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| CMS-B2G-13-006 ; CERN-PH-EP-2015-170 | ||
| Search for pair-produced vectorlike B quarks in proton-proton collisions at $\sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 25 July 2015 | ||
| Phys. Rev. D 93 (2016) 112009 | ||
| Abstract: A search for the production of a heavy B quark, having electric charge -1/3 and vector couplings to W, Z, and H bosons, is carried out using proton-proton collision data recorded at the CERN LHC by the CMS experiment, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The B quark is assumed to be pair produced and to decay in one of three ways: to $\mathrm{ t }\mathrm{ W }$, $\mathrm{ b }\mathrm{ Z }$, or $\mathrm{ b }\mathrm{ H }$. The search is carried out in final states with one, two, and more than two charged leptons, as well as in fully hadronic final states. Each of the channels in the exclusive final-state topologies is designed to be sensitive to specific combinations of the B quark-antiquark pair decays. The observed event yields are found to be consistent with the standard model expectations in all the final states studied. A statistical combination of these results is performed, and upper limits are set on the cross section of the strongly produced B quark-antiquark pairs as a function of the B quark mass. Lower limits on the B quark mass between 740 and 900 GeV are set at a 95% confidence level, depending on the values of the branching fractions of the B quark to $\mathrm{ t }\mathrm{ W }$, $\mathrm{ b }\mathrm{ Z }$, and $\mathrm{ b }\mathrm{ H }$. Overall, these limits are the most stringent to date. | ||
| Links: e-print arXiv:1507.07129 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1-a:
Feynman diagrams for the dominant B quark pair-production process (top) and for the B quark decay modes (bottom). |
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Figure 1-b:
Feynman diagrams for the dominant B quark pair-production process (top) and for the B quark decay modes (bottom). |
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Figure 1-c:
Feynman diagrams for the dominant B quark pair-production process (top) and for the B quark decay modes (bottom). |
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Figure 1-d:
Feynman diagrams for the dominant B quark pair-production process (top) and for the B quark decay modes (bottom). |
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Figure 2:
The reconstructed mass of the B quark candidate in the opposite-sign lepton pair channel, using the invariant mass of the dilepton and b-jet in simulated events. |
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Figure 3-a:
The $ {S_{\mathrm {T}}} $ distributions in the 0, 1, and ${\geq} $2 V-tag categories in the electron+jets channel (a,c,e) and muon+jets channel (b,d,f). The uncertainty bands shown include statistical and all systematic uncertainties, added in quadrature for each single bin. The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 3-b:
The $ {S_{\mathrm {T}}} $ distributions in the 0, 1, and ${\geq} $2 V-tag categories in the electron+jets channel (a,c,e) and muon+jets channel (b,d,f). The uncertainty bands shown include statistical and all systematic uncertainties, added in quadrature for each single bin. The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 3-c:
The $ {S_{\mathrm {T}}} $ distributions in the 0, 1, and ${\geq} $2 V-tag categories in the electron+jets channel (a,c,e) and muon+jets channel (b,d,f). The uncertainty bands shown include statistical and all systematic uncertainties, added in quadrature for each single bin. The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 3-d:
The $ {S_{\mathrm {T}}} $ distributions in the 0, 1, and ${\geq} $2 V-tag categories in the electron+jets channel (a,c,e) and muon+jets channel (b,d,f). The uncertainty bands shown include statistical and all systematic uncertainties, added in quadrature for each single bin. The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 3-e:
The $ {S_{\mathrm {T}}} $ distributions in the 0, 1, and ${\geq} $2 V-tag categories in the electron+jets channel (a,c,e) and muon+jets channel (b,d,f). The uncertainty bands shown include statistical and all systematic uncertainties, added in quadrature for each single bin. The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 3-f:
The $ {S_{\mathrm {T}}} $ distributions in the 0, 1, and ${\geq} $2 V-tag categories in the electron+jets channel (a,c,e) and muon+jets channel (b,d,f). The uncertainty bands shown include statistical and all systematic uncertainties, added in quadrature for each single bin. The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 4-a:
The ${S_{\mathrm {T}}}$ distributions used for signal discrimination in the same-sign dilepton channel. The distribution is shown for the three dilepton categories used: dielectron (a), dimuon (b), and electron-muon (c). The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 4-b:
The ${S_{\mathrm {T}}}$ distributions used for signal discrimination in the same-sign dilepton channel. The distribution is shown for the three dilepton categories used: dielectron (a), dimuon (b), and electron-muon (c). The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 4-c:
The ${S_{\mathrm {T}}}$ distributions used for signal discrimination in the same-sign dilepton channel. The distribution is shown for the three dilepton categories used: dielectron (a), dimuon (b), and electron-muon (c). The horizontal bars on the data points indicate the bin width. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 5-a:
The event distribution in the plane of $N_\text {jets}$ vs the b-tagging discriminator value, used to define the regions $A$, $B$, $C$, and $D$ for the opposite-sign dilepton ${{\mathrm{ Z } } \to \mathrm{ e }^- \mathrm{ e }^+ }$ (a) and ${{\mathrm{ Z } } \to \mu^+ \mu^- }$ (b) channels. The region $B$ is the signal region while the others constitute the control regions. The regions $I$, $J$, $K$, and $L$ are used for estimation of systematic uncertainties. All other selection criteria used to select the B quark candidates have been applied. The area of each bar is proportional to the number of events in a given bin of the distribution of $N_\text {jets}$ vs b-tagging discriminator. |
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Figure 5-b:
The event distribution in the plane of $N_\text {jets}$ vs the b-tagging discriminator value, used to define the regions $A$, $B$, $C$, and $D$ for the opposite-sign dilepton ${{\mathrm{ Z } } \to \mathrm{ e }^- \mathrm{ e }^+ }$ (a) and ${{\mathrm{ Z } } \to \mu^+ \mu^- }$ (b) channels. The region $B$ is the signal region while the others constitute the control regions. The regions $I$, $J$, $K$, and $L$ are used for estimation of systematic uncertainties. All other selection criteria used to select the B quark candidates have been applied. The area of each bar is proportional to the number of events in a given bin of the distribution of $N_\text {jets}$ vs b-tagging discriminator. |
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Figure 6-a:
The invariant mass of reconstructed B quark candidates in the opposite-sign dilepton ${{\mathrm{ Z } } \to \mathrm{ e }^- \mathrm{ e }^+ }$ (a) and ${{\mathrm{ Z } } \to \mu^+ \mu^- } $ (b) channels. The estimated background is shown by the solid line, along with the total uncertainty (hatched area). The last bin of the histograms contains all events with $ {M(\ell \ell \mathrm{ b } )} > $ 1000 GeV. The signal contribution is shown for two B quark masses. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 6-b:
The invariant mass of reconstructed B quark candidates in the opposite-sign dilepton ${{\mathrm{ Z } } \to \mathrm{ e }^- \mathrm{ e }^+ }$ (a) and ${{\mathrm{ Z } } \to \mu^+ \mu^- } $ (b) channels. The estimated background is shown by the solid line, along with the total uncertainty (hatched area). The last bin of the histograms contains all events with $ {M(\ell \ell \mathrm{ b } )} > $ 1000 GeV. The signal contribution is shown for two B quark masses. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 7-a:
The transverse mass $M_{\mathrm{T}}$ distribution of events in a control sample of the multilepton analysis, enriched in $\mathrm{ W } {\mathrm{ Z } } $ by requiring an OSSF pair with $M(\ell \ell )$ in the Z boson mass window and 50 $ < {E_{\mathrm {T}}} < $ 100 GeV (a). The ${S_{\mathrm {T}}}$ distribution for events containing an opposite-sign $\mathrm{ e } \mu $ pair in the ${\mathrm{ t \bar{t} } }$ control region of the multilepton analysis (b). Uncertainties include both statistical and systematic contributions. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 7-b:
The transverse mass $M_{\mathrm{T}}$ distribution of events in a control sample of the multilepton analysis, enriched in $\mathrm{ W } {\mathrm{ Z } } $ by requiring an OSSF pair with $M(\ell \ell )$ in the Z boson mass window and 50 $ < {E_{\mathrm {T}}} < $ 100 GeV (a). The ${S_{\mathrm {T}}}$ distribution for events containing an opposite-sign $\mathrm{ e } \mu $ pair in the ${\mathrm{ t \bar{t} } }$ control region of the multilepton analysis (b). Uncertainties include both statistical and systematic contributions. The difference between the observed and expected events divided by the total statistical and systematic uncertainty of the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 8-a:
Distributions of ${S_{\mathrm {T}}}$ for two event categorizations in the multilepton channel: (a) three leptons, no tau leptons, at least one b-tagged jet, and no reconstructed Z boson candidate; (b) four leptons, one tau lepton, no b-tagged jets, and one reconstructed Z boson candidate. Uncertainties include both statistical and systematic contributions. The data-driven contribution includes contributions from two genuine leptons and one or more misidentified leptons. The horizontal bars on the data points indicate the bin width. |
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Figure 8-b:
Distributions of ${S_{\mathrm {T}}}$ for two event categorizations in the multilepton channel: (a) three leptons, no tau leptons, at least one b-tagged jet, and no reconstructed Z boson candidate; (b) four leptons, one tau lepton, no b-tagged jets, and one reconstructed Z boson candidate. Uncertainties include both statistical and systematic contributions. The data-driven contribution includes contributions from two genuine leptons and one or more misidentified leptons. The horizontal bars on the data points indicate the bin width. |
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Figure 9-a:
Background closure test in the data control samples of the all-hadronic analysis with no b jets and exactly one AK5 jet with $ {p_{\mathrm {T}}} > $ 80 GeV (a) and with $\ge $2 AK5 jets with $ {p_{\mathrm {T}}} > $ 80 GeV (b). The red data points represent the actual background as derived from data. The blue dashed line represents the predicted background, with the hatched area depicting the corresponding uncertainty. The difference between the observed and predicted background divided by the total uncertainty in the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 9-b:
Background closure test in the data control samples of the all-hadronic analysis with no b jets and exactly one AK5 jet with $ {p_{\mathrm {T}}} > $ 80 GeV (a) and with $\ge $2 AK5 jets with $ {p_{\mathrm {T}}} > $ 80 GeV (b). The red data points represent the actual background as derived from data. The blue dashed line represents the predicted background, with the hatched area depicting the corresponding uncertainty. The difference between the observed and predicted background divided by the total uncertainty in the background prediction (pull) is shown for each bin in the lower panels. |
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Figure 10-a:
The ${H_{\mathrm {T}}}$ distributions in the 1 b jet (a) and the $\ge $2 b jets event categories (b) for the all-hadronic analysis. The blue solid line depicts the estimated background, with the hatched area showing the measured uncertainty in the background. The signal contributions for two B quark mass points, 500 and 800 GeV , are overlaid. The bin width is chosen to have a statistically significant number of events in every bin. The difference between the data and the estimated background divided by the total uncertainty in the background (pull) is shown for each bin in the lower panels. |
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Figure 10-b:
The ${H_{\mathrm {T}}}$ distributions in the 1 b jet (a) and the $\ge $2 b jets event categories (b) for the all-hadronic analysis. The blue solid line depicts the estimated background, with the hatched area showing the measured uncertainty in the background. The signal contributions for two B quark mass points, 500 and 800 GeV , are overlaid. The bin width is chosen to have a statistically significant number of events in every bin. The difference between the data and the estimated background divided by the total uncertainty in the background (pull) is shown for each bin in the lower panels. |
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Figure 11-a:
Comparison of individual channel limit results with those of the combination, for the expected limits only. Shown are the limit results, at 95% CL , for the three corners of the triangular parameter space, for 100% branching fractions for B to $\mathrm{ t } \mathrm{ W } $ (a), $\mathrm{ b } {\mathrm{ Z } } $ (b), and $\mathrm{ b } \mathrm{ H } $ (c). The same-sign and opposite-sign channels are denoted by ``SS'' and ``OS'', respectively. The step observed in the $\mathrm{ b } {\mathrm{ Z } } $ limit curve is due to two analysis channels (multilepton and OS dilepton) that do not contribute to the combination for B quark masses above 800 GeV. |
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Figure 11-b:
