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| CMS-FSQ-13-008 ; CERN-EP-2016-073 | ||
| Evidence for exclusive $\gamma\gamma \to \mathrm{ W^{+} } \mathrm{ W^{-} }$ production and constraints on anomalous quartic gauge couplings in pp collisions at $\sqrt{s}= $ 7 and 8 TeV | ||
| CMS Collaboration | ||
| 15 April 2016 | ||
| JHEP 08 (2016) 119 | ||
| Abstract: A search for exclusive or quasi-exclusive $\gamma\gamma\to\mathrm{ W^{+} }\mathrm{ W^{-} }$ production, via $\mathrm{ p }\mathrm{ p } \to \mathrm{ p }^{(*)}\mathrm{ W^{+} }\mathrm{ W^{-} }\mathrm{ p }^{(*)}\to \mathrm{ p }^{(*)}\mu^{\pm}\mathrm{ e }^{\mp}\mathrm{ p }^{(*)}$ at $\sqrt{s}= $ 8 TeV, is reported using data corresponding to an integrated luminosity of 19.7 fb$^{-1}$. Events are selected by requiring the presence of an electron-muon pair with large transverse momentum $p_{\mathrm{T}}(\mu^{\pm}\mathrm{ e }^{\mp})> $ 30 GeV, and no associated charged particles detected from the same vertex. The 8 TeV results are combined with the previous 7 TeV results (obtained for 5.05 fb$^{-1}$ of data). In the signal region, 13 (2) events are observed over an expected background of 3.9 $\pm$ 0.6 (0.84 $\pm$ 0.15) events for 8 (7) TeV, resulting in a combined excess of 3.4$ \sigma$ over the background-only hypothesis. The observed yields and kinematic distributions are compatible with the standard model prediction for exclusive and quasi-exclusive $\gamma\gamma \to \mathrm{ W^{+} }\mathrm{ W^{-} }$ production. Upper limits on the anomalous quartic gauge coupling operators $a^{\mathrm{ W }}_{0,C}$ (dimension-6) and $f_{M0,1,2,3}$ (dimension-8), the most stringent to date, are derived from the measured dilepton transverse momentum spectrum. | ||
| Links: e-print arXiv:1604.04464 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Quartic (left), $t$-channel (center), and $u$-channel (right) diagrams contributing to the $\gamma \gamma \to \mathrm{ W^{+} } \mathrm{ W^{-} } $ process at leading order in the SM. The $\mathrm{ p } ^{(*)}$ indicates that the final state proton(s) remain intact (``exclusive'' or ``elastic'' production), or dissociate (``quasi-exclusive'' production). |
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Figure 1-a:
Quartic diagram contributing to the $\gamma \gamma \to \mathrm{ W^{+} } \mathrm{ W^{-} } $ process at leading order in the SM. The $\mathrm{ p } ^{(*)}$ indicates that the final state proton(s) remain intact (``exclusive'' or ``elastic'' production), or dissociate (``quasi-exclusive'' production). |
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Figure 1-b:
$t$-channel diagram contributing to the $\gamma \gamma \to \mathrm{ W^{+} } \mathrm{ W^{-} } $ process at leading order in the SM. The $\mathrm{ p } ^{(*)}$ indicates that the final state proton(s) remain intact (``exclusive'' or ``elastic'' production), or dissociate (``quasi-exclusive'' production). |
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Figure 1-c:
$u$-channel diagram contributing to the $\gamma \gamma \to \mathrm{ W^{+} } \mathrm{ W^{-} } $ process at leading order in the SM. The $\mathrm{ p } ^{(*)}$ indicates that the final state proton(s) remain intact (``exclusive'' or ``elastic'' production), or dissociate (``quasi-exclusive'' production). |
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Figure 2:
Acoplanarity for the $ \mu^+ \mu^- $ (left) and $\mathrm{ e }^+ \mathrm{ e }^- $ (right) final states in the elastic $\gamma \gamma \to \ell^+ \ell^- $ control region ($ {| 1-\Delta \phi (\ell^+ \ell^- )/\pi | }<$ 0.01 and $m(\ell^+ \ell^- ) <$ 70 GeV or $m(\ell^+ \ell^- ) >$ 106 GeV ) and 0 additional tracks associated to the dilepton vertex. The data (points with error bars) are compared to the simulated samples (histograms) in the top panels. The data/MC ratios are shown in the bottom panels (the red line shows the extracted correction for the zero extra tracks efficiency). |
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Figure 2-a:
Acoplanarity for the $ \mu^+ \mu^- $ final state in the elastic $\gamma \gamma \to \ell^+ \ell^- $ control region ($ {| 1-\Delta \phi (\ell^+ \ell^- )/\pi | }<$ 0.01 and $m(\ell^+ \ell^- ) <$ 70 GeV or $m(\ell^+ \ell^- ) >$ 106 GeV ) and 0 additional tracks associated to the dilepton vertex. The data (points with error bars) are compared to the simulated samples (histograms) in the top panels. The data/MC ratios are shown in the bottom panels (the red line shows the extracted correction for the zero extra tracks efficiency). |
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Figure 2-b:
Acoplanarity for the $\mathrm{ e }^+ \mathrm{ e }^- $ final state in the elastic $\gamma \gamma \to \ell^+ \ell^- $ control region ($ {| 1-\Delta \phi (\ell^+ \ell^- )/\pi | }<$ 0.01 and $m(\ell^+ \ell^- ) <$ 70 GeV or $m(\ell^+ \ell^- ) >$ 106 GeV ) and 0 additional tracks associated to the dilepton vertex. The data (points with error bars) are compared to the simulated samples (histograms) in the top panels. The data/MC ratios are shown in the bottom panels (the red line shows the extracted correction for the zero extra tracks efficiency). |
