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| CMS-SUS-13-023 ; CERN-EP-2016-040 | ||
| Search for direct pair production of supersymmetric top quarks decaying to all-hadronic final states in pp collisions at $\sqrt{s} =$ 8 TeV | ||
| CMS Collaboration | ||
| 2 March 2016 | ||
| Eur. Phys. J. C 76 (2016) 460 | ||
| Abstract: Results are reported from a search for the pair production of top squarks, the supersymmetric partners of top quarks, in final states with jets and missing transverse momentum. The data sample used in this search was collected by the CMS detector and corresponds to an integrated luminosity of 18.9 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of 8 TeV produced by the LHC. The search features novel background suppression and prediction methods, including a dedicated top quark pair reconstruction algorithm. The data are found to be in agreement with the predicted backgrounds. Exclusion limits are set in simplified supersymmetry models with the top squark decaying to jets and an undetected neutralino, either through an on-shell top quark or through a bottom quark and chargino. Models with the top squark decaying via an on-shell top quark are excluded for top squark masses up to 755 GeV in the case of neutralino masses below 200 GeV. For decays via a chargino, top squark masses up to 620 GeV are excluded, depending on the masses of the chargino and neutralino. | ||
| Links: e-print arXiv:1603.00765 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1-a:
Diagrams representing the two simplified models of direct top squark pair production considered in this study: T2tt with top squark decay via an on-shell top quark (a) and T2bW with top squark decay via a chargino (b). |
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Figure 1-b:
Diagrams representing the two simplified models of direct top squark pair production considered in this study: T2tt with top squark decay via an on-shell top quark (a) and T2bW with top squark decay via a chargino (b). |
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Figure 2-a:
Efficiency as a function of generator level $ {p_{\mathrm {T}}} $ for picky jet clustering and CORRAL top quark pair reconstruction in all-hadronic T2tt events with $ {m_{\tilde{ \mathrm{t} } }} =$ 600 GeV and $ {m_{\tilde{\chi}^0_1 }} =$ 50 GeV. a: The efficiency to correctly cluster final state particles from each W boson and top quark decay into two and three picky jets, respectively, as a function of particle (top quark or W boson) $ {p_{\mathrm {T}}} $. b: The efficiency at each stage of the corral algorithm to reconstruct a hadronically decaying top quark pair as a function of the average $ {p_{\mathrm {T}}} $ of the two top quarks. They are the efficiency to correctly cluster the final state particles from top quark decays into six picky jets, labelled ``Picky jet clustering''; the efficiency to both carry out picky jet clustering and reconstruct the top quark pair with these six picky jets, labelled ``Top pair reconstruction''; and finally the efficiency to carry out picky jet clustering, top pair reconstruction, and then correctly select the reconstructed top quark pair for use in the analysis, labelled ``Correct pair selection''. |
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Figure 2-b:
Efficiency as a function of generator level $ {p_{\mathrm {T}}} $ for picky jet clustering and CORRAL top quark pair reconstruction in all-hadronic T2tt events with $ {m_{\tilde{ \mathrm{t} } }} =$ 600 GeV and $ {m_{\tilde{\chi}^0_1 }} =$ 50 GeV. a: The efficiency to correctly cluster final state particles from each W boson and top quark decay into two and three picky jets, respectively, as a function of particle (top quark or W boson) $ {p_{\mathrm {T}}} $. b: The efficiency at each stage of the corral algorithm to reconstruct a hadronically decaying top quark pair as a function of the average $ {p_{\mathrm {T}}} $ of the two top quarks. They are the efficiency to correctly cluster the final state particles from top quark decays into six picky jets, labelled ``Picky jet clustering''; the efficiency to both carry out picky jet clustering and reconstruct the top quark pair with these six picky jets, labelled ``Top pair reconstruction''; and finally the efficiency to carry out picky jet clustering, top pair reconstruction, and then correctly select the reconstructed top quark pair for use in the analysis, labelled ``Correct pair selection''. |
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Figure 3:
