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| CMS-PAS-TOP-16-019 | ||
| Bounding the top quark width using final states with two charged leptons and two jets at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| September 2016 | ||
| Abstract: A direct bound on the top quark decay width is presented, obtained by analysing 12.9 fb$^{-1}$ of proton-proton collision data collected at $ \sqrt{s} = $ 13 TeV by the CMS experiment at the LHC. The measurement is performed by partially reconstructing the kinematics of top quark candidates from final states containing at least two charged leptons (electrons or muons) and at least two jets, where at least one jet is identified as stemming from the fragmentation and hadronization of a b quark. The observable is compared to the simulated expectations for different top quark width scenarios using a likelihood technique. Under the hypothesis of a standard model-like top quark the measurement yields limits at the 95% CL of 0.6 $ \leq \Gamma_{\rm t} \leq $ 2.5 GeV, with an expected limit at 0.6 $ \leq \Gamma_{\rm t} \leq $ 2.4 GeV for a top quark mass of 172.5 GeV. | ||
| Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Distributions of the inclusive lepton-b-jet transverse momentum, $ {p_{\mathrm {T}}} (\ell ,\mathrm{ b } )$, for events with $=1$ (left) or $\geq $2 (right) b-tagged jets. All dilepton channels are combined. The top panels compare the distribution in the data to the simulated expectations, while the bottom panels display the ratio of the two. The shaded band in the bottom panel represents the uncertainty in the prediction due to the limited statistics in the simulation and due to the integrated luminosity. |
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Figure 1-a:
Distribution of the inclusive lepton-b-jet transverse momentum, $ {p_{\mathrm {T}}} (\ell ,\mathrm{ b } )$, for events with $=1$ b-tagged jet. All dilepton channels are combined. The top panel compares the distribution in the data to the simulated expectations, while the bottom panel displays the ratio of the two. The shaded band in the bottom panel represents the uncertainty in the prediction due to the limited statistics in the simulation and due to the integrated luminosity. |
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Figure 1-b:
Distribution of the inclusive lepton-b-jet transverse momentum, $ {p_{\mathrm {T}}} (\ell ,\mathrm{ b } )$, for events with $\geq $2 b-tagged jets. All dilepton channels are combined. The top panel compares the distribution in the data to the simulated expectations, while the bottom panel displays the ratio of the two. The shaded band in the bottom panel represents the uncertainty in the prediction due to the limited statistics in the simulation and due to the integrated luminosity. |
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Figure 2:
Distribution of the $M_{\ell \mathrm{ b } }$ observable calculated using up to two leptons and two b-tagged jets per event, for all dilepton channels combined. The plots on the top (bottom) refer to the boosted (non-boosted) category, and the plots on the left (right) correspond to the =1-b ($\geq $2-b ) event category. In each plot the top panels compare the distribution in the data to the simulated expectations, while the bottom panels display the ratio of the two. The last bin includes the overflow of the distributions. The shaded band in the bottom panel represents the uncertainty in the prediction due to the limited statistics in the simulation and due to the integrated luminosity. |
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Figure 2-a:
Distribution of the $M_{\ell \mathrm{ b } }$ observable calculated using up to two leptons and two b-tagged jets per event, for all dilepton channels combined. The plot refers to the boosted category, in the 1-b event category. The top panel compares the distribution in the data to the simulated expectations, while the bottom panel displays the ratio of the two. The last bin includes the overflow of the distributions. The shaded band in the bottom panel represents the uncertainty in the prediction due to the limited statistics in the simulation and due to the integrated luminosity. |
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Figure 2-b:
Distribution of the $M_{\ell \mathrm{ b } }$ observable calculated using up to two leptons and two b-tagged jets per event, for all dilepton channels combined. The plot refers to the boosted category, in the $\geq $2-b event category. The top panel compares the distribution in the data to the simulated expectations, while the bottom panel displays the ratio of the two. The last bin includes the overflow of the distributions. The shaded band in the bottom panel represents the uncertainty in the prediction due to the limited statistics in the simulation and due to the integrated luminosity. |
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Figure 2-c:
