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Search for single production of vector-like quarks decaying into $W(\ellν)b$ in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

ATLAS Collaboration

TL;DR

This work targets single VLQ production in $pp$ collisions at $\sqrt{s}=13~\mathrm{TeV}$ using ATLAS Run 2 data, focusing on $Q= T$ or $Y$ decaying to $Wb$ with leptonic $W$ decays. It implements a sophisticated event selection with SRs/CRs/VRs and reconstructs a VLQ candidate via $m_{\mathrm{VLQ}}$, incorporating interference with SM backgrounds in a binned likelihood fit. The analysis sets 95% CL limits on the coupling $\kappa$ across VLQ masses, with $T$-singlet limits $0.22$–$0.52$ for $m_T=1150$–$2300$ GeV and $(T,B,Y)$ triplet limits $0.14$–$0.46$ for $m_Y=1150$–$2600$ GeV, including theory cross-sections and finite-width corrections. No significant deviation from the SM is observed, and the results complement ATLAS’s fully hadronic VLQ searches, informing future combinations and extending the parameter space probed for VLQs at the LHC.

Abstract

A search for single production of a vector-like quark $Q$, which could be either a singlet $T$, with charge $\tfrac23$, or a $Y$ from a $(T,B,Y)$ triplet, with charge $-\tfrac43$, is performed using data from proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to the full integrated luminosity of 140 fb$^{-1}$ recorded with the ATLAS detector during Run~2 of the Large Hadron Collider. The analysis targets $Q \to Wb$ decays where the $W$ boson decays leptonically. The data are found to be consistent with the expected Standard Model background, so upper limits are set on the cross-section times branching ratio, and on the coupling of the $Q$ to the Standard Model sector for these two benchmark models. Effects of interference with the Standard Model background are taken into account. For the singlet $T$, the 95% confidence level limit on the coupling strength $κ$ ranges between 0.22 and 0.52 for masses from 1150 to 2300 GeV. For the $(T,B,Y)$ triplet, the limits on $κ$ vary from 0.14 to 0.46 for masses from 1150 to 2600 GeV.

Search for single production of vector-like quarks decaying into $W(\ellν)b$ in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

TL;DR

This work targets single VLQ production in collisions at using ATLAS Run 2 data, focusing on or decaying to with leptonic decays. It implements a sophisticated event selection with SRs/CRs/VRs and reconstructs a VLQ candidate via , incorporating interference with SM backgrounds in a binned likelihood fit. The analysis sets 95% CL limits on the coupling across VLQ masses, with -singlet limits – for – GeV and triplet limits – for – GeV, including theory cross-sections and finite-width corrections. No significant deviation from the SM is observed, and the results complement ATLAS’s fully hadronic VLQ searches, informing future combinations and extending the parameter space probed for VLQs at the LHC.

Abstract

A search for single production of a vector-like quark , which could be either a singlet , with charge , or a from a triplet, with charge , is performed using data from proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to the full integrated luminosity of 140 fb recorded with the ATLAS detector during Run~2 of the Large Hadron Collider. The analysis targets decays where the boson decays leptonically. The data are found to be consistent with the expected Standard Model background, so upper limits are set on the cross-section times branching ratio, and on the coupling of the to the Standard Model sector for these two benchmark models. Effects of interference with the Standard Model background are taken into account. For the singlet , the 95% confidence level limit on the coupling strength ranges between 0.22 and 0.52 for masses from 1150 to 2300 GeV. For the triplet, the limits on vary from 0.14 to 0.46 for masses from 1150 to 2600 GeV.

Paper Structure

This paper contains 30 sections, 3 equations, 6 figures, 8 tables.

Table of Contents

  1. Introduction
  2. VLQ phenomenology
  3. Previous studies
  4. Analysis overview
  5. ATLAS detector
  6. Background and signal simulation
  7. Simulated signal samples
  8. Simulated background samples
  9. Object definitions
  10. Lepton selection
  11. Jet selection
  12. Jet flavour-tagging and selection
  13. Overlap removal
  14. definition and $W$-boson reconstruction
  15. Reconstruction of the VLQ candidate
  16. ...and 15 more sections

Figures (6)

  • Figure 1: Leading-order Feynman diagram for single $Y$/$T$ production in $Wb$ fusion and subsequent decay into $Wb$.
  • Figure 2: Pre-fit VLQ mass distribution in the signal region. The signal shown here is for a $T/Y$ VLQ with $\kappa = 0.5$ and a mass of 1600 $\text{Ge V}$, and is presented multiplying by ten its expected cross-section for clarity. "Other" includes diboson and other final states containing a top quark. The error band contains the statistical and systematic uncertainties added in quadrature. Weight corrections are applied to the and $W\text{+jets}$ MC samples (see Section \ref{['sec:reweighting']}).
  • Figure 3: $m_\mathrm{VLQ}\xspace\xspace$ distribution in the CR \ref{['tt_CR2:NoRW_reweight_mVLQ_B']} before and \ref{['tt_CR2:FinalRW_reweight_mVLQ_B']} after reweighting. Shaded bands include statistical and systematic uncertainties, but not those related to the reweighting procedure. The last bin includes overflow.
  • Figure 4: Leading-jet distribution in the CR \ref{['tt_CR2:NoRW_reweight_leadjetpt']} before and \ref{['tt_CR2:FinalRW_reweight_leadjetpt']} after reweighting. Shaded bands include statistical and systematic uncertainties, but not those related to the reweighting procedure. The last bin includes overflow.
  • Figure 5: Distributions of the VLQ-candidate mass, $m_\mathrm{VLQ}\xspace$, in the (a--c) SRs, (d--f) $W\text{+jets}$ CRs and (g--i) CRs after the fit to the background-only hypothesis. The columns correspond from left to right to the low-, middle-, and high-$p_{\mathrm{T}}^W$ bins in each region. "Other" includes remaining backgrounds from top quarks or that contain two $W/Z$ bosons. The last bin includes overflow. The lower panels show the ratios of data to the fitted total prediction. The error bars represent the statistical uncertainty of the data. The band represents the total systematic uncertainty after the maximum-likelihood fit. The dashed line represents the data-to-background ratio before the fit. The worst agreement is in the low-$p_{\mathrm{T}}^W$ subregion of the $W\text{+jets}$ CR, where the $\chi^2$ probabilities before and after the fit are 0.018 and 0.57, respectively.
  • ...and 1 more figures