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| CMS-PAS-B2G-17-007 | ||
| Search for single production of a vector-like T quark decaying to a Z boson and a top quark in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| March 2017 | ||
| Abstract: A search for the single production of a vector-like quark, T, decaying into a Z boson and a top quark is presented. The Z boson decays leptonically while the top quark decays hadronically. The search is performed using data collected by the CMS experiment at the LHC in proton-proton collisions at $\sqrt{s} =$ 13 TeV in 2016, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The presence of jets produced in the forward region of the detector is a particular characteristic of the single production of vector-like quarks and is utilized in the analysis. Limits are set for different hypotheses, from negligible width to a width equal to 30% of the resonance mass. An additional production mode is also studied for the T quark, through the decay of a heavy Z' resonance that decays to Tt. The product of cross section and branching fraction above values in the range 0.27-0.04 pb is excluded at 95% confidence level for the range of resonance mass considered, which is between 0.7 and 1.7 TeV in the case of a T quark with negligible width. A similar sensitivity is observed for widths of up to 30% of the resonance mass. In the case that the T comes from the decay of a Z', limits on the product of cross section and branching fraction are set between 0.13 and 0.06 pb for Z' boson masses in the range from 1.5 to 2.5 TeV. | ||
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inSPIRE record ;
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These preliminary results are superseded in this paper, PLB 781 (2018) 574. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures & Tables | References | CMS Publications |
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| Figures | |
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Figure 1:
Leading order Feynman diagram for the production of a single T vector-like quark and its decay to a Z boson and a t quark, produced in association with a b quark (left) and for the production of a Z' boson decaying to Tt (right). |
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Figure 1-a:
Leading order Feynman diagram for the production of a single T vector-like quark and its decay to a Z boson and a t quark, produced in association with a b quark. |
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Figure 1-b:
Leading order Feynman diagram for the production of a Z' boson decaying to Tt. |
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Figure 2:
Comparison between the background estimate (as derived from control regions in the data) and data for the 2 categories where the T is reconstructed in the fully merged topology for events with the Z boson decaying into muons (left) and electrons (right). Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel in each plot shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 2-a:
Comparison between the background estimate (as derived from control regions in the data) and data for the 2 categories where the T is reconstructed in the fully merged topology for events with the Z boson decaying into muons. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 2-b:
Comparison between the background estimate (as derived from control regions in the data) and data for the 2 categories where the T is reconstructed in the fully merged topology for events with the Z boson decaying into electrons. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 3:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the partially merged topology for events with the Z boson decaying into muons (left) and electrons (right) and zero (at least one) forward jets on the top (bottom). Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel in each plot shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 3-a:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the partially merged topology for events with the Z boson decaying into muons and zero forward jet. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 3-b:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the partially merged topology for events with the Z boson decaying into electrons and zero forward jet. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 3-c:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the partially merged topology for events with the Z boson decaying into muons and at least one forward jet. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 3-d:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the partially merged topology for events with the Z boson decaying into electrons and at least one forward jet. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 4:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the resolved topology for events with the Z boson decaying into muons (left) and electrons (right) and zero (at least one) forward jets on the top (bottom). Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel in each plot shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 4-a:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the resolved topology for events with the Z boson decaying into muons and zero forward jet. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 4-b:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the resolved topology for events with the Z boson decaying into electrons and zero forward jet. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 4-c:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the resolved topology for events with the Z boson decaying into muons and at least one forward jet. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 4-d:
Comparison between the background estimate (as derived from control regions in the data) and data for the 4 categories where the T is reconstructed in the resolved topology for events with the Z boson decaying into electrons and at least one forward jet. Background composition is taken from simulation. The uncertainties in the background estimate method include both statistical and systematic components, as described in Section 6. The lower panel shows the ratio of the data and the background estimation, with the shaded band representing the uncertainties in the background estimate. |
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Figure 5:
Observed and expected 95% CL upper limit on the product of cross section and branching fraction for the singlet LH T(b) (left) and doublet RH T(t) (right) production modes, with the T decaying to tZ, where the T has a negligible width. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at next-to-leading order in Ref. [4] are shown. The branching fraction $\mathcal {B}(\mathrm{T\rightarrow tZ})$ is 0.25 (0.5) for the left (right) plot. |
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Figure 5-a:
Observed and expected 95% CL upper limit on the product of cross section and branching fraction for the singlet LH T(b) production modes, with the T deaying to tZ, where the T has a negligible width. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at next-to-leading order in Ref. [4] are shown. The branching fraction $\mathcal {B}(\mathrm{T\rightarrow tZ})$ is 0.25 (0.5) for the left (right) plot. |
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Figure 5-b:
Observed and expected 95% CL upper limit on the product of cross section and branching fraction for the doublet RH T(t) production modes, with the T deaying to tZ, where the T has a negligible width. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at next-to-leading order in Ref. [4] are shown. The branching fraction $\mathcal {B}(\mathrm{T\rightarrow tZ})$ is 0.25 (0.5) for the left (right) plot. |
