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| CMS-PAS-B2G-16-011 | ||
| Search for vector-like quark pair production in final states with leptons and boosted Higgs bosons at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| September 2016 | ||
| Abstract: We present a search for pair-produced vector-like top partners (``T quark'') using data from pp collisions at a center-of-mass energy of $\sqrt{s}= $ 13 TeV that were recorded with the CMS detector in the 2015 data-taking period. The search is carried out in the lepton+jets channel and is most sensitive for final states in which at least one T quark decays to a top quark and a Higgs boson. Since the final decay products tend to be collimated in the detector, jet-substructure techniques are used to identify boosted Higgs boson decays to $\mathrm{ b\overline{b} }$. No deviation from the Standard Model (SM) background prediction is observed in the data and upper 95% confidence level (CL) exclusion limits on the $\mathrm{T\bar{T}}$ production cross section are calculated for multiple branching fraction scenarios of the T quark. Assuming a branching fraction of 100% for the $\mathrm{T}\rightarrow \mathrm{tH}$ decay, T quark masses below 860 GeV (870 GeV expected) can be excluded. | ||
| Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Representative Feynman diagram for production of a $\mathrm {T\bar{T}}$pair with one T quark decaying to tH. |
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Figure 2:
Left: Distribution of the number of b-tagged subjets of the ${p_{\mathrm {T}}} $-leading Higgs candidate jet ($ {p_{\mathrm {T}}} > $ 300 GeV, $M_{\text{jet}} \in $ [60, 160 GeV]) without the subjet b-tag requirement. Right: Distribution of $ M_{text{jet}} $ of the ${p_{\mathrm {T}}} $-leading Higgs candidate jet ($ {p_{\mathrm {T}}} > $ 300 GeV) with at least two subjet b-tags before applying the mass cut. Both distributions are shown after the event selection described in Sec. 5.1 and all corrections described in Sec. 6 are applied to the MC. For the signal, the $ M_{\mathrm{T}} = $ 1200 GeV mass point is shown and the distributions are normalized to the corresponding theory cross section times the number behind the legend entry. A branching fraction of 33% to all three decay channels is assumed. The blue curve contains $\mathrm {T\bar{T}}$ events with at least one Higgs boson in the decay chain, the red curve shows $\mathrm {T\bar{T}}$ events where this is not the case. |
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Figure 2-a:
Distribution of the number of b-tagged subjets of the ${p_{\mathrm {T}}} $-leading Higgs candidate jet ($ {p_{\mathrm {T}}} > $ 300 GeV, $M_{\text{jet}} \in $ [60, 160 GeV]) without the subjet b-tag requirement. The distribution is shown after the event selection described in Sec. 5.1 and all corrections described in Sec. 6 are applied to the MC. For the signal, the $ M_{\mathrm{T}} = $ 1200 GeV mass point is shown and the distribution is normalized to the corresponding theory cross section times the number behind the legend entry. A branching fraction of 33% to all three decay channels is assumed. The blue curve contains $\mathrm {T\bar{T}}$ events with at least one Higgs boson in the decay chain, the red curve shows $\mathrm {T\bar{T}}$ events where this is not the case. |
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Figure 2-b:
Distribution of $ M_{text{jet}} $ of the ${p_{\mathrm {T}}} $-leading Higgs candidate jet ($ {p_{\mathrm {T}}} > $ 300 GeV) with at least two subjet b-tags before applying the mass cut. The distribution is shown after the event selection described in Sec. 5.1 and all corrections described in Sec. 6 are applied to the MC. For the signal, the $ M_{\mathrm{T}} = $ 1200 GeV mass point is shown and the distribution is normalized to the corresponding theory cross section times the number behind the legend entry. A branching fraction of 33% to all three decay channels is assumed. The blue curve contains $\mathrm {T\bar{T}}$ events with at least one Higgs boson in the decay chain, the red curve shows $\mathrm {T\bar{T}}$ events where this is not the case. |
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Figure 3:
Post-fit $S_{\mathrm {T}}$ distributions in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ (left) and W+jets (right) control regions after applying all corrections and performing the maximum-likelihood fit described in the main text. The $\mathrm {T\bar{T}}$ signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. |
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Figure 3-a:
Post-fit $S_{\mathrm {T}}$ distribution in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ control region after applying all corrections and performing the maximum-likelihood fit described in the main text. The $\mathrm {T\bar{T}}$ signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. |
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Figure 3-b:
Post-fit $S_{\mathrm {T}}$ distribution in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ W+jets control region after applying all corrections and performing the maximum-likelihood fit described in the main text. The $\mathrm {T\bar{T}}$ signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. |
