CMS-B2G-17-012 ; CERN-EP-2018-290 | ||
Search for vector-like quarks in events with two oppositely charged leptons and jets in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
CMS Collaboration | ||
24 December 2018 | ||
Eur. Phys. J. C 79 (2019) 364 | ||
Abstract: A search for the pair production of heavy vector-like partners T and B of the top and bottom quarks has been performed by the CMS experiment at the CERN LHC using proton-proton collisions at $\sqrt{s} = $ 13 TeV. The data sample was collected in 2016 and corresponds to an integrated luminosity of 35.9 fb$^{-1}$. Final states studied for ${{\mathrm{T}} } {\overline{{\mathrm{T}} }}$ production include those where one of the T quarks decays via $ {{\mathrm{T}} \to\mathrm{t}\mathrm{Z}} $ and the other via $ {{\mathrm{T}} \to\mathrm{b}\mathrm{W}} $, $ {\mathrm{t}\mathrm{Z}} $, or $ {\mathrm{t}\mathrm{H}} $, where H is a Higgs boson. For the $ {{\mathrm{B}} } {\overline{{\mathrm{B}} }} $ case, final states include those where one of the B quarks decays via $ {{\mathrm{B}} \to\mathrm{b}\mathrm{Z}} $ and the other $ {{\mathrm{B}} \to\mathrm{t}\mathrm{W}} $, $ {\mathrm{b}\mathrm{Z}} $, or $ {\mathrm{b}\mathrm{H}} $. Events with two oppositely charged electrons or muons, consistent with coming from the decay of a Z boson, and jets are investigated. The number of observed events is consistent with standard model background estimations. Lower limits at 95% confidence level are placed on the masses of the T and B quarks for a range of branching fractions. Assuming 100% branching fractions for $ {{\mathrm{T}} \to\mathrm{t}\mathrm{Z}} $, and $ {{\mathrm{B}} \to\mathrm{b}\mathrm{Z}} $, T and B quark mass values below 1280 and 1130 GeV, respectively, are excluded. | ||
Links: e-print arXiv:1812.09768 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
Leading-order Feynman diagrams for the pair production and decay of T (left) and B (right) VLQs relevant to final states considered in this analysis. |
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Figure 1-a:
Leading-order Feynman diagrams for the pair production and decay of T (left) and B (right) VLQs relevant to final states considered in this analysis. |
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Figure 1-b:
Leading-order Feynman diagrams for the pair production and decay of T (left) and B (right) VLQs relevant to final states considered in this analysis. |
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Figure 2:
The ${S_{\mathrm {T}}}$ distributions for the CR1 b+low-${S_{\mathrm {T}}}$ (left) and CR0 b+high-${S_{\mathrm {T}}}$ (right) control regions for the data (points) and the background simulations (shaded histograms) after applying the scale factors given in Table xxxxx. The vertical bars on the points represent the statistical uncertainties in the data. The hatched bands indicate the total uncertainties in the simulated background contributions added in quadrature. The lower plots show the difference between the data and the simulated background, divided by the total uncertainty. |
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Figure 2-a:
The ${S_{\mathrm {T}}}$ distributions for the CR1 b+low-${S_{\mathrm {T}}}$ (left) and CR0 b+high-${S_{\mathrm {T}}}$ (right) control regions for the data (points) and the background simulations (shaded histograms) after applying the scale factors given in Table xxxxx. The vertical bars on the points represent the statistical uncertainties in the data. The hatched bands indicate the total uncertainties in the simulated background contributions added in quadrature. The lower plots show the difference between the data and the simulated background, divided by the total uncertainty. |
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Figure 2-b:
The ${S_{\mathrm {T}}}$ distributions for the CR1 b+low-${S_{\mathrm {T}}}$ (left) and CR0 b+high-${S_{\mathrm {T}}}$ (right) control regions for the data (points) and the background simulations (shaded histograms) after applying the scale factors given in Table xxxxx. The vertical bars on the points represent the statistical uncertainties in the data. The hatched bands indicate the total uncertainties in the simulated background contributions added in quadrature. The lower plots show the difference between the data and the simulated background, divided by the total uncertainty. |
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Figure 3:
The ${S_{\mathrm {T}}}$ distributions for groups A, B, C, D (left to right, upper to lower) from data (points with vertical and horizontal bars), the expected SM backgrounds (shaded histograms), and the expected signal, scaled up by a factor 2, for $ {{{\mathrm {T}}} {\overline {{\mathrm {T}}}} \to {\mathrm {t}} {\mathrm {Z}} {\mathrm {t}} {\mathrm {Z}}} $ with $m_{{\mathrm {T}}} = $ 1200 GeV (dotted lines). The vertical bars on the points show the central 68% CL intervals for Poisson-distributed data. The horizontal bars give the bin widths. The hatched bands represent the statistical and systematic uncertainties in the total background contribution added in quadrature. The lower plots give the difference between the data and the total expected background, divided by the total background uncertainty. |
