CMSEXO17003 ; CERNEP2018218  
Search for pair production of secondgeneration leptoquarks at $\sqrt{s} = $ 13 TeV  
CMS Collaboration  
15 August 2018  
Phys. Rev. D 99 (2019) 032014  
Abstract: A search for pair production of secondgeneration leptoquarks is performed using protonproton collision data collected at $\sqrt{s} = $ 13 TeV in 2016 with the CMS detector at the CERN LHC, corresponding to an integrated luminosity of 35.9 fb$^{1}$. Final states with two muons and two jets, or with one muon, two jets, and missing transverse momentum are considered. Secondgeneration scalar leptoquarks with masses less than 1530 (1285) GeV are excluded for $\beta = $ 1.0 (0.5), where $\beta$ is the branching fraction for the decay of a leptoquark to a charged lepton and a quark. The results of the search are also interpreted as limits on the pair production of longlived top squarks in an $R$parity violating supersymmetry model that has a final state with two muons and two jets. These limits represent the most stringent limits to date on these models.  
Links: eprint arXiv:1808.05082 [hepex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; 
Figures  
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Figure 1:
Dominant leadingorder Feynman diagrams for the pair production of LQs at the LHC. 
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Figure 1a:
A leadingorder Feynman diagram for the pair production of LQs at the LHC. 
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Figure 1b:
A leadingorder Feynman diagram for the pair production of LQs at the LHC. 
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Figure 1c:
A leadingorder Feynman diagram for the pair production of LQs at the LHC. 
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Figure 1d:
A leadingorder Feynman diagram for the pair production of LQs at the LHC. 
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Figure 2:
Comparison of data and background at the preselection level for the ${{{\mu} {\mu} \mathrm {jj}}}$ channel, for the variables used for final the selection optimization: ${{m_{{\mu} {\mu}}}}$ (upper), ${{m_{{\mu} \mathrm {j}}^{\mathrm {min}}}}$ (lower left), and ${{S_{\mathrm {T}}^{{\mu} {\mu} \mathrm {jj}}}}$ (lower right). `Other background' includes W+jets, single top quark, and diboson backgrounds. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 2a:
Comparison of data and background for the ${{m_{{\mu} {\mu}}}}$ variable, at the preselection level for the ${{{\mu} {\mu} \mathrm {jj}}}$ channel. `Other background' includes W+jets, single top quark, and diboson backgrounds. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 2b:
Comparison of data and background for the ${{m_{{\mu} \mathrm {j}}^{\mathrm {min}}}}$ variable, at the preselection level for the ${{{\mu} {\mu} \mathrm {jj}}}$ channel. `Other background' includes W+jets, single top quark, and diboson backgrounds. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 2c:
Comparison of data and background for the ${{S_{\mathrm {T}}^{{\mu} {\mu} \mathrm {jj}}}}$ variable, at the preselection level for the ${{{\mu} {\mu} \mathrm {jj}}}$ channel. `Other background' includes W+jets, single top quark, and diboson backgrounds. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 3:
Comparison of data and background at the preselection level for the ${{{\mu} {\nu}\mathrm {jj}}}$ channel, for the variables used for final selection criteria optimization: ${{m_{\mathrm {T}}^{{\mu} {\nu}}}}$ (upper), ${{m_{{\mu} \mathrm {j}}}}$ (lower left), and ${{S_{\mathrm {T}}^{{\mu} {\nu}\mathrm {jj}}}}$ (lower right). `Other background' includes $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 3a:
Comparison of data and background for the ${{m_{\mathrm {T}}^{{\mu} {\nu}}}}$ variable, at the preselection level for the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. `Other background' includes $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 3b:
Comparison of data and background for the ${{m_{{\mu} \mathrm {j}}}}$ variable, at the preselection level for the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. `Other background' includes $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 3c:
Comparison of data and background for the ${{S_{\mathrm {T}}^{{\mu} {\nu}\mathrm {jj}}}}$ variable, at the preselection level for the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. `Other background' includes $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 4:
Lower bounds of the final selection criteria for the three variables for the ${{{\mu} {\mu} \mathrm {jj}}}$ (left) and ${{{\mu} {\nu}\mathrm {jj}}}$ (right) channels as a function of scalar ${{m_{\mathrm {LQ}}}}$. 
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Figure 4a:
Lower bounds of the final selection criteria for the three variables for the ${{{\mu} {\mu} \mathrm {jj}}}$ channel as a function of scalar ${{m_{\mathrm {LQ}}}}$. 
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Figure 4b:
Lower bounds of the final selection criteria for the three variables for the ${{{\mu} {\nu}\mathrm {jj}}}$ channel as a function of scalar ${{m_{\mathrm {LQ}}}}$. 
