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CMS-SMP-20-013 ; CERN-EP-2021-225
Evidence for WW/WZ vector boson scattering in the decay channel $ \ell\nu $qq produced in association with two jets in proton-proton collisions at $ \sqrt{s}= $ 13 TeV
Phys. Lett. B 834 (2022) 137438
Abstract: Evidence is reported for electroweak (EW) vector boson scattering in the decay channel $ \ell\nu $qq of two weak vector bosons WV (V = W or Z), produced in association with two parton jets. The search uses a data set of proton-proton collisions at 13 TeV collected with the CMS detector during 2016--2018 with an integrated luminosity of 138 fb$ ^{-1} $. Events are selected requiring one lepton (electron or muon), moderate missing transverse momentum, two jets with a large pseudorapidity separation and a large dijet invariant mass, and a signature consistent with the hadronic decay of a W/Z boson. The cross section is computed in a fiducial phase space defined at parton level requiring all parton transverse momenta $ p_{\mathrm{T}} > $ 10 GeV and at least one pair of outgoing partons with invariant mass $ m_{\mathrm{q}\mathrm{q}} > $ 100 GeV. The measured and expected EW WV production cross sections are 1.90 $ ^{+0.53}_{-0.46} $ pb and 2.23 $ ^{+0.08}_{-0.11} $ (scale) $ \pm $ 0.05 (PDF) pb, respectively, where PDF is the parton distribution function. The observed EW signal strength is $ \mu_{\mathrm{EW}} = $ 0.85 $ \pm $ 0.12 (stat) $ ^{+0.19}_{-0.17} $ (syst), corresponding to a signal significance of 4.4 standard deviations with 5.1 expected, and it is measured keeping the quantum chromodynamics (QCD) associated diboson production fixed to the standard model prediction. This is the first evidence of vector boson scattering in the $ \ell\nu $qq decay channel at LHC. The simultaneous measurement of the EW and QCD associated diboson production agrees with the standard model prediction.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Examples of Feynman diagrams contributing to the analyzed final state: general schema of purely EW VBS signal process contributions (upper left diagram), the $ s $-channel Higgs boson contribution (upper right diagram), the purely EW nonresonant diboson production (lower left diagram); and an example of non-EW nonresonant diboson production (lower right diagram), which is part of the irreducible background.

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Figure 1-a:
Example of Feynman diagram for the purely EW VBS process contribution to the analyzed final state.

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Figure 1-b:
Example of Feynman diagram for the $s$-channel Higgs boson contribution to the analyzed final state.

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Figure 1-c:
Example of Feynman diagram for the purely EW nonresonant diboson production contribution to the analyzed final state.

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Figure 1-d:
Example of Feynman diagram for non-EW nonresonant diboson production, which is part of the irreducible background.

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Figure 2:
Postfit distributions of the ${m_{\mathrm {jj}}^{\mathrm {VBS}}}$ observable in the resolved (left) and boosted (right) signal regions. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 2-a:
Postfit distributions of the ${m_{\mathrm {jj}}^{\mathrm {VBS}}}$ observable in the resolved signal region. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 2-b:
Postfit distributions of the ${m_{\mathrm {jj}}^{\mathrm {VBS}}}$ observable in the boosted signal region. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 3:
The DNN discriminator distribution, taken from simulation, for VBS signal and backgrounds in the resolved (left) and boosted (right) signal regions normalized to unity.

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Figure 3-a:
The DNN discriminator distribution, taken from simulation, for VBS signal and backgrounds in the resolved signal region normalized to unity.

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Figure 3-b:
The DNN discriminator distribution, taken from simulation, for VBS signal and backgrounds in the boosted signal region normalized to unity.

