CMS-SMP-18-007

CMS-SMP-18-007 ; CERN-EP-2020-007
Measurement of the cross section for electroweak production of a Z boson, a photon and two jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV and constraints on anomalous quartic couplings
CMS Collaboration
23 February 2020
JHEP 06 (2020) 076
Abstract: A measurement is presented of the cross section for electroweak production of a Z boson and a photon in association with two jets ( $\mathrm{Z \gamma jj}$ ) in proton-proton collisions. The Z boson candidates are selected through their decay into a pair of electrons or muons. The process of interest, electroweak $\mathrm{Z \gamma jj}$ production, is isolated by selecting events with a large dijet mass and a large pseudorapidity gap between the two jets. The measurement is based on data collected at the CMS experiment at $\sqrt{s} =$ 13 TeV, corresponding to an integrated luminosity of 35.9 fb $^{-1}$ . The observed significance of the signal is 3.9 standard deviations, where a significance of 5.2 standard deviations is expected in the standard model. These results are combined with published results by CMS at $\sqrt{s} =$ 8 TeV, which leads to observed and expected respective significances of 4.7 and 5.5 standard deviations. From the 13 TeV data, a value is obtained for the signal strength of electroweak $\mathrm{Z \gamma jj}$ production and bounds are given on quartic vector boson interactions in the framework of dimension-eight effective field theory operators.
Links: e-print arXiv:2002.09902 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagrams for $\mathrm{Z \gamma jj}$ production. The diagrams except (lower right) reflect EW origin: (upper left) bremsstrahlung, (upper center) multiperipheral, (upper right) VBF with TGCs, (lower left) VBS via W boson, (lower center) VBS, including QGCs, while (lower right) is a QCD-induced diagram.
png pdf	Figure 1-a: Representative Feynman diagram for $\mathrm{Z \gamma jj}$ production. The diagram reflects EW origin: bremsstrahlung.
png pdf	Figure 1-b: Representative Feynman diagram for $\mathrm{Z \gamma jj}$ production. The diagram reflects EW origin: multiperipheral.
png pdf	Figure 1-c: Representative Feynman diagram for $\mathrm{Z \gamma jj}$ production. The diagram reflects EW origin: VBF with TGCs.
png pdf	Figure 1-d: Representative Feynman diagram for $\mathrm{Z \gamma jj}$ production. The diagram reflects EW origin: VBS via W boson..
png pdf	Figure 1-e: Representative Feynman diagram for $\mathrm{Z \gamma jj}$ production. The diagram reflects EW origin: VBS, including QGCs.
png pdf	Figure 1-f: Representative Feynman diagram for $\mathrm{Z \gamma jj}$ production: QCD-induced diagram.
png pdf	Figure 2: The pre-fit $m_\mathrm {jj}$ distributions for the dilepton + $\gamma _{\text {barrel}}$ events are shown on the left for the dielectron and on the right for the dimuon categories. The data are compared to the sum of the signal and the background contribution. "Other'' represents the contribution from the top quark and $\mathrm{W} \gamma$ processes. The black points with error bars represent the data and their uncertainties, while the hatched bands represent the statistical uncertainty on the combined signal and background expectations. The last bin includes overflow events. The bottom plots show the ratio of the data to the expectation.
png pdf	Figure 2-a: The pre-fit $m_\mathrm {jj}$ distributions for the dilepton + $\gamma _{\text {barrel}}$ events are shown for the dielectron category. The data are compared to the sum of the signal and the background contribution. "Other'' represents the contribution from the top quark and $\mathrm{W} \gamma$ processes. The black points with error bars represent the data and their uncertainties, while the hatched bands represent the statistical uncertainty on the combined signal and background expectations. The last bin includes overflow events. The bottom plot shows the ratio of the data to the expectation.
png pdf	Figure 2-b: The pre-fit $m_\mathrm {jj}$ distributions for the dilepton + $\gamma _{\text {barrel}}$ events are shown for the dimuon category. The data are compared to the sum of the signal and the background contribution. "Other'' represents the contribution from the top quark and $\mathrm{W} \gamma$ processes. The black points with error bars represent the data and their uncertainties, while the hatched bands represent the statistical uncertainty on the combined signal and background expectations. The last bin includes overflow events. The bottom plot shows the ratio of the data to the expectation.
png pdf	Figure 3: The pre-fit $m_\mathrm {jj}$ distributions for the dilepton + $\gamma _{\text {end}}$ events are shown on the left for the dielectron and on the right for the dimuon categories. The data are compared to the sum of the signal and the background contribution. "Other'' represents the contribution from the top quark and $\mathrm{W} \gamma$ processes. The black points with error bars represent the data and their uncertainties, while the hatched bands represent the statistical uncertainty on the combined signal and background expectations. The last bin includes overflow events. The bottom plots show the ratio of the data to the expectation.
