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| CMS-EXO-16-025 ; CERN-EP-2016-300 | ||
| Search for high-mass $\mathrm{ Z }\gamma$ resonances in proton-proton collisions at $\sqrt{s}=$ 8 and 13 TeV using jet substructure techniques | ||
| CMS Collaboration | ||
| 30 December 2016 | ||
| Phys. Lett. B 772 (2017) 363 | ||
| Abstract: A search for massive resonances decaying to a Z boson and a photon is performed in events with a hadronically decaying Z boson candidate, separately in light-quark and b quark decay modes, identified using jet substructure and advanced b tagging techniques. Results are based on samples of proton-proton collisions collected with the CMS detector at the LHC at center-of-mass energies of 8 and 13 TeV, corresponding to integrated luminosities of 19.7 and 2.7 fb$^{-1}$, respectively. The results of the search are combined with those of a similar search in the leptonic decay modes of the Z boson, based on the same data sets. Spin-0 resonances with various widths and with masses in a range between 0.2 and 3.0 TeV are considered. No significant excess is observed either in the individual analyses or the combination. The results are presented in terms of upper limits on the production cross section of such resonances and constitute the most stringent limits to date for a wide range of masses. | ||
| Links: e-print arXiv:1612.09516 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Full selection and reconstruction efficiency (including $\mathcal {B}(\mathrm{ Z } \to {\mathrm{ q } \mathrm{ \bar{q} } } $)) of the two search categories for a narrow resonance signal as a function of its mass in the 8 TeV analysis (left) and 13 TeV analysis (right). |
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Figure 1-a:
Full selection and reconstruction efficiency (including $\mathcal {B}(\mathrm{ Z } \to {\mathrm{ q } \mathrm{ \bar{q} } } $)) of the two search categories for a narrow resonance signal as a function of its mass in the 8 TeV analysis. |
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Figure 1-b:
Full selection and reconstruction efficiency (including $\mathcal {B}(\mathrm{ Z } \to {\mathrm{ q } \mathrm{ \bar{q} } } $)) of the two search categories for a narrow resonance signal as a function of its mass in the 13 TeV analysis. |
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Figure 2:
Fits to the $ {M_{{\mathrm{ Z } } \gamma }} $ invariant mass spectra in the search region for the antitagged (left column) and b-tagged (right column) categories. Upper (lower) row corresponds to 8 (13) TeV data. The results of the fits to the two categories with the parametric background shape are shown. The lower panels show the difference between the data and the fit, divided by the statistical uncertainty in data $\sigma _\text { stat}$. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals [52]. The upper error bars for bins with zero data entries are shown only in the region up to the highest nonzero entry. |
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Figure 2-a:
Fits to the $ {M_{{\mathrm{ Z } } \gamma }} $ invariant mass spectra in the search region for the antitagged category in to 8 TeV data. The results of the fits to the two categories with the parametric background shape are shown. The lower panel shows the difference between the data and the fit, divided by the statistical uncertainty in data $\sigma _\text { stat}$. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals [52]. The upper error bars for bins with zero data entries are shown only in the region up to the highest nonzero entry. |
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Figure 2-b:
Fits to the $ {M_{{\mathrm{ Z } } \gamma }} $ invariant mass spectra in the search region for the b-tagged category in to 8 TeV data. The results of the fits to the two categories with the parametric background shape are shown. The lower panel shows the difference between the data and the fit, divided by the statistical uncertainty in data $\sigma _\text { stat}$. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals [52]. The upper error bars for bins with zero data entries are shown only in the region up to the highest nonzero entry. |
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Figure 2-c:
Fits to the $ {M_{{\mathrm{ Z } } \gamma }} $ invariant mass spectra in the search region for the antitagged category in to 13 TeV data. The results of the fits to the two categories with the parametric background shape are shown. The lower panel shows the difference between the data and the fit, divided by the statistical uncertainty in data $\sigma _\text { stat}$. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals [52]. The upper error bars for bins with zero data entries are shown only in the region up to the highest nonzero entry. |
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Figure 2-d:
Fits to the $ {M_{{\mathrm{ Z } } \gamma }} $ invariant mass spectra in the search region for the b-tagged category in to 13 TeV data. The results of the fits to the two categories with the parametric background shape are shown. The lower panel shows the difference between the data and the fit, divided by the statistical uncertainty in data $\sigma _\text { stat}$. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals [52]. The upper error bars for bins with zero data entries are shown only in the region up to the highest nonzero entry. |
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Figure 3:
Expected and observed upper limits on the product of the cross section and branching fraction ${\cal B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for the production of a narrow (left) or broad (right) spin-0 resonance, obtained from the combination of antitagged and b-tagged categories in 8 TeV (upper) and 13 TeV (lower) data. |