Distribution of the $M_{\ell \mathrm{ b } }$ observable calculated using up to two leptons and two b-tagged jets per event, for all dilepton channels combined. The plot refers to the non-boosted category, in the 1-b event category. The top panel compares the distribution in the data to the simulated expectations, while the bottom panel displays the ratio of the two. The last bin includes the overflow of the distributions. The shaded band in the bottom panel represents the uncertainty in the prediction due to the limited statistics in the simulation and due to the integrated luminosity. |
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Figure 2-d:
Distribution of the $M_{\ell \mathrm{ b } }$ observable calculated using up to two leptons and two b-tagged jets per event, for all dilepton channels combined. The plot refers to the non-boosted category, in the $\geq $2-b event category. The top panel compares the distribution in the data to the simulated expectations, while the bottom panel displays the ratio of the two. The last bin includes the overflow of the distributions. The shaded band in the bottom panel represents the uncertainty in the prediction due to the limited statistics in the simulation and due to the integrated luminosity. |
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Figure 3:
Distribution of the $M_{\ell \mathrm{ b } }$ variable for simulated POWHEG+PYTHIA-8 events where the top quark mass is varied by $\pm $3 GeV with respect to 172.5 GeV and the width is varied by a factor of 4 with respect to the SM value. The upper (lower) row of plots describes boosted (unboosted) events, while the left (right) column displays information in the 1-b ($\geq $2-b) category. The top panels show the distributions with the last bin displaying the overflow of the histograms while the bottom plot show the ratio with respect to the $m_\mathrm{ t } =$ 172.5 GeV and $\Gamma _\mathrm{ t } =\Gamma _{\rm SM}$ scenario. |
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Figure 3-a:
Distribution of the $M_{\ell \mathrm{ b } }$ variable for simulated POWHEG+PYTHIA-8 events where the top quark mass is varied by $\pm $3 GeV with respect to 172.5 GeV and the width is varied by a factor of 4 with respect to the SM value. The plot describes boosted events, in the 1-b category. The top panel shows the distribution with the last bin displaying the overflow of the histograms while the bottom plot shows the ratio with respect to the $m_\mathrm{ t } =$ 172.5 GeV and $\Gamma _\mathrm{ t } =\Gamma _{\rm SM}$ scenario. |
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Figure 3-b:
Distribution of the $M_{\ell \mathrm{ b } }$ variable for simulated POWHEG+PYTHIA-8 events where the top quark mass is varied by $\pm $3 GeV with respect to 172.5 GeV and the width is varied by a factor of 4 with respect to the SM value. The plot describes boosted events, in the $\geq $2-b category. The top panel shows the distribution with the last bin displaying the overflow of the histograms while the bottom plot shows the ratio with respect to the $m_\mathrm{ t } =$ 172.5 GeV and $\Gamma _\mathrm{ t } =\Gamma _{\rm SM}$ scenario. |
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Figure 3-c:
Distribution of the $M_{\ell \mathrm{ b } }$ variable for simulated POWHEG+PYTHIA-8 events where the top quark mass is varied by $\pm $3 GeV with respect to 172.5 GeV and the width is varied by a factor of 4 with respect to the SM value. The plot describes unboosted events, in the 1-b category. The top panel shows the distribution with the last bin displaying the overflow of the histograms while the bottom plot shows the ratio with respect to the $m_\mathrm{ t } =$ 172.5 GeV and $\Gamma _\mathrm{ t } =\Gamma _{\rm SM}$ scenario. |
png pdf |
Figure 3-d:
Distribution of the $M_{\ell \mathrm{ b } }$ variable for simulated POWHEG+PYTHIA-8 events where the top quark mass is varied by $\pm $3 GeV with respect to 172.5 GeV and the width is varied by a factor of 4 with respect to the SM value. The plot describes unboosted events, in the $\geq $2-b category. The top panel shows the distribution with the last bin displaying the overflow of the histograms while the bottom plot shows the ratio with respect to the $m_\mathrm{ t } =$ 172.5 GeV and $\Gamma _\mathrm{ t } =\Gamma _{\rm SM}$ scenario. |
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Figure 4:
Scan of the likelihood as function of $x$ (the fraction of the alternative width hypothesis) for different top quark width hypothesis. The plot on the left (right) displays the scan expected when $\Gamma _\mathrm{ t } =\Gamma _{\rm SM}$ ($\Gamma _\mathrm{ t } =4 \Gamma _{\rm SM}$) pseudo-data is used. In both cases the result of the fit to $x$ in the data, after profiling $\mu $, is overlaid for comparison. |
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Figure 4-a:
Scan of the likelihood as function of $x$ (the fraction of the alternative width hypothesis) for different top quark width hypothesis. The plot displays the scan expected when $\Gamma _\mathrm{ t } =\Gamma _{\rm SM}$ pseudo-data is used. The result of the fit to $x$ in the data, after profiling $\mu $, is overlaid for comparison. |