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Figure 3-a:
The ${S_{\mathrm {T}}}$ distributions for groups A, B, C, D (left to right, upper to lower) from data (points with vertical and horizontal bars), the expected SM backgrounds (shaded histograms), and the expected signal, scaled up by a factor 2, for $ {{{\mathrm {T}}} {\overline {{\mathrm {T}}}} \to {\mathrm {t}} {\mathrm {Z}} {\mathrm {t}} {\mathrm {Z}}} $ with $m_{{\mathrm {T}}} = $ 1200 GeV (dotted lines). The vertical bars on the points show the central 68% CL intervals for Poisson-distributed data. The horizontal bars give the bin widths. The hatched bands represent the statistical and systematic uncertainties in the total background contribution added in quadrature. The lower plots give the difference between the data and the total expected background, divided by the total background uncertainty. |
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Figure 3-b:
The ${S_{\mathrm {T}}}$ distributions for groups A, B, C, D (left to right, upper to lower) from data (points with vertical and horizontal bars), the expected SM backgrounds (shaded histograms), and the expected signal, scaled up by a factor 2, for $ {{{\mathrm {T}}} {\overline {{\mathrm {T}}}} \to {\mathrm {t}} {\mathrm {Z}} {\mathrm {t}} {\mathrm {Z}}} $ with $m_{{\mathrm {T}}} = $ 1200 GeV (dotted lines). The vertical bars on the points show the central 68% CL intervals for Poisson-distributed data. The horizontal bars give the bin widths. The hatched bands represent the statistical and systematic uncertainties in the total background contribution added in quadrature. The lower plots give the difference between the data and the total expected background, divided by the total background uncertainty. |
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Figure 3-c:
The ${S_{\mathrm {T}}}$ distributions for groups A, B, C, D (left to right, upper to lower) from data (points with vertical and horizontal bars), the expected SM backgrounds (shaded histograms), and the expected signal, scaled up by a factor 2, for $ {{{\mathrm {T}}} {\overline {{\mathrm {T}}}} \to {\mathrm {t}} {\mathrm {Z}} {\mathrm {t}} {\mathrm {Z}}} $ with $m_{{\mathrm {T}}} = $ 1200 GeV (dotted lines). The vertical bars on the points show the central 68% CL intervals for Poisson-distributed data. The horizontal bars give the bin widths. The hatched bands represent the statistical and systematic uncertainties in the total background contribution added in quadrature. The lower plots give the difference between the data and the total expected background, divided by the total background uncertainty. |
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Figure 3-d:
The ${S_{\mathrm {T}}}$ distributions for groups A, B, C, D (left to right, upper to lower) from data (points with vertical and horizontal bars), the expected SM backgrounds (shaded histograms), and the expected signal, scaled up by a factor 2, for $ {{{\mathrm {T}}} {\overline {{\mathrm {T}}}} \to {\mathrm {t}} {\mathrm {Z}} {\mathrm {t}} {\mathrm {Z}}} $ with $m_{{\mathrm {T}}} = $ 1200 GeV (dotted lines). The vertical bars on the points show the central 68% CL intervals for Poisson-distributed data. The horizontal bars give the bin widths. The hatched bands represent the statistical and systematic uncertainties in the total background contribution added in quadrature. The lower plots give the difference between the data and the total expected background, divided by the total background uncertainty. |
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Figure 4:
The observed (solid line) and expected (dashed line) 95% CL upper limits on the $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ cross section as a function of the T quark mass assuming (upper left) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = $ 100%, (upper right) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) = $ 50%, and (lower) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 50%. The dotted-dashed curve displays the theoretical $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ production cross section. The inner and outer bands show the one and two standard deviation uncertainties in the expected limits, respectively. |
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Figure 4-a:
The observed (solid line) and expected (dashed line) 95% CL upper limits on the $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ cross section as a function of the T quark mass assuming (upper left) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = $ 100%, (upper right) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) = $ 50%, and (lower) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 50%. The dotted-dashed curve displays the theoretical $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ production cross section. The inner and outer bands show the one and two standard deviation uncertainties in the expected limits, respectively. |
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Figure 4-b:
The observed (solid line) and expected (dashed line) 95% CL upper limits on the $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ cross section as a function of the T quark mass assuming (upper left) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = $ 100%, (upper right) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) = $ 50%, and (lower) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 50%. The dotted-dashed curve displays the theoretical $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ production cross section. The inner and outer bands show the one and two standard deviation uncertainties in the expected limits, respectively. |
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Figure 4-c:
The observed (solid line) and expected (dashed line) 95% CL upper limits on the $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ cross section as a function of the T quark mass assuming (upper left) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = $ 100%, (upper right) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) = $ 50%, and (lower) $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 50%. The dotted-dashed curve displays the theoretical $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ production cross section. The inner and outer bands show the one and two standard deviation uncertainties in the expected limits, respectively. |
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Figure 5:
The observed (left) and expected (right) 95% CL lower limits on the mass of the T (upper) and B (lower) quark, in GeV, for various branching fraction scenarios, assuming $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 1 and $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 1, respectively. |
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Figure 5-a:
The observed (left) and expected (right) 95% CL lower limits on the mass of the T (upper) and B (lower) quark, in GeV, for various branching fraction scenarios, assuming $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 1 and $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 1, respectively. |
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Figure 5-b:
The observed (left) and expected (right) 95% CL lower limits on the mass of the T (upper) and B (lower) quark, in GeV, for various branching fraction scenarios, assuming $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 1 and $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 1, respectively. |
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Figure 5-c:
The observed (left) and expected (right) 95% CL lower limits on the mass of the T (upper) and B (lower) quark, in GeV, for various branching fraction scenarios, assuming $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 1 and $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 1, respectively. |
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Figure 5-d:
The observed (left) and expected (right) 95% CL lower limits on the mass of the T (upper) and B (lower) quark, in GeV, for various branching fraction scenarios, assuming $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 1 and $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) + \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 1, respectively. |
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Figure 6:
The ${S_{\mathrm {T}}}$ distributions for the 1 b, 2 b, boosted t, boosted H and boosted Z (left to right, upper to lower) event categories for the data (points with vertical and horizontal bars), and the expected background (shaded histograms). The vertical bars give the statistical uncertainty in the data, and the horizontal bars show the bin widths. The expected signal for $ {{{\mathrm {B}}} {\overline {{\mathrm {B}}}} \to {\mathrm {b}} {\mathrm {Z}} {\mathrm {b}} {\mathrm {Z}}} $ with $m_{{\mathrm {B}}} = $ 1200 GeV multiplied by a factor of 5 is shown by the dashed line. The statistical and systematic uncertainties in the SM background prediction, added in quadrature, are represented by the hatched bands. The lower panel in each plot show the difference between the data and the expected background, divided by the total uncertainty. |
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Figure 6-a:
The ${S_{\mathrm {T}}}$ distributions for the 1 b, 2 b, boosted t, boosted H and boosted Z (left to right, upper to lower) event categories for the data (points with vertical and horizontal bars), and the expected background (shaded histograms). The vertical bars give the statistical uncertainty in the data, and the horizontal bars show the bin widths. The expected signal for $ {{{\mathrm {B}}} {\overline {{\mathrm {B}}}} \to {\mathrm {b}} {\mathrm {Z}} {\mathrm {b}} {\mathrm {Z}}} $ with $m_{{\mathrm {B}}} = $ 1200 GeV multiplied by a factor of 5 is shown by the dashed line. The statistical and systematic uncertainties in the SM background prediction, added in quadrature, are represented by the hatched bands. The lower panel in each plot show the difference between the data and the expected background, divided by the total uncertainty. |
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Figure 6-b:
The ${S_{\mathrm {T}}}$ distributions for the 1 b, 2 b, boosted t, boosted H and boosted Z (left to right, upper to lower) event categories for the data (points with vertical and horizontal bars), and the expected background (shaded histograms). The vertical bars give the statistical uncertainty in the data, and the horizontal bars show the bin widths. The expected signal for $ {{{\mathrm {B}}} {\overline {{\mathrm {B}}}} \to {\mathrm {b}} {\mathrm {Z}} {\mathrm {b}} {\mathrm {Z}}} $ with $m_{{\mathrm {B}}} = $ 1200 GeV multiplied by a factor of 5 is shown by the dashed line. The statistical and systematic uncertainties in the SM background prediction, added in quadrature, are represented by the hatched bands. The lower panel in each plot show the difference between the data and the expected background, divided by the total uncertainty. |
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Figure 6-c:
The ${S_{\mathrm {T}}}$ distributions for the 1 b, 2 b, boosted t, boosted H and boosted Z (left to right, upper to lower) event categories for the data (points with vertical and horizontal bars), and the expected background (shaded histograms). The vertical bars give the statistical uncertainty in the data, and the horizontal bars show the bin widths. The expected signal for $ {{{\mathrm {B}}} {\overline {{\mathrm {B}}}} \to {\mathrm {b}} {\mathrm {Z}} {\mathrm {b}} {\mathrm {Z}}} $ with $m_{{\mathrm {B}}} = $ 1200 GeV multiplied by a factor of 5 is shown by the dashed line. The statistical and systematic uncertainties in the SM background prediction, added in quadrature, are represented by the hatched bands. The lower panel in each plot show the difference between the data and the expected background, divided by the total uncertainty. |
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Figure 6-d:
The ${S_{\mathrm {T}}}$ distributions for the 1 b, 2 b, boosted t, boosted H and boosted Z (left to right, upper to lower) event categories for the data (points with vertical and horizontal bars), and the expected background (shaded histograms). The vertical bars give the statistical uncertainty in the data, and the horizontal bars show the bin widths. The expected signal for $ {{{\mathrm {B}}} {\overline {{\mathrm {B}}}} \to {\mathrm {b}} {\mathrm {Z}} {\mathrm {b}} {\mathrm {Z}}} $ with $m_{{\mathrm {B}}} = $ 1200 GeV multiplied by a factor of 5 is shown by the dashed line. The statistical and systematic uncertainties in the SM background prediction, added in quadrature, are represented by the hatched bands. The lower panel in each plot show the difference between the data and the expected background, divided by the total uncertainty. |
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Figure 6-e:
The ${S_{\mathrm {T}}}$ distributions for the 1 b, 2 b, boosted t, boosted H and boosted Z (left to right, upper to lower) event categories for the data (points with vertical and horizontal bars), and the expected background (shaded histograms). The vertical bars give the statistical uncertainty in the data, and the horizontal bars show the bin widths. The expected signal for $ {{{\mathrm {B}}} {\overline {{\mathrm {B}}}} \to {\mathrm {b}} {\mathrm {Z}} {\mathrm {b}} {\mathrm {Z}}} $ with $m_{{\mathrm {B}}} = $ 1200 GeV multiplied by a factor of 5 is shown by the dashed line. The statistical and systematic uncertainties in the SM background prediction, added in quadrature, are represented by the hatched bands. The lower panel in each plot show the difference between the data and the expected background, divided by the total uncertainty. |
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Figure 7:
The observed (solid line) and expected (dashed line) 95% CL upper limits on the $ {{\mathrm {B}}} {\overline {{\mathrm {B}}}} $ production cross section versus the B quark mass for (upper left) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = $ 100%, (upper right) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) = $ 50%, and (lower) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 50%. The dotted-dashed line displays the theoretical cross section. The inner and outer bands show the one and two standard deviation uncertainties in the expected limits, respectively. |
