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| CMS-PAS-TOP-13-006 | ||
| Determination of the normalised invariant mass distribution of $\mathrm{t\bar{t}}$+jet and extraction of the top quark mass | ||
| CMS Collaboration | ||
| May 2016 | ||
| Abstract: A measurement of the top quark mass from top quark pair ($\mathrm{t\bar{t}}$) events produced in association with additional hard jets is performed in pp collisions at $\sqrt{s}= $ 8 TeV with the CMS detector using data recorded in 2012, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The mass is extracted from the normalised invariant mass distribution of the $\mathrm{t\bar{t}}$+jet system at reconstruction level as well as from the related normalised differential cross section. Both measurements are performed in the dileptonic decay channels ($\mathrm{e^+e^-}$, $\mu^+\mu^-$ and $\mathrm{e^{\pm}}\mu^{\pm}$) of the $\mathrm{t\bar{t}}$ quark pairs. | ||
| Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1-a:
Invariant mass of the ${\mathrm{ t \bar{t} } }$ system (a), transverse momentum of the leading additional jet (b) and $\rho _s$ (c) at reconstruction level for the combined dilepton channel. The ${\mathrm{ t \bar{t} } }$ sample is simulated using MadGraph and is assuming a top mass of $ {m_{\mathrm {t}}} = $ 172.5 GeV. The label `` ${\mathrm{ t \bar{t} } }$ signal'' refers to the events decaying dileptonically, while `` ${\mathrm{ t \bar{t} } }$ other'' refers to the other decay modes including ${\mathrm{ t \bar{t} } }$ decays into prompt $\tau $-leptons. The hatched regions correspond to all shape uncertainties of the simulation (cf. Section 5). |
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Figure 1-b:
Invariant mass of the ${\mathrm{ t \bar{t} } }$ system (a), transverse momentum of the leading additional jet (b) and $\rho _s$ (c) at reconstruction level for the combined dilepton channel. The ${\mathrm{ t \bar{t} } }$ sample is simulated using MadGraph and is assuming a top mass of $ {m_{\mathrm {t}}} = $ 172.5 GeV. The label `` ${\mathrm{ t \bar{t} } }$ signal'' refers to the events decaying dileptonically, while `` ${\mathrm{ t \bar{t} } }$ other'' refers to the other decay modes including ${\mathrm{ t \bar{t} } }$ decays into prompt $\tau $-leptons. The hatched regions correspond to all shape uncertainties of the simulation (cf. Section 5). |
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Figure 1-c:
Invariant mass of the ${\mathrm{ t \bar{t} } }$ system (a), transverse momentum of the leading additional jet (b) and $\rho _s$ (c) at reconstruction level for the combined dilepton channel. The ${\mathrm{ t \bar{t} } }$ sample is simulated using MadGraph and is assuming a top mass of $ {m_{\mathrm {t}}} = $ 172.5 GeV. The label `` ${\mathrm{ t \bar{t} } }$ signal'' refers to the events decaying dileptonically, while `` ${\mathrm{ t \bar{t} } }$ other'' refers to the other decay modes including ${\mathrm{ t \bar{t} } }$ decays into prompt $\tau $-leptons. The hatched regions correspond to all shape uncertainties of the simulation (cf. Section 5). |
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Figure 2:
Global $\chi ^2$ distribution obtained as the sum of the $\chi ^2_i$ distributions of the individual bins for the dilepton combined channel. The most probable top quark mass is extracted from the minimum, the statistical uncertainty from a $\chi ^{2}+1$ variation around the minimum. |
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Figure 3:
Top quark mass obtained from pseudo data generated from each of the MadGraph samples with varied mass values. Neither a favoured mass value nor a bias towards higher or lower masses can be observed. |
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Figure 4:
Normalized differential ${\mathrm{ t \bar{t} } }$ cross section in the visible phase space after unfolding as a function of the observable $\rho _s$ in the dilepton channels, compared to the predictions from POWHEG $\mathrm{ t \bar{t} }$+jet simulated with a top quark mass of 172.5 GeV as well as $\pm$3 and 6 GeV variations with respect to the central value. The grey band represents the statistical uncertainty, the yellow band corresponds to the total systematic uncertainty. |
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Figure 5-a:
Distributions of the differential cross section for simulation and data for all mass samples in the different bins of the $\rho _s$ distribution, shown for the three dilepton final states combined. The error bands correspond to the statistical error on data and the confidence interval of the second order polynomial for the simulation. |
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Figure 5-b:
Distributions of the differential cross section for simulation and data for all mass samples in the different bins of the $\rho _s$ distribution, shown for the three dilepton final states combined. The error bands correspond to the statistical error on data and the confidence interval of the second order polynomial for the simulation. |
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Figure 5-c:
Distributions of the differential cross section for simulation and data for all mass samples in the different bins of the $\rho _s$ distribution, shown for the three dilepton final states combined. The error bands correspond to the statistical error on data and the confidence interval of the second order polynomial for the simulation. |
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Figure 5-d:
Distributions of the differential cross section for simulation and data for all mass samples in the different bins of the $\rho _s$ distribution, shown for the three dilepton final states combined. The error bands correspond to the statistical error on data and the confidence interval of the second order polynomial for the simulation. |
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Figure 6:
Global $\chi ^2$ distribution for the normalized differential cross section as a function of $\rho _s$ in the dilepton combined channel. |
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Figure 7:
Top quark mass obtained by using each of the MadGraph samples with varied mass values as pseudo data, for the dilepton combined channel. |
| Tables | |
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Table 1:
Breakdown of the systematic uncertainties for the top quark mass extracted from the normalized event yield for the dilepton combined channels. All systematic uncertainties are found to be statistically significant. For the asymmetric uncertainties due to scale variations, the first reported value corresponds to an increase of the corresponding scale and the second one to a decrease. |
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Table 2:
Breakdown of the systematic uncertainties for the top quark mass measured from the dileptonic channel. All systematic uncertainties are found to be statistically significant. For the asymmetric uncertainties due to scale variations, the first reported value corresponds to an increase of the corresponding scale and the second one to a decrease. |
| Summary |
|
The top quark mass is measured from the inverse of the invariant mass of the $\mathrm{t\bar{t}}$+jet system, an observable proposed in [3]. The mass extraction has been performed with a global template fit using the shape of the distribution at reconstruction level as well as using the normalised differential cross section in the visible phase space. The first approach avoids statistical correlations and uncertainties arising from the unfolding procedure, however it cannot be compared to predictions given at generator level, while the second eases the comparisons with theory models. The top quark mass obtained from the normalized differential $\mathrm{t\bar{t}}$+jet cross section using an NLO calculation interfaced with parton shower yields 169.9 $\pm$ 1.1 (stat) $^{+2.5}_{-3.1}$ (syst) $^{+3.6}_{-1.6}$ (theo) GeV. The precision is mostly limited by the systematic uncertainties arising from modelling sources and the theory uncertainties in the POWHEG $\mathrm{t\bar{t}}$+jet simulation. The result is in agreement within the uncertainties with other measurements performed following the same approach [4] as well as complementary measurements of the mass from the inclusive $\mathrm{ t \bar{t} }$ production cross section [39-41]. |
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