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Figure 4-b:
Scan of the likelihood as function of $x$ (the fraction of the alternative width hypothesis) for different top quark width hypothesis. The plot displays the scan expected when $\Gamma _\mathrm{ t } =4 \Gamma _{\rm SM}$ pseudo-data is used. The result of the fit to $x$ in the data, after profiling $\mu $, is overlaid for comparison. |
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Figure 5:
Distributions of the test statistic for pseudoexperiments run on the full with two different alternate hypotheses assuming $\Gamma _t = 0.2 \times \Gamma _{\rm SM}$ (left) and $\Gamma _t = 2.6 \times \Gamma _{\rm SM}$ (right). In both cases the distributions are shown when obtained with the pre-fit model at fixed signal strength and when obtained with the post-fit model after profiling the signal strength. The values of the test statistics observed in the data are represented by arrows. In both hypotheses, $m_\mathrm{ t } = $ 172.5 GeV is assumed. |
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Figure 5-a:
Distribution of the test statistic for pseudoexperiments run on the full with two different alternate hypotheses assuming $\Gamma _t = 0.2 \times \Gamma _{\rm SM}$. The distribution is shown when obtained with the pre-fit model at fixed signal strength and when obtained with the post-fit model after profiling the signal strength. The values of the test statistics observed in the data are represented by arrows. In both hypotheses, $m_\mathrm{ t } = $ 172.5 GeV is assumed. |
png pdf |
Figure 5-b:
Distribution of the test statistic for pseudoexperiments run on the full with two different alternate hypotheses assuming $\Gamma _t = 2.6 \times \Gamma _{\rm SM}$. The distribution is shown when obtained with the pre-fit model at fixed signal strength and when obtained with the post-fit model after profiling the signal strength. The values of the test statistics observed in the data are represented by arrows. In both hypotheses, $m_\mathrm{ t } = $ 172.5 GeV is assumed. |
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Figure 6:
Evolution of the quantiles of the distributions of the test statistics, as a function of the top quark width. The 68%, 95% and 99% CL quantiles are represented by different shades of orange (blue) for the SM (alternative) hypothesis. The plot on the top (bottom) shows the quantiles obtained with the pre-fit (post-fit) model. The values of the test statistics observed in data are represented by the black points in the bottom plot. |
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Figure 6-a:
Evolution of the quantiles of the distributions of the test statistics, as a function of the top quark width. The 68%, 95% and 99% CL quantiles are represented by different shades of orange (blue) for the SM (alternative) hypothesis. The plot shows the quantiles obtained with the pre-fit model. |
png pdf |
Figure 6-b:
Evolution of the quantiles of the distributions of the test statistics, as a function of the top quark width. The 68%, 95% and 99% CL quantiles are represented by different shades of orange (blue) for the SM (alternative) hypothesis. The plot shows the quantiles obtained with the post-fit model. The values of the test statistics observed in data are represented by the black points in the bottom plot. |
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Figure 7:
Evolution of the CL$_{\rm S}$ as a function of the top quark width. The derived limits at the 95% (99%) confidence level are represented as the intersection of the fits to the line at CL$_{\rm S}= $ 0.05 (0.01). The expectations from both the pre-fit and post-fit models are represented by means of a best-fit spline curve. |
| Summary |
| We have presented limits on the top quark width using 12.9 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = $ 13 TeV collected by the CMS experiment. Using $\mathrm{ t \bar{t} }$ and $\mathrm{t }\mathrm{ W }$ decay events with two charged leptons in the final state, we reconstruct the $M_{\ell\mathrm{b }}$ observable inclusively using up to two jets identified as stemming from the fragmentation and hadronization of a $\mathrm{b }$ quark. The observable is fit for deviations of the width with respect to the SM prediction. Different event categories are included in the fit to improve the sensitivity of the measurement and partially constraint some of the uncertainties. Binary hypothesis tests are then used to bound an SM-like top quark width to 0.6 $ \leq \Gamma_\mathrm{t } \leq $ 2.5 GeV at the 95% CL, with corresponding expected bounds of 0.6 $ \leq \Gamma_\mathrm{t } \leq $ 2.4 GeV for $m_\mathrm{t }= $ 172.5 GeV. This constitutes the first such direct measurement at the LHC and the most precise direct bound of the top quark width performed to date. |
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