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| CMS-PAS-FTR-18-014 | ||
| Vector Boson Scattering prospective studies in the ZZ fully leptonic decay channel for the High-Luminosity and High-Energy LHC upgrades | ||
| CMS Collaboration | ||
| December 2018 | ||
| Abstract: Prospective studies for the vector boson scattering (VBS) in the ZZ channel at the HL-LHC are presented, where the Z bosons are identified and measured through their leptonic decays, $\ell =$ e, $\mu$. The results obtained from the 2016 analysis with an integrated luminosity of 36 fb$^{-1}$ are projected to the HL-LHC luminosity of 3000 fb$^{-1}$ and center-of-mass energy of 14 TeV, taking into account the increased acceptance of the CMS detector. The projected uncertainty in the VBS ZZ cross section is 8.5-10.3% depending on the lepton $\eta$ coverage and assumptions made for the systematic uncertainties. A study is performed to separate the longitudinal polarization ($\mathrm{Z_L}$) from the dominant transverse polarizations. The expected sensitivity for the VBS $\mathrm{Z_LZ_L}$ fraction is 1.4 standard deviations. The foreseen upgrade coverage of up to $\vert \eta \vert = $ 3(2.8) for electrons (muons) leads to a 13% improvement in sensitivity compared to the Run 2 acceptance. Extending the coverage for electrons up to $\vert \eta \vert = $ 4 would result in a modest increase in the sensitivity. Finally, the HE-LHC option would allow to bring the sensitivity at the 5$\sigma$ level for this process. | ||
| Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Representative Feynman diagrams for the EW- (top row and bottom left) and QCD-induced production (bottom right) of the $ {\mathrm {Z}} {\mathrm {Z}} \textrm {jj}\rightarrow \ell \ell \ell ^{\prime}\,\ell ^{\prime} \textrm {jj}$ ($\ell,\, \ell ^{\prime}\,= {\mathrm {e}}$ or $\mu $) final state. The scattering of massive gauge bosons as depicted in the top row is unitarized by the interference with amplitudes that feature the Higgs boson (bottom left). |
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Figure 2:
Expected distribution of the BDT output for 3000 fb$^{-1}$. The points represent pseudo data generated from the sum of the expected contributions for each process. The purple filled histogram represents the EW signal, the dark blue the QCD ggZZ background, the light blue the QCD qqZZ background, the yellow the ttZ plus WWZ backgrounds and the green the reducible background. |
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Figure 3:
Projected significance for a 10% uncertainty in the QCD ggZZ background yield as a function of the integrated luminosity and for all other systematic uncertainties according to the Run 2 scenario (blue line and circles), and according to YR18 scenario (red line and triangles). The magenta line and squares show the results with only the statistical uncertainties included. The dashed line shows the projected significance as obtained scaling the 2016 result with statistical uncertainty only by the luminosity ratio. |
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Figure 4:
Projected relative uncertainty in the cross section for 3000 fb$^{-1}$ as a function of the uncertainty in the QCD ggZZ background yield (right). The YR18 scenario is used for the other systematic uncertainties. |
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Figure 5:
Projected relative uncertainty in the cross section as a function of the integrated luminosity and for all other systematic uncertainties according to the Run 2 scenario (blue line and circles), and according to the YR18 scenario (red line and triangles). Results are shown for 10% uncertainty uncertainty in the QCD ggZZ background yield. The magenta line and filled squares show the results with only the statistical uncertainties included. |
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Figure 6:
Distributions of some the discriminant variables for the $\mathrm {VBS Z_LZ_L}$ signal, the $\mathrm {VBS Z_LZ_T}$ and $\mathrm {Z_TZ_T}$ background and the QCD backgrounds from Delphes simulation and for the ZZjj inclusive selection that requires $m_{jj} > $ 100 GeV. The distributions are normalized to unity for shape comparison. |
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Figure 6-a:
Distribution of $m_{\text{jj}}$ for the $\mathrm {VBS Z_LZ_L}$ signal, the $\mathrm {VBS Z_LZ_T}$ and $\mathrm {Z_TZ_T}$ background and the QCD backgrounds from Delphes simulation and for the ZZjj inclusive selection that requires $m_{jj} > $ 100 GeV. The distribution is normalized to unity for shape comparison. |
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Figure 6-b:
Distribution of $\Delta \eta _{\text{jj}}$ for the $\mathrm {VBS Z_LZ_L}$ signal, the $\mathrm {VBS Z_LZ_T}$ and $\mathrm {Z_TZ_T}$ background and the QCD backgrounds from Delphes simulation and for the ZZjj inclusive selection that requires $m_{jj} > $ 100 GeV. The distribution is normalized to unity for shape comparison. |
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Figure 6-c:
Distribution of $\cos{\theta^{*}}(Z_1)$ for the $\mathrm {VBS Z_LZ_L}$ signal, the $\mathrm {VBS Z_LZ_T}$ and $\mathrm {Z_TZ_T}$ background and the QCD backgrounds from Delphes simulation and for the ZZjj inclusive selection that requires $m_{jj} > $ 100 GeV. The distribution is normalized to unity for shape comparison. |
