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| CMS-TOP-12-031 ; CERN-EP-2016-249 | ||
| Measurement of the mass difference between top quark and antiquark in pp collisions at $ \sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 29 October 2016 | ||
| Phys. Lett. B 770 (2017) 50 | ||
| Abstract: The invariance of the standard model (SM) under the CPT transformation predicts equality of particle and antiparticle masses. This prediction is tested by measuring the mass difference between the top quark and antiquark ($\Delta m_{\mathrm{t}} = m_{\mathrm{t}} - m_{\overline{\mathrm{t}}}$) that are produced in pp collisions at a center-of-mass energy of 8 TeV, using events with a muon or an electron and at least four jets in the final state. The analysis is based on data corresponding to an integrated luminosity of 19.6 fb$^{-1}$ collected by the CMS experiment at the LHC, and yields a value of $\Delta m_{\mathrm{t}} = -0.15 \pm 0.19\, \mathrm{(stat)} \pm 0.09\, \mathrm{(syst)}$ GeV, which is consistent with the SM expectation. This result is significantly more precise than previously reported measurements. | ||
| Links: e-print arXiv:1610.09551 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Comparison of the data to simulation for the transverse momenta of the four leading jets in each event for $\ell ^+$+jets events (left) and $\ell ^-$+jets events (right). The last bin of each distribution includes all jets with $ {p_{\mathrm {T}}} > $ 530 GeV. The bin-by-bin ratio of the observed to the simulated spectra one is shown at the bottom of each plot. The uncertainties are purely statistical. The total simulated event yields are normalized to the observed yields in the data, while keeping the relative fractions of the individual components fixed. |
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Figure 1-a:
Comparison of the data to simulation for the transverse momenta of the four leading jets in each event for $\ell ^+$+jets events. The last bin of each distribution includes all jets with $ {p_{\mathrm {T}}} > $ 530 GeV. The bin-by-bin ratio of the observed to the simulated spectra one is shown at the bottom of each plot. The uncertainties are purely statistical. The total simulated event yields are normalized to the observed yields in the data, while keeping the relative fractions of the individual components fixed. |
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Figure 1-b:
Comparison of the data to simulation for the transverse momenta of the four leading jets in each event for $\ell ^-$+jets events. The last bin of each distribution includes all jets with $ {p_{\mathrm {T}}} > $ 530 GeV. The bin-by-bin ratio of the observed to the simulated spectra one is shown at the bottom of each plot. The uncertainties are purely statistical. The total simulated event yields are normalized to the observed yields in the data, while keeping the relative fractions of the individual components fixed. |
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Figure 2:
Comparison between the data and simulation for the fitted top quark mass of the jet-quark assignment with the smallest $\chi ^2$ (top) and these smallest $\chi ^2$ values (bottom), for $\ell ^+$+jets events (left) and $\ell ^-$+jets events (right). The last bin of the top quark mass distributions includes all masses above 980 GeV. The bin-by-bin ratio of the observed spectrum to the simulated one is shown at the bottom of each plot. The uncertainties are purely statistical. |
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Figure 2-a:
Comparison between the data and simulation for the fitted top quark mass of the jet-quark assignment with the smallest $\chi ^2$, for $\ell ^+$+jets events. The last bin of the distribution includes all masses above 980 GeV. The bin-by-bin ratio of the observed spectrum to the simulated one is shown at the bottom of the plot. The uncertainties are purely statistical. |
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Figure 2-b:
Comparison between the data and simulation for the fitted top quark mass of the jet-quark assignment with the smallest $\chi ^2$, for $\ell ^-$+jets events. The last bin of the distribution includes all masses above 980 GeV. The bin-by-bin ratio of the observed spectrum to the simulated one is shown at the bottom of the plot. The uncertainties are purely statistical. |
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Figure 2-c:
Comparison between the data and simulation for the smallest $\chi ^2$ values, for $\ell ^+$+jets events. The bin-by-bin ratio of the observed distribution to the simulated one is shown at the bottom of the plot. The uncertainties are purely statistical. |
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Figure 2-d:
Comparison between the data and simulation for the smallest $\chi ^2$ values, for $\ell ^-$+jets events. The bin-by-bin ratio of the observed distribution to the simulated one is shown at the bottom of the plot. The uncertainties are purely statistical. |
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Figure 3:
Width of the pull distribution (left) and bias on the estimated top quark mass (right) as a function of the generated top quark mass for $\ell $+jets events. The dashed blue line represents the ideal outcome. |
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Figure 3-a:
Width of the pull distribution as a function of the generated top quark mass for $\ell $+jets events. The dashed blue line represents the ideal outcome. |
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Figure 3-b:
Bias on the estimated top quark mass as a function of the generated top quark mass for $\ell $+jets events. The dashed blue line represents the ideal outcome. |
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Figure 4:
Residual bias on the estimated top quark mass as a function of the generated top quark mass using $\ell ^+$+jets events (left) and $\ell ^-$+jets events (right) after the inclusive $\ell $+jets calibration. The dashed blue line represents the ideal outcome. |
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Figure 4-a:
Residual bias on the estimated top quark mass as a function of the generated top quark mass using $\ell ^+$+jets events after the inclusive $\ell $+jets calibration. The dashed blue line represents the ideal outcome. |
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Figure 4-b:
Residual bias on the estimated top quark mass as a function of the generated top quark mass using $\ell ^-$+jets events after the inclusive $\ell $+jets calibration. The dashed blue line represents the ideal outcome. |
| Tables | |
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Table 1:
Expected and observed yield of events passing the full selection of $\mathrm{ e }^{+} $+jets, $\mathrm{ e }^{-} $+jets, $\mu^{+} $+jets, and $\mu^{-} $+jets channels. Simulations are used to obtain the expected number of events except for the QCD multijet background, which is derived from data, as described in the text. The uncertainties on the event numbers are statistical and reflect the limited number of events in simulation or data for the individual processes. |
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Table 2:
Summary of systematic uncertainties on $\Delta m_{\mathrm{ t } }$. For each contribution, the first value is the observed systematic shift, whereas the second number is the uncertainty of the shift due to the limited number of generated events. In all cases, the larger among the two is considered as the final systematic uncertainty and is indicated in the bold font. The total uncertainty is obtained from the sum in quadrature of the individual terms. |
| Summary |
| Data collected by the CMS experiment in pp collisions at $ \sqrt{s} = $ 8 TeV and corresponding to an integrated luminosity of 19.6$\pm$0.5 fb$^{-1}$ have been used to measure the difference in mass between the top quark and antiquark. The measured value is $\Delta m_{\mathrm{t}} = -0.15 \pm 0.19\, \mathrm{(stat)} \pm 0.09\, \mathrm{(syst)}$ GeV. This result improves in precision upon previously reported measurements [9-12] by more than a factor of two. It is in agreement with the expectations from CPT invariance, requiring equal particle and antiparticle masses. |
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