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| CMS-SUS-14-015 ; CERN-PH-EP-2016-004 | ||
| Search for direct pair production of scalar top quarks in the single- and dilepton channels in proton-proton collisions at $ \sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 10 February 2016 | ||
| JHEP 07 (2016) 027 [Erratum] | ||
| Abstract: Results are reported from a search for the top squark $ \tilde{ \mathrm{t} }_1 $, the lighter of the two supersymmetric partners of the top quark. The data sample corresponds to 19.7 fb$^{-1}$ of proton-proton collisions at $ \sqrt{s} = $ 8 TeV collected with the CMS detector at the LHC. The search targets $ \tilde{ \mathrm{t} }_1 \to \mathrm{b } \tilde{ \chi }^{\pm}_1$ and $ \tilde{ \mathrm{t} }_1 \to \mathrm{t }^{(*)} \tilde{\chi}^0_1$ decay modes, where $\tilde{ \chi }^{\pm}_1$ and $\tilde{\chi}^0_1$ are the lightest chargino and neutralino, respectively. The reconstructed final state consists of jets, b jets, missing transverse energy, and either one or two leptons. Leading backgrounds are determined from data. No significant excess in data is observed above the expectation from standard model processes. The results exclude a region of the two-dimensional plane of possible $ \tilde{ \mathrm{t} }_1 $ and ${\tilde{\chi}^0_1} $ masses. The highest excluded $ \tilde{ \mathrm{t} }_1 $ and ${\tilde{\chi}^0_1} $ masses are about 700 GeV and 250 GeV, respectively. | ||
| Links: e-print arXiv:1602.03169 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Top squark direct pair production at the LHC. Left: tt decay mode. Right: bbWW decay mode. |
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Figure 2-a:
Distribution of the transverse momentum of the lepton versus the missing transverse energy at the preselection, for the simulated ${\mathrm {t}\overline {\mathrm {t}}}$ background (a) and for the bbWW decay mode ($x=$ 0.50) of the signal with $ m( \tilde{ \mathrm{t} }_{1} )-m( { {\tilde{\chi}^{0}_{1}} } ) \geq $ 625 GeV (b). |
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Figure 2-b:
Distribution of the transverse momentum of the lepton versus the missing transverse energy at the preselection, for the simulated ${\mathrm {t}\overline {\mathrm {t}}}$ background (a) and for the bbWW decay mode ($x=$ 0.50) of the signal with $ m( \tilde{ \mathrm{t} }_{1} )-m( { {\tilde{\chi}^{0}_{1}} } ) \geq $ 625 GeV (b). |
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Figure 2-c:
Distribution of the transverse momentum of the lepton versus the missing transverse energy at the preselection, for the simulated ${\mathrm {t}\overline {\mathrm {t}}}$ background (a) and for the bbWW decay mode ($x=$ 0.50) of the signal with $ m( \tilde{ \mathrm{t} }_{1} )-m( { {\tilde{\chi}^{0}_{1}} } ) \geq $ 625 GeV (b). |
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Figure 3-a:
Distribution of some discriminating variables for the bbWW ($x=$ 0.75) decay mode at the preselection level, for the main $ {\mathrm {t}\overline {\mathrm {t}}} $ background and benchmark signal mass points grouped in bands of constant width $ {\Delta m} =$ (150 $\pm$ 25 ), (350 $\pm$ 25 ), (550 $\pm$ 25 ), and (750 $\pm$ 25 ) GeV. Distributions are normalized to the same area. From (a) to (d): ${M_{{\mathrm T}2}^{ {\mathrm {W}}}} $, ${M(\text {3 jet})} $, ${M(\ell {\mathrm {b}})} and {N(\text {jets})} $. |
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Figure 3-b:
Distribution of some discriminating variables for the bbWW ($x=$ 0.75) decay mode at the preselection level, for the main $ {\mathrm {t}\overline {\mathrm {t}}} $ background and benchmark signal mass points grouped in bands of constant width $ {\Delta m} =$ (150 $\pm$ 25 ), (350 $\pm$ 25 ), (550 $\pm$ 25 ), and (750 $\pm$ 25 ) GeV. Distributions are normalized to the same area. From (a) to (d): ${M_{{\mathrm T}2}^{ {\mathrm {W}}}} $, ${M(\text {3 jet})} $, ${M(\ell {\mathrm {b}})} and {N(\text {jets})} $. |
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Figure 3-c:
Distribution of some discriminating variables for the bbWW ($x=$ 0.75) decay mode at the preselection level, for the main $ {\mathrm {t}\overline {\mathrm {t}}} $ background and benchmark signal mass points grouped in bands of constant width $ {\Delta m} =$ (150 $\pm$ 25 ), (350 $\pm$ 25 ), (550 $\pm$ 25 ), and (750 $\pm$ 25 ) GeV. Distributions are normalized to the same area. From (a) to (d): ${M_{{\mathrm T}2}^{ {\mathrm {W}}}} $, ${M(\text {3 jet})} $, ${M(\ell {\mathrm {b}})} and {N(\text {jets})} $. |
