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| CMS-PAS-TOP-18-011 | ||
| Measurement of the $\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}$ production cross section in the all-jet final state in pp collisions at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| May 2019 | ||
| Abstract: A measurement of the production cross section of top quark pairs in association with two b jets ($\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}$) is presented, using data collected in pp collisions at $\sqrt{s}= $ 13 TeV by the CMS experiment at the LHC corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The cross section is measured in the all-jet decay channel of the top quark pairs by selecting events containing at least eight jets, of which two are identified as originating from the hadronisation of b quarks. A combination of multivariate analysis techniques is used to reduce the large background consisting uniquely of jets produced through the strong interaction, and to discriminate the jets originating from the top quark decays and additional jets. The cross section is measured for the visible $\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}$ phase space, as well as for the full phase space, for which it is determined to be 5.5 $\pm$ 0.3 (stat) $^{+1.6}_{-1.3}$ (syst) pb. The measured cross sections are compared with predictions of several event generators and are found to be generally higher than the theoretical predictions. | ||
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These preliminary results are superseded in this paper, PLB 803 (2020) 135285. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Left: distribution of the QGLR. Right: distribution of the CWoLa BDT. Both distributions are shown after preselection, requiring $\chi ^{2} < $ 33.38, and at least eight selected jets. All processes are taken from the simulation. The multijet contribution is scaled to match the total yields in data, after the other processes including the ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{b} {}\mathrm{\bar{b}}}$ signal have been normalised to their corresponding theoretical cross sections. The small backgrounds include ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm {V}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets and diboson production. The lower panels show the ratio between the observed data and the predictions. Hatched bands in the upper and grey bands in the lower panel indicate the statistical uncertainty in the predictions, dominated by the uncertainties in the simulated multijet background. |
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Figure 1-a:
Distribution of the QGLR. The distribution is shown after preselection, requiring $\chi ^{2} < $ 33.38, and at least eight selected jets. All processes are taken from the simulation. The multijet contribution is scaled to match the total yields in data, after the other processes including the ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{b} {}\mathrm{\bar{b}}}$ signal have been normalised to their corresponding theoretical cross sections. The small backgrounds include ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm {V}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets and diboson production. The lower panel shows the ratio between the observed data and the predictions. Hatched bands in the upper and grey bands in the lower panel indicate the statistical uncertainty in the predictions, dominated by the uncertainties in the simulated multijet background. |
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Figure 1-b:
Distribution of the CWoLa BDT. The distribution is shown after preselection, requiring $\chi ^{2} < $ 33.38, and at least eight selected jets. All processes are taken from the simulation. The multijet contribution is scaled to match the total yields in data, after the other processes including the ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{b} {}\mathrm{\bar{b}}}$ signal have been normalised to their corresponding theoretical cross sections. The small backgrounds include ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm {V}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets and diboson production. The lower panel shows the ratio between the observed data and the predictions. Hatched bands in the upper and grey bands in the lower panel indicate the statistical uncertainty in the predictions, dominated by the uncertainties in the simulated multijet background. |
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Figure 2:
Distribution of the 2DCSV in the CR1 (upper left), SR (upper right), CR2 (lower left) and CR3 (lower right) regions. For visualisation purposes, the two-dimensional distribution of the largest and second-largest b tagging discriminator scores of the additional jets has been unrolled to one dimension, and the resulting bins have been ordered to increasing values of the ratio between expected signal and background yields in each bin in the SR. The contribution due to QCD multijet production is estimated from the data in the four regions according to the method described in Section 6. As a result, the multijet contributions in the CR1, CR2 and CR3 match the differences between the yields in data and from the other processes. The small backgrounds include ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm {V}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets and diboson production. Hatched and grey bands correspond to post fit uncertainties, and bottom panels show the ratio between data and post fit predictions. |
