A combination of searches for the single production of vector-like top quarks ($T$) is presented. These analyses are based on proton$-$proton collisions at $\sqrt{s}=13$ TeV recorded in 2015$-$2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb$^{-1}$. The $T$-quark decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a $Z$ boson ($T \to Ht$ and $T \to Zt$). The individual searches used in the combination are differentiated by the number of leptons ($e$, $\mu$) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vector-like top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vector-like top quark SU2 singlet with electroweak coupling, $\kappa$, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.
Observed and expected 95% CL upper limits on the total cross-section σ($pp$ → $T$ → $Ht/Zt$) as a function of $T$-quark mass in the SU(2) singlet representation assuming $\kappa$=0.3. The expected limits for the individual analyses are shown. The $HtZt$ analysis is only included in the limit calculation for $m_{\mathrm{T}}$ < 2.1 TeV.
Observed and expected 95% CL upper limits on the total cross-section σ($pp$ → $T$ → $Ht/Zt$) as a function of $T$-quark mass in the SU(2) singlet representation assuming $\kappa$=0.5. The expected limits for the individual analyses are shown. The $HtZt$ analysis is only included in the limit calculation for $m_{\mathrm{T}}$ < 2.1 TeV.
Observed and expected 95% CL upper limits on the total cross-section σ($pp$ → $T$ → $Ht/Zt$) as a function of $T$-quark mass in the SU(2) doublet representation assuming $\kappa$=0.3. The expected limits for the individual analyses are shown. The $HtZt$ analysis is only included in the limit calculation for $m_{\mathrm{T}}$ < 2.1 TeV.
A search for the pair-production of vector-like quarks optimized for decays into a $Z$ boson and a third-generation Standard Model quark is presented, using the full Run 2 dataset corresponding to 139 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=13$ TeV, collected in 2015-2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a $Z$ boson with high transverse momentum, reconstructed from a pair of same-flavour leptons with opposite-sign charges, as well as by the presence of $b$-tagged jets and high-transverse-momentum large-radius jets reconstructed from calibrated smaller-radius jets. Events with exactly two or at least three leptons are used, which are further categorized by the presence of boosted $W$, $Z$, and Higgs bosons and top quarks. The categorization is performed using a neural-network-based boosted object tagger to enhance the sensitivity to signal relative to the background. No significant excess above the background expectation is observed and exclusion limits at 95% confidence level are set on the masses of the vector-like partners $T$ and $B$ of the top and bottom quarks, respectively. In the singlet model, the limits allow $m_T > 1.27$ TeV and $m_B > 1.20$ TeV. In the doublet model, allowed masses are $m_T > 1.46$ TeV and $m_B >1.32$ TeV. In the case of 100% branching ratio for $T\rightarrow Zt$ and 100% branching ratio for $B\rightarrow Zb$, the limits allow $m_T > 1.60$ TeV and $m_B > 1.42$ TeV, respectively.
Expected and observed lower limits on B masses at 95% CL in the BR plane from the combination of the two analysis channels for all BR configurations when assuming a total BR of 100% for H, W and Z.
Expected and observed lower limits on T masses at 95% CL in the BR plane from the combination of the two analysis channels for all BR configurations when assuming a total BR of 100% for H, W and Z.
Expected and observed combined limits at 95% CL on the production cross-section of vector-like T for 100% $T\rightarrow Zt$.
A search for the pair production of heavy vector-like partners T and B of the top and bottom quarks has been performed by the CMS experiment at the CERN LHC using proton-proton collisions at $\sqrt{s} =$ 13 TeV. The data sample was collected in 2016 and corresponds to an integrated luminosity of 35.9 fb$^{-1}$. Final states studied for $\mathrm{T\overline{T}}$ production include those where one of the T quarks decays via T $\to$ tZ and the other via T $\to$ bW, tZ, or tH, where H is a Higgs boson. For the $\mathrm{B\overline{B}}$ case, final states include those where one of the B quarks decays via B $\to$ bZ and the other B $\to$ tW, bZ, or bH. Events with two oppositely charged electrons or muons, consistent with coming from the decay of a Z boson, and jets are investigated. The number of observed events is consistent with standard model background estimations. Lower limits at 95% confidence level are placed on the masses of the T and B quarks for a range of branching fractions. Assuming 100% branching fractions for T $\to$ tZ, and B $\to$ bZ, T and B quark mass values below 1280 and 1130 GeV, respectively, are excluded.
The $S_{\rm T}$ distribution for group A before fitting.
The $S_{\rm T}$ distribution for group B before fitting.
The $S_{\rm T}$ distribution for group C before fitting.
A search is presented for the pair production of heavy vector-like $T$ and $B$ quarks, primarily targeting the $T$ quark decays to a $W$ boson and a $b$-quark. The search is based on $36.1$ fb$^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton-plus-jets final state, including at least one $b$-tagged jet and a large-radius jet identified as originating from the hadronic decay of a high-momentum $W$ boson. No significant deviation from the Standard Model expectation is observed in the reconstructed $T$ mass distribution. The observed 95% confidence level lower limit on the $T$ mass are 1350 GeV assuming 100% branching ratio to $Wb$. In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. This search is also sensitive to a heavy vector-like $B$ quark decaying to $Wt$ and other final states. The results are thus reinterpreted to provide a 95% confidence level lower limit on the $B$ quark mass at 1250 GeV assuming 100% branching ratio to $Wt$; in the SU(2) singlet scenario, the limit is 1080 GeV. Mass limits on both $T$ and $B$ production are also set as a function of the decay branching ratios. The 100% branching ratio limits are found to be applicable to heavy vector-like $Y$ and $X$ production that decay to $Wb$ and $Wt$, respectively.
The leptonic VLQ candidate mass distributions in the signal region after the maximum likelihood fit in the signal region and control region.
Expected and observed upper limits at the 95% CL on the TT cross section as a function of T quark mass under the assumption of BR(T->Wb)=1.
Expected and observed upper limits at the 95% CL on the TT cross section as a function of T quark mass for an SU(2) singlet T.