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Figure 3-d:
Distribution of some discriminating variables for the bbWW ($x=$ 0.75) decay mode at the preselection level, for the main $ {\mathrm {t}\overline {\mathrm {t}}} $ background and benchmark signal mass points grouped in bands of constant width $ {\Delta m} =$ (150 $\pm$ 25 ), (350 $\pm$ 25 ), (550 $\pm$ 25 ), and (750 $\pm$ 25 ) GeV. Distributions are normalized to the same area. From (a) to (d): ${M_{{\mathrm T}2}^{ {\mathrm {W}}}} $, ${M(\text {3 jet})} $, ${M(\ell {\mathrm {b}})} and {N(\text {jets})} $. |
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Figure 4-a:
Distributions of different variables in both data and simulation, for both e and $\mu $ final states at the preselection level without the ${M_{\mathrm {T}}}$ requirement. From (a,c) to (b,d): ${M_{{\mathrm T}2}^{ {\mathrm {W}}}} $, ${M(\text {3 jet})} $, ${M(\ell {\mathrm {b}})}$ and ${N(\text {jets})} $. The hatched region represents the quadratic sum of statistical and JES simulation uncertainties. The lower panel shows the ratio of data to total simulation background, with the red band representing the uncertainties mentioned in the text. Two signal mass points of the bbWW decay mode ($x=$ 0.75) are represented by open histograms, dashed and solid, with their cross sections scaled by 100; the two mass points ${ ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )}$ are (300, 200) and (500, 200) GeV. |
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Figure 4-b:
Distributions of different variables in both data and simulation, for both e and $\mu $ final states at the preselection level without the ${M_{\mathrm {T}}}$ requirement. From (a,c) to (b,d): ${M_{{\mathrm T}2}^{ {\mathrm {W}}}} $, ${M(\text {3 jet})} $, ${M(\ell {\mathrm {b}})}$ and ${N(\text {jets})} $. The hatched region represents the quadratic sum of statistical and JES simulation uncertainties. The lower panel shows the ratio of data to total simulation background, with the red band representing the uncertainties mentioned in the text. Two signal mass points of the bbWW decay mode ($x=$ 0.75) are represented by open histograms, dashed and solid, with their cross sections scaled by 100; the two mass points ${ ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )}$ are (300, 200) and (500, 200) GeV. |
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Figure 4-c:
Distributions of different variables in both data and simulation, for both e and $\mu $ final states at the preselection level without the ${M_{\mathrm {T}}}$ requirement. From (a,c) to (b,d): ${M_{{\mathrm T}2}^{ {\mathrm {W}}}} $, ${M(\text {3 jet})} $, ${M(\ell {\mathrm {b}})}$ and ${N(\text {jets})} $. The hatched region represents the quadratic sum of statistical and JES simulation uncertainties. The lower panel shows the ratio of data to total simulation background, with the red band representing the uncertainties mentioned in the text. Two signal mass points of the bbWW decay mode ($x=$ 0.75) are represented by open histograms, dashed and solid, with their cross sections scaled by 100; the two mass points ${ ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )}$ are (300, 200) and (500, 200) GeV. |
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Figure 4-d:
Distributions of different variables in both data and simulation, for both e and $\mu $ final states at the preselection level without the ${M_{\mathrm {T}}}$ requirement. From (a,c) to (b,d): ${M_{{\mathrm T}2}^{ {\mathrm {W}}}} $, ${M(\text {3 jet})} $, ${M(\ell {\mathrm {b}})}$ and ${N(\text {jets})} $. The hatched region represents the quadratic sum of statistical and JES simulation uncertainties. The lower panel shows the ratio of data to total simulation background, with the red band representing the uncertainties mentioned in the text. Two signal mass points of the bbWW decay mode ($x=$ 0.75) are represented by open histograms, dashed and solid, with their cross sections scaled by 100; the two mass points ${ ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )}$ are (300, 200) and (500, 200) GeV. |
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Figure 5-a:
Signal regions (SRs) defined as functions of the chosen BDT trainings in the ${ ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )}$ plane for tt (a), bbWW $x=$ 0.25 (b), 0.50 (c), and 0.75 (d) decay modes. The SRs are delimited by continuous red lines, and the final selections within the different SRs are delimited by dashed red lines. The attributes ``low / high $m( { {\tilde{\chi}^{0}_{1}} } )$'' and ``low / high ${\Delta m} $'' indicate that in these regions different thresholds are applied for the same BDT training. |
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Figure 5-b:
Signal regions (SRs) defined as functions of the chosen BDT trainings in the ${ ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )}$ plane for tt (a), bbWW $x=$ 0.25 (b), 0.50 (c), and 0.75 (d) decay modes. The SRs are delimited by continuous red lines, and the final selections within the different SRs are delimited by dashed red lines. The attributes ``low / high $m( { {\tilde{\chi}^{0}_{1}} } )$'' and ``low / high ${\Delta m} $'' indicate that in these regions different thresholds are applied for the same BDT training. |
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Figure 5-c:
Signal regions (SRs) defined as functions of the chosen BDT trainings in the ${ ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )}$ plane for tt (a), bbWW $x=$ 0.25 (b), 0.50 (c), and 0.75 (d) decay modes. The SRs are delimited by continuous red lines, and the final selections within the different SRs are delimited by dashed red lines. The attributes ``low / high $m( { {\tilde{\chi}^{0}_{1}} } )$'' and ``low / high ${\Delta m} $'' indicate that in these regions different thresholds are applied for the same BDT training. |
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Figure 5-d:
Signal regions (SRs) defined as functions of the chosen BDT trainings in the ${ ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )}$ plane for tt (a), bbWW $x=$ 0.25 (b), 0.50 (c), and 0.75 (d) decay modes. The SRs are delimited by continuous red lines, and the final selections within the different SRs are delimited by dashed red lines. The attributes ``low / high $m( { {\tilde{\chi}^{0}_{1}} } )$'' and ``low / high ${\Delta m} $'' indicate that in these regions different thresholds are applied for the same BDT training. |
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Figure 6-a:
The BDT output distributions of the bbWW ($x=$ 0.75) decay mode in both final states at the preselection level for data and predicted background, with $\mathrm {BDT1} >$ 0.025 (a) and $\mathrm {BDT5}>$ 0 (b). Two representative signal mass points are shown: $ { ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )} =$ (300,200) and (500,200) GeV. In each panel the final selection is indicated by the vertical black dashed line. The normalization and $ {M_{\mathrm {T}}}$ correction (see Section 4.2.2), computed in the tail of the BDT output, i.e. to the right of the dashed line, are here propagated to the full distribution. The uncertainties are statistical. The plots on the bottom represent the ratio of Data over the predicted background, where we quadratically add statistical uncertainties with the uncertainties on the scale factors. |
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Figure 6-b:
The BDT output distributions of the bbWW ($x=$ 0.75) decay mode in both final states at the preselection level for data and predicted background, with $\mathrm {BDT1} >$ 0.025 (a) and $\mathrm {BDT5}>$ 0 (b). Two representative signal mass points are shown: $ { ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )} =$ (300,200) and (500,200) GeV. In each panel the final selection is indicated by the vertical black dashed line. The normalization and $ {M_{\mathrm {T}}}$ correction (see Section 4.2.2), computed in the tail of the BDT output, i.e. to the right of the dashed line, are here propagated to the full distribution. The uncertainties are statistical. The plots on the bottom represent the ratio of Data over the predicted background, where we quadratically add statistical uncertainties with the uncertainties on the scale factors. |
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Figure 7-a:
Full ${M_{\mathrm {T}}}$ distribution in the control region with zero b jets, without any extra signal selection. a: without the tail correction factors applied; b: with $SFR_{ {\mathrm {W}}}$ and $SFR_{\text {1$\ell $}}$ corrections applied. The plots on the bottom represent the ratio of Data over the predicted background. |
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Figure 7-b:
Full ${M_{\mathrm {T}}}$ distribution in the control region with zero b jets, without any extra signal selection. a: without the tail correction factors applied; b: with $SFR_{ {\mathrm {W}}}$ and $SFR_{\text {1$\ell $}}$ corrections applied. The plots on the bottom represent the ratio of Data over the predicted background. |
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Figure 8-a:
a: Comparison of data and simulation in the ${M^\prime _{\ell {\mathrm {b}}}}$ distributions for events with 50 $ < {M_{\mathrm {T}}} < $ 8 GeV and zero b jets. b: Shape comparison between ${ {\mathrm {t}\overline {\mathrm {t}}} \to 1\ell }$ and W+jets for $ {M_{\mathrm {T}}} > $ 100 GeV. |
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Figure 8-b:
a: Comparison of data and simulation in the ${M^\prime _{\ell {\mathrm {b}}}}$ distributions for events with 50 $ < {M_{\mathrm {T}}} < $ 8 GeV and zero b jets. b: Shape comparison between ${ {\mathrm {t}\overline {\mathrm {t}}} \to 1\ell }$ and W+jets for $ {M_{\mathrm {T}}} > $ 100 GeV. |
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Figure 9:
a: Data, expected background, and signal contributions in the ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution at the preselection level. Background processes are estimated as in Section 5.2. The uncertainty bands are calculated from the full list of uncertainties discussed in Section 5.4. The same signal mass point $ { ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )} =$ (400,50) GeV is represented for the tt and bbWW ($x=$ 0.75) decay modes. b: ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution for the $ {\mathrm {t}\overline {\mathrm {t}}} $ background and different signal mass points of the tt decay mode regrouped in constant $\Delta m$ bands; distributions are normalized to the same area. |
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Figure 9-a:
a: Data, expected background, and signal contributions in the ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution at the preselection level. Background processes are estimated as in Section 5.2. The uncertainty bands are calculated from the full list of uncertainties discussed in Section 5.4. The same signal mass point $ { ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )} =$ (400,50) GeV is represented for the tt and bbWW ($x=$ 0.75) decay modes. b: ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution for the $ {\mathrm {t}\overline {\mathrm {t}}} $ background and different signal mass points of the tt decay mode regrouped in constant $\Delta m$ bands; distributions are normalized to the same area. |
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Figure 9-b:
a: Data, expected background, and signal contributions in the ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution at the preselection level. Background processes are estimated as in Section 5.2. The uncertainty bands are calculated from the full list of uncertainties discussed in Section 5.4. The same signal mass point $ { ( {{m}}( \tilde{ \mathrm{t} }_{1} ), {{m}}( { {\tilde{\chi}^{0}_{1}} } ) )} =$ (400,50) GeV is represented for the tt and bbWW ($x=$ 0.75) decay modes. b: ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution for the $ {\mathrm {t}\overline {\mathrm {t}}} $ background and different signal mass points of the tt decay mode regrouped in constant $\Delta m$ bands; distributions are normalized to the same area. |
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Figure 10:
Data and expected background contributions for the ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution in a control region enriched in $\mathrm{Z} \to \ell \ell $ events. This control region is similar to the preselection, except that the Z boson veto and b jet requirements have been inverted. Background processes are estimated as in Section 5.2. The uncertainty bands are calculated from the full list of uncertainties discussed in Section 5.4. |
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Figure 10-a:
Data and expected background contributions for the ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution in a control region enriched in $\mathrm{Z} \to \ell \ell $ events. This control region is similar to the preselection, except that the Z boson veto and b jet requirements have been inverted. Background processes are estimated as in Section 5.2. The uncertainty bands are calculated from the full list of uncertainties discussed in Section 5.4. |
