CMS-TOP-18-005 ; CERN-EP-2019-255 | ||
Measurement of the top quark pair production cross section in dilepton final states containing one $\tau$ lepton in pp collisions at $\sqrt{s} = $ 13 TeV | ||
CMS Collaboration | ||
29 November 2019 | ||
JHEP 02 (2020) 191 | ||
Abstract: The cross section of top quark pair production is measured in the $\mathrm{t\bar{t}}\to (\ell\nu_{\ell})({\tau_\mathrm{h}}\nu_{\tau})\mathrm{b\bar{b}}$ final state, where ${\tau_\mathrm{h}}$ refers to the hadronic decays of the $\tau$ lepton, and $\ell$ is either an electron or a muon. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$ collected in proton-proton collisions at $\sqrt{s} = $ 13 TeV with the CMS detector. The measured cross section is $\sigma_{\mathrm{t\bar{t}}} = $ 781 $\pm$ 7 (stat) $\pm$ 62 (syst) $\pm$ 20 (lumi) pb, and the ratio of the partial width $\Gamma(\mathrm{t}\to\tau\nu_{\tau}\mathrm{b})$ to the total decay width of the top quark is measured to be 0.1050 $\pm$ 0.0009 (stat) $\pm$ 0.0071 (syst). This is the first measurement of the $\mathrm{t\bar{t}}$ production cross section in proton-proton collisions at $\sqrt{s} = $ 13 TeV that explicitly includes $\tau$ leptons. The ratio of the cross sections in the $\ell{\tau_\mathrm{h}}$ and $\ell\ell$ final states yields a value $R_{\ell{\tau_\mathrm{h}}/\ell\ell}=$ 0.973 $\pm$ 0.009 (stat) $\pm$ 0.066 (syst), consistent with lepton universality. | ||
Links: e-print arXiv:1911.13204 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
The $ {\tau _\mathrm {h}}$ $ {p_{\mathrm {T}}}$ distributions for events of the e$ {\tau _\mathrm {h}} $ (left) and $\mu {\tau _\mathrm {h}} $ (right) final states observed prior to fitting. Distributions obtained from data (filled circles) are compared with simulation (shaded histograms). The last bin includes overflow events. The simulated contributions are normalized to the cross section values predicted in the SM. The main processes are shown: the signal, the other ${\mathrm{t} \mathrm{\bar{t}}}$ processes grouped together, single top quark production, W+jets, DY processes, diboson, and multijet production. The ratio of the data to the total SM prediction is shown in the lower panel. The hatched bands indicate the systematic uncertainties and the statistical uncertainties of all simulated samples. Statistical uncertainties on the data points are not visible because of the scale of the figure. |
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Figure 1-a:
The $ {\tau _\mathrm {h}}$ $ {p_{\mathrm {T}}}$ distributions for events of the e$ {\tau _\mathrm {h}} $ final state observed prior to fitting. Distributions obtained from data (filled circles) are compared with simulation (shaded histograms). The last bin includes overflow events. The simulated contributions are normalized to the cross section values predicted in the SM. The main processes are shown: the signal, the other ${\mathrm{t} \mathrm{\bar{t}}}$ processes grouped together, single top quark production, W+jets, DY processes, diboson, and multijet production. The ratio of the data to the total SM prediction is shown in the lower panel. The hatched bands indicate the systematic uncertainties and the statistical uncertainties of all simulated samples. Statistical uncertainties on the data points are not visible because of the scale of the figure. |
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Figure 1-b:
The $ {\tau _\mathrm {h}}$ $ {p_{\mathrm {T}}}$ distributions for events of the $\mu {\tau _\mathrm {h}} $ final state observed prior to fitting. Distributions obtained from data (filled circles) are compared with simulation (shaded histograms). The last bin includes overflow events. The simulated contributions are normalized to the cross section values predicted in the SM. The main processes are shown: the signal, the other ${\mathrm{t} \mathrm{\bar{t}}}$ processes grouped together, single top quark production, W+jets, DY processes, diboson, and multijet production. The ratio of the data to the total SM prediction is shown in the lower panel. The hatched bands indicate the systematic uncertainties and the statistical uncertainties of all simulated samples. Statistical uncertainties on the data points are not visible because of the scale of the figure. |