Masses of the top quarks and W bosons reconstructed with picky jets that are matched at particle level in simulation, as discussed in the text, in all-hadronic T2tt events with $ {m_{\tilde{ \mathrm{t} } }} =$ 600 GeV and $ {m_{\tilde{\chi}^0_1 }} =$ 50 GeV. The labels ``before PU corr.'' and ``after PU corr.'' refer to results obtained before and after application of the trimming procedure used to correct for pileup effects. |
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Figure 4-a:
Properties of the reconstructed top quark pair used in the analysis are compared to their true properties in all-hadronic T2tt events with $ {m_{\tilde{ \mathrm{t} } }} =$ 600 GeV and $ {m_{\tilde{\chi}^0_1 }} =$ 50 GeV. The label ``Correct pair selection'' corresponds to events in which the two top quark decays are each resolved into three distinct picky jets and these jets are used to reconstruct the two top quarks. The label ``Incorrect clustering or pair selection'' is used for all other events. The two figures (a,b) show comparisons of the angular separation between the two top quarks in rapidity, $y\equiv -(1/2)\ln[(E+p_z)/(E-p_z)]$, and azimuthal angle $\phi $. The figure (c) compares the relative $ {p_{\mathrm {T}}} $ of the two top quarks. In all cases, $\mathrm{t} _1$ refers to the top quark with the highest $ {p_{\mathrm {T}}} $. |
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Figure 4-b:
Properties of the reconstructed top quark pair used in the analysis are compared to their true properties in all-hadronic T2tt events with $ {m_{\tilde{ \mathrm{t} } }} =$ 600 GeV and $ {m_{\tilde{\chi}^0_1 }} =$ 50 GeV. The label ``Correct pair selection'' corresponds to events in which the two top quark decays are each resolved into three distinct picky jets and these jets are used to reconstruct the two top quarks. The label ``Incorrect clustering or pair selection'' is used for all other events. The two figures (a,b) show comparisons of the angular separation between the two top quarks in rapidity, $y\equiv -(1/2)\ln[(E+p_z)/(E-p_z)]$, and azimuthal angle $\phi $. The figure (c) compares the relative $ {p_{\mathrm {T}}} $ of the two top quarks. In all cases, $\mathrm{t} _1$ refers to the top quark with the highest $ {p_{\mathrm {T}}} $. |
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Figure 4-c:
Properties of the reconstructed top quark pair used in the analysis are compared to their true properties in all-hadronic T2tt events with $ {m_{\tilde{ \mathrm{t} } }} =$ 600 GeV and $ {m_{\tilde{\chi}^0_1 }} =$ 50 GeV. The label ``Correct pair selection'' corresponds to events in which the two top quark decays are each resolved into three distinct picky jets and these jets are used to reconstruct the two top quarks. The label ``Incorrect clustering or pair selection'' is used for all other events. The two figures (a,b) show comparisons of the angular separation between the two top quarks in rapidity, $y\equiv -(1/2)\ln[(E+p_z)/(E-p_z)]$, and azimuthal angle $\phi $. The figure (c) compares the relative $ {p_{\mathrm {T}}} $ of the two top quarks. In all cases, $\mathrm{t} _1$ refers to the top quark with the highest $ {p_{\mathrm {T}}} $. |
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Figure 5-a:
Distributions of properties of reconstructed top quark pairs for data together with signal and background MC data samples after the baseline selection for two choices of $ {m_{\tilde{ \mathrm{t} } }} $ and $ {m_{\tilde{\chi}^0_1 }} $. For the case $ {m_{\tilde{ \mathrm{t} } }} =$ 775 GeV, $ {m_{\tilde{\chi}^0_1 }} =$ 25 GeV the expected signal is multiplied by a factor of 25. The plot (a) shows the minimum separation in the $\eta $-$\phi $ plane between any two jets in the leading reconstructed top quark, defined as the one with the highest discriminator value, while the plot (b) shows the separation in $\phi $ between ${{\vec{p}}_{\mathrm {T}}^{\text {miss}}} $ and the jet in the sub-leading reconstructed top quark for which this separation is the smallest. Both variables are inputs to the T2tt search region BDT discriminators, which are described in Section 6. |
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Figure 5-b:
Distributions of properties of reconstructed top quark pairs for data together with signal and background MC data samples after the baseline selection for two choices of $ {m_{\tilde{ \mathrm{t} } }} $ and $ {m_{\tilde{\chi}^0_1 }} $. For the case $ {m_{\tilde{ \mathrm{t} } }} =$ 775 GeV, $ {m_{\tilde{\chi}^0_1 }} =$ 25 GeV the expected signal is multiplied by a factor of 25. The plot (a) shows the minimum separation in the $\eta $-$\phi $ plane between any two jets in the leading reconstructed top quark, defined as the one with the highest discriminator value, while the plot (b) shows the separation in $\phi $ between ${{\vec{p}}_{\mathrm {T}}^{\text {miss}}} $ and the jet in the sub-leading reconstructed top quark for which this separation is the smallest. Both variables are inputs to the T2tt search region BDT discriminators, which are described in Section 6. |
