CMS-PAS-SMP-20-001

CMS-PAS-SMP-20-001
Evidence for vector boson scattering in events with four leptons and two jets in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
May 2020

Abstract: A search for the electroweak production of two Z bosons in association with two jets and constraints on anomalous quartic gauge couplings are presented. The analysis is based on a data sample of proton-proton collisions at $\sqrt{s}=$ 13 TeV collected with the CMS detector in 2016, 2017 and 2018, and corresponding to an integrated luminosity of 137.1 fb$^{-1}$. The search is performed in the fully leptonic final state $\mathrm{ZZ}\to\ell\ell\ell'\ell'$, where $\ell,\ell' = $ e, $\mu$. The electroweak production of two jets in association with two Z bosons is measured with an observed (expected) significance of 4.0 (3.5) standard deviations. Fiducial cross sections for the electroweak production are measured and the fiducial cross section is $\sigma_{\mathrm{EW}}(\mathrm{pp}\rightarrow ZZ\mathrm{jj}\rightarrow\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. Limits on anomalous quartic gauge couplings are derived in terms of the effective field theory operators T0-2 and T8-9.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 812 (2020) 135992. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Distribution of $m_{\textrm {jj}}$ (left) and $\|\Delta \eta _{\textrm {jj}}\|$ (right) for events satisfying the ${\mathrm{Z} \mathrm{Z} \mathrm {jj}}$ inclusive selection. Points represent the data, filled histograms the expected signal and background contributions (stacked). The unfilled magenta histograms represent the EW contribution (not stacked), scaled by a factor of 30. The bottom panels show the ratio of the number of events in the data to the total number of expected background events.
png pdf	Figure 1-a: Distribution of $m_{\textrm {jj}}$ (left) and $\|\Delta \eta _{\textrm {jj}}\|$ (right) for events satisfying the ${\mathrm{Z} \mathrm{Z} \mathrm {jj}}$ inclusive selection. Points represent the data, filled histograms the expected signal and background contributions (stacked). The unfilled magenta histograms represent the EW contribution (not stacked), scaled by a factor of 30. The bottom panels show the ratio of the number of events in the data to the total number of expected background events.
png pdf	Figure 1-b: Distribution of $m_{\textrm {jj}}$ (left) and $\|\Delta \eta _{\textrm {jj}}\|$ (right) for events satisfying the ${\mathrm{Z} \mathrm{Z} \mathrm {jj}}$ inclusive selection. Points represent the data, filled histograms the expected signal and background contributions (stacked). The unfilled magenta histograms represent the EW contribution (not stacked), scaled by a factor of 30. The bottom panels show the ratio of the number of events in the data to the total number of expected background events.
png pdf	Figure 2: Distributions of the matrix-element discriminant with fit normalizations for events satisfying the ${\mathrm{Z} \mathrm{Z} \mathrm {jj}}$ 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 bottom 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 on the bottom panel.
png pdf	Figure 3: Distributions of the ZZ invariant mass with fit normalizations for $f_{{\rm T}9}/\Lambda ^4$ and for events satisfying the ${\mathrm{Z} \mathrm{Z} \mathrm {jj}}$ inclusive selection. Points represent the data, filled histograms the fitted signal and background contributions, the gray band the uncertainties derived from the fit covariance matrix. The expected distribution for an example value of $f_{{\rm T}9}/\Lambda ^4 = $ 2 TeV$ ^{-4}$ is also shown. The last bin includes all contributions with $m_{\mathrm{Z} \mathrm{Z}} > $ 1200 GeV.

Tables
png pdf	Table 1: Signal and background yields for the ${\mathrm{Z} \mathrm{Z} \mathrm {jj}}$ 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: SM cross sections in the fiducial regions and the fitted value of the measured cross sections.
png pdf	Table 4: Expected and observed lower and upper 95% CL limits 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.

Limits on anomalous quartic gauge couplings are set at the 95% confidence level in terms of effective field theory operators, with units in TeV$^{-4}$: $-0.24 < f_{T_{0}}/\Lambda^4 < 0.22$
$-0.31 < f_{T_{1}}/\Lambda^4 < 0.31$
$-0.63 < f_{T_{2}}/\Lambda^4 < 0.59$
$-0.43 < f_{T_{8}}/\Lambda^4 < 0.43$
$-0.92 < f_{T_{9}}/\Lambda^4 < 0.92$

These are the most stringent limits to date on the neutral current operators T8 and T9.

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