Catalog · Collider RS · CR008

CR008 $pp \to T \bar{T} \,\text{(singlet)}\, \to tZ/tH/bW$

Pair production of singlet VLQ T (non-custodial)

Status REVIEWED VERIFIED High Code: NO Priority Low

PDG / equivalent values

Observable	Value	Year	Experiment / source	Provenance
$m_T$ lower limit for pair-produced isospin-singlet vector-like T quark	> 1.36 TeV 95% CL (lower_limit)	2024	ATLAS	source ↑
$m_T$ lower limit envelope for T $\bar{T}$ decays to third-generation quarks	> 1.48 TeV for all bW/tZ/tH third-generation decay branching patterns 95% CL (lower_limit)	2023	CMS	source ↑

Why this constrains the RS scan

In warped models, a charge-$2/3$ top partner is a direct handle on the fermionic top-sector resonance spectrum. A singlet $T$ search constrains a specific light-partner limit: QCD pair production is essentially set by $m_T$, while the search interpretation depends on the assumed standard decays $T\to bW,tZ,tH$ and on whether exotic or cascade decays are absent. For a non-custodial singlet top partner, the ATLAS benchmark maps to $m_T>1.36$ TeV at 95\% CL under the standard singlet branching ratios. This is not competitive with the anarchic-flavor mass reach already implied by the low-energy catalog. The quark-scan methodology note quotes the this repository headline crossing $M_{KK}^{\mathrm{min},p50}=47.26$ TeV at $g_*=3$. A direct singlet-top-partner bound near 1.4--1.5 TeV therefore works mainly as a collider cross-check, as leverage for non-anarchic or alignment-tuned RS variants, or as a constraint on scenarios where the lightest fermion partner is parametrically lighter than the gauge KK scale.

What's changed since the original paper

The post-2010 history is driven by the LHC moving from Run-1 branching-fraction triangles to full Run-2 multichannel combinations. The ATLAS 8 TeV lepton+jets search set $T$-mass lower limits over the $bW/tZ/tH$ plane at weaker Run-1 mass scales. CMS then used the first 13 TeV data with boosted jet-substructure techniques, and later combined single-lepton, same-sign dilepton, and multilepton channels to extend the reach across branching fractions. ATLAS full Run-2 searches added complementary lepton+jets and large-$E_T^{\rm miss}$ selections; the current singlet $bW/tH/tZ=1/2:1/4:1/4$ point is 1.36 TeV from the 2024 lepton+jets search, while the CMS full Run-2 leptonic search gives the stronger 1.48 TeV all-branching-envelope statement.

Validity and model dependence

The mass limits are collider exclusions, not generic RS bounds. They assume prompt pair-produced narrow vector-like quarks, standard decays to third-generation quarks plus $W/Z/H$, and no sizable exotic modes such as cascade decays through additional composite-sector states. Pair production is less coupling-dependent than single production, but the final-state acceptance is still branching-ratio dependent. A model with displaced decays, large widths, light neutral states, non-third-generation couplings, or a compressed spectrum would need a dedicated reinterpretation.

Code coverage in this repo

NO. Required greps over quarkConstraints/, qcd/, flavorConstraints/, neutrinos/, yukawa/, warpConfig/, solvers/, scanParams/, and tests/ found no VLQ, top-partner, ATLAS/CMS direct-search, or recast implementation. Adjacent evidence confirms the existing pipeline computes $M_{KK}$ and low-energy filters only: scanParams/scan.py:399 constructs $M_{KK}=\xi_{KK}\Lambda_{\rm IR}$, scanParams/scan.py:523 through scanParams/scan.py:535 applies the $\mu\to e\gamma$ filter, and quarkConstraints/scan.py:377-- quarkConstraints/scan.py:380 evaluates the quark $\Delta F=2$ constraints. There is no collider likelihood, cross-section limit, or branching-fraction-triangle filter.

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

Implementation difficulty

HIGH. A live constraint would require an external collider-recast layer: signal generation or tabulated cross sections, branching-ratio bookkeeping, acceptance and efficiency maps or full likelihoods, and validation against ATLAS/CMS exclusions. Practical implementation would likely use or interface to CheckMATE, MadAnalysis5, SModelS, Rivet/Contur, or collaboration HEPData likelihood material rather than a lightweight in-repo formula.

Reason: A live LHC-search constraint needs cross-section inputs, branching-fraction scans, detector acceptances or likelihoods, and external reinterpretation machinery such as CheckMATE, MadAnalysis5, SModelS, Rivet/Contur, or collaboration HEPData likelihood material.

Key references

PDG2025\_TprimeVLQ; ATLAS2024\_TsingletPair; CMS2023\_VLQPairLeptonic; ATLAS2015\_VLQPair8TeV; CMS2017\_VLQPairSingleLepton; CMS2018\_VLQPairLeptonic; ATLAS2023\_VLQPairMET; AguilarSaavedra2009\_TopPartners; AguilarSaavedraEtAl2013\_VLQHandbook; LariEtAl2008\_ColliderFlavourHighQ.

value_limit_converted_fallback > 1360 GeV

RESOLVED

Match snapshot line 26

L23: like $T$-quarks with 100% branching ratio to $Wb$ are excluded at 95% CL for masses
L24: below 1700 GeV. These limits are also applied to vector-like $Y$-quarks, which decay
L25: exclusively into a $W$ boson and a $b$-quark. Isospin singlets with $ {\cal B}(T \to
L26: Wb:Ht:Zt)={1/2}:{1/4}:{1/4}$ are excluded for masses below 1360 GeV.

display > 1.48 TeV for all bW/tZ/tH third-generation decay branching patterns

RESOLVED

Match snapshot line 28

L25: TeV, depending on the branching fractions assumed, with maximal sensitivity to decay
L26: modes that include multiple top quarks. The limits obtained in this search are the
L27: strongest limits to date for $\mathrm{T\overline{T}}$ production, excluding masses below
L28: 1.48 TeV for all decays to third generation quarks, and are the strongest limits to date
L29: for $\mathrm{B\overline{B}}$ production with B quark decays to tW.