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Figure 2:
Comparison of the signal (${\mathrm{t} \mathrm{\bar{t}}} \to \ell \nu _{\ell} {\tau _\mathrm {h}} \nu _{\tau} \mathrm{b} \mathrm{\bar{b}} $) and the main background of misidentified $ {\tau _\mathrm {h}} $ (${\mathrm{t} \mathrm{\bar{t}}} \to \ell \nu _{\ell} \mathrm{q} \mathrm{\bar{q}} ' \mathrm{b} \mathrm{\bar{b}} $) in the shapes of the normalized distributions of the transverse mass $ {m_{\mathrm {T}}} $ between the lepton and ${{p_{\mathrm {T}}} ^\text {miss}}$ (left), and the $D^\mathrm {min}_{\mathrm {jjb}}$ parameter (see text) of the event categories (right). In the $ {m_{\mathrm {T}}} $ distribution, the signal may extend beyond the W boson mass endpoint because of the two-neutrino final state, whereas the background process cannot. The last bin in both distributions includes overflow events. In the $D^\mathrm {min}_{\mathrm {jjb}}$ distribution, the downward arrow points at the threshold of the cut used ($D^\mathrm {min}_{\mathrm {jjb}} > $ 60 GeV), and the panel on the right shows the fraction of events in the "signal-like'' category where there is only one untagged jet. |
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Figure 2-a:
Comparison of the signal (${\mathrm{t} \mathrm{\bar{t}}} \to \ell \nu _{\ell} {\tau _\mathrm {h}} \nu _{\tau} \mathrm{b} \mathrm{\bar{b}} $) and the main background of misidentified $ {\tau _\mathrm {h}} $ (${\mathrm{t} \mathrm{\bar{t}}} \to \ell \nu _{\ell} \mathrm{q} \mathrm{\bar{q}} ' \mathrm{b} \mathrm{\bar{b}} $) in the shapes of the normalized distributions of the transverse mass $ {m_{\mathrm {T}}} $ between the lepton and ${{p_{\mathrm {T}}} ^\text {miss}}$. The signal may extend beyond the W boson mass endpoint because of the two-neutrino final state, whereas the background process cannot. The last bin in the distribution includes overflow events. |
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Figure 2-b:
Comparison of the signal (${\mathrm{t} \mathrm{\bar{t}}} \to \ell \nu _{\ell} {\tau _\mathrm {h}} \nu _{\tau} \mathrm{b} \mathrm{\bar{b}} $) and the main background of misidentified $ {\tau _\mathrm {h}} $ (${\mathrm{t} \mathrm{\bar{t}}} \to \ell \nu _{\ell} \mathrm{q} \mathrm{\bar{q}} ' \mathrm{b} \mathrm{\bar{b}} $) in the shapes of the normalized distributions of the $D^\mathrm {min}_{\mathrm {jjb}}$ parameter (see text) of the event categories. The last bin in the distribution includes overflow events. The downward arrow points at the threshold of the cut used ($D^\mathrm {min}_{\mathrm {jjb}} > $ 60 GeV), and the panel on the right shows the fraction of events in the "signal-like'' category where there is only one untagged jet. |
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Figure 3:
The transverse mass distributions between lepton (e or $\mu $) and ${{p_{\mathrm {T}}} ^\text {miss}}$, $ {m_{\mathrm {T}}} $, in the signal-like (upper) and background-like (lower) event categories for the e$ {\tau _\mathrm {h}} $ (left) and $\mu {\tau _\mathrm {h}} $ (right) final states observed prior to fitting. Distributions obtained from data (filled circles) are compared with simulation (shaded histograms). The last bin includes overflow events. The simulated contributions are normalized to the cross section values predicted in the SM. The main processes are shown: the signal, the other ${\mathrm{t} \mathrm{\bar{t}}}$ processes grouped together, single top quark production, W+jets, DY processes, diboson, and multijet production. The ratio of the data to the total SM prediction is shown in the lower panel. The vertical bars on the data points indicate the statistical uncertainties, the hatched band indicates the systematic uncertainties and the statistical uncertainties in all simulated samples. |
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Figure 3-a:
The transverse mass distributions between lepton (e or $\mu $) and ${{p_{\mathrm {T}}} ^\text {miss}}$, $ {m_{\mathrm {T}}} $, in the signal-like event category for the e$ {\tau _\mathrm {h}} $ final state observed prior to fitting. The distribution obtained from data (filled circles) is compared with simulation (shaded histograms). The last bin includes overflow events. The simulated contributions are normalized to the cross section values predicted in the SM. The main processes are shown: the signal, the other ${\mathrm{t} \mathrm{\bar{t}}}$ processes grouped together, single top quark production, W+jets, DY processes, diboson, and multijet production. The ratio of the data to the total SM prediction is shown in the lower panel. The vertical bars on the data points indicate the statistical uncertainties, the hatched band indicates the systematic uncertainties and the statistical uncertainties in all simulated samples. |
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Figure 3-b:
The transverse mass distributions between lepton (e or $\mu $) and ${{p_{\mathrm {T}}} ^\text {miss}}$, $ {m_{\mathrm {T}}} $, in the signal-like event category for the $\mu {\tau _\mathrm {h}} $ final state observed prior to fitting. The distribution obtained from data (filled circles) is compared with simulation (shaded histograms). The last bin includes overflow events. The simulated contributions are normalized to the cross section values predicted in the SM. The main processes are shown: the signal, the other ${\mathrm{t} \mathrm{\bar{t}}}$ processes grouped together, single top quark production, W+jets, DY processes, diboson, and multijet production. The ratio of the data to the total SM prediction is shown in the lower panel. The vertical bars on the data points indicate the statistical uncertainties, the hatched band indicates the systematic uncertainties and the statistical uncertainties in all simulated samples. |
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Figure 3-c:
The transverse mass distributions between lepton (e or $\mu $) and ${{p_{\mathrm {T}}} ^\text {miss}}$, $ {m_{\mathrm {T}}} $, in the background-like event category for the e$ {\tau _\mathrm {h}} $ final state observed prior to fitting. The distribution obtained from data (filled circles) is compared with simulation (shaded histograms). The last bin includes overflow events. The simulated contributions are normalized to the cross section values predicted in the SM. The main processes are shown: the signal, the other ${\mathrm{t} \mathrm{\bar{t}}}$ processes grouped together, single top quark production, W+jets, DY processes, diboson, and multijet production. The ratio of the data to the total SM prediction is shown in the lower panel. The vertical bars on the data points indicate the statistical uncertainties, the hatched band indicates the systematic uncertainties and the statistical uncertainties in all simulated samples. |
png pdf |
Figure 3-d:
The transverse mass distributions between lepton (e or $\mu $) and ${{p_{\mathrm {T}}} ^\text {miss}}$, $ {m_{\mathrm {T}}} $, in the background-like event category for the $\mu {\tau _\mathrm {h}} $ final state observed prior to fitting. The distribution obtained from data (filled circles) is compared with simulation (shaded histograms). The last bin includes overflow events. The simulated contributions are normalized to the cross section values predicted in the SM. The main processes are shown: the signal, the other ${\mathrm{t} \mathrm{\bar{t}}}$ processes grouped together, single top quark production, W+jets, DY processes, diboson, and multijet production. The ratio of the data to the total SM prediction is shown in the lower panel. The vertical bars on the data points indicate the statistical uncertainties, the hatched band indicates the systematic uncertainties and the statistical uncertainties in all simulated samples. |
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Figure 4:
The expected and observed dependence of the likelihood on the total ${\mathrm{t} \mathrm{\bar{t}}}$ cross section $\sigma _{{\mathrm{t} \mathrm{\bar{t}}}}$. It is derived from the fiducial phase space by a simple extrapolation. The arrow points at the cross section measured in the light dilepton final state. The goodness of the fit determined with a Kolmogorov-Smirnov method yields a $p$ value of 0.24. |