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Figure 4:
The DNN discriminator distribution for the resolved (left) and boosted (right) phase space region in the top quark (upper plots) and W+jets (lower plots) control regions. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 4-a:
The DNN discriminator distribution for the resolved phase space region in the top quark control region. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 4-b:
The DNN discriminator distribution for the boosted phase space region in the top quark control region. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 4-c:
The DNN discriminator distribution for the resolved phase space region in the W+jets control region. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 4-d:
The DNN discriminator distribution for the boosted phase space region in the W+jets control region. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 5:
Results for the EW-signal-only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN discriminator distributions for the resolved (left) and the boosted (right) signal regions. The signal contribution is plotted both stacked on top of the background processes and also overlaid to show the signal postfit distribution. The expected yield is the sum of signal and background. Lower plots: background-subtracted DNN discriminator distribution for the resolved (left) and the boosted (right) categories. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 5-a:
Results for the EW-signal-only fit, keeping the QCD WV contribution fixed to the SM prediction: post-fit DNN discriminator distributions for the resolved signal region. The signal contribution is plotted both stacked on top of the background processes and also overlaid to show the signal postfit distribution. The expected yield is the sum of signal and background.

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Figure 5-b:
Results for the EW-signal-only fit, keeping the QCD WV contribution fixed to the SM prediction: post-fit DNN discriminator distributions for the boosted signal region. The signal contribution is plotted both stacked on top of the background processes and also overlaid to show the signal postfit distribution. The expected yield is the sum of signal and background.

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Figure 5-c:
Results for the EW-signal-only fit, keeping the QCD WV contribution fixed to the SM prediction: background-subtracted DNN discriminator distribution for the resolved category. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 5-d:
Results for the EW-signal-only fit, keeping the QCD WV contribution fixed to the SM prediction: background-subtracted DNN discriminator distribution for the boosted category. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. Vertical bars on data points show the statistical error, whereas the gray band is the post-fit uncertainty on MC with all systematic uncertainties included.

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Figure 6:
Simultaneous EW and QCD WV production fit: the expected and observed 68 and 95% CL contours on the signal strengths. The best fit result is compatible with the SM prediction within the 68% CL area.
Tables

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Table 1:
Variables used as input to the DNN for the resolved and boosted models. They are ranked by their contributions to the signal discrimination power of the DNN model using the SHAP [64,65] technique and their rank is shown in the table for the resolved and boosted categories models.

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Table 2:
Breakdown of the uncertainties in the EW WV VBS signal strength measurement.
Summary
The first evidence for the electroweak (EW) production of a WV (V = W or Z ) pair plus two jets in the $ \ell\nu $qq decay channel is reported. Events are separated into two categories: either the hadronically decaying W or Z boson is reconstructed as one large-radius jet, or it is identified as a pair of jets with dijet mass close to the boson mass. Multivariate machine learning discriminators are optimized to separate the signal from the background in each category and their outputs are exploited in the statistical analysis. The large background from single W boson production accompanied by jets is estimated from control samples in the data to reduce the impact of Monte Carlo mismodeling in this multijet phase space region. Tabulated results are provided in the HEPData record for this analysis [81]. The EW-only WV signal strength, measured keeping the QCD-associated diboson production fixed to the standard model prediction, is: $ \mu_{\mathrm{EW}} = \sigma^{\text{obs}} / \sigma^{\text{SM}} = $ 0.85 $ \pm $ 0.12 (stat) $ ^{+0.19}_{-0.17} $ (syst) $ = $ 0.85 $ ^{+0.23}_{-0.21} $ at 1.00 $ ^{+0.24}_{-0.22} $ expected, where $ \sigma^{\text{obs}} $ and $ \sigma^{\text{SM}} $ are the observed and predicted cross sections, respectively. The observed significance for the SM EW WV signal is 4.4 standard deviations with 5.1 expected., it is measured keeping the quantum chromodynamics (QCD) associated diboson production fixed to the standard model prediction. When we consider the signal as the total EW and QCD-associated diboson yield, the overall signal strength $ \mu_{\mathrm{EW}+\mathrm{QCD}} $ is measured as: 0.97 $ \pm $ 0.06 (stat) $ ^{+0.19}_{-0.21} $ (syst) $ = $ 0.97 $ ^{+0.20}_{-0.22} $ with an expectation of 1.00 $ ^{+0.21}_{-0.20} $. Finally, a simultaneous two-dimensional fit of the EW and QCD WV production components is performed. Overall, both the WV EW-only measurement and the simultaneous EW and QCD WV measurements are in agreement with the SM predictions within the 68% confidence level.
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