CMS-PAS-SMP-18-001 | ||

Measurement of electroweak WZ production and search for new physics in pp collisions at $\sqrt{s} = $ 13 TeV | ||

CMS Collaboration | ||

July 2018 | ||

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Abstract:
A measurement of WZ electroweak vector boson scattering is presented. The analysis is based on a data sample of proton-proton collisions at $\sqrt{s} = $ 13 TeV collected with the CMS detector and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The measurement is performed in the leptonic decay modes $\mathrm{WZ} \to \ell\nu\ell^\prime \ell^\prime$, where $\ell, \ell^\prime = \mathrm{e}$, $\mu$. Electroweak WZ production in association with two jets is measured with an observed (expected) significance of 1.9 (2.7) standard deviations. The total WZ plus two jet production cross section is measured in fiducial regions with enhanced contributions from electroweak production and found to be consistent with the standard model prediction. Constraints on charged Higgs production and on anomalous quartic gauge couplings in terms of dimension-eight effective field theory operators are also presented.
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, PLB 795 (2019) 281.The superseded preliminary plots can be found here. |

Figures | |

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Figure 1:
Representative Feynman diagrams for ${\mathrm {W}} {\mathrm {Z}}{jj}$ production in the SM and BSM. EW-induced WZ production includes quartic interactions (a) of the vector bosons. This is distinguishable from QCD-induced production (b) through kinematic variables. New physics in the EW sector modifying the quartic coupling can be parameterized in terms of dimension-eight effective field theory operators (c). Specific models modifying this interaction include those predicting charged Higgs bosons (d). |

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Figure 1-a:
Representative Feynman diagrams for ${\mathrm {W}} {\mathrm {Z}}{jj}$ production in the SM and BSM. EW-induced WZ production includes quartic interactions (a) of the vector bosons. This is distinguishable from QCD-induced production (b) through kinematic variables. New physics in the EW sector modifying the quartic coupling can be parameterized in terms of dimension-eight effective field theory operators (c). Specific models modifying this interaction include those predicting charged Higgs bosons (d). |

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Figure 1-b:
Representative Feynman diagrams for ${\mathrm {W}} {\mathrm {Z}}{jj}$ production in the SM and BSM. EW-induced WZ production includes quartic interactions (a) of the vector bosons. This is distinguishable from QCD-induced production (b) through kinematic variables. New physics in the EW sector modifying the quartic coupling can be parameterized in terms of dimension-eight effective field theory operators (c). Specific models modifying this interaction include those predicting charged Higgs bosons (d). |

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Figure 1-c:
Representative Feynman diagrams for ${\mathrm {W}} {\mathrm {Z}}{jj}$ production in the SM and BSM. EW-induced WZ production includes quartic interactions (a) of the vector bosons. This is distinguishable from QCD-induced production (b) through kinematic variables. New physics in the EW sector modifying the quartic coupling can be parameterized in terms of dimension-eight effective field theory operators (c). Specific models modifying this interaction include those predicting charged Higgs bosons (d). |

png pdf |
Figure 1-d:
Representative Feynman diagrams for ${\mathrm {W}} {\mathrm {Z}}{jj}$ production in the SM and BSM. EW-induced WZ production includes quartic interactions (a) of the vector bosons. This is distinguishable from QCD-induced production (b) through kinematic variables. New physics in the EW sector modifying the quartic coupling can be parameterized in terms of dimension-eight effective field theory operators (c). Specific models modifying this interaction include those predicting charged Higgs bosons (d). |

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Figure 2:
The distributions of the dijet mass (left) and pseudorapidity separation between the two leading jets (right) for events satisfying the EW signal selection. The last bin also contains all events with dijet mass greater than 2500 GeV (left) and pseudorapidty separation greater than 7.5 (right). The solid symbols represent the number of events measured in data with statistical uncertainties, the stacked filled histograms show different background contributions. The dashed line represents the EW WZ contribution stacked on top of the backgrounds. The bottom panel shows the ratio of the number of events measured in data to the total number of expected events, the shaded band at 1 represents the size of the statistical uncertainties on the predicted yields. Normalizations are shown as values used for the input to the fit. |