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Figure 7-a:
The observed (solid line) and expected (dashed line) 95% CL upper limits on the $ {{\mathrm {B}}} {\overline {{\mathrm {B}}}} $ production cross section versus the B quark mass for (upper left) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = $ 100%, (upper right) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) = $ 50%, and (lower) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 50%. The dotted-dashed line displays the theoretical cross section. The inner and outer bands show the one and two standard deviation uncertainties in the expected limits, respectively. |
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Figure 7-b:
The observed (solid line) and expected (dashed line) 95% CL upper limits on the $ {{\mathrm {B}}} {\overline {{\mathrm {B}}}} $ production cross section versus the B quark mass for (upper left) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = $ 100%, (upper right) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) = $ 50%, and (lower) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 50%. The dotted-dashed line displays the theoretical cross section. The inner and outer bands show the one and two standard deviation uncertainties in the expected limits, respectively. |
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Figure 7-c:
The observed (solid line) and expected (dashed line) 95% CL upper limits on the $ {{\mathrm {B}}} {\overline {{\mathrm {B}}}} $ production cross section versus the B quark mass for (upper left) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = $ 100%, (upper right) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) = $ 50%, and (lower) $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 50%. The dotted-dashed line displays the theoretical cross section. The inner and outer bands show the one and two standard deviation uncertainties in the expected limits, respectively. |
Tables | |
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Table 1:
Event selection criteria. |
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Table 2:
The different event groups used for the $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ search, classified according to the number of b-tagged jets $N_{{\mathrm {b}}}$ and the number of $ {{\mathrm {V}} \to {{\mathrm {q}} {\overline {\mathrm {q}}}}} $, $ {{\mathrm {H}} \to {{\mathrm {b}} {\overline {\mathrm {b}}}}} $, and $ {\mathrm {t}}\to {{\mathrm {q}} {\overline {\mathrm {q}}} ^\prime} {\mathrm {b}}$ candidates in the event, $N_{{\mathrm {V}}}$, $N_{{\mathrm {H}}}$ and $N_{{\mathrm {t}}}$, respectively, identified using both the jet substructure and resolved tagger algorithms. |
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Table 3:
The different event categories used for the $ {{\mathrm {B}}} {\overline {{\mathrm {B}}}} $ search, classified according to the number of AK4 b-tagged jets $N_{{\mathrm {b}}}$ and the number of $ {{\mathrm {V}} \to {{\mathrm {q}} {\overline {\mathrm {q}}}}} $, $ {{\mathrm {H}} \to {{\mathrm {b}} {\overline {\mathrm {b}}}}} $, and $ {\mathrm {t}}\to {{\mathrm {q}} {\overline {\mathrm {q}}} ^\prime} {\mathrm {b}}$ candidates in the event, $N_{{\mathrm {V}}}$, $N_{{\mathrm {H}}}$, and $N_{{\mathrm {t}}}$, respectively, identified using the jet substructure algorithm. |
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Table 4:
The scale factors determined from data for correcting the AK4 jet multiplicity distribution in the simulation. The quoted uncertainties in the scale factors are statistical only. |
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Table 5:
The number of observed events and the predicted number of SM background events in the $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ search using $ {{\mathrm {Z}} \to {{\mathrm {e}^+} {\mathrm {e}^-}}} $ channel in the four event groups. The expected numbers of signal events for T quark masses of 800 and 1200 GeV for three different decay scenarios with assumed branching fractions $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = $ 100% (tZtZ), $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) = $ 50% (tZtH), and $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 50% (tZbW) are also shown. The uncertainties in the number of expected background events include the statistical and systematic uncertainties added in quadrature. |
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Table 6:
The number of observed events and the predicted number of SM background events in the $ {{\mathrm {T}}} {\overline {{\mathrm {T}}}} $ search using $ {{\mathrm {Z}} \to {{{\mu ^+}} {{\mu ^-}}}} $ channel in the four event groups. The expected numbers of signal events for T quark masses of 800 and 1200 GeV for three different decay scenarios with assumed branching fractions $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = $ 100% (tZtZ), $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {H}}}) = $ 50% (tZtH), and $\mathcal {B} ({{\mathrm {T}} \to {\mathrm {t}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {T}} \to {\mathrm {b}} {\mathrm {W}}}) = $ 50% (tZbW) are also shown. The uncertainties in the number of expected background events include the statistical and systematic uncertainties added in quadrature. |
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Table 7:
The numbers of observed events and the predicted number of SM background events in the $ {{\mathrm {B}}} {\overline {{\mathrm {B}}}} $ search for the five event categories using $ {{\mathrm {Z}} \to {{\mathrm {e}^+} {\mathrm {e}^-}}} $ channel. The expected numbers of signal events for B masses of 800 and 1200 GeV with branching fraction hypotheses for the three decay channels, $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = $ 100% (bZbZ), $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) = $ 50% (bZbH), and $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 50% (bZtW) are also shown. The uncertainties in the number of expected background events include the statistical and systematic uncertainties added in quadrature. |
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Table 8:
The number of observed events and the predicted number of SM background events in the $ {{\mathrm {B}}} {\overline {{\mathrm {B}}}} $ search for the five event categories using $ {{\mathrm {Z}} \to {{{\mu ^+}} {{\mu ^-}}}} $ channel. The expected numbers of signal events for B masses of 800 and 1200 GeV with branching fraction hypotheses for the three decay channels, $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = $ 100% (bZbZ), $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {H}}}) = $ 50% (bZbH), and $\mathcal {B} ({{\mathrm {B}} \to {\mathrm {b}} {\mathrm {Z}}}) = \mathcal {B} ({{\mathrm {B}} \to {\mathrm {t}} {\mathrm {W}}}) = $ 50% (bZtW) are also shown. The uncertainties in the number of expected background events include the statistical and systematic uncertainties added in quadrature. |
Summary |
The results of a search have been presented for the pair production of vector-like top (T) and bottom (B) quark partners in proton-proton collisions at $\sqrt{s} = $ 13 TeV, using data collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The ${{\mathrm{T}} } {\overline{{\mathrm{T}} }}$ search is performed by looking for events in which one T quark decays via $ {{\mathrm{T}} \to\mathrm{t}\mathrm{Z}} $ and the other decays via $ {{\mathrm{T}} \to\mathrm{b}\mathrm{W}} $, $ {\mathrm{t}\mathrm{Z}} $, $ {\mathrm{t}\mathrm{H}} $, where H refers to the Higgs boson. The $ {{\mathrm{B}} } {\overline{{\mathrm{B}} }} $ search looks for events in which one B quark decays via $ {{\mathrm{B}} \to\mathrm{b}\mathrm{Z}} $ and the other via $ {{\mathrm{B}} \to\mathrm{t}\mathrm{W}} $, $ {\mathrm{b}\mathrm{Z}} $, or $ {\mathrm{b}\mathrm{H}} $. Events with two oppositely charged electrons or muons, consistent with coming from the decay of a Z boson, and jets are investigated, and are categorized according to the numbers of top quark and W, Z, and Higgs boson candidates. These categories are individually optimized for ${{\mathrm{T}} } {\overline{{\mathrm{T}} }}$ and $ {{\mathrm{B}} } {\overline{{\mathrm{B}} }} $ event topologies. The data are in agreement with the standard model background predictions for all the event categories. Upper limits at 95% confidence level on the ${{\mathrm{T}} } {\overline{{\mathrm{T}} }}$ and $ {{\mathrm{B}} } {\overline{{\mathrm{B}} }} $ production cross sections are obtained from a simultaneous binned maximum-likelihood fit to the observed distributions for the different event categories, under the assumption of various T and B quark branching fractions. Comparing these upper limits to the theoretical predictions for the ${{\mathrm{T}} } {\overline{{\mathrm{T}} }}$ and $ {{\mathrm{B}} } {\overline{{\mathrm{B}} }} $ cross sections as a function of the T and B quark masses, lower limits on the masses at 95% confidence level are determined for different branching fraction scenarios. In the case of a T quark decaying exclusively via $ {{\mathrm{T}} \to\mathrm{t}\mathrm{Z}} $, the lower mass limit is 1280 GeV, while for a B quark decaying only via $ {{\mathrm{B}} \to\mathrm{b}\mathrm{Z}} $, it is 1130 GeV. These lower limits are comparable with those measured by the ATLAS Collaboration [20], also using the Z boson dilepton decay channel. The results of the analysis presented in this paper are complementary to previous CMS measurements [21,22,23], and have extended sensitivity in reaching higher mass limits for T and B quarks. |
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