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| CMS-B2G-16-001 ; CERN-EP-2016-326 | ||
| Search for single production of vector-like quarks decaying to a Z boson and a top or a bottom quark in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 25 January 2017 | ||
| JHEP 05 (2017) 029 | ||
| Abstract: A search for single production of vector-like quarks, T and B, decaying into a Z boson and a top or a bottom quark, respectively, is presented. The search is performed using data collected by the CMS experiment at the LHC in proton-proton collisions at $\sqrt{s}=$ 13 TeV, corresponding to an integrated luminosity of 2.3 fb$^{-1}$. An exotic T quark production mode through the decay of a heavy Z' resonance is also considered. The search is performed in events with a Z boson decaying leptonically, accompanied by a bottom or a top quark decaying hadronically. No excess of events is observed over the standard model background expectation. Products of production cross section and branching fraction for T and B quarks from 1.26 and 0.13 pb are excluded at 95% confidence level for the range of resonance mass considered, which is between 0.7 and 1.7 TeV. Limits on the product of the Z' boson production cross section and branching fraction, with the Z' boson decaying to the Tt final state, are set between 0.31 and 0.13 pb, for Z' boson masses in the range from 1.5 to 2.5 TeV. This is the first search at 13 TeV for single production of vector-like quarks in events with a Z boson decaying leptonically accompanied by boosted jets. | ||
| Links: e-print arXiv:1701.07409 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading order Feynman diagrams for the production of a single T (B) vector-like quark and its decay to a Z boson and a t (b) quark on the left (center) and production of a Z' boson decaying to Tt on the 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. |
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Figure 1-b:
Leading order Feynman diagram for the production of a single B vector-like quark and its decay to a Z boson and a b quark. |
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Figure 1-c:
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 and data for the T categories: fully merged region (upper-left), partially merged region (upper right), and resolved region (lower) for events with the Z boson decaying into muons (left) and electrons (right). For the fully and partially merged topologies, the sets of events with the Z boson decaying to muons and electrons are combined. For the fully merged region a shape analysis is not performed because of the small number of events, and a single bin is shown. 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 and data for the fully merged region. The sets of events with the Z boson decaying to muons and electrons are combined. A shape analysis is not performed because of the small number of events, and a single bin is shown. 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 and data for the partially merged region. The sets of events with the Z boson decaying to muons and electrons are combined. 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-c:
Comparison between the background estimate and data for the resolved region for events with the Z boson decaying into muons. 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-d:
Comparison between the background estimate and data for the resolved region for events with the Z boson decaying into electrons. 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 and data for the B search categories: events with the Z boson decaying to muons (left) and to electrons (right). The uncertainties in the background estimate 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 and data for the B search categories: events with the Z boson decaying to muons. The uncertainties in the background estimate 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 and data for the B search categories: events with the Z boson decaying to electrons. The uncertainties in the background estimate 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:
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. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at NLO in Ref. [12] are shown. The branching fraction $\mathcal {B} (\mathrm{ T\to tZ} )$ is 0.25 (0.5) for the left (right) plot. |
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Figure 4-a:
Observed and expected 95% CL upper limit on the product of cross section and branching fraction for the singlet LH T(b) production mode, with the T decaying to tZ. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at NLO in Ref. [12] are shown. The branching fraction $\mathcal {B} (\mathrm{ T\to tZ} )$ is taken as 0.25. |
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Figure 4-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. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at NLO in Ref. [12] are shown. The branching fraction $\mathcal {B} (\mathrm{ T\to tZ} )$ is taken as 0.5. |
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Figure 5:
Observed and expected 95% CL upper limit on the product of cross section and branching fraction for the B(t) (left) and B(b) (right) signals in the singlet LH scenario, with the B decaying to bZ. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at NLO in Ref. [12] are shown. The branching fraction $\mathcal {B}(\mathrm{B} \to \mathrm{ b } {\mathrm{ Z } } )$ is 0.25. |
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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 B(t) signal in the singlet LH scenario, with the B decaying to bZ. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at NLO in Ref. [12] are shown. The branching fraction $\mathcal {B}(\mathrm{B} \to \mathrm{ b } {\mathrm{ Z } } )$ is 0.25. |
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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 B(b) signal in the singlet LH scenario, with the B decaying to bZ. The 68% and 95% expected bands are shown. Theoretical cross sections as calculated at NLO in Ref. [12] are shown. The branching fraction $\mathcal {B}(\mathrm{B} \to \mathrm{ b } {\mathrm{ Z } } )$ is 0.25. |
| Tables | |
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Table 1:
Theoretical cross sections for T(b), B(t), B(b), and T(t) processes for the different benchmark mass points considered in the analysis, with the couplings set to 0.5 as calculated at NLO in Ref. [12]. Cross sections do not depend on the chirality of the new particle (T or B). |
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Table 2:
Summary of the final event selection for the four categories of the T search. In each category exactly two oppositely charged leptons are required. |
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Table 3:
Summary of the final event selection for the two categories of the B search. In each category exactly two oppositely charged leptons are required. |
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Table 4:
The numbers of estimated background events compared to the measured numbers of events for the four categories of the T search. 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) are also shown for three benchmark mass points. |
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Table 5:
The numbers of estimated background events compared to the measured numbers of events for the two categories of the B search. 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) are also shown for three benchmark mass points. |
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Table 6:
Observed and expected 95% CL upper limit on $\sigma \mathcal {B}$ for the $\mathrm{ Z }' \to {\mathrm {T}} \mathrm{ t } $ signal. The branching fraction $\mathcal {B}( {\mathrm {T}} \to \mathrm{ t } {\mathrm{ Z } } )$ 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 and of a search for single production of a B quark with a charge of $-1/3$ decaying to a b quark and a Z boson have been presented. No deviations from the expected standard model background are observed. Limits on the product of the cross section and branching fraction for a left-handed T(b), with the T quark decaying to tZ, vary between 0.98 and 0.15 pb at 95% confidence level and between 0.60 and 0.13 pb for a right-handed T(t) signal, for the range of resonance mass considered, which is between 0.7 and 1.7 TeV. For a left-handed B quark produced in association with a top quark and decaying to bZ, products of the cross section and branching fraction between 0.68 and 0.15 pb are excluded in the same mass range, while for a B quark produced in association with a bottom quark, products of the cross section and branching fraction between 1.26 and 0.28 pb are excluded. Additionally, products of the cross section and branching fraction for T quarks from the decay $\mathrm{ Z }'\to\mathrm{ T }\mathrm{ t }$ are excluded between 0.31 and 0.13 pb, for the range of Z' (T) mass considered, which is between 1.5 to 2.5 (0.7 to 1.5) TeV. This is the first search at 13 TeV for single production of vector-like quarks in events with a Z boson decaying leptonically accompanied by boosted jets. |
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Compact Muon Solenoid LHC, CERN |
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