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CMS-PAS-SMP-16-019

Measurements of differential cross sections and search for the electroweak production of two Z bosons in association with jets

CMS Collaboration

May 2017

Abstract: This analysis reports measurements of differential cross sections for the production of two Z bosons in association with jets in pp collisions and a search for the electroweak production of two Z bosons in association with two jets at $\sqrt{s}= $ 13 TeV. The analysis is based on a data sample collected with the CMS detector corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The measurements are performed in the leptonic decay modes $\mathrm{ZZ}\to\ell\ell\ell'\ell'$, where $\ell,\ell' = e, \mu$. The differential cross section as a function of the jet multiplicity, the transverse momentum and pseudorapidity of the leading and subleading jets, as well as the invariant mass of the two leading jets and their pseudorapidity separation are presented. The measured differential cross sections are compared to theoretical predictions. The electroweak production (EW) of two Z bosons in association with two jets is measured with an observed (expected) significance of 2.7 (1.6) standard deviations. A fiducial cross section for the electroweak production is measured to be $\sigma_{\textrm{EW}} = $ 0.40$^{+0.21}_{-0.16}$ (stat) $^{+0.13}_{-0.09}$ (syst) fb, in agreement with the standard model prediction. Limits on anomalous quartic gauge couplings are derived in terms of the effective field theory operators T0, T1, T2, T8 and T9.

Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures
png pdf	Figure 1: Distribution of reconstructed jet multiplicity. Points represent data, shaded histograms represent Monte Carlo predictions and background estimate while the hatched band on them represent systematic uncertainty on the prediction. Reducible background is obtained with data driven method.
png pdf	Figure 2: Differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes as a function of the multiplicity of jets with $|\eta ^{\mathrm {jet}}|< $ 4.7 (left) and $|\eta ^{\mathrm {jet}}|< $ 2.4 (right). The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description.
png pdf	Figure 2-a: Differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes as a function of the multiplicity of jets with $|\eta ^{\mathrm {jet}}|< $ 4.7. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description.
png pdf	Figure 2-b: Differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes as a function of the multiplicity of jets with $|\eta ^{\mathrm {jet}}|< $ 2.4. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description.
png pdf	Figure 3: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes as a function of the multiplicity of jets with $|\eta ^{\mathrm {jet}}|< $ 4.7 (left) and $|\eta ^{\mathrm {jet}}|< $ 2.4 (right). The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d \mathrm {N_{jets}}}$ distribution.
png pdf	Figure 3-a: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes as a function of the multiplicity of jets with $|\eta ^{\mathrm {jet}}|< $ 4.7. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d \mathrm {N_{jets}}}$ distribution.
png pdf	Figure 3-b: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes as a function of the multiplicity of jets with $|\eta ^{\mathrm {jet}}|< $ 2.4. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d \mathrm {N_{jets}}}$ distribution.
png pdf	Figure 4: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $N_{\mathrm {jets}}\ge $ 1 as a function of the ${p_{\mathrm {T}}} $-leading jet transverse momentum (left) and absolute value of pseudorapidity (right). Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{ d p_T^{\mathrm {jet1}}}$ and $\frac {d \sigma }{d |\eta ^{\mathrm {jet1}}|}$ distributions, for the ${p_{\mathrm {T}}} $-leading jet transverse momentum and pseudorapidity respectively.
png pdf	Figure 4-a: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $N_{\mathrm {jets}}\ge $ 1 as a function of the ${p_{\mathrm {T}}} $-leading jet transverse momentum. Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{ d p_T^{\mathrm {jet1}}}$ and $\frac {d \sigma }{d |\eta ^{\mathrm {jet1}}|}$ distributions, for the ${p_{\mathrm {T}}} $-leading jet transverse momentum and pseudorapidity respectively.
png pdf	Figure 4-b: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $N_{\mathrm {jets}}\ge $ 1 as a function of the absolute value of pseudorapidity. Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{ d p_T^{\mathrm {jet1}}}$ and $\frac {d \sigma }{d |\eta ^{\mathrm {jet1}}|}$ distributions, for the ${p_{\mathrm {T}}} $-leading jet transverse momentum and pseudorapidity respectively.
png pdf	Figure 5: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $N_{\mathrm {jets}}\ge $ 2 as a function of the ${p_{\mathrm {T}}} $-subleading jet transverse momentum (left) and absolute value of pseudorapidity (right). Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d p_T^{\mathrm {jet2}}}$ and $\frac {d \sigma }{d |\eta ^{\mathrm {jet2}}|}$ distributions, for the ${p_{\mathrm {T}}} $-subleading jet transverse momentum and pseudorapidity respectively.
png pdf	Figure 5-a: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $N_{\mathrm {jets}}\ge $ 2 as a function of the ${p_{\mathrm {T}}} $-subleading jet transverse momentum. Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d p_T^{\mathrm {jet2}}}$ and $\frac {d \sigma }{d |\eta ^{\mathrm {jet2}}|}$ distributions, for the ${p_{\mathrm {T}}} $-subleading jet transverse momentum and pseudorapidity respectively.
png pdf	Figure 5-b: Normalized differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $N_{\mathrm {jets}}\ge $ 2 as a function of the absolute value of pseudorapidity. Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d p_T^{\mathrm {jet2}}}$ and $\frac {d \sigma }{d |\eta ^{\mathrm {jet2}}|}$ distributions, for the ${p_{\mathrm {T}}} $-subleading jet transverse momentum and pseudorapidity respectively.
png pdf	Figure 6: Differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $\mathrm {N_{jets}}\ge $ 2 as a function of the invariant mass of the two $ {p_{\mathrm {T}}} $-leading jets (left) and their pseudorapidity separation (right). Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d m_{jj}}$ and $\frac {d \sigma }{d \Delta \eta _{jj}}$ distributions, for the invariant mass of the two $ {p_{\mathrm {T}}} $-leading jets and their pseudorapidity separation respectively.
png pdf	Figure 6-a: Differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $\mathrm {N_{jets}}\ge $ 2 as a function of the invariant mass of the two $ {p_{\mathrm {T}}} $-leading jets. Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d m_{jj}}$ and $\frac {d \sigma }{d \Delta \eta _{jj}}$ distributions, for the invariant mass of the two $ {p_{\mathrm {T}}} $-leading jets and their pseudorapidity separation respectively.
png pdf	Figure 6-b: Differential cross sections of $ \mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to 4\ell $ processes with $\mathrm {N_{jets}}\ge $ 2 as a function of the pseudorapidity separation of the two $ {p_{\mathrm {T}}} $-leading jets. Each bin of the distribution is divided by its width. The cross sections are compared to the predictions from the MadGraph5_aMC@NLO and powheg sets of samples. All generators are interfaced to PYTHIA 8. The total experimental uncertainties are shown with hatched regions, while colored bands display the effect of varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description. The $\sigma _{\mathrm {fid}}$ on $y$ axis refers to the integral of the $\frac {d \sigma }{d m_{jj}}$ and $\frac {d \sigma }{d \Delta \eta _{jj}}$ distributions, for the invariant mass of the two $ {p_{\mathrm {T}}} $-leading jets and their pseudorapidity separation respectively.
png pdf	Figure 7: Distribution of the dijet pseudorapidity separation (left) and dijet invariant mass (right) for events passing the ZZjj selection, which requires $m_{jj} > $ 100 GeV. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Figure 7-a: Distribution of the dijet pseudorapidity separation for events passing the ZZjj selection, which requires $m_{jj} > $ 100 GeV. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Figure 7-b: Distribution of the dijet invariant mass for events passing the ZZjj selection, which requires $m_{jj} > $ 100 GeV. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Figure 8: Distribution of the BDT score in the control region obtained by selecting ZZjj events with $m_{jj}< $ 400 GeV or $|\Delta \eta _{jj}|< $ 2.4 (left) and for the full ZZjj selection (right). Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Figure 8-a: Distribution of the BDT score in the control region obtained by selecting ZZjj events with $m_{jj}< $ 400 GeV or $|\Delta \eta _{jj}|< $ 2.4. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Figure 8-b: Distribution of the BDT score for the full ZZjj selection. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Figure 9: The $m_{ZZ}$ distribution in the ZZjj selection together with the SM prediction and two hypotheses for the aQGC coupling strengths. Points represent the data, filled histograms the expected signal and background contributions.