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Figure 6-d:
Distribution of $\eta( Z_2)$ for the $\mathrm {VBS Z_LZ_L}$ signal, the $\mathrm {VBS Z_LZ_T}$ and $\mathrm {Z_TZ_T}$ background and the QCD backgrounds from Delphes simulation and for the ZZjj inclusive selection that requires $m_{jj} > $ 100 GeV. The distribution is normalized to unity for shape comparison. |
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Figure 7:
Expected significance for the VBS $\mathrm {Z_LZ_L}$ fraction as a function of the integrated luminosity and for systematic uncertainties according to the Run 2 scenario (blue line and circles), and according to the YR18 scenario (red line and triangles). Results are shown for 10% uncertainty in the QCD ggZZ background yield. The magenta line and squares show the results with only the statistical uncertainties included. |
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Figure 8:
Expected relative uncertainty in the VBS $\mathrm {Z_LZ_L}$ fraction as a function of the integrated luminosity and for systematic uncertainties according to the YR18 scenario. Results are shown for 10% uncertainty in the QCD ggZZ background yield. |
| Tables | |
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Table 1:
Cross section ratios $\sigma _{14 TeV}$ / $\sigma _{13 TeV}$ and $\sigma _{27 TeV}$ / $\sigma _{14 TeV}$ for the EW signal and the QCD background processes. |
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Table 2:
Acceptance ratios for the Phase-2 detector with respect to Run 2 for various $\eta $ coverage configurations. The first number denotes the cut value for electrons while the number in parentheses denotes the cut value for muons. The numbers are for the center-of-mass energy of 13 TeV. |
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Table 3:
Effect of the systematic uncertainties on the signal and backgrounds yields for the two considered scenarios. |
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Table 4:
Signal and background yields projections for the ZZjj inclusive selection used in the statistical analysis and for a VBS cut-based selection also requiring $m_{jj} > $ 400 GeV and $| \Delta \eta _{jj} | > 2.4$. Quoted uncertainties correspond to the systematic uncertainties for the Run 2 scenario together with a 40% uncertainty in the QCD ggZZ background yield, as used for the Run 2 analysis. |
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Table 5:
Significance and measurement uncertainty in the VBS $\mathrm {Z_LZ_L}$ fraction for different lepton coverage configurations. The first configuration corresponds to the Run 2 configuration, the second to the Phase-2 upgrade and the third to an option for which the electron coverage would be extended up to $| \eta | = $ 4. In the quoted $\eta $ coverages, the first number corresponds to electrons, while the number in parentheses corresponds to muons. |
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Table 6:
Expected significance and measurement uncertainty for the measurement of the VBS $\mathrm {Z_LZ_L}$ fraction at HL-LHC and HE-LHC, with and without systematic uncertainties included. |
| Summary |
|
We presented prospective studies for the vector boson scattering at the HL-LHC in the ZZ fully leptonic decay channel. The analysis is based on the measurement performed using data recorded by the CMS experiment in 2016. The results previously obtained are projected to the expected integrated luminosity at HL-LHC of 3000 fb$^{-1}$ at the center-of-mass energy of 14 TeV, taking into account the increased acceptance of the new detector for the leptons and considering two scenario for the systematic uncertainties. The projected relative uncertainty in the VBS ZZ cross section measurement is 9.8% (8.8%) for the Run 2 (YR18) scenario and for a 10% uncertainty in the QCD ggZZ background yield, for an integrated luminosity of 3000 fb$^{-1}$ and a coverage of up to $\vert \eta \vert = $ 3 for electrons. Extending the coverage up to $\vert \eta \vert = 4$ for electrons, the projected measurement uncertainty would be 9.5% and 8.5%, respectively. The sensitivity for the longitudinal scattering VV $\rightarrow \mathrm{Z_L}\mathrm{Z_L}$ is assessed. The VBS $\mathrm{Z_LZ_L}$ signal is separated from the VBS and QCD backgrounds by means of a multivariate discriminant that combines observables that discriminate VBS from QCD processes as well as observables that discriminate longitudinal from transverse Z boson polarizations. The expected significance for the VBS $\mathrm{Z_LZ_L}$ fraction is 1.4$\sigma$ for an integrated luminosity of 3000 fb$^{-1}$. With such integrated luminosity we enter measurement era for the VBS $\mathrm{Z_LZ_L}$ fraction, with relative uncertainty below 100%. The measurement of such rare processes will of course benefit greatly of the highest luminosities. The lepton pseudorapidity coverage foreseen for the CMS detector upgrade leads to a significant improvement of the significance and cross section uncertainty for the VBS $\mathrm{Z_L}\mathrm{Z_L}$ process. Extending the coverage for electrons up to $\vert \eta \vert = $ 4 would result in a modest improvement in the performance. Finally, the HE-LHC option would allow to bring the sensitivity at the 5$\sigma$ level for this process. |
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