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Figure 3:
Dilepton invariant mass for the $ \mu^+ \mu^- $ (left) and $\mathrm{ e }^+ \mathrm{ e }^- $ (right) final states with an acoplanarity requirement, $ {| 1-\Delta \phi (\ell^+ \ell^- )/\pi | }<$ 0.01, and zero additional tracks associated to the dilepton vertex. The data (points with error bars) are compared to the simulated samples (histograms) in the top panels, and the data/MC ratios are shown in the bottom panels. The exclusive-production simulated samples are scaled to the number of events in data for $m(\ell^+ \ell^- ) < $ 70 GeV or $m(\ell^+ \ell^- ) >$ 106 GeV. The Drell-Yan simulation is scaled to the number of events in data for 70 $< m(\ell^+ \ell^- ) <$ 106 GeV. The last bin in both plots is an overflow bin and includes all events with invariant mass greater than 200 GeV. |
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Figure 3-a:
Dilepton invariant mass for the $ \mu^+ \mu^- $ final state with an acoplanarity requirement, $ {| 1-\Delta \phi (\ell^+ \ell^- )/\pi | }<$ 0.01, and zero additional tracks associated to the dilepton vertex. The data (points with error bars) are compared to the simulated samples (histograms) in the top panels, and the data/MC ratios are shown in the bottom panels. The exclusive-production simulated samples are scaled to the number of events in data for $m(\ell^+ \ell^- ) < $ 70 GeV or $m(\ell^+ \ell^- ) >$ 106 GeV. The Drell-Yan simulation is scaled to the number of events in data for 70 $< m(\ell^+ \ell^- ) <$ 106 GeV. The last bin in both plots is an overflow bin and includes all events with invariant mass greater than 200 GeV. |
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Figure 3-b:
Dilepton invariant mass for the $\mathrm{ e }^+ \mathrm{ e }^- $ (b) final state with an acoplanarity requirement, $ {| 1-\Delta \phi (\ell^+ \ell^- )/\pi | }<$ 0.01, and zero additional tracks associated to the dilepton vertex. The data (points with error bars) are compared to the simulated samples (histograms) in the top panels, and the data/MC ratios are shown in the bottom panels. The exclusive-production simulated samples are scaled to the number of events in data for $m(\ell^+ \ell^- ) < $ 70 GeV or $m(\ell^+ \ell^- ) >$ 106 GeV. The Drell-Yan simulation is scaled to the number of events in data for 70 $< m(\ell^+ \ell^- ) <$ 106 GeV. The last bin in both plots is an overflow bin and includes all events with invariant mass greater than 200 GeV. |
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Figure 4:
Dilepton invariant mass for the $ \mu^+ \mu^- $ final state in the $\gamma \gamma \to \ell^+ \ell^- $ proton dissociation control region with no additional tracks associated to the dilepton vertex, for data (points with error bars) and simulated samples (histograms, with the efficiency correction applied to the exclusive samples). The last bin is both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio where the denominator includes the sum of all simulated samples except the double-dissociation contribution (shown as the blue dotted line in the top plots). |
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Figure 4-a:
Dilepton invariant mass for the $ \mu^+ \mu^- $ final state in the $\gamma \gamma \to \ell^+ \ell^- $ proton dissociation control region with no additional tracks associated to the dilepton vertex, for data (points with error bars) and simulated samples (histograms, with the efficiency correction applied to the exclusive samples). The last bin is both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio where the denominator includes the sum of all simulated samples except the double-dissociation contribution (shown as the blue dotted line in the top plots). |
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Figure 4-b:
Dilepton invariant mass for the $\mathrm{ e }^+ \mathrm{ e }^- $ final states in the $\gamma \gamma \to \ell^+ \ell^- $ proton dissociation control region with no additional tracks associated to the dilepton vertex, for data (points with error bars) and simulated samples (histograms, with the efficiency correction applied to the exclusive samples). The last bin is both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio where the denominator includes the sum of all simulated samples except the double-dissociation contribution (shown as the blue dotted line in the top plots). |
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Figure 5:
Distributions of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ invariant mass (left) and acoplanarity (right) for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) > $ 30 GeV and 1-6 extra tracks (inclusive $\mathrm{ W }^+ \mathrm{ W }^- $ control region). The last bin in the invariant mass plot is an overflow bin and includes all events with $m(\mu \mathrm{ e } )> $ 360 GeV. The bottom panels show the data/MC ratio. |