Tables
png pdf	Table 1: The contributions of each source of signal source of systematic uncertainty in the differential cross section measurements and in the normalized cross section measurement. Uncertainties that depend on jet multiplicity are listed as a range.
png pdf	Table 2: Fiducial phase space definition.
png pdf	Table 3: The $\mathrm{pp} \to {\mathrm{ Z } } {\mathrm{ Z } } \to \ell \ell \ell '\ell '$ production cross section as a function of the jet multiplicity. Luminosity uncertainty for number of jets $\ge $ 3 is smaller than 0.1 pb and is not quoted. The cross sections are compared to the theoretical predictions (last column) from the MadGraph5_aMC@NLO sets of samples. Theoretical uncertainty are obtained varying the renormalization and factorization scales, PDFs and $\alpha _s$ on Monte Carlo (matrix element only) event description.
png pdf	Table 4: Signal and background yields for the ZZjj selection and for a VBS signal-enriched selection that requires $m_{jj}> $ 400 GeV and $|\Delta \eta _{jj}|> $ 2.4.
png pdf	Table 5: Observed and expected 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$^{-4}$, the unitarity bounds in TeV.

Summary

We presented results for a study of ZZ production in the four-lepton final state in association with jets in proton-proton collisions at 13 TeV. The data correspond to an integrated luminosity of 35.9 fb$^{-1}$ collected with the CMS detector at the LHC. Cross sections for the production of a pair of Z bosons in association with jets as a function of the number of jets, the transverse momentum and pseudorapidity of the leading and subleading jets, as well distributions of the invariant mass of the two leading jets and of the pseudorapidity separation between the two leading jets are reported. The results are corrected for detector effects by means of an iterative unfolding technique, and compared with particle-level predictions. A good agreement is observed overall with theoretical predictions. The more recent Monte Carlo ME calculations and parton-shower models predictions adopted in this analysis show a better agreement up to higher jet multiplicities with respect to the results at 8 TeV. The electroweak production of a pair of Z bosons in association with two jets is measured with an observed (expected) significance of 2.7 (1.6) standard deviations. The fiducial cross section is measured to be $\sigma_{\textrm{fid}} = $ 0.40$^{+0.21}_{-0.16}$ (stat) $^{+0.13}_{-0.09}$ (syst) fb, which is consistent with the SM prediction. Limits on anomalous quartic gauge couplings are set at 95% confidence level in terms of effective field theory operators, with units in TeV$^{-4}$: $-0.46 < f_{T_{0}}/\Lambda^4 <0.44$, $-0.61 < f_{T_{1}}/\Lambda^4 <0.61$, $-1.2 < f_{T_{2}}/\Lambda^4 < 1.2$, $-0.84 < f_{T_{8}}/\Lambda^4 <0.84$, $-1.8 < f_{T_{9}}/\Lambda^4 <1.8$.

