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| CMS-PAS-SMP-20-013 | ||
| Search for vector boson scattering at the LHC Run 2 with CMS data in the semi-leptonic $\ell\nu \mathrm{qq}$ final state | ||
| CMS Collaboration | ||
| July 2021 | ||
| Abstract: A search for electroweak (EW) vector boson scattering in the semi-leptonic decay $\ell\nu \mathrm{qq}$ channel is reported. The search uses the full Run-II CMS dataset of proton-proton collisions at 13 TeV corresponding to an integrated luminosity of 137 fb$^{-1}$. Events are selected requiring one lepton (electron or muon), moderate missing transverse momentum, two jets with large pseudorapidity separation and dijet mass, and an additional hadronic signature consistent with the decay of a W/Z boson. Events are separated in two categories: either the hadronically decaying W/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. The signal strength is measured fitting the shape of multivariate machine learning discriminators, implemented to separate the signal from the backgrounds in each category. The observed EW signal strength is $\mu_{\text{EW}} = $ 0.85 $ ^{+0.24}_{-0.20}$ $=$ 0.85$^{+0.21}_{-0.17}$ (syst) $^{+0.12}_{-0.12}$ (stat), corresponding to a signal significance of 4.4 standard deviations (5.1 expected): the first evidence of vector boson scattering in the semileptonic channel at LHC. A simultaneous measurement of the EW and QCD WW and WZ production is also performed in the same phase space and found to be in agreement within uncertainty with the standard model prediction. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, PLB 834 (2022) 137438. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Example of a Feynman diagram contributing to the EW component of the process under study. |
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Figure 2:
Analysis workflow: objects and categories selection followed by control regions (CR) and signal regions definition. |
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Figure 3:
Postfit distributions of ${m_{jj}^{\text{VBS}}}$ observable in the resolved (left) and boosted (right) signal regions. |
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Figure 3-a:
Postfit distributions of ${m_{jj}^{\text{VBS}}}$ observable in the resolved (left) and boosted (right) signal regions. |
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Figure 3-b:
Postfit distributions of ${m_{jj}^{\text{VBS}}}$ observable in the resolved (left) and boosted (right) signal regions. |
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Figure 4:
The DNN distribution for VBS signal and backgrounds in the resolved (left) and boosted (right) signal regions normalized to 1. |
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Figure 4-a:
The DNN distribution for VBS signal and backgrounds in the resolved (left) and boosted (right) signal regions normalized to 1. |
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Figure 4-b:
The DNN distribution for VBS signal and backgrounds in the resolved (left) and boosted (right) signal regions normalized to 1. |
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Figure 5:
The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included. |
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Figure 5-a:
The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included. |
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Figure 5-b:
The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included. |
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Figure 5-c:
The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included. |
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Figure 5-d:
The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included. |
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Figure 6:
Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. 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 backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included. |
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Figure 6-a:
Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. 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 backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included. |
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Figure 6-b:
Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. 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 backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included. |
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Figure 6-c:
Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. 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 backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included. |
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Figure 6-d:
Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. 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 backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included. |
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Figure 7:
Simultaneous EW and QCD WV production fit: the expected and observed 68% and 95% confidence level (CL) contours on the signal strengths are shown in the plot. The best-fit result is compatible with the SM prediction within the 68% confidence level area. |
| Tables | |
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Table 1:
Variables used as input of the DNN for the resolved and boosted models. The Zeppenfeld variable of a particle X is defined as $Z_{X} = \frac {\eta ^{X} - \bar{\eta}^{\text{VBS}}}{{\Delta \eta ^{\text{VBS}}}}$, where $\bar{\eta}^{\text{VBS}}$ is the mean $\eta $ of VBS tag-jets, while the centrality [49,6] is $C_{VW} = \min(\Delta \eta _-, \Delta \eta _+) $, with $\Delta \eta _{+} = \max(\eta ^{\text{VBS}}) - \max(\eta ^{{V_{had}}}, \eta ^{W}) $ and $\Delta \eta _{-} = \min(\eta ^{\text{VBS}}) - \min(\eta ^{{V_{had}}}, \eta ^{W}) $. The $\eta ^{W}$ is built assuming the W-mass from the lepton and ${{p_{\mathrm {T}}} ^\text {miss}}$ kinematics. |
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Table 2:
Uncertainties breakdown on electroweak SM signal measurement |
| Summary |
|
The first evidence for the EW vector boson scattering of a WV pair plus two jets in the semi-leptonic channel is reported. Events are separated in two categories: either the hadronically decaying W/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 background in each category and their output is exploited in the statistical analysis. The large background from single W boson production accompanied by jets is estimated in a data-driven way to reduce the impact of MC mismodeling in this multi-jet phase space. The observed EW signal strength is 0.85 $^{+0.24}_{-0.20}$ $=$ 0.85 $^{+0.21}_{-0.17}$ (syst) $^{+0.12}_{-0.12}$ (stat), 1$^{+0.24}_{-0.22}$ expected, corresponding to an observed significance of the signal of 4.4 standard deviations, 5.1 expected. The electroweak WV fiducial signal cross section defined at parton level requiring all partons to have ${p_{\mathrm{T}}} > $ 10 GeV and at least one pair of outgoing quarks with invariant mass $m_{\mathrm{qq}} > $ 100 GeV, is measured to be 1.9 $\pm$ 0.5 pb, 2.23$^{+0.08}_{-0.11}$ (scale) $^{+0.05}_{-0.05}$ (pdf) pb expected. Considering instead as signal the overall EW and QCD WV plus two jets production, the signal strength is extracted as 0.98 $ ^{+0.20}_{-0.17}$ $ = $ 0.98$^{+0.19}_{-0.16}$ (syst) $^{+0.07}_{-0.07}$ (stat), (1$^{+0.19}_{-0.18}$ expected), corresponding to a cross-section, in the same fiducial phasespace as the EW only fit, of 16.6$^{+3.4}_{-2.9}$ pb, 16.9$^{+2.9}_{-2.1}$ (scale) $ ^{+0.5}_{-0.5}$ (pdf) pb expected. Finally a simultaneous two-dimensional fit of the EW and QCD WV production components is also performed. Overall, both the WV EW only measurement and the simultaneous EW and QCD WV ones are consistent between each other and in agreement with the SM predictions within the 68% confidence interval. |
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