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CMS-SMP-19-008 ; CERN-EP-2020-143
Observation of electroweak production of W$\gamma$ with two jets in proton-proton collisions at $\sqrt{s} = $ 13 TeV
Phys. Lett. B 811 (2020) 135988
Abstract: A first observation is presented for the electroweak production of a W boson, a photon, and two jets in proton-proton collisions. The W boson decays are selected by requiring one identified electron or muon and an imbalance in transverse momentum. The two jets are required to have a high dijet mass and a large separation in pseudorapidity. The measurement is based on data collected with the CMS detector at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The observed (expected) significance for this process is 4.9 (4.6) standard deviations. After combining with previously reported CMS results at 8 TeV, the observed (expected) significance is 5.3 (4.8) standard deviations. The cross section for the electroweak W$\gamma$jj production in a restricted fiducial region is measured as 20.4 $\pm$ 4.5 fb and the total cross section for W$\gamma$ production in association with 2 jets in the same fiducial region is 108 $\pm$ 16 fb. All results are in good agreement with recent theoretical predictions. Constraints are placed on anomalous quartic gauge couplings in terms of dimension-8 effective field theory operators.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Representative diagrams for $\ell\nu\gamma$jj production at the LHC for EW production (left), EW production through triple (middle left) and quartic (middle right) gauge boson couplings, and QCD-induced processes (right).

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Figure 1-a:
Representative diagram for $\ell\nu\gamma$jj EW production at the LHC.

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Figure 1-b:
Representative diagram for $\ell\nu\gamma$jj EW production through triple gauge boson couplings at the LHC.

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Figure 1-c:
Representative diagram for $\ell\nu\gamma$jj EW production through quartic gauge boson couplings at the LHC.

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Figure 1-d:
Representative diagram for $\ell\nu\gamma$jj QCD-induced processes at the LHC.

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Figure 2:
The photon ${p_{\mathrm {T}}}$ distribution in the muon barrel control region for data and background estimations. The misID backgrounds are derived from data, whereas the remaining backgrounds are estimated from simulation. All events with photon $ {p_{\mathrm {T}}} > $ 195 GeV are included in the last bin. The hatched bands represent the total and relative statistical uncertainties on the predicted yields. The bottom graph shows the data divided by the prediction.

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Figure 3:
The 2D distributions used in the fit for the signal strength of EW W$\gamma$+2 jets for events in the electron barrel (upper left), electron endcap (upper right), muon barrel (lower left), and muon endcap (lower right). The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 3-a:
The 2D distributions used in the fit for the signal strength of EW W$\gamma$+2 jets for events in the electron barrel. The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 3-b:
The 2D distributions used in the fit for the signal strength of EW W$\gamma$+2 jets for events in the electron endcap. The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 3-c:
The 2D distributions used in the fit for the signal strength of EW W$\gamma$+2 jets for events in the muon barrel. The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 3-d:
The 2D distributions used in the fit for the signal strength of EW W$\gamma$+2 jets for events in the muon endcap. The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 4:
The 2D distributions used in the fit for the signal strength of EW+QCD W$\gamma$+2 jets in the electron barrel (upper left), electron endcap (upper right), muon barrel (lower left) and muon endcap (lower right). The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 4-a:
The 2D distributions used in the fit for the signal strength of EW+QCD W$\gamma$+2 jets in the electron barrel. The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 4-b:
The 2D distributions used in the fit for the signal strength of EW+QCD W$\gamma$+2 jets in the electron endcap. The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 4-c:
The 2D distributions used in the fit for the signal strength of EW+QCD W$\gamma$+2 jets in the muon barrel. The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 4-d:
The 2D distributions used in the fit for the signal strength of EW+QCD W$\gamma$+2 jets in the muon endcap. The hatched bands represent the total and relative systematic uncertainties on the predicted yields. The predicted yields are shown with their best-fit normalizations.

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Figure 5:
The ${m_{{\mathrm{W} {}\gamma} {}}}$ distribution of events satisfying the aQGC region selection, which is used to set constraints on the anomalous coupling parameters. The orange line represents a nonzero ${f_{\text {T,0}}/\Lambda ^{4}}$ setting. All events with $ {m_{{\mathrm{W} {}\gamma} {}}} > $ 950 GeV are included in the last bin. The hatched bands represent the total and relative statistical uncertainties in the predicted yields.

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Figure 6:
Observed 95% CL interval on the aQGC parameter ${f_{\text {T,0}}/\Lambda ^{4}}$.
Tables

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Table 1:
Relative systematic uncertainties in the estimated signal and background yields in units of percent. The ranges reflect the dependence of the specified uncertainty on ${m_{{\text {jj}}}}$ and ${m_{\ell\nu\gamma}}$.

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Table 2:
Signal, background, and data yields after the final selection. Statistical and systematic uncertainties (before the fitting) are added in quadrature.

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Table 3:
The exclusion limits at 95% CL on each aQGC coefficient, parameterized using the distribution in ${m_{{\mathrm{W} {}\gamma} {}}}$, and listed along with the unitarity bound. All coupling parameter limits are in ${{\text {TeV}}}^{-4}$, while the ${U_{\text {bound}}}$ values are in TeV.
Summary
The cross section for the electroweak production of a W boson, a photon, and two jets is measured in proton-proton collisions at a center-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 35.9 fb$^{-1}$ collected with the CMS detector. Events are selected by requiring one identified lepton (electron or muon), a moderate missing transverse momentum, one photon, and two jets with a large rapidity separation and a large dijet mass. The observed significance is 4.9 standard deviations, where a significance of 4.6 standard deviations is expected based on the standard model. After combination with previously reported CMS results based on 8 TeV data, the observed (expected) signal significance is 5.3 (4.8) standard deviations. This constitutes the first observation of electroweak W$\gamma$jj production in proton-proton collisions. The cross section for the electroweak W$\gamma$jj production in a restricted fiducial region is measured as 20.4 $\pm$ 4.5 fb and the total cross section for W$\gamma$ production in association with 2 jets in the same fiducial region is 108 $\pm$ 16 fb, consistent with standard model predictions. Constraints placed on anomalous quartic gauge couplings in terms of dimension-8 effective field theory operators are competitive with previous results. For the parameters $f_{\mathrm{M},2-5}$ and $f_{\mathrm{M},6-7}$, the constraints are the most stringent to date.
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Compact Muon Solenoid
LHC, CERN