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Figure 5-a:
Distribution of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ invariant mass for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) > $ 30 GeV and 1-6 extra tracks (inclusive $\mathrm{ W }^+ \mathrm{ W }^- $ control region). The last bin in the invariant mass plot is an overflow bin and includes all events with $m(\mu \mathrm{ e } )> $ 360 GeV. The bottom panels show the data/MC ratio. |
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Figure 5-b:
Distributions of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ acoplanarity for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) > $ 30 GeV and 1-6 extra tracks (inclusive $\mathrm{ W }^+ \mathrm{ W }^- $ control region). The last bin in the invariant mass plot is an overflow bin and includes all events with $m(\mu \mathrm{ e } )> $ 360 GeV. The bottom panels show the data/MC ratio. |
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Figure 6:
Distributions of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ invariant mass (left) and acoplanarity (right) for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) <$ 30 GeV and 1-6 extra tracks (Drell-Yan $\tau ^{+}\tau ^{-}$ control region). The last bin in both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio. |
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Figure 6-a:
Distributions of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ invariant mass for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) <$ 30 GeV and 1-6 extra tracks (Drell-Yan $\tau ^{+}\tau ^{-}$ control region). The last bin in both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio. |
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Figure 6-b:
Distributions of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ acoplanarity for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) <$ 30 GeV and 1-6 extra tracks (Drell-Yan $\tau ^{+}\tau ^{-}$ control region). The last bin in both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio. |
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Figure 7:
Distributions of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ invariant mass (left) and acoplanarity (right) for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) <$ 30 GeV and no additional tracks ($\gamma \gamma \to \tau ^{+}\tau ^{-}$ control region). The last bin in both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio. |
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Figure 7-a:
Distributions of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ invariant mass for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) <$ 30 GeV and no additional tracks ($\gamma \gamma \to \tau ^{+}\tau ^{-}$ control region). The last bin in both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio. |
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Figure 7-b:
Distributions of $\mu ^{\pm }\mathrm{ e } ^{\mp }$ acoplanarity for data (points with error bars) and expected backgrounds (histograms) for $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) <$ 30 GeV and no additional tracks ($\gamma \gamma \to \tau ^{+}\tau ^{-}$ control region). The last bin in both plots is an overflow bin and includes all events above the maximum value in the plot. The bottom panels show the data/MC ratio. |
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Figure 8:
Distributions of muon-electron transverse momentum for events with zero associated tracks (left), and extra-tracks multiplicity for events with $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) >$ 30 GeV (right). The data are shown by the points with error bars; the histograms indicate the expected SM signal and backgrounds. Two representative values for anomalous couplings are shown stacked on top of the backgrounds. The last bin in the $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp })$ distribution is an overflow bin and includes all events with $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) >$ 210 GeV. |
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Figure 8-a:
Distribution of muon-electron transverse momentum for events with zero associated tracks. The data are shown by the points with error bars; the histograms indicate the expected SM signal and backgrounds. Two representative values for anomalous couplings are shown stacked on top of the backgrounds. The last bin in the $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp })$ distribution is an overflow bin and includes all events with $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) >$ 210 GeV. |
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Figure 8-b:
Distribution of extra-tracks multiplicity for events with $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) >$ 30 GeV (b). The data are shown by the points with error bars; the histograms indicate the expected SM signal and backgrounds. Two representative values for anomalous couplings are shown stacked on top of the backgrounds. The last bin in the $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp })$ distribution is an overflow bin and includes all events with $ {p_{\mathrm {T}}} (\mu ^{\pm }\mathrm{ e } ^{\mp }) >$ 210 GeV. |
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Figure 9:
Muon-electron invariant mass (top left), acoplanarity (top right), and missing transverse energy (bottom) in the $\gamma \gamma \to \mathrm{ W }^+ \mathrm{ W }^- $ signal region. The data are shown by the points with error bars; the histograms indicate the expected SM signal and backgrounds. The last bin in the invariant mass and missing transverse energy plots is an overflow bin and includes also all events above the maximum value in the plot. |
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Figure 9-a:
Muon-electron invariant mass in the $\gamma \gamma \to \mathrm{ W }^+ \mathrm{ W }^- $ signal region. The data are shown by the points with error bars; the histograms indicate the expected SM signal and backgrounds. The last bin is an overflow bin and includes also all events above the maximum value in the plot. |
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Figure 9-b:
Muon-electron acoplanarity in the $\gamma \gamma \to \mathrm{ W }^+ \mathrm{ W }^- $ signal region. The data are shown by the points with error bars; the histograms indicate the expected SM signal and backgrounds. |
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Figure 9-c:
Muon-electron missing transverse energy in the $\gamma \gamma \to \mathrm{ W }^+ \mathrm{ W }^- $ signal region. The data are shown by the points with error bars; the histograms indicate the expected SM signal and backgrounds. The last bin is an overflow bin and includes also all events above the maximum value in the plot. |
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Figure 10:
Excluded values of the anomalous coupling parameters $a^{\mathrm{ W } }_{0}/\Lambda ^{2}$ and $a^{\mathrm{ W } }_{C}/\Lambda ^{2}$ with $\Lambda _{\text {cutoff}}=$ 500 GeV. The exclusion regions are shown for the CMS measurements of $\gamma \gamma \to \mathrm{ W }^+ \mathrm{ W }^- $ at 7 TeV (outer contour), 8 TeV (middle contour), and the 7+8 TeV combination (innermost contour). The areas outside the solid contours are excluded by each measurement at 95% CL. The cross indicates the one-dimensional limits obtained for each parameter from the 7 and 8 TeV combination, with the other parameter fixed to zero. |
| Tables | |
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Table 1:
Number of expected signal and background events in simulation passing each selection step, normalized to an integrated luminosity of 19.7 fb$^{-1}$. The preselection includes events with an opposite-charge muon and electron associated with the same vertex, each with $ {p_{\mathrm {T}}} >$ 20 GeV and $ {| \eta | }<$ 2.4, and $<$16 additional tracks at the vertex. Uncertainties are statistical only. |
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Table 2:
Summary of systematic uncertainties affecting the signal. |
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Table 3:
Summary of all 95% CL AQGC limits derived from the measured $ {p_{\mathrm {T}}} (\mu \mathrm{ e } )$ distributions in the $\gamma \gamma \to \mathrm{ W^{+} } \mathrm{ W^{-} } $ signal region production in CMS at 7 and 8 TeV. The second column lists the 7 TeV limits on dimension-6 operators taken from Ref. [5], as well as their conversion to dimension-8 operators at 7 TeV. The third column contains the 8 TeV results described in this paper. The final column shows the combined 7 and 8 TeV limits. |
| Summary |
| Results are presented for exclusive and quasi-exclusive $\gamma\gamma \to \mathrm{ W^{+} }\mathrm{ W^{-} }$ production in the $\mu^{\pm}\mathrm{ e }^{\mp}$ final state in pp collisions at $\sqrt{s} = $ 8 (7) TeV, using data samples corresponding to integrated luminosities of 19.7 (5.05) fb$^{-1}$. In the signal region with $p_{\mathrm{T}}(\mu^{\pm}\mathrm{ e }^{\mp}) > $ 30 GeV and no additional charged particles associated with the $\mu^{\pm}\mathrm{ e }^{\mp}$ vertex, we observe 13 (2) events with an expected background of 3.9 $\pm$ 0.6 (0.84 $\pm$ 0.15) events in the 8 (7) TeV data. The observed yields and kinematic distributions are consistent with the SM prediction, with a combined significance over the background-only hypothesis of 3.4$\sigma$. No significant deviations from the SM are observed in the $p_{\mathrm{T}}(\mu^{\pm}\mathrm{ e }^{\mp})$ distribution, and the combined 7+8 TeV limits are interpreted in terms of improved constraints on dimension-6 and dimension-8 anomalous quartic gauge operator couplings. |
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