Catalog · Top / Higgs / EW · T002

T002 $\mathcal{B}(t\to uZ)$

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

Status SUBTLETY-ADDED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
B(t $ \to $ Z u), left-handed t-u-Z benchmark	< 6.2e-5 branching fraction, 95% CL upper limit	2025	PDG2025:T002:tZu_left	source ↑
B(t $ \to $ Z u), right-handed t-u-Z benchmark	< 6.6e-5 branching fraction, 95% CL upper limit	2025	PDG2025:T002:tZu_right	source ↑
ATLAS dataset for tZq FCNC search	139 fb^-1 at sqrt(s) = 13 TeV integrated luminosity and collision energy	2023	ATLAS2023:T002:dataset	source ↑
B(t $ \to $ Z u), ATLAS left-handed t-u-Z benchmark	< 6.2e-5 branching fraction, 95% CL upper limit	2023	ATLAS2023:T002:tZu_left	source ↑
B(t $ \to $ Z u), ATLAS right-handed t-u-Z benchmark	< 6.6e-5 branching fraction, 95% CL upper limit	2023	ATLAS2023:T002:tZu_right	source ↑
CMS dataset for tZq FCNC search	19.7 fb^-1 at sqrt(s) = 8 TeV integrated luminosity and collision energy	2017	CMS2017:T002:dataset	source ↑
B(t $ \to $ Z u), CMS tZ-FCNC comparison limit	< 0.022% branching fraction, 95% CL upper limit	2017	CMS2017:T002:tZu	source ↑
Standard Model estimate for B(t $ \to $ u Z)	8e-17 Standard Model branching-fraction estimate	2004	AguilarSaavedra2004:T002:SM	source ↑
Minimal-RS scaling of t $ \to $ u Z relative to t $ \to $ c Z	$approximately two orders of magnitude below t \to c Z$ minimal-RS scaling statement	2008	Casagrande2008:T002:RS_suppression	source ↑
Generic warped-flavor KK-gluon lower-scale statement	about 21 TeV generic warped-flavor KK-gluon lower-scale statement	2008	CFW2008:T002:KK_gluon_scale	source ↑

Why this constrains the RS scan

In warped models with bulk fermions, non-universal zero-mode profiles and mass-basis rotations can induce flavor off-diagonal neutral-current $Z$ couplings. The $t\to uZ$ channel probes the first--third generation up-sector rotation and is therefore complementary to charm and neutral-meson constraints. In anarchic RS language it is sensitive to the same profile and Yukawa structures that control $u$-$c$-$t$ mixing, while remaining a direct top observable rather than a low-energy hadronic matrix-element constraint.

What's changed since the original paper

The CFW reference point is arXiv:0804.1954, which framed generic warped-flavor bounds before the full LHC top-FCNC program (CsakiFalkowskiWeiler2008WarpedFlavor). Shortly after it, the RS-specific Casagrande--Goertz--Haisch--Neubert--Pfoh analysis included $t\to c(u)Z$, noting that $t\to uZ$ is typically about two orders of magnitude below $t\to cZ$ in the minimal RS model (Casagrande2008:T002:RS\_suppression). Since then the key experimental development is flavor-separated LHC sensitivity. CMS used $19.7~\mathrm{fb}^{-1}$ at $\sqrt{s}=8~\mathrm{TeV}$ (CMS2017:T002:dataset) and found $\mathcal{B}(t\to Zu)<0.022\%$ (CMS2017:T002:tZu). ATLAS later used $139~\mathrm{fb}^{-1}$ at $\sqrt{s}=13~\mathrm{TeV}$ (ATLAS2023:T002:dataset), combining single-top $tZ$ production and $t\bar t$ FCNC-decay topologies, and improved the $t\to Zu$ reach to the $6\times10^{-5}$ level (ATLAS2023:T002:tZu\_left, ATLAS2023:T002:tZu\_right).

Validity and model dependence

Experimentally this is a clean 95\% CL upper limit; there is no Standard Model signal at the observable level. Aguilar-Saavedra's benchmark SM estimate is $\mathcal{B}(t\to uZ)\simeq 8\times10^{-17}$ (AguilarSaavedra2004:T002:SM). The interpretation is operator-dependent: ATLAS quotes left- and right-handed tensor-SMEFT benchmarks, while an RS model can generate vector-like $Ztu$ couplings, KK-mediated contact effects, or correlated shifts in other electroweak vertices. A live constraint therefore needs a matching convention before the branching-ratio limit can be used as a hard scan 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 $t\to uZ$, $tZu$, $tuZ$, $tZq$, and generic FCNC text across quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/. The only targeted code-directory hits were generic FCNC prose/comments, not an observable implementation, 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 $tuZ$ decay-width or single-production reinterpretation, and no top-sector flavor-rotation interface. Implementing it honestly would require new RS-to-SMEFT or direct $Ztu$ matching before comparing this limit 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 tuZ decay-width/production interpretation, and top-sector flavor-rotation plumbing.

Key references

PDG2025TopFCNCTZQ; ATLAS2023TopFCNCTZQ; CMS2017TZQFCNC; AguilarSaavedra2004TopFCNC; CsakiFalkowskiWeiler2008WarpedFlavor; CasagrandeGoertzHaischNeubertPfoh2008RSFlavor.

pdg_units 0.062

RESOLVED

Match snapshot line 15

L12: Test for Delta T = 1 weak neutral current. Allowed by higher-order electroweak interaction.
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

pdg_units 0.066

RESOLVED

Match snapshot line 17

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
L20: < 0.49, 95, SIRUNYAN 17E, CMS, t -> Z 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 < 6.2e-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 < 6.6e-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

RESOLVED

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 < 0.022%

RESOLVED

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 8e-17

RESOLVED

Match snapshot line 31

L28: corresponding charm modes by |V_ub / V_cb|^2 approximately 0.0079.
L29: 
L30: In its model-comparison table, the Standard Model entry for t -> u Z is
L31: 8 x 10^-17. The same table lists new-physics examples with much larger
L32: rates, e.g. models with extra quarks at 1.1 x 10^-4, MSSM examples around
L33: 2 x 10^-6, and R-parity-violating SUSY around 3 x 10^-5.

value about 21 TeV

RESOLVED

Match snapshot line 21

L18: The paper studies the flavor structure of 5D warped models that provide a
L19: dual description of a composite pseudo-Goldstone Higgs. It re-examines flavor
L20: constraints in standard Randall-Sundrum-type scenarios and finds that the KK
L21: gluon mass should generically be heavier than about 21 TeV. The authors find
L22: additional sources of flavor violation in the composite-Higgs setup, including
L23: kinetic mixing among SM fields, and conclude that fully anarchic warped flavor
L24: is strongly constrained unless partial flavor symmetries remove some sources