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Figure 4:
Post-fit distributions of $S_{\mathrm {T}}$ in the 0H (top left), H1b (top right) and H2b (bottom) category after performing the maximum-likelihood fit described in Sec. 6. Signal samples are normalized to a cross section of 5 pb in the 0H and H1b categories and 1 pb in the H2b category. The $ \mathrm {T\bar{T}} $signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. |
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Figure 4-a:
Post-fit distribution of $S_{\mathrm {T}}$ in the 0H category after performing the maximum-likelihood fit described in Sec. 6. Signal samples are normalized to a cross section of 5 pb in the 0H and H1b categories and 1 pb in the H2b category. The $ \mathrm {T\bar{T}} $signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. |
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Figure 4-b:
Post-fit distribution of $S_{\mathrm {T}}$ in the H1b category after performing the maximum-likelihood fit described in Sec. 6. Signal samples are normalized to a cross section of 5 pb in the 0H and H1b categories and 1 pb in the H2b category. The $ \mathrm {T\bar{T}} $signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. |
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Figure 4-c:
Post-fit distribution of $S_{\mathrm {T}}$ in the H2b category after performing the maximum-likelihood fit described in Sec. 6. Signal samples are normalized to a cross section of 5 pb in the 0H and H1b categories and 1 pb in the H2b category. The $ \mathrm {T\bar{T}} $signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. |
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Figure 5:
Exclusion limits on the total cross-section of pair-produced T's with a branching fraction of 100% to tH. The theory cross section (dashed line) is computed at next-to-next-to-leading order. |
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Figure 6:
Expected (left) and observed (right) upper mass limits in GeV for different combinations of $ \mathrm{ T \rightarrow tH } $ and $ \mathrm{ T \rightarrow tZ } $ branching fractions. The branching fraction $ \mathrm{ T \rightarrow bW } $ is, for each point in the triangle, 1 $-$ BR($ \mathrm{ T \rightarrow tH } $) $-$ BR($ \mathrm{ T \rightarrow tZ } $). |
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Figure 6-a:
Expected upper mass limit in GeV for different combinations of $ \mathrm{ T \rightarrow tH } $ and $ \mathrm{ T \rightarrow tZ } $ branching fractions. The branching fraction $ \mathrm{ T \rightarrow bW } $ is, for each point in the triangle, 1 $-$ BR($ \mathrm{ T \rightarrow tH } $) $-$ BR($ \mathrm{ T \rightarrow tZ } $). |
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Figure 6-b:
Observed upper mass limit in GeV for different combinations of $ \mathrm{ T \rightarrow tH } $ and $ \mathrm{ T \rightarrow tZ } $ branching fractions. The branching fraction $ \mathrm{ T \rightarrow bW } $ is, for each point in the triangle, 1 $-$ BR($ \mathrm{ T \rightarrow tH } $) $-$ BR($ \mathrm{ T \rightarrow tZ } $). |
| Tables | |
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Table 1:
Signal efficiencies in the three event categories for two example mass points, split into the six possible final states. Uncertainties include both systematic and statistical uncertainties on the MC samples. |
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Table 2:
Summary of all systematic uncertainties, their sizes, types and to which processes they apply. |
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Table 3:
Event counts in the three final categories after performing the maximum-likelihood fit described in Sec. 6. Uncertainties comprise both statistical and systematic uncertainties. For the $\mathrm {T\bar{T}}$signal, the theoretically predicted production cross section with a branching fraction of 100% $ \mathrm{ T \rightarrow tH } $ is assumed. |
| Summary |
| We present a search for pair-produced vector-like T quarks analyzing data from pp collisions at a center-of-mass energy of $ \sqrt{s} = $ 13 TeV. The data were recorded by the CMS detector during the 2015 data-taking period and correspond to integrated luminosities of 2.6 fb$^{-1}$ and 2.7 fb$^{-1}$ in the electron and muon channel, respectively. The analysis requires at least one lepton in the final state and is optimized for the case that at least one of the T quarks decays to a Higgs boson and a top quark where the Higgs boson decays to $\mathrm{ b \bar{b} } $. Events are selected using substructure techniques to identify boosted Higgs bosons and the statistical interpretation of the results is conducted using $ S_{\mathrm{T}} $ as final discriminating variable. No excess above the Standard Model background is observed and upper 95% CL exclusion limits on the cross section of $ \mathrm{ T \bar{T} } $ production are calculated for various branching fraction scenarios. For a branching fraction of 100% $ \mathrm{ T \to tH } $, T quarks with masses up to 860 GeV can be excluded (870 GeV expected) which already exceeds the exclusion limits set by the 8 TeV analyses both by ATLAS and CMS [16-18] for this decay mode. |
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