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Figure 10-b:
Data and expected background contributions for the ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution in a control region enriched in $\mathrm{Z} \to \ell \ell $ events. This control region is similar to the preselection, except that the Z boson veto and b jet requirements have been inverted. Background processes are estimated as in Section 5.2. The uncertainty bands are calculated from the full list of uncertainties discussed in Section 5.4. |
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Figure 10-c:
Data and expected background contributions for the ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution in a control region enriched in $\mathrm{Z} \to \ell \ell $ events. This control region is similar to the preselection, except that the Z boson veto and b jet requirements have been inverted. Background processes are estimated as in Section 5.2. The uncertainty bands are calculated from the full list of uncertainties discussed in Section 5.4. |
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Figure 10-d:
Data and expected background contributions for the ${M_{\mathrm {T2}}^{\ell \ell }}$ distribution in a control region enriched in $\mathrm{Z} \to \ell \ell $ events. This control region is similar to the preselection, except that the Z boson veto and b jet requirements have been inverted. Background processes are estimated as in Section 5.2. The uncertainty bands are calculated from the full list of uncertainties discussed in Section 5.4. |
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Figure 11-a:
Exclusion limit at 95% CL obtained with a statistical combination of the results from the single-lepton and dilepton searches, for the tt (a), bbWW $x=$ 0.25 (b), bbWW $x=$ 0.50 (c) and bbWW $x=$ 0.75 (d) decay modes. The red and black lines represent the expected and observed limits, respectively; the dotted lines represent in each case the $\pm $1$\sigma $ variations of the contours. For all decay modes, we show the kinematic limit $m( \tilde{ \mathrm{t} }_{1} )=m( {\mathrm {b}})+m( {\mathrm {W}})+m( { {\tilde{\chi}^{0}_{1}} } )$ on the left side of the $(m( \tilde{ \mathrm{t} }_{1} ),m( { {\tilde{\chi}^{0}_{1}} } ))$ plane; for the tt decay mode, we show the $ {\Delta m} =m( {\mathrm {t}})$ line; and for the bbWW decay mode, we show the $m( {\tilde{\chi}^\pm _{1}} )-m( { {\tilde{\chi}^{0}_{1}} } )=m( {\mathrm {W}})$ line. |
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Figure 11-b:
Exclusion limit at 95% CL obtained with a statistical combination of the results from the single-lepton and dilepton searches, for the tt (a), bbWW $x=$ 0.25 (b), bbWW $x=$ 0.50 (c) and bbWW $x=$ 0.75 (d) decay modes. The red and black lines represent the expected and observed limits, respectively; the dotted lines represent in each case the $\pm $1$\sigma $ variations of the contours. For all decay modes, we show the kinematic limit $m( \tilde{ \mathrm{t} }_{1} )=m( {\mathrm {b}})+m( {\mathrm {W}})+m( { {\tilde{\chi}^{0}_{1}} } )$ on the left side of the $(m( \tilde{ \mathrm{t} }_{1} ),m( { {\tilde{\chi}^{0}_{1}} } ))$ plane; for the tt decay mode, we show the $ {\Delta m} =m( {\mathrm {t}})$ line; and for the bbWW decay mode, we show the $m( {\tilde{\chi}^\pm _{1}} )-m( { {\tilde{\chi}^{0}_{1}} } )=m( {\mathrm {W}})$ line. |
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Figure 11-c:
Exclusion limit at 95% CL obtained with a statistical combination of the results from the single-lepton and dilepton searches, for the tt (a), bbWW $x=$ 0.25 (b), bbWW $x=$ 0.50 (c) and bbWW $x=$ 0.75 (d) decay modes. The red and black lines represent the expected and observed limits, respectively; the dotted lines represent in each case the $\pm $1$\sigma $ variations of the contours. For all decay modes, we show the kinematic limit $m( \tilde{ \mathrm{t} }_{1} )=m( {\mathrm {b}})+m( {\mathrm {W}})+m( { {\tilde{\chi}^{0}_{1}} } )$ on the left side of the $(m( \tilde{ \mathrm{t} }_{1} ),m( { {\tilde{\chi}^{0}_{1}} } ))$ plane; for the tt decay mode, we show the $ {\Delta m} =m( {\mathrm {t}})$ line; and for the bbWW decay mode, we show the $m( {\tilde{\chi}^\pm _{1}} )-m( { {\tilde{\chi}^{0}_{1}} } )=m( {\mathrm {W}})$ line. |
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Figure 11-d:
Exclusion limit at 95% CL obtained with a statistical combination of the results from the single-lepton and dilepton searches, for the tt (a), bbWW $x=$ 0.25 (b), bbWW $x=$ 0.50 (c) and bbWW $x=$ 0.75 (d) decay modes. The red and black lines represent the expected and observed limits, respectively; the dotted lines represent in each case the $\pm $1$\sigma $ variations of the contours. For all decay modes, we show the kinematic limit $m( \tilde{ \mathrm{t} }_{1} )=m( {\mathrm {b}})+m( {\mathrm {W}})+m( { {\tilde{\chi}^{0}_{1}} } )$ on the left side of the $(m( \tilde{ \mathrm{t} }_{1} ),m( { {\tilde{\chi}^{0}_{1}} } ))$ plane; for the tt decay mode, we show the $ {\Delta m} =m( {\mathrm {t}})$ line; and for the bbWW decay mode, we show the $m( {\tilde{\chi}^\pm _{1}} )-m( { {\tilde{\chi}^{0}_{1}} } )=m( {\mathrm {W}})$ line. |
| Tables | |
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Table 1:
Kinematic conditions for the $ \tilde{ \mathrm{t} }_{1} $ decay modes explored in this paper. |
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Table 2:
Final selection variables chosen as input for the BDT training, as functions of the decay modes bbWW and tt, and kinematic regions. Column headings ${\Delta R}$ and ${\Delta \phi }$ refer to ${\Delta R (\ell , {\mathrm {b}}_1)}$ and ${\Delta \phi (\mathrm {j}_{1,2}, {\vec{p}}_{\mathrm {T}}^{\text {miss}} )} $. |
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Table 3:
Summary of the relative systematic uncertainties in the total background, at the preselection level, and the range of variation over the BDT selections. |
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Table 4:
Background prediction without signal contamination and observed data for the BDT selections. The total systematic uncertainties are reported for the predicted background. |
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Table 5:
The relevant sources of systematic uncertainty in the background estimate for each signal region used in the limit setting. From left to right, the systematic uncertainty sources are: lepton energy scale ($\ell $ ES), jet energy scale (JES), unclustered energy scale (Uncl.), ${E_{\mathrm {T}}^{\text {miss}}}$ energy resolution from jets (JER), uncertainty in b tagging scale factors (b tag), lepton selection efficiency ($\ell $ eff.), ISR reweighting (ISR), the misidentified lepton estimate (ML), and the combined normalization uncertainty in the $ {\mathrm {t}\overline {\mathrm {t}}} $ , DY, and other electroweak backgrounds ($\sigma $). |
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Table 6:
Data yields and background expectation for five different ${M_{\mathrm {T2}}^{\ell \ell }}$ threshold values. The asymmetric uncertainties quoted for the background indicate the total systematic uncertainty, including the statistical uncertainty in the background expectation. |
| Summary |
| Using up to 19.7 fb$^{-1}$ of pp collision data taken at $\sqrt{s}=$ 8 TeV, we search for direct top squark pair production in both single-lepton and dilepton final states. In both searches the standard model background, dominated by the $\mathrm{ t \bar{t} }$ process, is predicted using control samples in data. In this single-lepton search, we improve the results of Ref. [12] by employing an upgraded multivariate tool for signal selection, fed by both kinematic and topological variables and specifically trained for different decay modes and kinematic regions. This systematic approach to the signal selection, where the discriminating power of each selection variable is quantitatively assessed, is a key feature of the single-lepton search. The background determination method has also been improved compared to Ref. [12]. In the dilepton search the signal selection is based on the ${M_{\mathrm{T2}}}$ $S{\ell}{\ell}$ variable. In both searches, the effect of the signal contamination is accounted for. No excess above the predicted background is observed in either search. Simplified models (Fig. 1) are used to interpret the results in terms of a region in the ${ ( {{m}}( \tilde{ \mathrm{t} }_1), {{m}}({\tilde{\chi}^0_1} ) )}$ plane, excluded at 95% CL. We combine the results of both searches for maximal sensitivity; the sensitivity depends on the decay mode, and on the ${ ( {{m}}( \tilde{ \mathrm{t} }_1 ), {{m}}({\tilde{\chi}^0_1} ) )}$ signal point. The highest excluded $ \tilde{ \mathrm{t} }_1 $ and ${\tilde{\chi}^0_1} $ masses are about 700 GeV and 250 GeV, respectively. |
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