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| CMS-SMP-24-009 ; CERN-EP-2024-314 | ||
| Measurement of the W boson decay branching fraction ratio $ \mathcal{B}(\mathrm{ W \to c q })/\mathcal{B}(\mathrm{{W}\to q \bar{q}{}^{'}}) $ in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 20 December 2024 | ||
| PLB 868 (2025) 139754 | ||
| Abstract: The most precise measurement to date of the W boson hadronic decay branching fraction ratio $ R_\mathrm{c}^{\mathrm{W}}=\mathcal{B}(\mathrm{ W \to c q })/\mathcal{B}(\mathrm{{W}\to q \bar{q}{}^{'}}) $ is presented. The measurement is based on a sample of proton-proton collision data from the CERN LHC collected by the CMS experiment at a center-of-mass energy of 13 TeV in 2016-2018 with an integrated luminosity of 138 fb$^{-1}$. The large cross section of top quark-antiquark production at the LHC offers a sizable high-purity sample of W bosons suitable for this measurement. Events with one charged lepton (electron or muon) and at least four jets, two tagged as bottom quark jets, are analyzed. Charm jets are tagged using the presence of a muon inside the jet. The result, $ R_\mathrm{c}^{\mathrm{W}}= $ 0.489 $ \pm $ 0.020, is consistent with the standard model prediction and is twice as precise as the current world-average value. | ||
| Links: e-print arXiv:2412.16296 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Kinematic distributions of the muon inside the c-tagged jets: transverse momentum $ p_{\mathrm{T}}^{\mu} $ (upper left), pseudorapidity $ \eta^{\mu} $ (upper right), isolation $ I^{\mu} $ (lower left), and $ p_{\mathrm{T}}^{\mu}/p_{\mathrm{T}}^{\text{jet}} $ (lower right). Histograms show OS-SS yields for both data and simulations. Events with a prompt electron or muon are considered. The data are displayed as points with statistical error bars. The gray band in the predictions represents the systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel, together with the statistical and systematic uncertainties. |
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Figure 1-a:
Kinematic distributions of the muon inside the c-tagged jets: transverse momentum $ p_{\mathrm{T}}^{\mu} $ (upper left), pseudorapidity $ \eta^{\mu} $ (upper right), isolation $ I^{\mu} $ (lower left), and $ p_{\mathrm{T}}^{\mu}/p_{\mathrm{T}}^{\text{jet}} $ (lower right). Histograms show OS-SS yields for both data and simulations. Events with a prompt electron or muon are considered. The data are displayed as points with statistical error bars. The gray band in the predictions represents the systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel, together with the statistical and systematic uncertainties. |
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Figure 1-b:
Kinematic distributions of the muon inside the c-tagged jets: transverse momentum $ p_{\mathrm{T}}^{\mu} $ (upper left), pseudorapidity $ \eta^{\mu} $ (upper right), isolation $ I^{\mu} $ (lower left), and $ p_{\mathrm{T}}^{\mu}/p_{\mathrm{T}}^{\text{jet}} $ (lower right). Histograms show OS-SS yields for both data and simulations. Events with a prompt electron or muon are considered. The data are displayed as points with statistical error bars. The gray band in the predictions represents the systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel, together with the statistical and systematic uncertainties. |
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Figure 1-c:
Kinematic distributions of the muon inside the c-tagged jets: transverse momentum $ p_{\mathrm{T}}^{\mu} $ (upper left), pseudorapidity $ \eta^{\mu} $ (upper right), isolation $ I^{\mu} $ (lower left), and $ p_{\mathrm{T}}^{\mu}/p_{\mathrm{T}}^{\text{jet}} $ (lower right). Histograms show OS-SS yields for both data and simulations. Events with a prompt electron or muon are considered. The data are displayed as points with statistical error bars. The gray band in the predictions represents the systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel, together with the statistical and systematic uncertainties. |
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Figure 1-d:
Kinematic distributions of the muon inside the c-tagged jets: transverse momentum $ p_{\mathrm{T}}^{\mu} $ (upper left), pseudorapidity $ \eta^{\mu} $ (upper right), isolation $ I^{\mu} $ (lower left), and $ p_{\mathrm{T}}^{\mu}/p_{\mathrm{T}}^{\text{jet}} $ (lower right). Histograms show OS-SS yields for both data and simulations. Events with a prompt electron or muon are considered. The data are displayed as points with statistical error bars. The gray band in the predictions represents the systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel, together with the statistical and systematic uncertainties. |
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Figure 2:
Comparison of the measured value of $ R_\mathrm{c}^{\mathrm{W}} $ with previous LEP2 measurements [2,3], and the world-average value [1]. Horizontal bars represent the total uncertainty of the measurements. |
