CMS-PAS-SMP-24-009

CMS-PAS-SMP-24-009
Measurement of the $\mathrm{W}\to\mathrm{c}\mathrm{q}/\mathrm{W}\to\mathrm{q}\bar{\mathrm{q}}^{'}$ decay branching fraction ratio in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
18 July 2024

Abstract: The most precise measurement to date of the W boson hadronic decay branching fraction ratio $R_\mathrm{c}^\mathrm{W} = \mathcal{B}(\mathrm{W}\to\mathrm{c}\mathrm{q})/\mathcal{B}(\mathrm{W}\to\mathrm{q}\bar{\mathrm{q}}^{'})$ 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 and corresponding to 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 of them 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 improves the precision of the current world average value by a factor of two. From this measurement, the sum of squared elements in the second row of the CKM matrix, 0.970 $\pm$ 0.041, and the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $\|V_{\mathrm{c}\mathrm{s}}\|=$ 0.959 $\pm$ 0.021 are derived.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PLB. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Kinematic distributions of the muon inside the c-tagged jets after OS-SS subtraction (see text): transverse momentum $p^{\mu}_T$ (top-left), pseudorapidity $\eta^{\mu}$ (top-right), isolation (bottom-left), and $p^{\mu}_T/p^{\text{jet}}_T$ (bottom-right). Events with a prompt electron or muon are considered. The grey 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.
png pdf	Figure 1-a: Kinematic distributions of the muon inside the c-tagged jets after OS-SS subtraction (see text): transverse momentum $p^{\mu}_T$ (top-left), pseudorapidity $\eta^{\mu}$ (top-right), isolation (bottom-left), and $p^{\mu}_T/p^{\text{jet}}_T$ (bottom-right). Events with a prompt electron or muon are considered. The grey 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.
png pdf	Figure 1-b: Kinematic distributions of the muon inside the c-tagged jets after OS-SS subtraction (see text): transverse momentum $p^{\mu}_T$ (top-left), pseudorapidity $\eta^{\mu}$ (top-right), isolation (bottom-left), and $p^{\mu}_T/p^{\text{jet}}_T$ (bottom-right). Events with a prompt electron or muon are considered. The grey 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.
png pdf	Figure 1-c: Kinematic distributions of the muon inside the c-tagged jets after OS-SS subtraction (see text): transverse momentum $p^{\mu}_T$ (top-left), pseudorapidity $\eta^{\mu}$ (top-right), isolation (bottom-left), and $p^{\mu}_T/p^{\text{jet}}_T$ (bottom-right). Events with a prompt electron or muon are considered. The grey 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.
png pdf	Figure 1-d: Kinematic distributions of the muon inside the c-tagged jets after OS-SS subtraction (see text): transverse momentum $p^{\mu}_T$ (top-left), pseudorapidity $\eta^{\mu}$ (top-right), isolation (bottom-left), and $p^{\mu}_T/p^{\text{jet}}_T$ (bottom-right). Events with a prompt electron or muon are considered. The grey 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.
png pdf	Figure 2: Invariant mass of the two jets reconstructing the W boson, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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 2-a: Invariant mass of the two jets reconstructing the W boson, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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 2-b: Invariant mass of the two jets reconstructing the W boson, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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 2-c: Invariant mass of the two jets reconstructing the W boson, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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 2-d: Invariant mass of the two jets reconstructing the W boson, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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: Invariant mass of the three jets reconstructing the top quark, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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-a: Invariant mass of the three jets reconstructing the top quark, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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 three jets reconstructing the top quark, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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 three jets reconstructing the top quark, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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-d: Invariant mass of the three jets reconstructing the top quark, for the four event categories entering the fit: prompt muon and no charm tag (top-left), prompt electron and no charm tag (top-right), prompt muon and charm tag (bottom-left), prompt electron and charm tag (bottom-right). The measured data correspond to OS events, while the predictions from the simulations are OS-SS subtracted yields. The background prediction using the SS data is also shown. The grey 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
png pdf	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 charm tag, and the last column reflects the impacts in percentage of the measured $R_\mathrm{c}^{\mathrm{W}}$ value from each uncertainty source.

Summary

The most precise measurement to date of the W boson hadronic decay branching fraction ratio

$R_\mathrm{c}^{\mathrm{W}}$ =

$\mathcal{B}(\mathrm{W}\to\mathrm{c}\mathrm{q})/\mathcal{B}(\mathrm{W}\to\mathrm{q}\overline{\mathrm{q}}^{'})$ has been performed. 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 and corresponding to 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 b-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 pure 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 precision of the measurement of 4%, limited by the systematic uncertainty in the charm tagging efficiency, is improved by a factor of two compared to the current world average value. From the

$R_\mathrm{c}^{\mathrm{W}}$ measurement, the sum of squared elements in the second row of the CKM matrix, 0.970

$\pm$ 0.041, and the Cabibbo--Kobayashi--Maskawa (CKM) matrix element

$|V_{\mathrm{c}\mathrm{s}}|=$ 0.959

$\pm$ 0.021 are derived. These results provide a consistency test of CKM unitarity and a measurement of

$|V_{\mathrm{c}\mathrm{s}}|$ from hadronic W boson decays.

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