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Figure 5:
Comparison of data and background distributions of ${{S_{\mathrm {T}}^{{\mu} {\mu} \mathrm {jj}}}}$ (left) and ${{m_{{\mu} \mathrm {j}}^{\mathrm {min}}}}$ (upper right) and ${{m_{{\mu} \mathrm {j}}}}$ (lower right), for the ${{{\mu} {\mu} \mathrm {jj}}}$ channel (upper plots) and the ${{{\mu} {\nu}\mathrm {jj}}}$ channel (lower plots). Events after final selections with $ {m_{\mathrm {LQ}}} = $ 1400 GeV are shown in the upper plots, and with $ {m_{\mathrm {LQ}}} = $ 1100 GeV in the lower plots. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. `Other background' includes W+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\mu} \mathrm {jj}}}$ channel, and $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. 
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Figure 5a:
Comparison of data and background distribution of ${{S_{\mathrm {T}}^{{\mu} {\mu} \mathrm {jj}}}}$. Events after final selections with $ {m_{\mathrm {LQ}}} = $ 1400 GeV are shown. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. `Other background' includes W+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\mu} \mathrm {jj}}}$ channel, and $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. 
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Figure 5b:
Comparison of data and background distribution of ${{m_{{\mu} \mathrm {j}}^{\mathrm {min}}}}$ for the ${{{\mu} {\mu} \mathrm {jj}}}$ channel. Events after final selections with $ {m_{\mathrm {LQ}}} = $ 1400 GeV are shown. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. `Other background' includes W+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\mu} \mathrm {jj}}}$ channel, and $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. 
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Figure 5c:
Comparison of data and background distribution of ${{S_{\mathrm {T}}^{{\nu} {\mu} \mathrm {jj}}}}$. Events after final selections with $ {m_{\mathrm {LQ}}} = $ 1100 GeV are shown. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. `Other background' includes W+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\mu} \mathrm {jj}}}$ channel, and $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. 
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Figure 5d:
Comparison of data and background distribution of ${{m_{{\mu} \mathrm {j}}}}$ for the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. Events after final selections with $ {m_{\mathrm {LQ}}} = $ 1100 GeV are shown. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. `Other background' includes W+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\mu} \mathrm {jj}}}$ channel, and $ {\mathrm {Z}}/\gamma ^*$+jets, single top quark, and diboson backgrounds in the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. 
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Figure 6:
Data and background event yields after final selections for the ${{{\mu} {\mu} \mathrm {jj}}}$ analysis, as a function of scalar ${m_{\mathrm {LQ}}}$. `Other background' includes W+jets and single top quark. The selection criteria for each bin are detailed in Table 1. All the bins are correlated, as the events selected for each ${{m_{\mathrm {LQ}}}}$ are a strict subset of the events selected for the lower mass LQ. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 7:
Data and background event yields after final selections for the ${{{\mu} {\nu}\mathrm {jj}}}$ analysis, as a function of ${m_{\mathrm {LQ}}}$. `Other background' includes $ {\mathrm {Z}}/\gamma ^*$+jets and single top quark. The selection criteria for each bin are detailed in Table 2. All the bins are correlated, as the events selected for each ${m_{\mathrm {LQ}}}$ are a strict subset of the events selected for the lower mass LQ. The hashed band represents the combined statistical and systematic uncertainty in the full background estimate. 
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Figure 8:
The expected and observed upper limits at 95% CL on the product of the scalar LQ pair production cross section and the branching fractions $\beta ^2$ or $2\beta (1\beta)$ as a function of the secondgeneration ${m_{\mathrm {LQ}}}$ obtained with the ${{{\mu} {\mu} \mathrm {jj}}}$ (left) and ${{{\mu} {\nu}\mathrm {jj}}}$ (right) analysis. The solid lines represent the observed limits, the dashed lines represent the median expected limits, and the inner darkgreen and outer lightyellow bands represent the 68% and 95% confidence intervals. The $\sigma _{\text {theory}}$ curves and their blue bands represent the theoretical scalar LQ pair production cross sections and the uncertainties on the cross sections due to the choice of PDF and renormalization and factorization scales, respectively. 
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Figure 8a:
The expected and observed upper limits at 95% CL on the product of the scalar LQ pair production cross section and the branching fractions $\beta ^2$ or $2\beta (1\beta)$ as a function of the secondgeneration ${m_{\mathrm {LQ}}}$ obtained with the ${{{\mu} {\mu} \mathrm {jj}}}$ analysis. The solid line represents the observed limit, the dashed line represent the median expected limit, and the inner darkgreen and outer lightyellow bands represent the 68% and 95% confidence intervals. The $\sigma _{\text {theory}}$ curve and its blue bands represent the theoretical scalar LQ pair production cross sections and the uncertainties on the cross sections due to the choice of PDF and renormalization and factorization scales, respectively. 
