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| CMS-PAS-EXO-22-013 | ||
| Search for $ t $-channel scalar and vector leptoquark exchange in the high mass dimuon and dielectron spectrum in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 25 July 2024 | ||
| Abstract: A search for $ t $-channel exchange of leptoquarks (LQs) is performed using proton-proton collision data collected at $ \sqrt{s}= $ 13 TeV with the CMS detector at the CERN LHC. The data correspond to an integrated luminosity of 138 fb$ ^{-1} $. The search spans scenarios with scalar and vector LQs that couple up and down quarks to electrons and muons. Dielectron and dimuon final states are considered, with dilepton invariant masses above 500 GeV. The differential distributions of dilepton events are fit to templates built from reweighted samples of simulated standard model events. This method is able to probe higher LQ masses than previous pair-production and single-production searches. Limits are set on LQ-fermion coupling strengths for LQ masses up to 5 TeV. Based on the results, scalar LQs are excluded for masses up to 5 TeV for a coupling strength of 1.2, and vector LQs are excluded for masses up to 5 TeV for a coupling strength of 1.5. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading order diagrams for SM DY production (left) and $ t $-channel LQ exchange (right). The LQ amplitude interferes with the $ \mathrm{Z}/\gamma^{*} $ amplitude. |
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Figure 2:
A comparison of data and expected background distributions in dilepton invariant mass (upper row), $ c_\mathrm{R} $ (middle row) and dilepton rapidity (lower row). The left (right) plots show the $ \mu\mu $ ($ \mathrm{e}\mathrm{e} $) channel. The blue histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{S}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{S}_{\mu\mathrm{u}} $) with $ y_{\mathrm{e}\mathrm{u}}(y_{\mu\mathrm{u}})= $ 2.0, while the yellow histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{V}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{V}_{\mu\mathrm{u}} $) with $ g_{\mathrm{e}\mathrm{u}}(g_{\mu\mathrm{u}})= $ 1.0. The black points with error bars represent the data and their statistical uncertainties. The background expectation is shown as stacked histograms. The hatched band shows the total systematic uncertainty in the expected background yield. The lower panels show the ratio of the data to the expectation. The gray bands represents the normalized uncertainty in the predicted yield. The error bars in the ratio plot represent the normalized statistical uncertainty of the data. |
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Figure 2-a:
A comparison of data and expected background distributions in dilepton invariant mass (upper row), $ c_\mathrm{R} $ (middle row) and dilepton rapidity (lower row). The left (right) plots show the $ \mu\mu $ ($ \mathrm{e}\mathrm{e} $) channel. The blue histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{S}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{S}_{\mu\mathrm{u}} $) with $ y_{\mathrm{e}\mathrm{u}}(y_{\mu\mathrm{u}})= $ 2.0, while the yellow histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{V}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{V}_{\mu\mathrm{u}} $) with $ g_{\mathrm{e}\mathrm{u}}(g_{\mu\mathrm{u}})= $ 1.0. The black points with error bars represent the data and their statistical uncertainties. The background expectation is shown as stacked histograms. The hatched band shows the total systematic uncertainty in the expected background yield. The lower panels show the ratio of the data to the expectation. The gray bands represents the normalized uncertainty in the predicted yield. The error bars in the ratio plot represent the normalized statistical uncertainty of the data. |
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Figure 2-b:
A comparison of data and expected background distributions in dilepton invariant mass (upper row), $ c_\mathrm{R} $ (middle row) and dilepton rapidity (lower row). The left (right) plots show the $ \mu\mu $ ($ \mathrm{e}\mathrm{e} $) channel. The blue histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{S}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{S}_{\mu\mathrm{u}} $) with $ y_{\mathrm{e}\mathrm{u}}(y_{\mu\mathrm{u}})= $ 2.0, while the yellow histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{V}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{V}_{\mu\mathrm{u}} $) with $ g_{\mathrm{e}\mathrm{u}}(g_{\mu\mathrm{u}})= $ 1.0. The black points with error bars represent the data and their statistical uncertainties. The background expectation is shown as stacked histograms. The hatched band shows the total systematic uncertainty in the expected background yield. The lower panels show the ratio of the data to the expectation. The gray bands represents the normalized uncertainty in the predicted yield. The error bars in the ratio plot represent the normalized statistical uncertainty of the data. |
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Figure 2-c:
A comparison of data and expected background distributions in dilepton invariant mass (upper row), $ c_\mathrm{R} $ (middle row) and dilepton rapidity (lower row). The left (right) plots show the $ \mu\mu $ ($ \mathrm{e}\mathrm{e} $) channel. The blue histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{S}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{S}_{\mu\mathrm{u}} $) with $ y_{\mathrm{e}\mathrm{u}}(y_{\mu\mathrm{u}})= $ 2.0, while the yellow histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{V}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{V}_{\mu\mathrm{u}} $) with $ g_{\mathrm{e}\mathrm{u}}(g_{\mu\mathrm{u}})= $ 1.0. The black points with error bars represent the data and their statistical uncertainties. The background expectation is shown as stacked histograms. The hatched band shows the total systematic uncertainty in the expected background yield. The lower panels show the ratio of the data to the expectation. The gray bands represents the normalized uncertainty in the predicted yield. The error bars in the ratio plot represent the normalized statistical uncertainty of the data. |
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Figure 2-d:
A comparison of data and expected background distributions in dilepton invariant mass (upper row), $ c_\mathrm{R} $ (middle row) and dilepton rapidity (lower row). The left (right) plots show the $ \mu\mu $ ($ \mathrm{e}\mathrm{e} $) channel. The blue histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{S}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{S}_{\mu\mathrm{u}} $) with $ y_{\mathrm{e}\mathrm{u}}(y_{\mu\mathrm{u}})= $ 2.0, while the yellow histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{V}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{V}_{\mu\mathrm{u}} $) with $ g_{\mathrm{e}\mathrm{u}}(g_{\mu\mathrm{u}})= $ 1.0. The black points with error bars represent the data and their statistical uncertainties. The background expectation is shown as stacked histograms. The hatched band shows the total systematic uncertainty in the expected background yield. The lower panels show the ratio of the data to the expectation. The gray bands represents the normalized uncertainty in the predicted yield. The error bars in the ratio plot represent the normalized statistical uncertainty of the data. |
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Figure 2-e:
A comparison of data and expected background distributions in dilepton invariant mass (upper row), $ c_\mathrm{R} $ (middle row) and dilepton rapidity (lower row). The left (right) plots show the $ \mu\mu $ ($ \mathrm{e}\mathrm{e} $) channel. The blue histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{S}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{S}_{\mu\mathrm{u}} $) with $ y_{\mathrm{e}\mathrm{u}}(y_{\mu\mathrm{u}})= $ 2.0, while the yellow histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{V}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{V}_{\mu\mathrm{u}} $) with $ g_{\mathrm{e}\mathrm{u}}(g_{\mu\mathrm{u}})= $ 1.0. The black points with error bars represent the data and their statistical uncertainties. The background expectation is shown as stacked histograms. The hatched band shows the total systematic uncertainty in the expected background yield. The lower panels show the ratio of the data to the expectation. The gray bands represents the normalized uncertainty in the predicted yield. The error bars in the ratio plot represent the normalized statistical uncertainty of the data. |
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Figure 2-f:
A comparison of data and expected background distributions in dilepton invariant mass (upper row), $ c_\mathrm{R} $ (middle row) and dilepton rapidity (lower row). The left (right) plots show the $ \mu\mu $ ($ \mathrm{e}\mathrm{e} $) channel. The blue histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{S}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{S}_{\mu\mathrm{u}} $) with $ y_{\mathrm{e}\mathrm{u}}(y_{\mu\mathrm{u}})= $ 2.0, while the yellow histogram represents the signal yield of a hypothetical 2.5 TeV $ \mathrm{V}_{\mathrm{e}\mathrm{u}} $ ($ \mathrm{V}_{\mu\mathrm{u}} $) with $ g_{\mathrm{e}\mathrm{u}}(g_{\mu\mathrm{u}})= $ 1.0. The black points with error bars represent the data and their statistical uncertainties. The background expectation is shown as stacked histograms. The hatched band shows the total systematic uncertainty in the expected background yield. The lower panels show the ratio of the data to the expectation. The gray bands represents the normalized uncertainty in the predicted yield. The error bars in the ratio plot represent the normalized statistical uncertainty of the data. |
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Figure 3:
The observed data in the dielectron channel and the fitted signal-plus-background templates, shown for the $ \mathrm{S}_{\mathrm{e}\mathrm{u}} $ scenario with a candidate LQ mass of 2.5 TeV. The black points are the observed data, the stacked histograms represent the backgrounds, and the yellow histogram shows the fitted LQ signal multiplied by 10. Distributions of events are binned in reconstructed $ m_{\ell\ell} $, $ |y| $ and $ c_\mathrm{R} $. |