Comparison of individual channel limit results with those of the combination, for the expected limits only. Shown are the limit results, at 95% CL , for the three corners of the triangular parameter space, for 100% branching fractions for B to $\mathrm{ t } \mathrm{ W } $ (a), $\mathrm{ b } {\mathrm{ Z } } $ (b), and $\mathrm{ b } \mathrm{ H } $ (c). The same-sign and opposite-sign channels are denoted by ``SS'' and ``OS'', respectively. The step observed in the $\mathrm{ b } {\mathrm{ Z } } $ limit curve is due to two analysis channels (multilepton and OS dilepton) that do not contribute to the combination for B quark masses above 800 GeV. |
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Figure 11-c:
Comparison of individual channel limit results with those of the combination, for the expected limits only. Shown are the limit results, at 95% CL , for the three corners of the triangular parameter space, for 100% branching fractions for B to $\mathrm{ t } \mathrm{ W } $ (a), $\mathrm{ b } {\mathrm{ Z } } $ (b), and $\mathrm{ b } \mathrm{ H } $ (c). The same-sign and opposite-sign channels are denoted by ``SS'' and ``OS'', respectively. The step observed in the $\mathrm{ b } {\mathrm{ Z } } $ limit curve is due to two analysis channels (multilepton and OS dilepton) that do not contribute to the combination for B quark masses above 800 GeV. |
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Figure 12-a:
Expected (a) and observed (b) limits for each combination of branching fractions to $\mathrm{ t } \mathrm{ W } $, $\mathrm{ b } {\mathrm{ Z } } $, and $\mathrm{ b } \mathrm{ H } $ obtained by the combination of channels. The color scale represents the mass exclusion limit obtained at each point. The branching fraction for the B quark decay to $\mathrm{ b } {\mathrm{ Z } } $ can be obtained through the relation $\mathcal {B}(\mathrm{ b } {\mathrm{ Z } } ) = 1 - \mathcal {B}(\mathrm{ t } \mathrm{ W } ) - \mathcal {B}(\mathrm{ b } \mathrm{ H } )$. |
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Figure 12-b:
Expected (a) and observed (b) limits for each combination of branching fractions to $\mathrm{ t } \mathrm{ W } $, $\mathrm{ b } {\mathrm{ Z } } $, and $\mathrm{ b } \mathrm{ H } $ obtained by the combination of channels. The color scale represents the mass exclusion limit obtained at each point. The branching fraction for the B quark decay to $\mathrm{ b } {\mathrm{ Z } } $ can be obtained through the relation $\mathcal {B}(\mathrm{ b } {\mathrm{ Z } } ) = 1 - \mathcal {B}(\mathrm{ t } \mathrm{ W } ) - \mathcal {B}(\mathrm{ b } \mathrm{ H } )$. |
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Figure 13-a:
Observed and expected cross section limit results as a function of B mass, for the combination of all channels. The limit results are shown for exclusive branching fractions of B to $\mathrm{ t } \mathrm{ W } $ (a), $\mathrm{ b } {\mathrm{ Z } } $ (b), and $\mathrm{ b } \mathrm{ H } $ (c). The step observed in the $\mathrm{ b } {\mathrm{ Z } } $ limit curve is due to two analysis channels (multilepton and OS dilepton) that do not contribute to the combination for B quark masses above 800 GeV. |
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Figure 13-b:
Observed and expected cross section limit results as a function of B mass, for the combination of all channels. The limit results are shown for exclusive branching fractions of B to $\mathrm{ t } \mathrm{ W } $ (a), $\mathrm{ b } {\mathrm{ Z } } $ (b), and $\mathrm{ b } \mathrm{ H } $ (c). The step observed in the $\mathrm{ b } {\mathrm{ Z } } $ limit curve is due to two analysis channels (multilepton and OS dilepton) that do not contribute to the combination for B quark masses above 800 GeV. |
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Figure 13-c:
Observed and expected cross section limit results as a function of B mass, for the combination of all channels. The limit results are shown for exclusive branching fractions of B to $\mathrm{ t } \mathrm{ W } $ (a), $\mathrm{ b } {\mathrm{ Z } } $ (b), and $\mathrm{ b } \mathrm{ H } $ (c). The step observed in the $\mathrm{ b } {\mathrm{ Z } } $ limit curve is due to two analysis channels (multilepton and OS dilepton) that do not contribute to the combination for B quark masses above 800 GeV. |
| Tables | |
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Table 1:
Production cross sections for $\mathrm{ p } \mathrm{ p } \to \mathrm{ B } \mathrm{ \bar{B} } $, used to normalize simulated signal samples to expected event yields. The cross sections are computed to NNLO with Top++2.0 [40]. |
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Table 2:
A summary of analysis channels entering the combination, along with the number of selected leptons, the variable used for signal discrimination, and the B quark decay mode providing the best sensitivity for the channel. |