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Figure 6:
Observed and expected 95% CL upper limit on the product of cross section and branching fraction for the singlet LH T(b) (left) and doublet RH T(t) (right) production modes, with the T decaying to tZ, where the T has a width of 10%, 20% and 30% of the resonance mass. Theoretical cross sections have been calculated at leading order using a private version of the model constructed by the authors of [5,31,32] and are reported in Table 2. |
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Figure 6-a:
Observed and expected 95% CL upper limit on the product of cross section and branching fraction for the singlet LH T(b) production modes, with the T decaying to tZ, where the T has a width of 10%, 20% and 30% of the resonance mass. Theoretical cross sections have been calculated at leading order using a private version of the model constructed by the authors of [5,31,32] and are reported in Table 2. |
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Figure 6-b:
Observed and expected 95% CL upper limit on the product of cross section and branching fraction for the doublet RH T(t) production modes, with the T decaying to tZ, where the T has a width of 10%, 20% and 30% of the resonance mass. Theoretical cross sections have been calculated at leading order using a private version of the model constructed by the authors of [5,31,32] and are reported in Table 2. |
| Tables | |
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Table 1:
Theoretical cross sections for T(b) and T(t) processes for the different benchmark mass points considered in the analysis, with the couplings set to 0.5 and a narrow width as calculated at next-to-leading order in Ref. [4]. The cross sections do not depend on the chirality of the new particle. For any value of the couplings below 0.5, the theoretical width of the VLQ is negligible compared to the experimental mass resolution. |
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Table 2:
Theoretical cross sections $\tilde{\sigma }_{FW}$ for T(b) and T(t) processes for the different benchmark mass points considered in the analysis for a T width of 10%, 20% and 30% of the resonance mass. In parentheses the leading order cross sections $\sigma $ are shown. |
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Table 3:
The numbers of estimated background events compared to the measured numbers of events for the two fully merged categories. The quoted uncertainties in the background estimates include both statistical and systematic components, as described in Section 6. Expected signal yields and signal efficiencies in parentheses, defined considering only events with the Z boson decaying to electrons or muons, are also shown for two benchmark mass points and two width ("w'') points for both T(b) and T(t) processes. |
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Table 4:
The numbers of estimated background events compared to the measured numbers of events for the four partially merged categories. The quoted uncertainties in the background estimates include both statistical and systematic components, as described in Section 6. Expected signal yields and signal efficiencies in parentheses, defined considering only events with the Z boson decaying to electrons or muons are also shown for two benchmark mass points and two width (``w'') points for both T(b) and T(t) processes. |
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Table 5:
The numbers of estimated background events compared to the measured numbers of events for the four resolved categories. The quoted uncertainties in the background estimates include both statistical and systematic components, as described in Section 6. Expected signal yields and signal efficiencies in parentheses, defined considering only events with the Z boson decaying to electrons or muons are also shown for two benchmark mass points and two width (``w'') points for both T(b) and T(t) processes. |
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Table 6:
Observed and expected 95% CL upper limit on $\sigma (\mathrm{ pp \rightarrow Z'}) \mathcal {B}(\mathrm{ Z'\rightarrow Tt}) \mathcal {B} (\mathrm{ T \rightarrow tZ}) $. The branching fraction $\mathcal {B}(\mathrm{T \rightarrow tZ})$ is taken to be 100%. In order to consider different branching fractions, the limits should be scaled by the corresponding branching fraction value. The 1 and 2 standard deviation bands are given. |
| Summary |
| Results of a search for single production of a T quark with a charge of $+2/3$ decaying to a Z boson and a top quark have been presented. No deviations from the expected standard model background are observed. Limits on the product of the cross section and branching fraction vary between 0.27 and 0.04 pb at 95% confidence level for a left-handed T(b) with the T quark decaying to tZ, and between 0.15 and 0.04 pb for a right-handed T(t) signal, for the ranges of resonance masses considered, which is between 0.7 and 1.7 TeV. This result has been obtained under the hypothesis of a narrow width resonance, allowing the interpretation of results using the simplified approach studied in [4]. In this case, left-handed T quarks produced in association with a b quark and with a coupling C(bW) set to 0.5 are excluded below the mass of 1.2 TeV. The effect of a non-negligible width is also studied by considering values of the width to be 10%, 20% and 30% of the resonance mass; a similar sensitivity is observed. In this case the results have been interpreted using a private version of the model constructed by the authors of [5,31,32] and a left-handed T(b) signal is excluded for masses below values in the range 1.35 to 1.45 TeV depending on the width, while a right-handed T(t) signal is excluded for masses below values in the range 0.85 to 0.95 TeV. Finally, the production of a Z' that decays to Tt is excluded for products of production cross sections and branching fractions below values in the range 0.13-0.06 pb for a Z' (T) mass range of 1.5 to 2.5 (0.7 to 1.5) TeV. |
| Additional Figures | |
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Additional Figure 1:
Reconstructed T mass after full event selection for $\mathrm{ pp \rightarrow Tb }$ signal, with T mass equal to 1.2 TeV and four values of the T width: negligible, 10%, 20% and 30%. |
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Additional Figure 2:
Comparison between background and signal for the $\Delta $R between the two leptons after event preselection. Signal is normalized to background yield. |
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Additional Figure 3:
Comparison between background and signal for the leading lepton ${p_{\mathrm {T}}}$ after event preselection. Signal is normalized to background yield. |
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Additional Figure 4:
Comparison between background and signal for the number of b-tagged jets (medium working point) after event preselection. Signal is normalized to background yield. |
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Additional Figure 5:
Comparison between background and signal for the number of forward jets after event preselection. Signal is normalized to background yield. |
| Additional Tables | |
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Additional Table 1:
Summary of the final event selection. In each category exactly two oppositely charged leptons are required. |
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Compact Muon Solenoid LHC, CERN |
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