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Figure 3-a:
Expected and observed upper limits on the product of the cross section and branching fraction ${\cal B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for the production of a narrow spin-0 resonance, obtained from the combination of antitagged and b-tagged categories in 8 TeV data. |
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Figure 3-b:
Expected and observed upper limits on the product of the cross section and branching fraction ${\cal B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for the production of a broad spin-0 resonance, obtained from the combination of antitagged and b-tagged categories in 8 TeV data. |
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Figure 3-c:
Expected and observed upper limits on the product of the cross section and branching fraction ${\cal B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for the production of a narrow spin-0 resonance, obtained from the combination of antitagged and b-tagged categories in 13 TeV data. |
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Figure 3-d:
Expected and observed upper limits on the product of the cross section and branching fraction ${\cal B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for the production of a broad spin-0 resonance, obtained from the combination of antitagged and b-tagged categories in 13 TeV data. |
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Figure 4:
Expected and observed limits on the product of the cross section at $ \sqrt{s} = $ 13 TeV and branching fraction $\mathcal {B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for the production of a narrow spin-0 resonance, obtained from the combination of the 8 and 13 TeV analyses in the hadronic decay channel, assuming a gluon fusion production mechanism. |
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Figure 5:
Left : Expected and observed limits on the product of the cross section at $ \sqrt{s} = $ 13 TeV and branching fraction $\mathcal {B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for the production of a narrow spin-0 resonance, obtained from the combination of the 8 and 13 TeV analyses in hadronic and leptonic [15] decay channels of the Z boson, assuming a gluon fusion production mechanism. Right : expected limits from the individual and combined analyses, showing the relative contribution of each channel. The discontinuities are due to the difference in the mass ranges used in the individual searches. |
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Figure 5-a:
Expected and observed limits on the product of the cross section at $ \sqrt{s} = $ 13 TeV and branching fraction $\mathcal {B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for the production of a narrow spin-0 resonance, obtained from the combination of the 8 and 13 TeV analyses in hadronic and leptonic [15] decay channels of the Z boson, assuming a gluon fusion production mechanism. |
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Figure 5-b:
Expected limits from the individual and combined analyses, showing the relative contribution of each channel. The discontinuities are due to the difference in the mass ranges used in the individual searches. |
| Tables | |
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Table 1:
Summary of event selection. |
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Table 2:
Summary of the sources of systematic uncertainties, their magnitudes, effects on the signal yield, and affected quantities. The third column indicates the magnitude of the yield variation. The last column indicates if the source of the uncertainty affects the total signal yield, signal shape, or introduces a category migration. The numbers in parentheses correspond to the 13 TeV analysis (whenever there is a difference from the 8 TeV numbers). |
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Table 3:
Observed (expected) limits on the production cross section times branching fraction ${\cal B}(\mathrm {X} \to \mathrm{ Z } \gamma)$ for narrow resonances from each of the two categories of the analysis, as well as from their combination. |
| Summary |
| We have presented a search for new spin-0 resonances decaying to a Z boson and a photon, where the Z boson decays hadronically, in the mass range from 0.65 to 3.0 TeV, using 2012 and 2015 proton-proton collision data at center-of-mass energies of 8 and 13 TeV, respectively. The search is carried out with two exclusive categories of events, with or without identification of the $\mathrm{ Z }\to\mathrm{ b \bar{b} }$ decay, and the final result is obtained from the combination of these two categories. Jet substructure and subjet b tagging techniques are used in order to enhance the sensitivity of the analysis. No significant deviation from the standard model prediction is found. Results are presented as upper limits at 95% confidence level on the product of the production cross section and the branching fraction of the $\mathrm{ Z }\gamma$ decay channel of a new resonance. The results of the searches at the two center-of-mass energies are combined assuming the mechanism for production of a new resonance is gluon fusion. These results are further combined with those of analogous searches in the leptonic decay channel of the Z boson. The limits set in this analysis are the most stringent limits to date on $\mathrm{ Z }\gamma$ resonances in a wide range of masses. |
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