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Figure 2-a:
Distribution of the 2DCSV in the CR1 region. For visualisation purposes, the two-dimensional distribution of the largest and second-largest b tagging discriminator scores of the additional jets has been unrolled to one dimension, and the resulting bins have been ordered to increasing values of the ratio between expected signal and background yields in each bin in the SR. The contribution due to QCD multijet production is estimated from the data in the four regions according to the method described in Section 6. As a result, the multijet contributions in the CR1, CR2 and CR3 match the differences between the yields in data and from the other processes. The small backgrounds include ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm {V}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets and diboson production. Hatched and grey bands correspond to post fit uncertainties, and the bottom panel shows the ratio between data and post fit predictions. |
png pdf |
Figure 2-b:
Distribution of the 2DCSV in the SR region. For visualisation purposes, the two-dimensional distribution of the largest and second-largest b tagging discriminator scores of the additional jets has been unrolled to one dimension, and the resulting bins have been ordered to increasing values of the ratio between expected signal and background yields in each bin in the SR. The contribution due to QCD multijet production is estimated from the data in the four regions according to the method described in Section 6. The small backgrounds include ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm {V}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets and diboson production. Hatched and grey bands correspond to post fit uncertainties, and the bottom panel shows the ratio between data and post fit predictions. |
png pdf |
Figure 2-c:
Distribution of the 2DCSV in the CR2 region. For visualisation purposes, the two-dimensional distribution of the largest and second-largest b tagging discriminator scores of the additional jets has been unrolled to one dimension, and the resulting bins have been ordered to increasing values of the ratio between expected signal and background yields in each bin in the SR. The contribution due to QCD multijet production is estimated from the data in the four regions according to the method described in Section 6. As a result, the multijet contributions in the CR1, CR2 and CR3 match the differences between the yields in data and from the other processes. The small backgrounds include ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm {V}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets and diboson production. Hatched and grey bands correspond to post fit uncertainties, and the bottom panel shows the ratio between data and post fit predictions. |
png pdf |
Figure 2-d:
Distribution of the 2DCSV in the CR3 region. For visualisation purposes, the two-dimensional distribution of the largest and second-largest b tagging discriminator scores of the additional jets has been unrolled to one dimension, and the resulting bins have been ordered to increasing values of the ratio between expected signal and background yields in each bin in the SR. The contribution due to QCD multijet production is estimated from the data in the four regions according to the method described in Section 6. As a result, the multijet contributions in the CR1, CR2 and CR3 match the differences between the yields in data and from the other processes. The small backgrounds include ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm {V}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets and diboson production. Hatched and grey bands correspond to post fit uncertainties, and the bottom panel shows the ratio between data and post fit predictions. |
png pdf |
Figure 3:
Comparison of the measured ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{b} {}\mathrm{\bar{b}}}$ production cross sections with predictions from several Monte Carlo generators, for the three definitions of the ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{b} {}\mathrm{\bar{b}}}$ phase space. Uncertainty bands in the theoretical cross sections include the statistical uncertainty as well as the uncertainties due to the PDFs and to the $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ scale variations. |
| Tables | |
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Table 1:
The various sources of systematic uncertainties and their respective contribution, quoted as a percentage of the measured cross section, to the total systematic uncertainty in the measured ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{b} {}\mathrm{\bar{b}}}$ cross section in the VPS for the two ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{b} {}\mathrm{\bar{b}}}$ definitions. |
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Table 2:
Measured and predicted cross sections (in pb) for the different definitions of the ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{b} {}\mathrm{\bar{b}}}$ phase space considered in this analysis. For the measurements, the first uncertainty is statistical, while the second is the systematic uncertainty. The uncertainties in the predicted cross sections include the statistical uncertainty, the PDF uncertainties, and the $\mu _R$ and $\mu _F$ scale variations. Parton shower scale uncertainties are not included, and amount to about 15% for POWHEG+PYTHIA. Unless specified otherwise, PYTHIA is used for the modelling of the parton shower, hadronisation and underlying event. |
| Summary |
| The first measurement of the $\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}$ cross section $\sigma_{\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}}$ in the all-jets final state, using 35.9 fb$^{-1}$ of data collected in pp collisions at $\sqrt{s} = $ 13 TeV, has been presented. The cross section is measured in the visible particle-level phase space using two definitions of $\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}$ events, as well as in the full phase space. One definition in the visible phase space does not rely on parton-level information, while the other uses parton-level information to identify the particle-level jets that do not originate from the decay of the top quarks. For both definitions, the cross section is measured to be $\sigma_{\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}}= $ 1.6 $\pm$ 0.1 (stat) $^{+0.5}_{-0.4}$ (syst) pb. The cross section in the full phase space is obtained by correcting this latter measurement for the experimental acceptance on the jets stemming from the top quarks, yielding 5.5 $\pm$ 0.3 (stat) $^{+1.6}_{-1.3}$ (syst) pb. This measurement provides valuable input to studies of the $\mathrm{t\bar{t}}\mathrm{H}$ process, where the Higgs boson decays into a pair of b quarks, and for which the normalisation and modelling of the $\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}$ process represent a leading source of systematic uncertainty. Furthermore, these results represent a stringent test for perturbative QCD predictions at the LHC. The tensions between measurements and theoretical predictions call for new developments in the modelling of the associated production of top quark pairs and b jets. |
| Additional Figures | |
png pdf |
Additional Figure 1:
Distribution of the quark-gluon likelihood value of the ${p_{\mathrm {T}}}$-leading jet, for events which pass the preselection, contain eight or more jets, and satisfy $\chi ^{2} < $ 33.38. All processes are taken from the simulation. The QCD multijet contribution is scaled to match the total yields in data, after the other processes including the ${{{\mathrm {t}\overline {\mathrm {t}}}} {{\mathrm {b}} {\overline {\mathrm {b}}}}}$ signal have been normalised to their corresponding theoretical cross sections. The small backgrounds include $ {{\mathrm {t}\overline {\mathrm {t}}}} {\mathrm {V}} $, $ {{\mathrm {t}\overline {\mathrm {t}}}} {\mathrm {H}} $, single top quark, V+jets and diboson production. The lower panel shows the ratio between the observed data and the predictions. Hatched and grey bands indicate the statistical uncertainty in the predictions, dominated by the uncertainties in the simulated multijet background. |
png pdf |
Additional Figure 2:
Prefit distribution of the ratio S/B between the signal and background yields in each bin of the 2DCSV in the SR, CR1, CR2, and CR3 regions. The two-dimensional distribution of the largest and second-largest b-tagging discriminator values of the additional jets has been unrolled to one dimension, and the resulting bins have been ordered to increasing values of S/B in the SR. The signal contribution includes ${{{\mathrm {t}\overline {\mathrm {t}}}} {{\mathrm {b}} {\overline {\mathrm {b}}}}}$, ${{{\mathrm {t}\overline {\mathrm {t}}}} {{\mathrm {b}} {\overline {\mathrm {b}}}}}$ (OOA), ${{{\mathrm {t}\overline {\mathrm {t}}}} 2 {\mathrm {b}}}$ and ${{{\mathrm {t}\overline {\mathrm {t}}}} {\mathrm {b}}}$. |
png pdf |
Additional Figure 3:
Comparison of the measured ${{{\mathrm {t}\overline {\mathrm {t}}}} {{\mathrm {b}} {\overline {\mathrm {b}}}}}$ production cross sections with predictions from several Monte Carlo generators, for parton-agnostic definition of the visible ${{{\mathrm {t}\overline {\mathrm {t}}}} {{\mathrm {b}} {\overline {\mathrm {b}}}}}$ phase space. Uncertainty bands in the theoretical cross sections include the statistical uncertainty as well as the uncertainties due to the PDFs and to the $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ scale variations. |
png pdf |
Additional Figure 4:
Comparison of the measured ${{{\mathrm {t}\overline {\mathrm {t}}}} {{\mathrm {b}} {\overline {\mathrm {b}}}}}$ production cross sections with predictions from several Monte Carlo generators, for parton-based definition of the visible ${{{\mathrm {t}\overline {\mathrm {t}}}} {{\mathrm {b}} {\overline {\mathrm {b}}}}}$ phase space. Uncertainty bands in the theoretical cross sections include the statistical uncertainty as well as the uncertainties due to the PDFs and to the $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ scale variations. |
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Compact Muon Solenoid LHC, CERN |
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