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| CMS-EXO-18-004 ; CERN-EP-2018-280 | ||
| Search for excited leptons in $ \ell \ell \gamma $ final states in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 7 November 2018 | ||
| JHEP 04 (2019) 015 | ||
| Abstract: A search is presented for excited electrons and muons in $ \ell \ell \gamma $ final states at the LHC. The search is based on a data sample corresponding to an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the CMS detector in 2016. This is the first search for excited leptons at $\sqrt{s} = $ 13 TeV. The observation is consistent with the standard model background prediction, and the most stringent exclusion limits to date are set on the excited lepton mass and the compositeness scale, at 95% confidence level. Excited electrons and muons are excluded for masses below 3.9 and 3.8 TeV, respectively, under the assumption that the excited lepton mass equals the compositeness scale. The best observed limit on the compositeness scale is obtained with an excited lepton mass of around 1.0 TeV, excluding values below 25 TeV for both excited electrons and muons. | ||
| Links: e-print arXiv:1811.03052 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The Feynman diagram of the production of excited leptons in $ {\ell \ell {\gamma}} $ final states. |
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Figure 2:
The distributions of $ {m_{\ell {\gamma}}^\text {min}} $ (left column) and $ {m_{\ell {\gamma}}^\text {max}} $ (right column) in the $ {{\mathrm {e}} {\mathrm {e}} {\gamma}} $ channel (upper row) and the $ {{{\mu}} {{\mu}} {\gamma}} $ channel (lower row). The points with error bars denote the data and the stacked histograms show the predictions for each of the backgrounds. The uncertainty bands of the SM prediction include only statistical uncertainties. Signal events for $ {m_{\ell ^*}} = $ 1 TeV at $\Lambda = $ 10 TeV are also shown as dotted lines. The last bin of each distribution includes overflow events. |
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Figure 2-a:
Distribution of $ {m_{\ell {\gamma}}^\text {min}} $ in the $ {{\mathrm {e}} {\mathrm {e}} {\gamma}} $ channel. The points with error bars denote the data and the stacked histograms show the predictions for each of the backgrounds. The uncertainty band of the SM prediction includes only statistical uncertainties. Signal events for $ {m_{\ell ^*}} = $ 1 TeV at $\Lambda = $ 10 TeV are also shown as a dotted line. The last bin includes overflow events. |
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Figure 2-b:
Distribution of $ {m_{\ell {\gamma}}^\text {max}} $ in the $ {{\mathrm {e}} {\mathrm {e}} {\gamma}} $ channel. The points with error bars denote the data and the stacked histograms show the predictions for each of the backgrounds. The uncertainty band of the SM prediction includes only statistical uncertainties. Signal events for $ {m_{\ell ^*}} = $ 1 TeV at $\Lambda = $ 10 TeV are also shown as a dotted line. The last bin includes overflow events. |
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Figure 2-c:
Distribution of $ {m_{\ell {\gamma}}^\text {min}} $ in the $ {{{\mu}} {{\mu}} {\gamma}} $ channel. The points with error bars denote the data and the stacked histograms show the predictions for each of the backgrounds. The uncertainty band of the SM prediction includes only statistical uncertainties. Signal events for $ {m_{\ell ^*}} = $ 1 TeV at $\Lambda = $ 10 TeV are also shown as a dotted line. The last bin includes overflow events. |
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Figure 2-d:
Distribution of $ {m_{\ell {\gamma}}^\text {max}} $ in the $ {{\mathrm {e}} {\mathrm {e}} {\gamma}} $ $ {{{\mu}} {{\mu}} {\gamma}} $ channel. The points with error bars denote the data and the stacked histograms show the predictions for each of the backgrounds. The uncertainty band of the SM prediction includes only statistical uncertainties. Signal events for $ {m_{\ell ^*}} = $ 1 TeV at $\Lambda = $ 10 TeV are also shown as a dotted line. The last bin includes overflow events. |
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Figure 3:
The two-dimensional distributions of ${m_{\ell {\gamma}}^\text {max}}$ versus ${m_{\ell {\gamma}}^\text {min}} $ of excited electrons with a mass of 500 GeV (left) and of excited muons with a mass of 750 GeV (right), after the event selection, normalized to the expected signal cross section at $\Lambda = $ 10 TeV. The red dashed lines denote the boundary of the L-shaped search window. |
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Figure 3-a:
The two-dimensional distributions of ${m_{\ell {\gamma}}^\text {max}}$ versus ${m_{\ell {\gamma}}^\text {min}} $ of excited electrons with a mass of 500 GeV, after the event selection, normalized to the expected signal cross section at $\Lambda = $ 10 TeV. The red dashed lines denote the boundary of the L-shaped search window. |
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Figure 3-b:
The two-dimensional distributions of ${m_{\ell {\gamma}}^\text {max}}$ versus ${m_{\ell {\gamma}}^\text {min}} $ of excited muons with a mass of 750 GeV, after the event selection, normalized to the expected signal cross section at $\Lambda = $ 10 TeV. The red dashed lines denote the boundary of the L-shaped search window. |
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Figure 4:
The two-dimensional distributions of ${m_{\ell {\gamma}}^\text {max}}$ versus ${m_{\ell {\gamma}}^\text {min}}$ of DY+$ {\gamma}$ background events in the ${{\mathrm {e}} {\mathrm {e}} {\gamma}}$ (left) and ${{{\mu}} {{\mu}} {\gamma}}$ (right) channels, after the event selection, normalized to the cross section for DY+$ {\gamma}$ production. |