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Figure 3:
Invariant mass of the two jets associated with the W boson, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
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Figure 3-a:
Invariant mass of the two jets associated with the W boson, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
png pdf |
Figure 3-b:
Invariant mass of the two jets associated with the W boson, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
png pdf |
Figure 3-c:
Invariant mass of the two jets associated with the W boson, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
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Figure 3-d:
Invariant mass of the two jets associated with the W boson, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
png pdf |
Figure 4:
Invariant mass of the three jets associated with the top quark, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
png pdf |
Figure 4-a:
Invariant mass of the three jets associated with the top quark, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
png pdf |
Figure 4-b:
Invariant mass of the three jets associated with the top quark, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
png pdf |
Figure 4-c:
Invariant mass of the three jets associated with the top quark, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
png pdf |
Figure 4-d:
Invariant mass of the three jets associated with the top quark, for the four event categories entering the fit: prompt muon and no charm tag (upper left), prompt electron and no charm tag (upper right), prompt muon and charm tag (lower left), prompt electron and charm tag (lower right). The measured data correspond to OS events, whereas the predictions from the simulations are OS-SS subtracted yields on top of the background prediction using the SS data. The data are displayed as points with statistical error bars. The gray band in the predictions represents the pre-fit systematic uncertainties. The ratios of data to the expected yields are given at the bottom of each panel together with the statistical and systematic uncertainties. |
| Tables | |
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Table 1:
Summary of the main systematic uncertainties affecting the $ R_\mathrm{c}^{\mathrm{W}} $ measurement. The quoted numbers are the percentage change in the predicted yields of the samples with no charm and with a charm tag, and the last column reflects the impacts in percentage of the measured $ R_\mathrm{c}^{\mathrm{W}} $ value from each uncertainty source. The total systematic uncertainty, calculated from the various sources of uncertainty and their correlations, is given in the last row. |
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Table 2:
Observed and predicted event yields input to the fit for the four categories. Predictions are separated by process. For the two categories with charm tag, the yields predicted by the simulations correspond to OS-SS subtracted events, the SS contamination is estimated with data, and the number of observed events in data corresponds to OS events. The relative uncertainties shown in parenthesis for the predictions are based on the statistical uncertainties of the MC samples and the systematic uncertainties and their correlations discussed in Section 5. Correlated systematic effects dominate the uncertainties. |
| Summary |
| The most precise measurement to date of the W boson hadronic decay branching fraction ratio $ R_\mathrm{c}^{\mathrm{W}} $ = $ \mathcal{B}(\mathrm{ W \to c q })/\mathcal{B}(\mathrm{{W}\to q \bar{q}{}^{'}}) $ is reported. The measurement is based on a data sample collected by the CMS experiment at a center-of-mass energy of 13 TeV in 2016-2018 at the CERN LHC with an integrated luminosity of 138 fb$ ^{-1} $. The large cross section of top quark-antiquark production at the LHC offers a sizable high-purity sample of W bosons suitable for this measurement. Events with one prompt charged lepton (electron or muon) and at least four jets, two of them tagged as bottom quark jets, are analyzed. Charm jets are tagged using the presence of a muon inside the jet. This charm tagging method enables the selection of a sample of charm jets with a low level of background that is precisely determined from data. The measured $ R_\mathrm{c}^{\mathrm{W}} $ value is 0.489 $ \pm $ 0.020, in good agreement with the standard model prediction. The relative precision of the measurement of 4%, limited by the systematic uncertainty in the charm tagging efficiency, is improved by a factor of two compared with the current world-average value. From the $ R_\mathrm{c}^{\mathrm{W}} $ measurement, the sum of squared elements in the second row of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, 0.970 $ \pm $ 0.041, and the CKM matrix element $ |V_{\mathrm{c}\mathrm{s}}|= $ 0.959 $ \pm $ 0.021 are derived. These results provide a consistency test of the CKM unitarity and a measurement of $ |V_{\mathrm{c}\mathrm{s}}| $ from hadronic W boson decays. |
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