Tables | |
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Table 1:
Expected and observed event yields in the $\ell {\tau _\mathrm {h}} $ ($\ell =\mathrm{e},\mu $) final state for signal and background processes for an integrated luminosity of 35.9 fb$^{-1}$. Statistical and systematic uncertainties are shown. The expected prefit contributions of all processes are presented separately for background-like and signal-like event categories. The statistical uncertainties of the modelling are shown for the processes estimated from the simulation. The multijet contribution and the corresponding statistical uncertainties are estimated using data, as described in Section 6. |
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Table 2:
Systematic and statistical uncertainties determined from the fit to the data in the e$ {\tau _\mathrm {h}} $ and $\mu {\tau _\mathrm {h}} $ final states, and their combination. Uncertainties are grouped by their origin: experimental, theoretical, and extrapolation. The uncertainties in the measurement in the dilepton final state [14] used in the partial width ratio estimate are also quoted (column "dileptons"), where the asymmetric extrapolation uncertainties are symmetrized by adding them in quadrature. As both measurements use the same data, some uncertainties are correlated, as shown in the last column. |
Summary |
A measurement of the top quark pair production cross section in the $\mathrm{t\bar{t}}\to (\ell \mathrm{g}n_{\ell}) ({\tau_\mathrm{h}} \mathrm{g}n_{\tau}) \mathrm{b\bar{b}}$ channel, where $\ell$ is either an electron or a muon, is performed by CMS in proton-proton collisions at LHC, using a data sample corresponding to an integrated luminosity of 35.9 fb$^{-1}$ obtained at $\sqrt{s} = $ 13 TeV. Events are selected by requiring the presence of an electron or a muon, and at least three jets, of which at least one is b tagged and one is identified as a $\tau$ lepton decaying to hadrons (${\tau_\mathrm{h}}$). The largest background contribution arises from $\mathrm{t\bar{t}}$ lepton+jets events, $\mathrm{t\bar{t}}\to (\ell\mathrm{g}n_{\ell})(\mathrm{q\bar{q}}')\mathrm{b\bar{b}}$, where one jet is misidentified as the ${\tau_\mathrm{h}}$. The background contribution is constrained in a fit to the distribution of the transverse mass of the light lepton and missing transverse momentum system in two event categories, constructed according to the kinematic properties of the jets in the $\mathrm{t\bar{t}}$ lepton+jets final state. The signal enters as a free parameter without constraining the kinematic properties of the $\tau$ lepton. Assuming a top quark mass of 172.5 GeV, the measured total $\mathrm{t\bar{t}}$ cross section $\sigma_{\mathrm{t\bar{t}}}(\ell{\tau_\mathrm{h}}) =$ 781 $\pm$ 7 (stat) $\pm$ 62 (syst) $\pm$ 20 (lumi) pb is in agreement with the standard model expectation. This is the first measurement of the $\mathrm{t\bar{t}}$ production cross section in proton-proton collisions at $\sqrt{s} = $ 13 TeV that explicitly includes hadronically decaying $\tau$ leptons, and it improves the relative precision with respect to the 7 and 8 TeV results [64,65]. The higher precision is achieved through a shape fit to the kinematic distributions of the events, thus better constraining the backgrounds. The measurement of the ratio of the cross sections in the $\ell{\tau_\mathrm{h}}$ final state to the light dilepton cross section [14] yields a value of $R_{\ell{\tau_\mathrm{h}}/\ell\ell}=$ 0.973 $\pm$ 0.009 (stat) $\pm$ 0.066 (syst), consistent with lepton universality. The ratio of the partial to the total width of the top quark $\Gamma(\mathrm{t}\to\tau\mathrm{g}n_{\tau}\mathrm{b})/\Gamma_{\mathrm{total}}=$ 0.1050 $\pm$ 0.0009 (stat) $\pm$ 0.0071 (syst) is measured with respect to the $\mathrm{t\bar{t}}$ inclusive cross section extrapolated from the light dilepton final state, improving the precision over the previous measurements [63,62]. |
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Compact Muon Solenoid LHC, CERN |