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Figure 4-a:
The two-dimensional distributions of ${m_{\ell {\gamma}}^\text {max}}$ versus ${m_{\ell {\gamma}}^\text {min}}$ of DY+$ {\gamma}$ background events in the ${{\mathrm {e}} {\mathrm {e}} {\gamma}}$ channel, after the event selection, normalized to the cross section for DY+$ {\gamma}$ production. |
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Figure 4-b:
The two-dimensional distributions of ${m_{\ell {\gamma}}^\text {max}}$ versus ${m_{\ell {\gamma}}^\text {min}}$ of DY+$ {\gamma}$ background events in the ${{{\mu}} {{\mu}} {\gamma}}$ channel, after the event selection, normalized to the cross section for DY+$ {\gamma}$ production. |
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Figure 5:
The product of signal acceptance and efficiency as a function of the generated resonance mass for the $ {{\mathrm {e}} {\mathrm {e}} {\gamma}} $ (lower) and $ {{{\mu}} {{\mu}} {\gamma}} $ (upper) channels. Each marker denotes the value measured from the simulated signal sample at a given mass point, and the lines represent polynomial fits to the measured values. |
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Figure 6:
Observed (solid) and expected (dashed) 95% CL upper limits on the product of the production cross section and branching fraction (left column) and lower limits on the compositeness scale (right column) as a function of signal mass $ {m_{\ell ^*}} $, together with the 68% (green, inner) and 95% (yellow, outer) quantiles of the expected limit, for $ {{\mathrm {e}}^*} $ (upper row) and ${{{\mu}}^*}$ (lower row). |
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Figure 6-a:
Observed (solid) and expected (dashed) 95% CL upper limits on the product of the production cross section and branching fraction as a function of signal mass $ {m_{{\mathrm {e}} ^*}} $, together with the 68% (green, inner) and 95% (yellow, outer) quantiles of the expected limit, for $ {{\mathrm {e}}^*} $. |
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Figure 6-b:
Observed (solid) and expected (dashed) 95% CL lower limits on the compositeness scale as a function of signal mass $ {m_{{\mathrm {e}} ^*}} $, together with the 68% (green, inner) and 95% (yellow, outer) quantiles of the expected limit, for $ {{\mathrm {e}}^*} $. |
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Figure 6-c:
Observed (solid) and expected (dashed) 95% CL upper limits on the product of the production cross section and branching fraction as a function of signal mass $ {m_{\mu ^*}} $, together with the 68% (green, inner) and 95% (yellow, outer) quantiles of the expected limit, for ${{{\mu}}^*}$. |
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Figure 6-d:
Observed (solid) and expected (dashed) 95% CL lower limits on the compositeness scale as a function of signal mass $ {m_{\mu ^*}} $, together with the 68% (green, inner) and 95% (yellow, outer) quantiles of the expected limit, for ${{{\mu}}^*}$. |
| Tables | |
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Table 1:
Summary of the systematic uncertainties (in %) in the signal yield, the prompt photon background prediction, and the jet background prediction. |
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Table 2:
The observed yield and the SM prediction in the search window of the given ${m_{\ell ^*}}$ in the ${{\mathrm {e}} {\mathrm {e}} {\gamma}}$ channel. The symbols ${N_\text {data}}$, ${N_\text {prompt}}$, and ${N_\text {jet}}$ represent the number of events in data, the prompt photon background prediction, and the jet background estimate, respectively, together with statistical and systematical uncertainties. |
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Table 3:
The observed yield and the SM prediction in the search window of the given ${m_{\ell ^*}}$ in the ${{{\mu}} {{\mu}} {\gamma}}$ channel. The symbols ${N_\text {data}}$, ${N_\text {prompt}}$, and ${N_\text {jet}}$ represent the number of events in data, the prompt photon background prediction, and the jet background estimate, respectively, together with statistical and systematical uncertainties. |
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Table 4:
Summary of the observed (expected) lower limits on $ {m_{\ell ^*}} $, assuming ${\Lambda = {m_{\ell ^*}}}$, and the best observed (expected) lower limits on $\Lambda $ in the mass range 0.5-1.0 TeV. |
| Summary |
| A search has been presented for excited electrons and muons in $ \ell \ell \gamma $ final states at the LHC. The search is based on a data sample corresponding to an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the CMS detector in 2016. No significant excess over the standard model prediction is observed in the data, and 95% confidence level upper and lower limits are set on the signal production cross sections and the compositeness scale, respectively, as a function of the excited lepton mass. The observed limits on the signal cross section range from 3.7 to 0.2 fb as a function of $ m_{\ell*} $. Excited electrons and muons are excluded for masses below 3.9 and 3.8 TeV, respectively, under the assumption that the excited lepton mass equals the compositeness scale. The best observed limit on the compositeness scale is obtained with an excited lepton mass of around 1.0 TeV, excluding a compositeness scale below 25 TeV for both excited electrons and muons. These are the first results of a search at $\sqrt{s} = $ 13 TeV for excited leptons and also the most stringent limits on the excited lepton mass and the compositeness scale to date. |
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