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Figure 2-a:
The distributions of the dijet mass (left) and pseudorapidity separation between the two leading jets (right) for events satisfying the EW signal selection. The last bin also contains all events with dijet mass greater than 2500 GeV (left) and pseudorapidty separation greater than 7.5 (right). The solid symbols represent the number of events measured in data with statistical uncertainties, the stacked filled histograms show different background contributions. The dashed line represents the EW WZ contribution stacked on top of the backgrounds. The bottom panel shows the ratio of the number of events measured in data to the total number of expected events, the shaded band at 1 represents the size of the statistical uncertainties on the predicted yields. Normalizations are shown as values used for the input to the fit. |

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Figure 2-b:
The distributions of the dijet mass (left) and pseudorapidity separation between the two leading jets (right) for events satisfying the EW signal selection. The last bin also contains all events with dijet mass greater than 2500 GeV (left) and pseudorapidty separation greater than 7.5 (right). The solid symbols represent the number of events measured in data with statistical uncertainties, the stacked filled histograms show different background contributions. The dashed line represents the EW WZ contribution stacked on top of the backgrounds. The bottom panel shows the ratio of the number of events measured in data to the total number of expected events, the shaded band at 1 represents the size of the statistical uncertainties on the predicted yields. Normalizations are shown as values used for the input to the fit. |

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Figure 3:
The 2D distribution of dijet mass and pseudorapidity separation, used for the EW signal significance extraction. The x-axis shows the dijet mass distribution in the indicated bins, split into three bins of ${{\Delta \eta _{jj}}}$: ${{\Delta \eta _{jj}}} \in$ [2.5, 4], [4, 5], $\ge 5$. The solid symbols represent the number of events measured in data with statistical uncertainties, the stacked filled histograms show different background contributions. The dashed line represents the EW WZ contribution stacked on top of the backgrounds. The bottom panel shows the ratio of the number of events measured in data to the total number of expected events, the shaded band at 1 represents the size of the systematic uncertainties on the predicted yields. Normalizations are shown as the best fit values. |

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Figure 4:
Transverse mass of the WZ system for events satisfying the EW signal selection, used to place constraints on anomalous coupling parameters. The EW WZ contribution, which is treated as background in the fit, is shown as a filled histogram. The dashed lines show predictions for several aQGC parameters values, which include the EW WZ process. Normalizations are shown as the best fit values from the background-only fit. The last bin also contains all events with transverse mass greater than 2000 GeV. Other details as in the caption of Fig. xxxxx. |

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Figure 5:
Two-dimensional observed 95% CL limits (solid contour) and expected 68 and 95% CL limits (dashed contour) on the selected aQGC parameters. The values of couplings outside of contours are excluded at the corresponding confidence level. |

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Figure 5-a:
Two-dimensional observed 95% CL limits (solid contour) and expected 68 and 95% CL limits (dashed contour) on the selected aQGC parameters. The values of couplings outside of contours are excluded at the corresponding confidence level. |

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Figure 5-b:
Two-dimensional observed 95% CL limits (solid contour) and expected 68 and 95% CL limits (dashed contour) on the selected aQGC parameters. The values of couplings outside of contours are excluded at the corresponding confidence level. |

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Figure 6:
Expected (dashed lines) and observed (solid lines) exclusion limits at 95% confidence level for the model independent $\sigma (H^+) \times BR(H^+\rightarrow {\mathrm {W}} {\mathrm {Z}})$ as a function of $m(H^\pm)$ (top) and on the vacuum expectation value ratio in the Georgi-Machacek model (bottom). |

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Figure 6-a:
Expected (dashed lines) and observed (solid lines) exclusion limits at 95% confidence level for the model independent $\sigma (H^+) \times BR(H^+\rightarrow {\mathrm {W}} {\mathrm {Z}})$ as a function of $m(H^\pm)$ (top) and on the vacuum expectation value ratio in the Georgi-Machacek model (bottom). |

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Figure 6-b:
Expected (dashed lines) and observed (solid lines) exclusion limits at 95% confidence level for the model independent $\sigma (H^+) \times BR(H^+\rightarrow {\mathrm {W}} {\mathrm {Z}})$ as a function of $m(H^\pm)$ (top) and on the vacuum expectation value ratio in the Georgi-Machacek model (bottom). |

