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| CMS-EXO-16-021 ; CERN-EP-2016-230 | ||
| Search for high-mass Z$\gamma$ resonances in $\mathrm{ e }^+\mathrm{ e }^-\gamma$ and $\mu^+\mu^-\gamma$ final states in proton-proton collisions at $\sqrt{s}= $ 8 and 13 TeV | ||
| CMS Collaboration | ||
| 10 October 2016 | ||
| JHEP 01 (2017) 076 | ||
| Abstract: This paper describes the search for a high-mass narrow-width scalar particle decaying into a Z boson and a photon. The analysis is performed using proton-proton collision data recorded 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 Z bosons are reconstructed from opposite-sign electron or muon pairs. No statistically significant deviation from the standard model predictions has been found in the 200-2000 GeV mass range. Upper limits at 95% confidence level have been derived on the product of the scalar particle production cross section and the branching fraction of the Z decaying into electrons or muons, which range from 280 to 20 fb for resonance masses between 200 and 2000 GeV. | ||
| Links: e-print arXiv:1610.02960 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Observed $ {{M}_{\ell \ell \gamma }} $ invariant mass spectra in the 8 TeV data, for the $ {\mathrm{ e } ^+\mathrm{ e } ^-\gamma } $(left) and the $ {\mu ^+\mu ^-\gamma } $ (right) channels. The fitted function is represented by a line, with the 68% uncertainty band as grey shading. The lower panels show the difference between the data and the fit, divided by the uncertainty $\sigma _\text {stat}$, that includes the statistical uncertainty in both the data and the fit. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals. |
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Figure 1-a:
Observed $ {{M}_{\mathrm{ e } ^+\mathrm{ e } ^- \gamma }} $ invariant mass spectrum in the 8 TeV data. The fitted function is represented by a line, with the 68% uncertainty band as grey shading. The lower panels show the difference between the data and the fit, divided by the uncertainty $\sigma _\text {stat}$, that includes the statistical uncertainty in both the data and the fit. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals. |
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Figure 1-b:
Observed $ {{M}_{\mu ^+\mu ^-\gamma }} $ invariant mass spectrum in the 8 TeV data. The fitted function is represented by a line, with the 68% uncertainty band as grey shading. The lower panels show the difference between the data and the fit, divided by the uncertainty $\sigma _\text {stat}$, that includes the statistical uncertainty in both the data and the fit. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals. |
png pdf |
Figure 2:
Observed $ {{M}_{\ell \ell \gamma }} $ invariant mass spectra in the 13 TeV data, for the $ {\mathrm{ e } ^+\mathrm{ e } ^-\gamma } $(left) and the $ {\mu ^+\mu ^-\gamma } $ (right) channels. The fitted function is represented by a line, with the 68% uncertainty band as gray shading. The lower panels show the difference between the data and the fit, divided by the uncertainty $\sigma _\text {stat}$, which includes the statistical uncertainty in both the data and the fit. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals. |
png pdf |
Figure 2-a:
Observed $ {{M}_{\mathrm{ e } ^+\mathrm{ e } ^- \gamma }} $ invariant mass spectrum in the 13 TeV data. The fitted function is represented by a line, with the 68% uncertainty band as gray shading. The lower panels show the difference between the data and the fit, divided by the uncertainty $\sigma _\text {stat}$, which includes the statistical uncertainty in both the data and the fit. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals. |
png pdf |
Figure 2-b:
Observed $ {{M}_{\mu ^+\mu ^-\gamma }} $ invariant mass spectrum in the 13 TeV data. The fitted function is represented by a line, with the 68% uncertainty band as gray shading. The lower panels show the difference between the data and the fit, divided by the uncertainty $\sigma _\text {stat}$, which includes the statistical uncertainty in both the data and the fit. For bins with a low number of data entries, the error bars correspond to the Garwood confidence intervals. |
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Figure 3:
Expected and observed upper limits, at 95% CL, on the cross section times branching fraction for ${\mathrm{X} }\to {\mathrm{ Z } } \gamma $ obtained with the searches performed at 8 TeV (left) and at 13 TeV (right). |
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Figure 3-a:
Expected and observed upper limits, at 95% CL, on the cross section times branching fraction for ${\mathrm{X} }\to {\mathrm{ Z } } \gamma $ obtained with the searches performed at 8 TeV. |
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Figure 3-b:
Expected and observed upper limits, at 95% CL, on the cross section times branching fraction for ${\mathrm{X} }\to {\mathrm{ Z } } \gamma $ obtained with the searches performed at 13 TeV. |
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Figure 4:
Left: expected and observed upper limits, at 95% CL, on the 13 TeV cross section $\sigma _{13 TeV }({\mathrm{X} } \to {\mathrm{ Z } } \gamma )$ for the scaled 8 TeV (blue, lighter) and 13 TeV (red, darker) searches, together with their combination(black). Expected limits are shown with dashed lines, observed ones with solid lines. Right: 95% CL upper limit for the combination of 8 TeV and 13 TeV data. The solid(dashed) line represents the observed(expected) limit, whereas the inner green(outer yellow) bands represent the 68%(95%) uncertainty bands. |
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Figure 4-a:
Expected and observed upper limits, at 95% CL, on the 13 TeV cross section $\sigma _{13 TeV }({\mathrm{X} } \to {\mathrm{ Z } } \gamma )$ for the scaled 8 TeV (blue, lighter) and 13 TeV (red, darker) searches, together with their combination(black). Expected limits are shown with dashed lines, observed ones with solid lines. |
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Figure 4-b:
95% CL upper limit for the combination of 8 TeV and 13 TeV data. The solid(dashed) line represents the observed(expected) limit, whereas the inner green(outer yellow) bands represent the 68%(95%) uncertainty bands. |
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Figure 5:
Observed background-only local $p$-values for the scaled 8 TeV search(blue, dotted), the 13 TeV search (red, dashed), and the combination (black, solid). |
| Tables | |
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Table 1:
Summary of considered systematic uncertainties in signal. |
| Summary |
| A search for heavy resonances decaying to Z$\gamma$, with further decay $\mathrm{ Z }\to \ell^+\ell^-$, with $\ell = \mathrm{ e }$ or $\mu$, has been presented. The search makes use of proton-proton data collected by the CMS detector at the LHC, corresponding to integrated luminosities of 19.7 and 2.7 fb at 8 and 13 TeV, respectively. The background is measured directly from data and localized excesses are looked for. No significant deviation with respect to the standard model expectation is found. Upper limits at 95% confidence level are set on the production cross section of narrow resonances, ranging from 280 to 20 fb for resonance masses from 200 to 2000 GeV. |
| Additional Tables | |
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Additional Table 1:
The signal efficiency of the selection used in 13 TeV data for the $ {\mu ^+\mu ^-\gamma} $ channel. The efficiency is measured by counting the number of the simulated events passing the selection requirements sequentially, in order from top to bottom. The following is the selection requirement at each step; Total: no selection requirements (base). Trigger: to fire at least one of the muon trigger paths. Muon identification: to have muons passing the muon identification criteria. Dimuon Reconstruction: to have two muons in $|\eta | < $ 2.4, the leading muon with $ {p_{\mathrm {T}}} > $ 25 GeV and the second-leading muon with $ {p_{\mathrm {T}}} > $ 20 GeV. Photon Identification: to have a photon passing the photon identification criteria. Photon $ {p_{\mathrm {T}}} $, $\eta $: to have a photon with $ {p_{\mathrm {T}}} > $ 40 GeV and $|\eta | < $ 2.5. $\Delta {R}_{\ell \gamma} > $ 0.4: $\Delta {R}$ between the photon and each of the two muons must be larger than 0.4. 50 GeV $ < {M}_{\ell \ell} < $ 130 GeV: the invariant mass of the dimuon system must be in the range from 50 GeV to 130 GeV. $ {p_{\mathrm {T}}} ^{\gamma} / {{M}_{\ell \ell \gamma}} > $ 0.27: The portion of the $ {p_{\mathrm {T}}} $ of the photon to the invariant mass of the dimuon and the photon must be larger than 0.27. |
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Additional Table 2:
The signal efficiency of the selection used in 13 TeV data for the $ {\mathrm{e} ^+\mathrm{e} ^-\gamma} $ channel. The efficiency is measured by counting the number of the simulated events passing the selection requirements sequentially, in order from top to bottom. The following is the selection requirement at each step; Total: no selection requirements (base). Trigger: to fire at least one of the electron trigger paths. Electron identification: to have electrons passing the electron identification criteria. Dielectron Reconstruction: to have two electrons in $|\eta | < $ 2.5, the leading electron with $ {p_{\mathrm {T}}} > $ 25 GeV and the second-leading electron with $ {p_{\mathrm {T}}} > $ 20 GeV. Photon Identification: to have a photon passing the photon identification criteria. Photon $ {p_{\mathrm {T}}} $, $\eta $: to have a photon with $ {p_{\mathrm {T}}} > $ 40 GeV and $|\eta | < $ 2.5. $\Delta {R}_{\ell \gamma} > $ 0.4: $\Delta {R}$ between the photon and each of the two electrons must be larger than 0.4. 50 GeV $ < {M}_{\ell \ell} < $ 130 GeV: the invariant mass of the dielectron system must be in the range from 50 GeV to 130 GeV. $ {p_{\mathrm {T}}} ^{\gamma} / {{M}_{\ell \ell \gamma}} > $ 0.27: The portion of the $ {p_{\mathrm {T}}} $ of the photon to the invariant mass of the dielectron and the photon must be larger than 0.27. |
| An example python configuration to generate signal events of the resoance mass 750 GeV is provided at this link. The python parameters are given as processParameters in the configuration file. |
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