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Figure 4:
The observed data in the dielectron channel and the fitted signal-plus-background templates, shown for the $ \mathrm{S}_{\mathrm{e}\mathrm{d}} $ scenario with a candidate LQ mass of 2.5 TeV. The black points are the observed data, the stacked histograms represent the backgrounds, and the yellow histogram shows the fitted LQ signal multiplied by 10. Distributions of events are binned in reconstructed $ m_{\ell\ell} $, $ |y| $ and $ c_\mathrm{R} $. |
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Figure 5:
The observed data in the dimuon channel and the fitted signal-plus-background templates, shown for the $ \mathrm{S}_{\mu\mathrm{u}} $ scenario with a candidate LQ mass of 2.5 TeV. The black points are the observed data, the stacked histograms represent the backgrounds, and the yellow histogram shows the fitted LQ signal multiplied by 10. Distributions of events are binned in reconstructed $ m_{\ell\ell} $, $ |y| $ and $ c_\mathrm{R} $. |
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Figure 6:
The observed data in the dimuon channel and the fitted signal-plus-background templates, shown for the $ \mathrm{S}_{\mu\mathrm{d}} $ scenario with a candidate LQ mass of 2.5 TeV. The black points are the observed data, the stacked histograms represent the backgrounds, and the yellow histogram shows the fitted LQ signal multiplied by 10. Distributions of events are binned in reconstructed $ m_{\ell\ell} $, $ |y| $ and $ c_\mathrm{R} $. |
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Figure 7:
The observed data in the dielectron channel and the fitted signal-plus-background templates, shown for the $ \mathrm{V}_{\mathrm{e}\mathrm{u}} $ scenario with a candidate LQ mass of 2.5 TeV. The black points are the observed data, the stacked histograms represent the backgrounds, and the yellow histogram shows the fitted LQ signal multiplied by 10. Distributions of events are binned in reconstructed $ m_{\ell\ell} $, $ |y| $ and $ c_\mathrm{R} $. |
png pdf |
Figure 8:
The observed data in the dielectron channel and the fitted signal-plus-background templates, shown for the $ \mathrm{V}_{\mathrm{e}\mathrm{d}} $ scenario with a candidate LQ mass of 2.5 TeV. The black points are the observed data, the stacked histograms represent the backgrounds, and the yellow histogram shows the fitted LQ signal multiplied by 10. Distributions of events are binned in reconstructed $ m_{\ell\ell} $, $ |y| $ and $ c_\mathrm{R} $. |
png pdf |
Figure 9:
The observed data in the dimuon channel and the fitted signal-plus-background templates, shown for the $ \mathrm{V}_{\mu\mathrm{u}} $ scenario with a candidate LQ mass of 2.5 TeV. The black points are the observed data, the stacked histograms represent the backgrounds, and the yellow histogram shows the fitted LQ signal multiplied by 10. Distributions of events are binned in reconstructed $ m_{\ell\ell} $, $ |y| $ and $ c_\mathrm{R} $. |
png pdf |
Figure 10:
The observed data in the dimuon channel and the fitted signal-plus-background templates, shown for the $ \mathrm{V}_{\mu\mathrm{d}} $ scenario with a candidate LQ mass of 2.5 TeV. The black points are the observed data, the stacked histograms represent the backgrounds, and the yellow histogram shows the fitted LQ signal multiplied by 10. Distributions of events are binned in reconstructed $ m_{\ell\ell} $, $ |y| $ and $ c_\mathrm{R} $. |
png pdf |
Figure 11:
Upper limits at 95% CL on the LQ-fermion couplings, $ y_{\mathrm{e}\mathrm{u}} $ (left) and $ y_{\mathrm{e}\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for scalar LQs coupling to electrons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
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Figure 11-a:
Upper limits at 95% CL on the LQ-fermion couplings, $ y_{\mathrm{e}\mathrm{u}} $ (left) and $ y_{\mathrm{e}\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for scalar LQs coupling to electrons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
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Figure 11-b:
Upper limits at 95% CL on the LQ-fermion couplings, $ y_{\mathrm{e}\mathrm{u}} $ (left) and $ y_{\mathrm{e}\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for scalar LQs coupling to electrons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
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Figure 12:
Upper limits at 95% CL on the LQ-fermion couplings, $ y_{\mu\mathrm{u}} $ (left) and $ y_{\mu\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for scalar LQs coupling to muons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
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Figure 12-a:
Upper limits at 95% CL on the LQ-fermion couplings, $ y_{\mu\mathrm{u}} $ (left) and $ y_{\mu\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for scalar LQs coupling to muons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