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Table 3:
Expected background yields and observed number of events in data in the opposite-sign dilepton channel. The background is obtained from data. The background uncertainties include both statistical and systematic components. |
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Table 4:
Expected signal event yields in the opposite-sign dilepton channel, shown for B quark masses $M(\mathrm{ B } )$ from 450 to 800 GeV and for two values of the branching fraction. |
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Table 5:
Closure test of the background estimation method for the all-hadronic channel, in the data using a control sample with no b jets. The background is estimated from the sideband regions $A$, $C$, and $D$. The product $N_A (N_C/N_D)$ is compared with the actual background observed in region $B$ in the data. The uncertainties correspond to the statistical uncertainties from limited sample sizes. The agreement obtained is within the uncertainties. |
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Table 6:
Estimated background and the event yields in the data for the 1 b jet, ${\ge }2$ b jets, and combined event categories in the all-hadronic channel. The uncertainties in the background yields are obtained by propagating the statistical uncertainties from the sideband samples. |
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Table 7:
Nuisance parameters applied to the statistical combination. They are listed separately for each individual channel, and the $v$ symbol is used if they are applied to that given channel. If a nuisance parameter is taken as correlated between channels, the $v$ symbol is shown. In some cases, several systematic uncertainties are combined into a single nuisance parameter (for example, in the case of combined lepton categories); in such instances, the $\bullet $ symbol is used to denote the presence of a systematic uncertainty combined with others in a distinct nuisance parameter. The $\sim $ symbol has been used to denote systematic uncertainties that have negligible effects on the analysis results. The ``Combined systematic uncertainty'' entry represents a contribution composed of other sources listed in the table, applied as a single nuisance parameter during limit extraction. |
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Table 8:
Cross section limits for various combinations of branching fractions and B quark masses. The expected cross section limits are given in the first row for each branching fraction combination, along with their corresponding uncertainties. The observed cross section limits are shown in the second row. All limits are given at 95% CL , and are shown in units of pb. |
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Table 9:
Expected and observed mass exclusion limits for the combined result, quoted at 95% CL , for the topologies where the B quark decays exclusively in one of the possible modes. |
| Summary |
|
A search for pair production of the B quark with vectorlike couplings to W, Z, and Higgs bosons has been performed, using data recorded by the CMS experiment from proton-proton collisions at a center-of-mass energy of 8 TeV at the CERN LHC in 2012. This hypothesized particle could decay in one of three ways: to $\mathrm{ t }\mathrm{ W }$, $\mathrm{ b }\mathrm{ Z }$, or $\mathrm{ b }\mathrm{ H }$. The search is performed using five distinct topologies to maintain sensitivity to each of these decay modes. The topologies included in this search are the lepton+jets final state, both the opposite-sign and same-sign lepton pair final states, the three or more leptons final state, and finally the all-hadronic final state targeting Higgs boson decays to pairs of bottom quarks. No evidence for the production of B quarks in any topology is found, and limits are set on the B quark-antiquark pair-production cross section. A scan over possible combinations of the branching fractions to $\mathrm{ t }\mathrm{ W }$, $\mathrm{ b }\mathrm{ Z }$, and $\mathrm{ b }\mathrm{ H }$ is performed. For a B quark decaying with a branching fraction of 100% to $\mathrm{ b }\mathrm{ H }$, B quarks with masses up to 900 GeV are excluded, at 95% confidence level. This branching fraction corresponds to the highest excluded B quark mass in this scan. Observed exclusion limit results from a scan of all possible branching fractions range from a minimum of 740 GeV to 900 GeV. The combination of these results provides the most stringent exclusion limit to date for the existence of a vectorlike B quark. |
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