CMS-SMP-20-001

CMS-SMP-20-001 ; CERN-EP-2020-127
Evidence for electroweak production of four charged leptons and two jets in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
17 August 2020
Phys. Lett. B 812 (2020) 135992
Abstract: Evidence is presented for the electroweak (EW) production of two jets (jj) in association with two Z bosons and constraints on anomalous quartic gauge couplings are set. The analysis is based on a data sample of proton-proton collisions at $\sqrt{s} = $ 13 TeV collected with the CMS detector in 2016-2018, and corresponding to an integrated luminosity of 137 fb$^{-1}$. The search is performed in the fully leptonic final state $\mathrm{Z}\mathrm{Z}\to\ell\ell\ell'\ell'$, where $\ell,\ell' =$ e, $\mu$. The EW production of two jets in association with two Z bosons is measured with an observed (expected) significance of 4.0 (3.5) standard deviations. The cross sections for the EW production are measured in three fiducial volumes and the result is $\sigma_{\mathrm{EW}}({\mathrm{p}}{\mathrm{p}}\to \mathrm{Z}\mathrm{Z}\mathrm{jj}\to\ell\ell\ell'\ell'\mathrm{jj}) = $ 0.33 $ ^{+0.11}_{-0.10}$ (stat) $ ^{+0.04}_{-0.03}$ (syst) fb in the most inclusive volume, in agreement with the standard model prediction of 0.275 $\pm$ 0.021 fb. Limits on anomalous quartic gauge couplings are derived in terms of the effective field theory operators T0, T1, T2, T8, and T9.
Links: e-print arXiv:2008.07013 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagrams for the EW- (top row and bottom left) and QCD-induced (bottom right) production of the $ {\mathrm{Z} \mathrm{Z} \mathrm {jj}} \to \ell \ell \ell '\ell '\mathrm {jj}$ ($\ell,\ell ' = $ e, $\mu $) final state. The scattering of massive gauge bosons as depicted in the top row is unitarized by the interference with amplitudes that feature the Higgs boson (bottom left).
png pdf	Figure 2: Distribution of ${m_{\mathrm {jj}}}$ (left) and $ {\| {\Delta \eta _{\mathrm {jj}}} \|}$ (right) for events satisfying the ZZjj inclusive selection. Points represent the data, filled histograms the expected signal and background contributions (stacked). The unfilled purple histograms represent the EW contribution (not stacked), scaled by a factor of 30. The lower panels show the ratio of the number of events in the data to the total number of expected background events.
png pdf	Figure 2-a: Distribution of ${m_{\mathrm {jj}}}$ for events satisfying the ZZjj inclusive selection. Points represent the data, filled histograms the expected signal and background contributions (stacked). The unfilled purple histograms represent the EW contribution (not stacked), scaled by a factor of 30. The lower panel shows the ratio of the number of events in the data to the total number of expected background events.
png pdf	Figure 2-b: Distribution of $ {\| {\Delta \eta _{\mathrm {jj}}} \|} $ for events satisfying the ZZjj inclusive selection. Points represent the data, filled histograms the expected signal and background contributions (stacked). The unfilled purple histograms represent the EW contribution (not stacked), scaled by a factor of 30. The lower panel shows the ratio of the number of events in the data to the total number of expected background events.
png pdf	Figure 3: Distributions of the matrix element discriminant with fit normalizations for events satisfying the ZZjj inclusive selection. Points represent the data, filled histograms the fitted signal and background contributions. The gray bands represent the uncertainties obtained from the fit covariance matrix. In the lower panel, points show the ratio of the number of events in the data to the total number of background events, with the red line indicating the ratio of the fitted total distribution to its background-only component. The observed significance is indicated in the lower panel.
png pdf	Figure 4: Distributions of the four-lepton invariant mass with fit normalizations for $f_{\mathrm {T}9}/\Lambda ^4$ and for events satisfying the ZZjj inclusive selection. Points represent the data, filled histograms the fitted signal and background contributions, and the gray band the uncertainties derived from the fit covariance matrix. The expected distribution for an example value of $f_{\mathrm {T}9}/\Lambda ^4 =$ 2 TeV$ ^{-4}$ is also shown. The last bin includes all contributions with ${m_{4\ell}} > $ 1200 GeV.

Tables
png pdf	Table 1: Predicted signal and background yields with total uncertainties, and observed number of events for the ZZjj inclusive selection and for the VBS loose and tight signal-enriched selections.
png pdf	Table 2: Particle-level selections used to define the fiducial regions for EW and EW+QCD cross sections.
png pdf	Table 3: Measured cross sections and corresponding SM predictions in the three fiducial regions. The reported SM predictions include those extracted from generated events in MC samples adopted for the analysis (LO), as well as higher-order calculations at NLO in QCD [55] (NLO QCD).
png pdf	Table 4: Expected and observed limits of the 2$\sigma $ CL intervals on the couplings of the quartic operators T0, T1, and T2, as well as the neutral current operators T8 and T9. The unitarity bounds are also listed. All coupling parameter limits are in TeV$^{-4}$, while the unitarity bounds are in TeV.

Summary

A search was performed for the electroweak production of two jets in association with two Z bosons in the four-lepton final state in proton-proton collisions at 13 TeV. The data correspond to an integrated luminosity of 137 fb$^{-1}$ collected with the CMS detector at the LHC.

The electroweak production of two jets in association with a pair of Z bosons is measured with an observed (expected) significance of 4.0 (3.5) standard deviations. The measured fiducial cross section is $\sigma_{\mathrm{fid}} = $ 0.33 $^{+0.11}_{-0.10}$ (stat) $^{+0.04}_{-0.03}$ (syst) fb, which is consistent with the standard model prediction of 0.275 $\pm$ 0.021 fb.

Limits on anomalous quartic gauge couplings are set at 95% confidence level in terms of effective field theory operators, with units in TeV$^{-4}$:

$-0.24 < f_{\mathrm{T}0}/\Lambda^{4} < 0.22$
$-0.31 < f_{\mathrm{T}1}/\Lambda^{4} < 0.31$
$-0.63 < f_{\mathrm{T}2}/\Lambda^{4} < 0.59$
$-0.43 < f_{\mathrm{T}8}/\Lambda^{4} < 0.43$
$-0.92 < f_{\mathrm{T}9}/\Lambda^{4} < 0.92$
These are the most stringent limits to date on the neutral current operators T8 and T9.

Additional Figures
png pdf	Additional Figure 1: Distribution of $K_D$ for events in the ZZjj control region. Points represent the data, filled histograms the expected signal and background contributions (stacked). The bottom panel shows the ratio of the number of events in the data to the total number of expected background events.
png pdf	Additional Figure 2: Distribution of $\|\Delta\eta_\textrm{jj}\|$ for events in the ZZjj control region. Points represent the data, filled histograms the expected signal and background contributions (stacked). The bottom panel shows the ratio of the number of events in the data to the total number of expected background events.
png pdf	Additional Figure 3: Distribution of $m_\textrm{jj}$ for events in the ZZjj control region. Points represent the data, filled histograms the expected signal and background contributions (stacked). The bottom panel shows the ratio of the number of events in the data to the total number of expected background events.

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