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Figure 6-a:
a: Selection efficiency for non-prompt leptons versus that for prompt leptons. The curves are produced by varying BDT discriminator values for electrons, muons, and taus. Prompt leptons are those matched to lepton candidates in semileptonic $ {\mathrm{t} \mathrm{\bar{t}} } $ events whereas non-prompt leptons are those that are matched to lepton candidates in all-hadronic $ {\mathrm{t} \mathrm{\bar{t}} } $ in the case of electrons and muons, or all-hadronic T2tt signal events in the case of $\tau $ leptons. b: The $ {m_{\mathrm {T}}} $ calculated from $ {{\vec{p}}_{\mathrm {T}}^{\text {miss}}} $ and the momentum of the visible $\tau $ lepton decay products, for $\tau $ lepton candidates matched to $\tau $ lepton decays in semileptonic $ {\mathrm{t} \mathrm{\bar{t}} } $ events, and all $\tau $ lepton candidates in a simulated all-hadronic T2tt signal sample ($ {m_{\tilde{ \mathrm{t} } }} =$ 620 GeV, $ {m_{\tilde{\chi}^0_1 }} =$ 40 GeV). |
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Figure 6-b:
a: Selection efficiency for non-prompt leptons versus that for prompt leptons. The curves are produced by varying BDT discriminator values for electrons, muons, and taus. Prompt leptons are those matched to lepton candidates in semileptonic $ {\mathrm{t} \mathrm{\bar{t}} } $ events whereas non-prompt leptons are those that are matched to lepton candidates in all-hadronic $ {\mathrm{t} \mathrm{\bar{t}} } $ in the case of electrons and muons, or all-hadronic T2tt signal events in the case of $\tau $ leptons. b: The $ {m_{\mathrm {T}}} $ calculated from $ {{\vec{p}}_{\mathrm {T}}^{\text {miss}}} $ and the momentum of the visible $\tau $ lepton decay products, for $\tau $ lepton candidates matched to $\tau $ lepton decays in semileptonic $ {\mathrm{t} \mathrm{\bar{t}} } $ events, and all $\tau $ lepton candidates in a simulated all-hadronic T2tt signal sample ($ {m_{\tilde{ \mathrm{t} } }} =$ 620 GeV, $ {m_{\tilde{\chi}^0_1 }} =$ 40 GeV). |
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Figure 7-a:
Search regions providing the most stringent limits in the $ {m_{\tilde{ \mathrm{t} } }} $-$ {m_{\tilde{\chi}^0_1 }} $ plane in the T2tt signal topology (a) and the T2bW signal topologies for mass splitting parameter values $x =$ 0.25, 0.50, 0.75. The T2tt\_LM, T2tt\_MM, T2tt\_HM, and T2tt\_VHM search regions are numbered 1, 2, 3, and 4, respectively. The T2bW\_LX, T2bW\_LM, T2bW\_MXHM, T2bW\_VHM, and T2bW\_HXHM search regions are numbered 1, 2, 3, 4, and 5 respectively. In regions with $ {m_{\tilde{\chi}^0_1 }} $ similar to $ {m_{\tilde{ \mathrm{t} } }} $, the different search regions can have similar sensitivity, which leads to the fluctuations in choice of search regions in neighboring bins that is seen in some areas. |
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Figure 7-b:
Search regions providing the most stringent limits in the $ {m_{\tilde{ \mathrm{t} } }} $-$ {m_{\tilde{\chi}^0_1 }} $ plane in the T2tt signal topology (a) and the T2bW signal topologies for mass splitting parameter values $x =$ 0.25, 0.50, 0.75. The T2tt\_LM, T2tt\_MM, T2tt\_HM, and T2tt\_VHM search regions are numbered 1, 2, 3, and 4, respectively. The T2bW\_LX, T2bW\_LM, T2bW\_MXHM, T2bW\_VHM, and T2bW\_HXHM search regions are numbered 1, 2, 3, 4, and 5 respectively. In regions with $ {m_{\tilde{\chi}^0_1 }} $ similar to $ {m_{\tilde{ \mathrm{t} } }} $, the different search regions can have similar sensitivity, which leads to the fluctuations in choice of search regions in neighboring bins that is seen in some areas. |
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Figure 7-c:
Search regions providing the most stringent limits in the $ {m_{\tilde{ \mathrm{t} } }} $-$ {m_{\tilde{\chi}^0_1 }} $ plane in the T2tt signal topology (a) and the T2bW signal topologies for mass splitting parameter values $x =$ 0.25, 0.50, 0.75. The T2tt\_LM, T2tt\_MM, T2tt\_HM, and T2tt\_VHM search regions are numbered 1, 2, 3, and 4, respectively. The T2bW\_LX, T2bW\_LM, T2bW\_MXHM, T2bW\_VHM, and T2bW\_HXHM search regions are numbered 1, 2, 3, 4, and 5 respectively. In regions with $ {m_{\tilde{\chi}^0_1 }} $ similar to $ {m_{\tilde{ \mathrm{t} } }} $, the different search regions can have similar sensitivity, which leads to the fluctuations in choice of search regions in neighboring bins that is seen in some areas. |
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Figure 7-d:
Search regions providing the most stringent limits in the $ {m_{\tilde{ \mathrm{t} } }} $-$ {m_{\tilde{\chi}^0_1 }} $ plane in the T2tt signal topology (a) and the T2bW signal topologies for mass splitting parameter values $x =$ 0.25, 0.50, 0.75. The T2tt\_LM, T2tt\_MM, T2tt\_HM, and T2tt\_VHM search regions are numbered 1, 2, 3, and 4, respectively. The T2bW\_LX, T2bW\_LM, T2bW\_MXHM, T2bW\_VHM, and T2bW\_HXHM search regions are numbered 1, 2, 3, 4, and 5 respectively. In regions with $ {m_{\tilde{\chi}^0_1 }} $ similar to $ {m_{\tilde{ \mathrm{t} } }} $, the different search regions can have similar sensitivity, which leads to the fluctuations in choice of search regions in neighboring bins that is seen in some areas. |
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Figure 8-a:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the four T2tt validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 8-b:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the four T2tt validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 8-c:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the four T2tt validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 8-d:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the four T2tt validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 9-a:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the five T2bW validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 9-b:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the five T2bW validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 9-c:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the five T2bW validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 9-d:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the five T2bW validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 9-e:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 1$\ell $ closure samples, with leptons removed, for the five T2bW validation regions. The three bins at the far right in each plot are used to validate the MC performance in the signal region and its two extensions. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms indicate the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the W+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 10-a:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All four T2tt validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 10-b:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All four T2tt validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 10-c:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All four T2tt validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 10-d:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All four T2tt validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 11-a:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All five T2bW validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 11-b:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All five T2bW validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
png pdf |
Figure 11-c:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All five T2bW validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
png pdf |
Figure 11-d:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All five T2bW validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
png pdf |
Figure 11-e:
Comparisons of BDT discriminator (D) outputs for data and corrected MC simulation for the 2$\ell $ closure samples, with leptons removed. All five T2bW validation regions are plotted. The points with error bars represent the event yields in data. The histogram labelled ``MC without corr." in the bottom pane of each figure plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC shown in the upper pane. The other histograms provide the contributions of the various background processes. The ``LF'' and ``HF'' labels denote the subsets of the Z+jets process in which the boson is produced in association with light and heavy flavour (b) quark jets, respectively. |
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Figure 12-a:
Observed and predicted event yields for each T2tt search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 12-b:
Observed and predicted event yields for each T2tt search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 12-c:
Observed and predicted event yields for each T2tt search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 12-d:
Observed and predicted event yields for each T2tt search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 13-a:
Observed and predicted event yields for each T2bW search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 13-b:
Observed and predicted event yields for each T2bW search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 13-c:
Observed and predicted event yields for each T2bW search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 13-d:
Observed and predicted event yields for each T2bW search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 13-e:
Observed and predicted event yields for each T2bW search region discriminator (D). The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 14-a:
Observed and predicted event yields for each T2tt search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
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Figure 14-b:
Observed and predicted event yields for each T2tt search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
png pdf |
Figure 14-c:
Observed and predicted event yields for each T2tt search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
png pdf |
Figure 14-d:
Observed and predicted event yields for each T2tt search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
png pdf |
Figure 15-a:
Observed and predicted event yields for each T2bW search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
png pdf |
Figure 15-b:
Observed and predicted event yields for each T2bW search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
png pdf |
Figure 15-c:
Observed and predicted event yields for each T2bW search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
png pdf |
Figure 15-d:
Observed and predicted event yields for each T2bW search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
png pdf |
Figure 15-e:
Observed and predicted event yields for each T2bW search region discriminator (D) before lepton vetoes are applied, which are used for the cross-checks discussed in the text. The bottom pane of each plot shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The filled bands represent the relative systematic uncertainties in the predictions. |
png pdf |
Figure 16-a:
Observed and expected 95% CL limits on the $\tilde{ \mathrm{t} } \tilde{ \bar{ \mathrm{t} } } $ production cross section and exclusion areas in the $ {m_{\tilde{ \mathrm{t} } }} $-$ {m_{\tilde{\chi}^0_1 }} $ plane for the T2tt (a) and T2bW signal topologies (with $x =$ 0.25, 0.50, 0.75). In the rare cases in which a statistical fluctuation leads to zero signal events for a particular set of masses, the limit is taken to be the average of the limits obtained for the neighboring bins. The $\pm$1$\sigma _{\text {theory}}$ lines indicate the variations in the excluded region due to the uncertainty in the theoretical prediction of the signal cross section. |
png pdf |
Figure 16-b:
Observed and expected 95% CL limits on the $\tilde{ \mathrm{t} } \tilde{ \bar{ \mathrm{t} } } $ production cross section and exclusion areas in the $ {m_{\tilde{ \mathrm{t} } }} $-$ {m_{\tilde{\chi}^0_1 }} $ plane for the T2tt (a) and T2bW signal topologies (with $x =$ 0.25, 0.50, 0.75). In the rare cases in which a statistical fluctuation leads to zero signal events for a particular set of masses, the limit is taken to be the average of the limits obtained for the neighboring bins. The $\pm$1$\sigma _{\text {theory}}$ lines indicate the variations in the excluded region due to the uncertainty in the theoretical prediction of the signal cross section. |
png pdf |
Figure 16-c:
Observed and expected 95% CL limits on the $\tilde{ \mathrm{t} } \tilde{ \bar{ \mathrm{t} } } $ production cross section and exclusion areas in the $ {m_{\tilde{ \mathrm{t} } }} $-$ {m_{\tilde{\chi}^0_1 }} $ plane for the T2tt (a) and T2bW signal topologies (with $x =$ 0.25, 0.50, 0.75). In the rare cases in which a statistical fluctuation leads to zero signal events for a particular set of masses, the limit is taken to be the average of the limits obtained for the neighboring bins. The $\pm$1$\sigma _{\text {theory}}$ lines indicate the variations in the excluded region due to the uncertainty in the theoretical prediction of the signal cross section. |
png pdf |
Figure 16-d:
Observed and expected 95% CL limits on the $\tilde{ \mathrm{t} } \tilde{ \bar{ \mathrm{t} } } $ production cross section and exclusion areas in the $ {m_{\tilde{ \mathrm{t} } }} $-$ {m_{\tilde{\chi}^0_1 }} $ plane for the T2tt (a) and T2bW signal topologies (with $x =$ 0.25, 0.50, 0.75). In the rare cases in which a statistical fluctuation leads to zero signal events for a particular set of masses, the limit is taken to be the average of the limits obtained for the neighboring bins. The $\pm$1$\sigma _{\text {theory}}$ lines indicate the variations in the excluded region due to the uncertainty in the theoretical prediction of the signal cross section. |
| Tables | |
png pdf |
Table 1:
Estimated SM background yields as obtained with the methods described in Section {sec:background}, and the observed data yields for the T2tt and T2bW baseline selections. The T2bW yield corresponds to the simplified model point with $( {m_{\tilde{\mathrm{t}} }},\, {m_{\tilde{\chi}^0_1 }} ; \, x ) = $ (600 GeV, 0 GeV; 0.75), and the T2tt yield is for the simplified model point with $ ( {m_{\tilde{\mathrm{t}} }}, \, {m_{\tilde{\chi}^0_1 }} ) =$ (700 GeV, 0 GeV). The uncertainties listed are statistical only. |
png pdf |
Table 2:
Search regions for the T2tt and T2bW channels. The table lists the SUSY particle masses used for the training of the BDTs, the cutoff on the BDT output, and the efficiency for the signal to pass the BDT selection relative to the baseline selection. The event counts of the T2bW discriminator training samples are limited and so four nearby mass points were used. They are the four combinations of the two $\tilde{\mathrm{t}} $ and two $\tilde{\chi}^0_1 $ masses listed. The signal efficiency in each row of the table is then that of the worst case of the four, which in every case is the point with the largest $ {m_{\tilde{\mathrm{t}} }} $ and smallest $ {m_{\tilde{\chi}^0_1 }} $ values of those indicated. |
png pdf |
Table 3:
Estimated contributions and uncertainties for the SM backgrounds in the T2tt search regions. The $ \mathrm{ t \bar{t} } $, W+jets, single top, Z+jets, and QCD multijet background estimates make use of MC simulated samples that have been weighted by scale factors obtained from data-MC comparisons as discussed in the text. The $ \mathrm{ t \bar{t} Z } $ background is estimated directly from simulation, with uncertainties assigned for sources of MC mismodelling. |
png pdf |
Table 4:
Estimated contributions and uncertainties for the SM backgrounds in the T2bW search regions. The $ \mathrm{ t \bar{t} } $, W+jets, single top, Z+jets, and QCD multijet background estimates make use of MC simulated samples that have been weighted by scale factors obtained from data-MC comparisons as discussed in the text. The $ \mathrm{ t \bar{t} Z } $ background is estimated directly from simulation, with uncertainties assigned for sources of MC mismodelling. |
png pdf |
Table 5:
Predicted and observed data yields in the T2tt search regions. The uncertainties in the background predictions are the combined systematic and statistical uncertainties. The T2tt yields correspond to the simplified model points with $( {m_{\tilde{\mathrm{t}} }} , \, {m_{\tilde{\chi}^0_1 }} ) =$ (500 GeV, 200 GeV ) and (700 GeV, 0 GeV ). The uncertainties in the signal yields are statistical only. |
png pdf |
Table 6:
Predicted and observed data yields in the T2bW search regions. The uncertainties in the background predictions are the combined systematic and statistical uncertainties. The T2bW yields correspond to the simplified model points with $( {m_{\tilde{\mathrm{t}} }} ,\, {m_{\tilde{\chi}^0_1 }} ; \, x) =$ (500 GeV, 175 GeV; 0.25) and (600 GeV, 0 GeV; 0.75). The uncertainties in the signal yields are statistical only. |
png pdf |
Table 7:
Summary of the systematic uncertainties in the signal selection efficiencies. The uncertainties can depend on signal topology, mass values, and search region. The quoted value ranges capture the variations associated with these dependencies. In all cases, the upper bound corresponds to the region in which $ {m_{\tilde{\chi}^0_1 }} $ is close to $ {m_{\tilde{\mathrm{t}} }} $. |
| Summary |
| We report a search for the direct pair production of top squarks in an all-hadronic final state containing jets and large missing transverse momentum. Two decay channels for the top squarks are considered. In the first channel, each top squark decays to a top quark and a neutralino, whereas in the second channel they each decay to a bottom quark and a chargino, with the chargino subsequently decaying to a W boson and a neutralino. A dedicated top quark pair reconstruction algorithm provides efficient identification of hadronically decaying top quarks. The search is carried out in several search regions based on the output of multivariate discriminators, where the standard model background yield is estimated with corrected simulation samples and validated in data control regions. The observed yields are statistically compatible with the standard model estimates and are used to restrict the allowed parameter space for these two signal topologies. The search is particularly sensitive to the production of top squarks that decay via an on-shell top quark. For models predicting such decays, a 95% CL lower limit of 755 GeV is found for the top squark mass when the neutralino is lighter than 200 GeV, extending the current limits on these models by 50-100 GeV. In models with top squarks that decay via a chargino, scenarios with a top squark mass up to 620 GeV are excluded. |
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