png pdf	Figure 3-a: The pre-fit $m_\mathrm {jj}$ distributions for the dilepton + $\gamma _{\text {end}}$ events are shown for the dielectron category. The data are compared to the sum of the signal and the background contribution. "Other'' represents the contribution from the top quark and $\mathrm{W} \gamma$ processes. The black points with error bars represent the data and their uncertainties, while the hatched bands represent the statistical uncertainty on the combined signal and background expectations. The last bin includes overflow events. The bottom plot shows the ratio of the data to the expectation.
png pdf	Figure 3-b: The pre-fit $m_\mathrm {jj}$ distributions for the dilepton + $\gamma _{\text {end}}$ events are shown for the dimuon category. The data are compared to the sum of the signal and the background contribution. "Other'' represents the contribution from the top quark and $\mathrm{W} \gamma$ processes. The black points with error bars represent the data and their uncertainties, while the hatched bands represent the statistical uncertainty on the combined signal and background expectations. The last bin includes overflow events. The bottom plot shows the ratio of the data to the expectation.
png pdf	Figure 4: The post-fit 2D distributions of the dielectron (left) and dimuon (right) + $\gamma _{\text {barrel}}$ categories as a function of $m_\mathrm {jj}$ in bins of $\Delta \eta _{\mathrm {jj}}$ . The horizontal axis is split into bins of $\Delta \eta _{\mathrm {jj}}$ of [2.5, 4.5], (4.5, 6.0], and $>$ 6.0. The data are compared to the signal and background predictions in the signal region. The "Other'' represents contributions from top quark and $\mathrm{W} \gamma$ events. The black points with error bars represent the data and statistical uncertainties of data, the hatched bands represent the full uncertainties of the predictions.
png pdf	Figure 4-a: The post-fit 2D distributions of the dielectron + $\gamma _{\text {barrel}}$ category as a function of $m_\mathrm {jj}$ in bins of $\Delta \eta _{\mathrm {jj}}$ . The horizontal axis is split into bins of $\Delta \eta _{\mathrm {jj}}$ of [2.5, 4.5], (4.5, 6.0], and $>$ 6.0. The data are compared to the signal and background predictions in the signal region. The "Other'' represents contributions from top quark and $\mathrm{W} \gamma$ events. The black points with error bars represent the data and statistical uncertainties of data, the hatched bands represent the full uncertainties of the predictions.
png pdf	Figure 4-b: The post-fit 2D distributions of the dimuon + $\gamma _{\text {barrel}}$ category as a function of $m_\mathrm {jj}$ in bins of $\Delta \eta _{\mathrm {jj}}$ . The horizontal axis is split into bins of $\Delta \eta _{\mathrm {jj}}$ of [2.5, 4.5], (4.5, 6.0], and $>$ 6.0. The data are compared to the signal and background predictions in the signal region. The "Other'' represents contributions from top quark and $\mathrm{W} \gamma$ events. The black points with error bars represent the data and statistical uncertainties of data, the hatched bands represent the full uncertainties of the predictions.
png pdf	Figure 5: The post-fit 2D distributions of the dielectron (left) and dimuon (right) + $\gamma _{\text {end}}$ categories as a function of $m_\mathrm {jj}$ in bins of $\Delta \eta _{\mathrm {jj}}$ . The horizontal axis is split into bins of $\Delta \eta _{\mathrm {jj}}$ of [2.5, 4.5], (4.5, 6.0], and $>$ 6.0. The data are compared to the signal and background predictions in the signal region. The "Other'' represents contributions from top quark and $\mathrm{W} \gamma$ events. The black points with error bars represent the data and statistical uncertainties of data, the hatched bands represent the full uncertainties of the predictions.
png pdf	Figure 5-a: The post-fit 2D distributions of the dielectron + $\gamma _{\text {end}}$ category as a function of $m_\mathrm {jj}$ in bins of $\Delta \eta _{\mathrm {jj}}$ . The horizontal axis is split into bins of $\Delta \eta _{\mathrm {jj}}$ of [2.5, 4.5], (4.5, 6.0], and $>$ 6.0. The data are compared to the signal and background predictions in the signal region. The "Other'' represents contributions from top quark and $\mathrm{W} \gamma$ events. The black points with error bars represent the data and statistical uncertainties of data, the hatched bands represent the full uncertainties of the predictions.
png pdf	Figure 5-b: The post-fit 2D distributions of the dimuon + $\gamma _{\text {end}}$ category as a function of $m_\mathrm {jj}$ in bins of $\Delta \eta _{\mathrm {jj}}$ . The horizontal axis is split into bins of $\Delta \eta _{\mathrm {jj}}$ of [2.5, 4.5], (4.5, 6.0], and $>$ 6.0. The data are compared to the signal and background predictions in the signal region. The "Other'' represents contributions from top quark and $\mathrm{W} \gamma$ events. The black points with error bars represent the data and statistical uncertainties of data, the hatched bands represent the full uncertainties of the predictions.
png pdf	Figure 6: The $m_{\mathrm{Z} \gamma}$ distribution of events satisfying the aQGC region selection, which is used to set constraints on the anomalous coupling parameters. The red line represents a nonzero $F_{\mathrm {T,8}}$ setting, which would significantly enhance the yields at high $m_{\mathrm{Z} \gamma}$ . The bins of $m_{\mathrm{Z} \gamma}$ are [100, 400, 600, 800, 1000, 1500] GeV, where the last bin includes overflow. The hatched bands represent the statistical uncertainties in the predictions.
png pdf	Figure 7: Observed (left) and expected (right) 95% CL intervals on the aQGC parameter $F_{\mathrm {T,8}}$ .
png pdf	Figure 7-a: Observed 95% CL intervals on the aQGC parameter $F_{\mathrm {T,8}}$ .
png pdf	Figure 7-b: Expected 95% CL intervals on the aQGC parameter $F_{\mathrm {T,8}}$ .