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Figure 8b:
The expected and observed upper limits at 95% CL on the product of the scalar LQ pair production cross section and the branching fractions $\beta ^2$ or $2\beta (1\beta)$ as a function of the secondgeneration ${m_{\mathrm {LQ}}}$ obtained with the ${{{\mu} {\nu}\mathrm {jj}}}$ analysis. The solid line represents the observed limit, the dashed line represent the median expected limit, and the inner darkgreen and outer lightyellow bands represent the 68% and 95% confidence intervals. The $\sigma _{\text {theory}}$ curve and its blue bands represent the theoretical scalar LQ pair production cross sections and the uncertainties on the cross sections due to the choice of PDF and renormalization and factorization scales, respectively. 
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Figure 9:
The expected and observed exclusion limits at 95% CL for secondgeneration ${m_{\mathrm {LQ}}}$ as a function of the branching fraction $\beta $ vs. ${m_{\mathrm {LQ}}}$. The inner darkgreen and outer lightyellow expected limit uncertainty bands represent the 68% and 95% confidence intervals on the combination. Limits for the individual ${{{\mu} {\mu} \mathrm {jj}}}$ and ${{{\mu} {\nu}\mathrm {jj}}}$ channels are also drawn. The solid lines represent the observed limits in each channel, and the dashed lines represent the expected limits. 
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Figure 10:
Expected and observed upper limits at 95% CL on the longlived RPV SUSY $\tilde{\mathrm{t}}$ pair production cross section as a function of $\tilde{\mathrm{t}}$ mass ($x$ axis) and lifetime ($y$ axis). The dashed line and the inner darkgreen and outer lightyellow uncertainty bands represent the median expected limits, and the 68% and 95% confidence intervals, respectively. Extrapolation has been performed to produce a limit plot extending down to the prompt kinematic range. 
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Figure 11:
The product of signal acceptance and efficiency for optimized final selections as a function of ${m_{\mathrm {LQ}}}$ in the ${{{\mu} {\mu} \mathrm {jj}}}$ (left) and ${{{\mu} {\nu}\mathrm {jj}}}$ (right) channels. 
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Figure 11a:
The product of signal acceptance and efficiency for optimized final selections as a function of ${m_{\mathrm {LQ}}}$ in the ${{{\mu} {\mu} \mathrm {jj}}}$ channel. 
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Figure 11b:
The product of signal acceptance and efficiency for optimized final selections as a function of ${m_{\mathrm {LQ}}}$ in the ${{{\mu} {\nu}\mathrm {jj}}}$ channel. 
Tables  
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Table 1:
Range of systematic uncertainties in the signal acceptance and background yields for the ${{{\mu} {\mu} \mathrm {jj}}}$ analysis. The last two lines show the total systematic uncertainty and the total statistical uncertainty in the simulated samples, respectively. 
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Table 2:
Range of systematic uncertainties in the signal acceptance and background yields for the ${{{\mu} {\nu}\mathrm {jj}}}$ analysis. The last two lines show the total systematic uncertainty and the total statistical uncertainty in the simulated samples, respectively. 
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Table 3:
Event yields after final selections for the ${{{\mu} {\mu} \mathrm {jj}}}$ analysis. `Other bkg.' includes W+jets and single top quark. Uncertainties are statistical unless otherwise indicated. 
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Table 4:
Event yields after final selections for the ${{{\mu} {\nu}\mathrm {jj}}}$ analysis. `Other bkg.' includes $ {\mathrm {Z}}/\gamma ^*$+jets and single top quark. Uncertainties are statistical unless otherwise indicated. 
Summary 
A search has been presented for pair production of secondgeneration leptoquarks using protonproton collision data collected at $\sqrt{s} = $ 13 TeV in 2016 with the CMS detector at the LHC, corresponding to an integrated luminosity of 35.9 fb$^{1}$. Limits are set at 95% confidence level on the product of the scalar leptoquark pair production cross section and $\beta^2$ ($2\beta(1\beta)$) in the $\mu\mu\mathrm{jj}$ ($\mu\nu\mathrm{jj}$) channels, for the branching fraction $\beta = $ 1.0 (0.5) as a function of the leptoquark mass ${m_{\mathrm{LQ}}}$. Secondgeneration leptoquarks with masses less than 1530 (1285) GeV are excluded for $\beta= $ 1.0 (0.5), an improvement of 370 (525) GeV compared to previously published results. Twodimensional limits are set in the $\beta$${{m_{\mathrm{LQ}}}}$ plane. The results in the $\mu\mu\mathrm{jj}$ search are interpreted in the context of an $R$parity violating supersymmetry model with longlived top squarks. These limits represent the most stringent limits to date on these models. 
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