, 95, SIRUNYAN 17E, CMS, t -> Z u

pdg_units 0.12

RESOLVED

Match snapshot line 18

L15: < 0.062, 95, AAD 23AS, ATLAS, B(t -> Z u), left-handed t u Z coupling
L16: < 0.13, 95, AAD 23AS, ATLAS, B(t -> Z c), left-handed t c Z coupling
L17: < 0.066, 95, AAD 23AS, ATLAS, B(t -> Z u), right-handed t u Z coupling
L18: < 0.12, 95, AAD 23AS, ATLAS, B(t -> Z c), right-handed t c Z coupling
L19: < 0.22, 95, SIRUNYAN 17E, CMS, t -> Z u
L20: < 0.49, 95, SIRUNYAN 17E, CMS, t -> Z c
L21: < 0.7, 95, AAD 16D, ATLAS, t -> Z q (q = u,c)

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

RESOLVED

Match snapshot line 24

L21: an up or charm quark and a Z boson is presented, using proton-proton collision
L22: data at sqrt(s) = 13 TeV collected by the ATLAS detector at the Large Hadron
L23: Collider. The analyzed dataset corresponds to an integrated luminosity of
L24: 139 fb^-1.
L25: 
L26: The search targets both single-top-quark events produced as gq -> tZ
L27: (q = u,c) and top-quark-pair events, with one top quark decaying through

value < 13e-5

RESOLVED

Match snapshot line 34

L31: The data are consistent with the background-only hypothesis. At 95% CL,
L32: the t -> Zq branching-ratio upper limits, assuming only tensor SMEFT
L33: operators contribute to the tZq vertices, are:
L34:   B(t -> Zu) < 6.2 x 10^-5 and B(t -> Zc) < 13 x 10^-5
L35:   for a left-handed tZq coupling;
L36:   B(t -> Zu) < 6.6 x 10^-5 and B(t -> Zc) < 12 x 10^-5
L37:   for a right-handed coupling.

value < 12e-5

RESOLVED

Match snapshot line 36

L33: operators contribute to the tZq vertices, are:
L34:   B(t -> Zu) < 6.2 x 10^-5 and B(t -> Zc) < 13 x 10^-5
L35:   for a left-handed tZq coupling;
L36:   B(t -> Zu) < 6.6 x 10^-5 and B(t -> Zc) < 12 x 10^-5
L37:   for a right-handed coupling.
L38: 
L39: The same abstract reports corresponding 95% CL Wilson-coefficient limits

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

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Match snapshot line 22

L19: A search for the production of a single top quark in association with a Z
L20: boson is presented, both to identify the expected standard model process and
L21: to search for flavour-changing neutral current interactions. The data sample
L22: corresponds to an integrated luminosity of 19.7 inverse femtobarns recorded
L23: by the CMS experiment at the LHC in proton-proton collisions at sqrt(s) =
L24: 8 TeV. Final states with three leptons and at least one jet are investigated.
L25:

value < 0.049%

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Match snapshot line 34

L31: 
L32: Exclusion limits at 95% confidence level on the branching fractions of a top
L33: quark decaying to a Z boson and an up or a charm quark are found to be
L34: B(t -> Z u) < 0.022% and B(t -> Z c) < 0.049%.

value 1e-14

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Match snapshot line 25

L22: In the source file rev.tex, Table 1 gives branching ratios for top FCN decays
L23: in the SM and in several SM extensions. The table entries include:
L24:   t -> u Z: SM 8 x 10^-17
L25:   t -> c Z: SM 1 x 10^-14
L26:   t -> c Z: quark-singlet model 1.1 x 10^-4
L27:   t -> c Z: 2HDM approximately 10^-7
L28:   t -> c Z: FC 2HDM approximately 10^-10

value order 1e-14

RESOLVED

Match snapshot line 29

L26: production in association with a Z boson and in the decay of a top quark in
L27: ttbar production. Events with three leptons plus at least one jet, one b-tag,
L28: and missing transverse momentum are used. The SM predictions of the
L29: corresponding branching ratios are of the order of 10^-14.
L30: 
L31: PDG note 2:
L32: SIRUNYAN 17E is based on 19.7 fb^-1 of pp data at sqrt(s) = 8 TeV. The final

value approximately 1e-6

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Match snapshot line 27

L24: The abstract reports that LHC currently improves on the limit set by previous
L25: machines, including LEP indirect sensitivity to heavy vectors. Future direct
L26: searches at HL-LHC may reach a level equivalent to
L27: BR(t -> c Z) approximately 10^-6. It also studies sensitivity at a Higgs
L28: factory with E_cm in [200,240] GeV and at a high-energy muon collider with
L29: E_cm = 10 TeV.