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| CMS-TOP-13-008 ; CERN-EP-2016-118 | ||
| Measurement of the W boson helicity fractions in the decays of top quark pairs to lepton+jets final states produced in pp collisions at $ \sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 29 May 2016 | ||
| Phys. Lett. B 762 (2016) 512 | ||
| Abstract: The W boson helicity fractions from top quark decays in $\mathrm{ t \bar{t} }$ events are measured using data from proton-proton collisions at a centre-of-mass energy of 8 TeV. The data were collected in 2012 with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.8 fb$^{-1}$. Events are reconstructed with either one muon or one electron, along with four jets in the final state, with two of the jets being identified as originating from b quarks. The measured helicity fractions from both channels are combined, yielding $F_{\rm 0}= 0.681 \pm 0.012 $ (stat) $\pm 0.023 $ (syst), $F_{\rm L}= 0.323 \pm 0.008 $ (stat) $\pm 0.014 $ (syst), and $F_{\rm R}= -0.004 \pm 0.005 $ (stat) $\pm 0.014 $ (syst) for the longitudinal, left-, and right-handed components of the helicity, respectively. These measurements of the W boson helicity fractions are the most accurate to date and they agree with the predictions from the standard model. | ||
| Links: e-print arXiv:1605.09047 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Predicted cos$\theta ^*$ distributions for the different helicity fractions. The distributions for the fractions $F_\mathrm {0},\ F_\mathrm {L}$, and $F_\mathrm {R}$ are shown as dashed, dotted, and dash-dotted lines, respectively, and the sum of the three contributions according to the SM predictions is displayed as a solid line. |
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Figure 2:
Distributions for the cosine of the helicity angle in the leptonic (upper row) and hadronic (lower row) branches, for the $\mathrm{ e } $+jets (left) and $\mu $+jets (right) decay channels. The combined $\ell $+jets post-fit measurements of the helicity fractions were used in the simulation of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ and single top quark events. The data are displayed as solid points, simulated samples of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ (signal) processes and the contribution from background processes as histograms. At the bottom of each plot, the ratio between MC simulation and data is displayed. The error bars correspond to the statistical uncertainties. |
png pdf |
Figure 2-a:
Distributions for the cosine of the helicity angle in the leptonic (upper row) and hadronic (lower row) branches, for the $\mathrm{ e } $+jets (left) and $\mu $+jets (right) decay channels. The combined $\ell $+jets post-fit measurements of the helicity fractions were used in the simulation of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ and single top quark events. The data are displayed as solid points, simulated samples of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ (signal) processes and the contribution from background processes as histograms. At the bottom of each plot, the ratio between MC simulation and data is displayed. The error bars correspond to the statistical uncertainties. |
png pdf |
Figure 2-b:
Distributions for the cosine of the helicity angle in the leptonic (upper row) and hadronic (lower row) branches, for the $\mathrm{ e } $+jets (left) and $\mu $+jets (right) decay channels. The combined $\ell $+jets post-fit measurements of the helicity fractions were used in the simulation of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ and single top quark events. The data are displayed as solid points, simulated samples of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ (signal) processes and the contribution from background processes as histograms. At the bottom of each plot, the ratio between MC simulation and data is displayed. The error bars correspond to the statistical uncertainties. |
png pdf |
Figure 2-c:
Distributions for the cosine of the helicity angle in the leptonic (upper row) and hadronic (lower row) branches, for the $\mathrm{ e } $+jets (left) and $\mu $+jets (right) decay channels. The combined $\ell $+jets post-fit measurements of the helicity fractions were used in the simulation of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ and single top quark events. The data are displayed as solid points, simulated samples of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ (signal) processes and the contribution from background processes as histograms. At the bottom of each plot, the ratio between MC simulation and data is displayed. The error bars correspond to the statistical uncertainties. |
png pdf |
Figure 2-d:
Distributions for the cosine of the helicity angle in the leptonic (upper row) and hadronic (lower row) branches, for the $\mathrm{ e } $+jets (left) and $\mu $+jets (right) decay channels. The combined $\ell $+jets post-fit measurements of the helicity fractions were used in the simulation of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ and single top quark events. The data are displayed as solid points, simulated samples of $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ (signal) processes and the contribution from background processes as histograms. At the bottom of each plot, the ratio between MC simulation and data is displayed. The error bars correspond to the statistical uncertainties. |
png pdf |
Figure 3:
Left: the measured W boson helicity fractions in the $(F_\mathrm {0},F_\mathrm {L})$ plane. The dashed and solid ellipses enclose the allowed two-dimensional 68% and 95% CL regions, for the combined $\ell $+jets measurement, taking into account the correlations on the total (including systematic) uncertainties. The error bars give the one-dimensional 68% CL interval for the separate $F_\mathrm {0}$ and $F_\mathrm {L}$ measurements, with the inner-tick (outer-tick) mark representing the statistical (total) uncertainty. Right: the corresponding allowed regions for the real components of the anomalous couplings $g_\mathrm {L}$ and $g_\mathrm {R}$ at 68% and 95% CL, for $V_\mathrm {L}=$ 1 and $V_\mathrm {R}= $ 0. A region near $\mathrm {Re}(g_\mathrm {L})= $ 0 and $\mathrm {Re}(g_\mathrm {R}) \gg 0$, allowed by the fit but excluded by the CMS single top quark production measurement, is omitted. The SM predictions are shown as stars. |
png pdf |
Figure 3-a:
Left: the measured W boson helicity fractions in the $(F_\mathrm {0},F_\mathrm {L})$ plane. The dashed and solid ellipses enclose the allowed two-dimensional 68% and 95% CL regions, for the combined $\ell $+jets measurement, taking into account the correlations on the total (including systematic) uncertainties. The error bars give the one-dimensional 68% CL interval for the separate $F_\mathrm {0}$ and $F_\mathrm {L}$ measurements, with the inner-tick (outer-tick) mark representing the statistical (total) uncertainty. Right: the corresponding allowed regions for the real components of the anomalous couplings $g_\mathrm {L}$ and $g_\mathrm {R}$ at 68% and 95% CL, for $V_\mathrm {L}=$ 1 and $V_\mathrm {R}= $ 0. A region near $\mathrm {Re}(g_\mathrm {L})= $ 0 and $\mathrm {Re}(g_\mathrm {R}) \gg 0$, allowed by the fit but excluded by the CMS single top quark production measurement, is omitted. The SM predictions are shown as stars. |
png pdf |
Figure 3-b:
Left: the measured W boson helicity fractions in the $(F_\mathrm {0},F_\mathrm {L})$ plane. The dashed and solid ellipses enclose the allowed two-dimensional 68% and 95% CL regions, for the combined $\ell $+jets measurement, taking into account the correlations on the total (including systematic) uncertainties. The error bars give the one-dimensional 68% CL interval for the separate $F_\mathrm {0}$ and $F_\mathrm {L}$ measurements, with the inner-tick (outer-tick) mark representing the statistical (total) uncertainty. Right: the corresponding allowed regions for the real components of the anomalous couplings $g_\mathrm {L}$ and $g_\mathrm {R}$ at 68% and 95% CL, for $V_\mathrm {L}=$ 1 and $V_\mathrm {R}= $ 0. A region near $\mathrm {Re}(g_\mathrm {L})= $ 0 and $\mathrm {Re}(g_\mathrm {R}) \gg 0$, allowed by the fit but excluded by the CMS single top quark production measurement, is omitted. The SM predictions are shown as stars. |
| Tables | |
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Table 1:
Systematic uncertainties on the measurements of the W boson helicity fractions from lepton+jets events. The cases in which the statistical precision of the limited sample size was assigned as systematic uncertainties are denoted by the symbol (*). |
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Table 2:
Measurements of the W boson helicity fractions from lepton+jets final states in $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $ decays. The helicity fractions $ F_\mathrm {0}$ and $ F_\mathrm {L}$ are measured simultaneously and are strongly anti-correlated, as indicated by a correlation coefficient $\rho _\mathrm {0,L}$, because $F_\mathrm {R}$ is derived from the unitarity condition. |
| Summary |
| A measurement of the W boson helicity fractions in top quark pair events decaying in the $\mathrm{ e }$+jets and $\mu$+jets channels has been presented, using proton-proton collision data at $ \sqrt{s} = $ 8 TeV, and corresponding to an integrated luminosity of 19.8 fb$^{-1}$. The helicity fractions $F_{\rm 0}$ and $F_{\rm L}$ are measured with a precision of better than 5%, yielding the most accurate experimental determination of the W boson helicity fractions in $\mathrm{ t \bar{t} }$ processes to date. The measured W boson helicity fractions are $F_{\rm 0}=0.681 \pm 0.012 $ (stat) $\pm 0.023 $ (syst), $F_{\rm L}=0.323 \pm 0.008 $ (stat) $\pm 0.014 $ (syst), and $F_{\rm R}=-0.004 \pm 0.005 $ (stat) $\pm 0.014 $ (syst), with a correlation coefficient of $-0.87$ between $F_{\rm 0}$ and $F_{\rm L}$, and they are consistent with the expectations from the standard model. |
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