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Figure 12-b:
Upper limits at 95% CL on the LQ-fermion couplings, $ y_{\mu\mathrm{u}} $ (left) and $ y_{\mu\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for scalar LQs coupling to muons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
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Figure 13:
Upper limits at 95% CL on the LQ-fermion couplings, $ g_{\mathrm{e}\mathrm{u}} $ (left) and $ g_{\mathrm{e}\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for vector LQs coupling to electrons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
png pdf |
Figure 13-a:
Upper limits at 95% CL on the LQ-fermion couplings, $ g_{\mathrm{e}\mathrm{u}} $ (left) and $ g_{\mathrm{e}\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for vector LQs coupling to electrons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
png pdf |
Figure 13-b:
Upper limits at 95% CL on the LQ-fermion couplings, $ g_{\mathrm{e}\mathrm{u}} $ (left) and $ g_{\mathrm{e}\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for vector LQs coupling to electrons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
png pdf |
Figure 14:
Upper limits at 95% CL on the LQ-fermion couplings, $ g_{\mu\mathrm{u}} $ (left) and $ g_{\mu\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for vector LQs coupling to muons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
png pdf |
Figure 14-a:
Upper limits at 95% CL on the LQ-fermion couplings, $ g_{\mu\mathrm{u}} $ (left) and $ g_{\mu\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for vector LQs coupling to muons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
png pdf |
Figure 14-b:
Upper limits at 95% CL on the LQ-fermion couplings, $ g_{\mu\mathrm{u}} $ (left) and $ g_{\mu\mathrm{d}} $ (right), versus $ m_{\rm LQ} $ for vector LQs coupling to muons. The black points show the observed limits, the red line shows the expected limits, and the yellow and blue bands show the variations on the expected limit at the 1 and 2 standard deviation levels. |
| Tables | |
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Table 1:
Properties of the $ R_2 $, $ \widetilde{R}_2 $, and $ U_3 $ LQs. Specifically, this analysis searches for $ R_2 $ LQs with RL couplings and charge 5/3, $ \widetilde{R}_2 $ LQs with RL couplings and charge 2/3, and $ U_3 $ LQs with charges 2/3 and 5/3. |
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Table 2:
The contribution of statistical uncertainty and individual sources of systematic uncertainty to the total variance on the fitted value of $ y_{\rm LQ}^2 $ for a scalar LQ mass of 2.5 TeV. For a given source of uncertainty, the impact is determined by fixing its associated nuisance parameter to the nominal expectation and evaluating the change in the total uncertainty. |
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Table 3:
The contribution of statistical uncertainty and individual sources of systematic uncertainty to the total variance on the fitted value of $ g_{\rm LQ}^2 $ for a vector LQ mass of 2.5 TeV. For a given source of uncertainty, the impact is determined by fixing its associated nuisance parameter to the nominal expectation and evaluating the change in the total uncertainty. |
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Table 4:
Best fit values of $ A_0 $, $ A_4 $ and $ y_{\rm LQ}^2 $ for scalar LQ models. Results are shown for a candidate $ m_{\rm LQ} $ of 2.5 TeV. |
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Table 5:
Best fit values of $ A_0 $, $ A_4 $ and $ g_{\rm LQ}^2 $ for vector LQ models. Results are shown for a candidate $ m_{\rm LQ} $ of 2.5 TeV. |
| Summary |
| A search for the $ t $-channel exchange of leptoquarks (LQs) coupling to first- and second-generation fermions has been presented. The search uses proton-proton collision data at $ \sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Both scalar and vector LQ scenarios are considered, with masses between 1-5 TeV for exclusive LQ couplings to an up or a down quark and an electron or a muon. The $ t $-channel exchange of a LQ modifies the dilepton angular distributions at masses well below the resonance mass of the LQ. A template fit to the angular and invariant mass distributions of high-mass dilepton events is used to distinguish the signal process from the dominant Drell-Yan background, incorporating interference effects. No evidence for such LQs is observed, and limits are set at 95% confidence level on the LQ-fermion couplings. Coupling values greater than 0.4-1.1 (0.3-1.0) are excluded for scalar LQs with masses from 1-5 TeV coupling to an electron (muon) and a first-generation quark. Similarly, for vector LQs, coupling values greater than 0.2-0.5 (0.1-0.4) are excluded. This search is sensitive to LQs with significantly higher masses than prior single- and pair-production searches, establishing the best limits on LQs with masses up to 5 TeV, and is also the first to set limits on vector LQs coupling to first- and second-generation fermions. |
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