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Figure 7:
The distribution of the dijet invariant mass of the two leading jets for events in the EW signal selection control region. The last bin also contains all events with dijet mass greater than 1900 GeV. The solid symbols represent the number of events measured in data with statistical uncertainties, the stacked filled histograms show different background contributions. The dashed purple line represents the EW WZ contribution stacked on top of the backgrounds. An alternative prediction for the QCD WZ contribution, using MadGraph5\_aMC@NLO with FxFx merging as described in the text of SMP-18-001, is shown in dashed blue. This distribution can be compared to the nominal prediction in filled purple. The bottom panel shows the ratio of the number of events measured in data to the total number of expected events for the nominal QCD WZ prediction. The shaded band at 1 represents the size of the statistical uncertainties on the predicted yields. The hatched blue band shows the sum of Monte Carlo predictions and statistical uncertainties with the alternative QCD WZ prediction divided by the sum of Monte Carlo predictions with the nominal QCD WZ prediction. Normalizations are shown as values used for the input to the fit. |

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Figure 8:
The distribution of the pseudorapidity separation between the two leading jets for events in the EW signal selection control region. The last bin also contains all events with pseduorapidty separation greater than 5. The solid symbols represent the number of events measured in data with statistical uncertainties, the stacked filled histograms show different background contributions. The dashed purple line represents the EW WZ contribution stacked on top of the backgrounds. An alternative prediction for the QCD WZ contribution, using MadGraph5\_aMC@NLO with FxFx merging as described in the text of SMP-18-001, is shown in dashed blue. This distribution can be compared to the nominal prediction in filled purple. The bottom panel shows the ratio of the number of events measured in data to the total number of expected events for the nominal QCD WZ prediction. The shaded band at 1 represents the size of the statistical uncertainties on the predicted yields. The hatched blue band shows the sum of Monte Carlo predictions and statistical uncertainties with the alternative QCD WZ prediction divided by the sum of Monte Carlo predictions with the nominal QCD WZ prediction. Normalizations are shown as values used for the input to the fit. |

Tables | |

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Table 1:
Summary of event selections and fiducial region definitions for the analysis. The selections in the first two columns are applied to data and reconstructed simulated events. The electroweak selection is used for all measurements except for charged Higgs boson studies, which use the selection indicated in the second column. The fiducial selections shown in the last two columns are applied to showered and hadronized events using the R0.8IVET framework [49] or to Born level leptons and partons. The ${\mathrm {W}} {\mathrm {Z}}{jj}$ cross section is reported in the fiducial regions indicated. |

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Table 2:
The major uncertainty contributions in the fiducial ${\mathrm {W}} {\mathrm {Z}}{jj}$ cross section measurement and in the measured EW WZ significance. The impact of each systematic uncertainty on the ${\mathrm {W}} {\mathrm {Z}}{jj}$ cross section measurement is obtained by freezing the set of nuisance parameters to their best fit values and comparing the total uncertainty on the signal strength to the result from the nominal fit. The effect on the EW WZ significance, shown in the last column, is defined as the relative increase in the observed significance when removing the nuisance term from the fit. |

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Table 3:
Postfit event yields after the signal extraction fit to events satisfying the EW signal selection. |

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Table 4:
Observed and expected 95% CL limits for one coupling parameter while all other parameters are set to zero. |

Summary |

A measurement of the production of a W and Z boson in association with two jets was presented, using events where both bosons decay leptonically. Results are based on data corresponding to an integrated luminosity of 35.9 fb$^{-1}$ recorded in proton-proton collisions at $\sqrt{s} = $ 13 TeV with the CMS detector at the CERN LHC in 2016. The cross section in the fiducial region with enhanced contributions from electroweak WZ production is $\sigma^{\mathrm{fid}}_{\mathrm{WZjj}} = $ 2.91$^{+0.67}_{-0.60}$ fb, consistent with the SM prediction. The dijet transverse mass and dijet rapidity separation are used to extract the significance of EW WZ production, which is found to be 1.9 standard deviations with 2.7 expected from the SM prediction. These are the first results for electroweak WZ production at 13 TeV. Constraints are placed on anomalous quartic gauge couplings in terms of dimension-eight effective field theory operators, and upper limits are given on the production cross section times branching fraction of charged Higgs bosons. Results are competitive or more stringent than previous constraints from the WZ channel or from other final states. |

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Compact Muon Solenoid LHC, CERN |