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| CMS-PAS-FTR-18-028 | ||
| Prospects for exclusion or discovery of a third generation leptoquark decaying into a $\tau$ lepton and a b quark with the upgraded CMS detector at the HL-LHC | ||
| CMS Collaboration | ||
| December 2018 | ||
| Abstract: Projections for searches for production of third generation leptoquarks (LQs) in proton-proton collisions at a center-of-mass energy of 14 TeV are presented. The projections use simulated data samples corresponding to integrated luminosities of 300 and 3000 fb$^{-1}$. The analysis utilizes the DELPHES simulation package for the upgraded CMS detector at the High-Luminosity LHC, and considers both the single production channel, with a final state consisting of one b quark and two $\tau$ leptons, and the pair production channel, producing two b quarks and two $\tau$ leptons. In both cases, only $\tau$ leptons that decay hadronically are considered. Assuming a Yukawa coupling of one for the LQ-b-$\tau$ vertex, the expected 95% confidence level mass limit is 732 and 1130 GeV for integrated luminosities of 300 and 3000 fb$^{-1}$ for singly-produced LQs. The corresponding limits for pair production are 1249 (1518) GeV. Discovery sensitivity (5$\sigma$) is expected to be reached for masses below 800 (1000) GeV for the single production channel and 1200 (1500) GeV for the pair production channel in the 300 (3000) fb$^{-1}$ scenario. Limits are calculated both assuming negligible systematic uncertainties and utilizing ones extrapolated from searches at 13 TeV. | ||
| Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading order Feynman diagrams for the production of a third-generation LQ in the single production s-channel (left) and the pair production channel via gluon fusion (center) and quark fusion (right). |
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Figure 1-a:
Leading order Feynman diagrams for the production of a third-generation LQ in the single production s-channel (left) and the pair production channel via gluon fusion (center) and quark fusion (right). |
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Figure 1-b:
Leading order Feynman diagrams for the production of a third-generation LQ in the single production s-channel (left) and the pair production channel via gluon fusion (center) and quark fusion (right). |
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Figure 1-c:
Leading order Feynman diagrams for the production of a third-generation LQ in the single production s-channel (left) and the pair production channel via gluon fusion (center) and quark fusion (right). |
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Figure 2:
(left) Scalar sum of the $p_{\text {T}}$ of the two selected $\tau $ leptons and the highest-$p_{\text {T}}$ jet in the single LQ selected sample. (right) Scalar sum of the $p_{\text {T}}$ of the two selected $\tau $ leptons and the two highest-$p_{\text {T}}$ jets in the LQ pair selected sample. The considered backgrounds are shown as stacked histograms, while empty histograms for signals for the single LQ and LQ pair channels (for $m_{LQ}=$ 1000 GeV) are overlaid to illustrate the sensitivity. Both signal and background are normalized to a luminosity of 3000 fb$^{-1}$. |
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Figure 2-a:
Scalar sum of the $p_{\text {T}}$ of the two selected $\tau $ leptons and the highest-$p_{\text {T}}$ jet in the single LQ selected sample. The considered backgrounds are shown as stacked histograms, while empty histograms for signals for the single LQ channel (for $m_{LQ}=$ 1000 GeV) are overlaid to illustrate the sensitivity. Both signal and background are normalized to a luminosity of 3000 fb$^{-1}$. |
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Figure 2-b:
Scalar sum of the $p_{\text {T}}$ of the two selected $\tau $ leptons and the two highest-$p_{\text {T}}$ jets in the LQ pair selected sample. The considered backgrounds are shown as stacked histograms, while empty histograms for signals for the LQ pair channel (for $m_{LQ}=$ 1000 GeV) are overlaid to illustrate the sensitivity. Both signal and background are normalized to a luminosity of 3000 fb$^{-1}$. |
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Figure 3:
Expected limits at 95% CL on the product of the cross section $\sigma $ and the branching fraction $\beta $ for the single (left) and pair (right) LQ production channels. Note that, in the case of pair LQ production, the limit is calculated for $\sigma \times \beta ^2$. Limits are calculated as a function of the LQ mass, for the two high luminosity projections, 300 fb$^{-1}$ (red) and 3000 fb$^{-1}$ (orange), for both the stat. only (dashed lines) and the stat. +syst. scenarios (solid lines). This is shown in conjunction with the theoretical predictions at NLO [47], in cyan. |
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Figure 3-a:
Expected limits at 95% CL on the product of the cross section $\sigma $ and the branching fraction $\beta $ for the single LQ production channel. Limits are calculated as a function of the LQ mass, for the two high luminosity projections, 300 fb$^{-1}$ (red) and 3000 fb$^{-1}$ (orange), for both the stat. only (dashed lines) and the stat. +syst. scenarios (solid lines). This is shown in conjunction with the theoretical predictions at NLO [47], in cyan. |
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Figure 3-b:
Expected limits at 95% CL on the product of the cross section $\sigma $ and the branching fraction $\beta $ for the pair LQ production channel. Note that the limit is calculated for $\sigma \times \beta ^2$. Limits are calculated as a function of the LQ mass, for the two high luminosity projections, 300 fb$^{-1}$ (red) and 3000 fb$^{-1}$ (orange), for both the stat. only (dashed lines) and the stat. +syst. scenarios (solid lines). This is shown in conjunction with the theoretical predictions at NLO [47], in cyan. |
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Figure 4:
Expected exclusion limits at 95% CL on the Yukawa coupling $\lambda $ at the LQ-lepton-quark vertex, as a function of the LQ mass. A unit branching fraction $\beta $ of the LQ to a $\tau $ lepton and a bottom quark is assumed. Future projections for 300 and 3000 fb$^{-1}$ are shown in red and blue respectively, for both the stat. only and stat. + syst. scenarios, shown as dashed and filled lines respectively, and for both the single LQ and LQ pair production, where the latter corresponds to the vertical line (since it does not depend on $\lambda $). The left hand side of the lines represents the exclusion region for each of the projections, whereas the region with diagonal blue hatching shows the parameter space preferred by one of the models proposed to explain anomalies observed in $ {{\mathrm {B}}}$ physics [32]. |
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Figure 5:
Expected local significance of a signal-like excess as a function of the LQ mass, for the two high luminosity projections, 300 fb$^{-1}$ (red) and 3000 fb$^{-1}$ (orange), assuming the theoretical prediction for the LQ cross section at NLO [47], calculated with $\lambda = $ 1 and $\beta = $ 1. Projections are calculated for both single LQ (left) and LQ pair production (right). |
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Figure 5-a:
Expected local significance of a signal-like excess as a function of the LQ mass, for the two high luminosity projections, 300 fb$^{-1}$ (red) and 3000 fb$^{-1}$ (orange), assuming the theoretical prediction for the LQ cross section at NLO [47], calculated with $\lambda = $ 1 and $\beta = $ 1. Projections are calculated for single LQ production. |
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Figure 5-b:
Expected local significance of a signal-like excess as a function of the LQ mass, for the two high luminosity projections, 300 fb$^{-1}$ (red) and 3000 fb$^{-1}$ (orange), assuming the theoretical prediction for the LQ cross section at NLO [47], calculated with $\lambda = $ 1 and $\beta = $ 1. Projections are calculated for LQ pair production. |
| Tables | |
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Table 1:
Summary of the main systematic uncertainties, where $\sigma _{\text {bkg}}$ represents the uncertainty in the cross section of the background bkg. Uncertainty in b misidentification refers to the tagging of light jets as b jets. |
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Table 2:
Lower limits on the LQ mass for each considered production mechanism, uncertainty scenario, and integrated luminosity hypothesis considered in the analysis. |
| Summary |
|
Expected limits on the cross section for singly and pair produced third-generation scalar leptoquarks (LQ), each of which decays to a $\tau$ lepton and a bottom quark, have been presented as a function of the LQ mass. Projections have been made using DELPHES simulated samples at 14 TeV, for two luminosity scenarios at 300 fb$^{-1}$ and 3000 fb$^{-1}$. Comparing the limits with theoretical predictions assuming unit Yukawa coupling $\lambda = $ 1, third-generation scalar leptoquarks are expected to be excluded at 95% confidence level for LQ masses below 732 and 1130 GeV for the single LQ production channel for the 300 and 3000 fb$^{-1}$ scenarios, considering both statistical and systematic uncertainties. The corresponding limits for LQ pair production are 1249 GeV and 1518 GeV. Limits on $\lambda$ are also placed as a function of the leptoquark mass. For the 300 (3000) fb$^{-1}$ luminosity scenario, the leptoquark pair production channel is more sensitive if $\lambda < $ 2.7 (2.3), while the single leptoquark production dominant otherwise. These results show that future LQ searches under higher luminosity conditions are promising, as they are expected to greatly increase the reach of the search. They also show that the pair production channel is expected to be the most sensitive. A significance of 5$\sigma$ is projected for LQ masses below 800 (1000) GeV for the single production channels and 1200 (1500) GeV for the pair production channel in the 300 (3000) fb$^{-1}$ scenario. |
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