Tables
png pdf	Table 1: Summary of the five sets of event-selection criteria used to define events in the common selection, control region selection, EW signal extraction, the fiducial cross section, and the search for an aQGC contribution.
png pdf	Table 2: The pre-fit systematic uncertainties in the measurement of the extracted signal. They are for signal or background (bkg) if the source is specified, or for both if the source is not specified.
png pdf	Table 3: Post-fit signal and background yields and observed event counts in data after the final selection in the search for EW signal. The $\gamma _{\text {barrel}}$ and $\gamma _{\text {end}}$ represent photons in the barrel and end-detector region, respectively. "Other bkgs.'' represents the contribution of diboson, top and $\mathrm{W} \gamma$ process. The uncertainties are the quadratic sum of statistical and systematic uncertainties.
png pdf	Table 4: 95% CL exclusion limits in units of TeV $^{-4}$ ; the unitarity bounds are also listed in units of TeV.

Summary

A new measurement has been made of vector boson scattering in the

$\mathrm{Z \gamma jj}$ channel. The data, collected in proton-proton collisions at

$\sqrt{s}$ = 13 TeV in the CMS detector in 2016, correspond to an integrated luminosity of 35.9 fb

$^{-1}$ . Events were selected by requiring two identified oppositely charged electrons or muons with invariant mass consistent with a Z boson, one identified photon, and two jets that have a large separation in pseudorapidity and a large dijet mass. The observed significance for a signal in the data is 3.9 standard deviations (s.d.), where a significance of 5.2 s.d. is expected based on the standard model. When this result is combined with previous CMS measurements at 8 TeV, the observed and expected significances become respectively 4.7 and 5.5 s.d. The fiducial cross section for electroweak

$\mathrm{Z \gamma jj}$ production is 3.2

$\pm$ 1.2 fb for the data at 13 TeV, and the fiducial cross section for the sum of sources from electroweak and from quantum chromodynamics is 14.3

$\pm$ 3.0 fb. Constraints placed on anomalous quartic gauge couplings in terms of dimension-eight operators in effective field theory are either competitive with or more stringent than those previously obtained.

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