Additional Figures
png pdf	Additional Figure 1: Distribution of reconstructed multiplicity of jets with $|\eta ^{\mathrm {jet}}| < $ 2.4. Points represent data, shaded histograms represent Monte Carlo predictions and background estimate while the hatched band on them represent systematic uncertainty on the prediction. The reducible background is obtained with a data driven method.
png pdf	Additional Figure 2: Distribution of reconstructed $p_{\mathrm{T}}$-leading jet transverse momentum. Points represent data, shaded histograms represent Monte Carlo predictions and background estimate while the hatched band on them represent systematic uncertainty on the prediction. The reducible background is obtained with a data driven method.
png pdf	Additional Figure 3: Distribution of reconstructed $p_{\mathrm{T}}$-leading jet pseudorapidity. Points represent data, shaded histograms represent Monte Carlo predictions and background estimate while the hatched band on them represent systematic uncertainty on the prediction. The reducible background is obtained with a data driven method.
png pdf	Additional Figure 4: Distribution of reconstructed $p_{\mathrm{T}}$-subleading jet transverse momentum. Points represent data, shaded histograms represent Monte Carlo predictions and background estimate while the hatched band on them represent systematic uncertainty on the prediction. The reducible background is obtained with a data driven method.
png pdf	Additional Figure 5: Distribution of reconstructed $p_{\mathrm{T}}$-subleading jet pseudorapidity. Points represent data, shaded histograms represent Monte Carlo predictions and background estimate while the hatched band on them represent systematic uncertainty on the prediction. The reducible background is obtained with a data driven method.
png pdf	Additional Figure 6: Distribution of reconstructed invariant mass of the two $p_{\mathrm{T}}$-leading jets. Points represent data, shaded histograms represent Monte Carlo predictions and background estimate while the hatched band on them represent systematic uncertainty on the prediction. The reducible background is obtained with a data driven method.
png pdf	Additional Figure 7: Distribution of reconstructed pseudorapidity separation between the two $p_{\mathrm{T}}$-leading jets. Points represent data, shaded histograms represent Monte Carlo predictions and background estimate while the hatched band on them represent systematic uncertainty on the prediction. The reducible background is obtained with a data driven method.
png pdf	Additional Figure 8: The following figure displays a real proton-proton collision event at 13 TeV in the CMS detector in which two high-energy electrons (light blue lines), two high-energy muons (red lines), and two high-energy hadronic jets (dark green cones) are observed. The presence of two opposite-sign same-flavour lepton pairs with mass close to the Z mass, of two hadronic jets in opposite hemispheres of the detector with a large pseudorapidity separation, as well as the absence of hadronic activity in the central region of the detector, are indicative of the electroweak production of two Z bosons and two jets.
png pdf	Additional Figure 9: Distribution of the Zeppenfeld variable of the leading Z boson, $\eta ^*_{Z_{1}}=\eta _{Z_{1}} - (\eta _{jet 1} + \eta _{jet 2})/2$, for events passing the ZZjj selection, which requires $m_{jj} > $ 100 GeV. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Additional Figure 10: Distribution of the Zeppenfeld variable of the subleading Z boson, $\eta ^*_{Z_{2}}=\eta _{Z_{2}} - (\eta _{jet 1} + \eta _{jet 2})/2$, for events passing the ZZjj selection, which requires $m_{jj} > $ 100 GeV. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Additional Figure 11: Distribution of the event balance observable for events passing the ZZjj selection, which requires $m_{jj}> 100 GeV $. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Additional Figure 12: Distribution of the ratio between the $ {p_{\mathrm {T}}} $ of the dijet system and the scalar sum of the tagging jets' $ {p_{\mathrm {T}}} $ for events passing the ZZjj selection, which requires $m_{jj} > $ 100 GeV. Points represent the data, filled histograms the expected signal and background contributions.
png pdf	Additional Figure 13: Signal versus background efficiency curves of the boosted decision tree (BDT) and matrix element likelihood (MELA) classifiers for separating the electroweak from the QCD-induced production of the $\ell \ell \ell '\ell 'jj$ final state. The efficiency of a cut-based selection on the dijet mass and dijet pseudorapidity separation is also shown.

Additional Tables
png pdf	Additional Table 1: Selected kinematic properties of signal-like events with BDT score $>$ 0.9 observed in the data

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