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| CMS-PAS-EXO-19-015 | ||
| Search for singly and pair-produced leptoquarks coupling to third-generation fermions in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| July 2020 | ||
| Abstract: A search for leptoquarks produced singly and in pairs in proton-proton collisions is presented. The leptoquark (LQ) may couple to a top quark plus a $\tau$ lepton (t$\tau$) or a bottom quark plus a neutrino (b$\nu$, scalar LQ), or else to t$\nu$ or b$\tau$ (vector LQ), leading to the final states t$\tau\nu$b and t$\tau\nu$. The data were recorded by the CMS experiment at the CERN LHC at $\sqrt{s} = $ 13 TeV and correspond to an integrated luminosity of 137 fb$^{-1}$. The data are found to be in agreement with the standard model background prediction. Lower limits at 95% CL are set on the LQ mass in the range 0.98-1.73 TeV, depending on the LQ spin and the LQ-lepton-quark vertex coupling $\lambda$, and assuming equal branching fractions for the two LQ decay modes considered. These are the most stringent constraints on the existence of leptoquarks in this scenario. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Accepted by PLB. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures & Tables | References | CMS Publications |
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| Figures | |
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Figure 1:
Main Feynman diagrams for LQ production: pairwise (left), and in combination with a lepton (right). |
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Figure 2:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection. Upper-left: boosted top (hadronically decaying top quark reconstructed in the fully or partially merged topology) and exactly one b jet; lower-left: boosted top and at least two b jets; upper-right: resolved top (hadronically decaying top quark reconstructed in the resolved topology) and exactly one b jet; lower-right: resolved top and at least two b jets. |
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Figure 2-a:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection, boosted top (hadronically decaying top quark reconstructed in the fully or partially merged topology) and exactly one b jet. |
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Figure 2-b:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection, resolved top (hadronically decaying top quark reconstructed in the resolved topology) and exactly one b jet. |
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Figure 2-c:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection, boosted top (hadronically decaying top quark reconstructed in the fully or partially merged topology) and at least two b jets. |
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Figure 2-d:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection, resolved top (hadronically decaying top quark reconstructed in the resolved topology) and at least two b jets. |
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Figure 3:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$ (upper), $\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 1.5 (center-left) and 2.5 (lower-left), and $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$+$\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 1.5 (center-right) and 2.5 (lower-right), as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pair-produced LQ$_{S}$, for which an NLO calculation [44] is shown. |
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Figure 3-a:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pair-produced LQ$_{S}$, for which an NLO calculation [44] is shown. |
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Figure 3-b:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pair-produced LQ$_{S}$, for which an NLO calculation [44] is shown. |
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Figure 3-c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pair-produced LQ$_{S}$, for which an NLO calculation [44] is shown. |
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Figure 3-d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$+$\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pair-produced LQ$_{S}$, for which an NLO calculation [44] is shown. |
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Figure 3-e:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$+$\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pair-produced LQ$_{S}$, for which an NLO calculation [44] is shown. |
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Figure 4:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$ (upper), $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5 (center-left) and 2.5 (lower-left), and $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5 (center-right) and 2.5 (lower-right), as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 4-a:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 4-b:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 4-c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 4-d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 4-e:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 5:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$ (upper), $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5 (center-left) and 2.5 (lower-left), and $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5 (center-right) and 2.5 (lower-right), as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 5-a:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 5-b:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 5-c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 5-d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 5-e:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. |
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Figure 6:
The observed and expected (solid and dotted lines) 95% CL LQ exclusion limits in the plane of the LQ-lepton-quark vertex coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The upper plot pertains to an LQ$_{S}$ with equal couplings to t$\tau $, b$\nu $, while the lower plots are for an LQ$_{V}$ assuming $k = $ 0 (left) and 1 (right) and equal couplings to t$\nu $, b$\tau $. For LQ$_{V}$, the gray area shows the band preferred 95% CL) by B-physics anomalies: $\lambda = \sqrt {0.7 \pm 0.2 } \times m_{\rm {LQ}}$ TeV [42]. |
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Figure 6-a:
The observed and expected (solid and dotted lines) 95% CL LQ exclusion limits in the plane of the LQ-lepton-quark vertex coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The plot pertains to an LQ$_{S}$ with equal couplings to t$\tau $, b$\nu $. |
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Figure 6-b:
The observed and expected (solid and dotted lines) 95% CL LQ exclusion limits in the plane of the LQ-lepton-quark vertex coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The plot pertains to an LQ$_{V}$ assuming $k = $ 0 and equal couplings to t$\nu $, b$\tau $. The gray area shows the band preferred 95% CL) by B-physics anomalies: $\lambda = \sqrt {0.7 \pm 0.2 } \times m_{\rm {LQ}}$ TeV [42]. |
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Figure 6-c:
The observed and expected (solid and dotted lines) 95% CL LQ exclusion limits in the plane of the LQ-lepton-quark vertex coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The plot pertains to an LQ$_{V}$ assuming $k = $ 1 and equal couplings to t$\nu $, b$\tau $. The gray area shows the band preferred 95% CL) by B-physics anomalies: $\lambda = \sqrt {0.7 \pm 0.2 } \times m_{\rm {LQ}}$ TeV [42]. |
| Tables | |
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Table 1:
Yields from the SM background estimation, expected signal, and data, for the selected events. |
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Table 2:
Lower limits on the mass in TeV of the leptoquarks LQ$_{S}$, LQ$_{V}$ $k = $ 0, and LQ$_{V}$ $k = $ 1, based on the pair- and single-production mechanisms taken separately and together. The searches that depend on the $\lambda $ parameter are given for values of 1.5 and 2.5. The expected limits are given in parentheses. |
| Summary |
| A search for leptoquarks, coupling to third-generation fermions, and produced in pairs and singly in association with a lepton, has been presented. The leptoquark LQ may couple to a top quark plus a $\tau$ lepton ($\mathrm{t}\tau$) or a bottom quark plus a neutrino ($\mathrm{b}\nu$, scalar LQ) or else to $\mathrm{t}\nu$ and $\mathrm{b}\tau$ (vector LQ), resulting in the final states $\mathrm{t}\tau\nu\mathrm{b}$ and $\mathrm{t}\tau\nu$. The channel in which both the top quark and the $\tau$ lepton decay hadronically is investigated, including the case of a large LQ-$\mathrm{t}$ mass splitting giving rise to a boosted top quark whose decay daughters may not be separated on the scale of the spatial resolution of the jet. Such a signature has not been previously examined in searches for physics beyond the standard model. The data used corresponds to an integrated luminosity of 137 fb$^{-1}$ collected with the CMS detector at the LHC in pp collisions at $\sqrt{s} = $ 13 TeV. The observations are found to be in agreement with the standard model prediction. Exclusion limits are given in the plane of the LQ-lepton-quark vertex coupling $\lambda$ and the LQ mass for scalar and vector leptoquarks. The range of lower limits on the LQ mass, at 95% CL, is 0.98-1.73 TeV, depending on $\lambda$ and the leptoquark spin. These results represent the most stringent limits to date on the presence of such leptoquarks for the case of a decay branching fraction of 0.5 to each lepton-quark pair. These results also probe the parameter space preferred by the B-physics anomalies in several models ([41], [42]), and exclude relevant portions. |
| Additional Figures | |
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Additional Figure 1:
Pair leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search. The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton, having the leptoquark decay a branching fraction of 0.5 to each fermion pair. |
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Additional Figure 2:
Single leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search, for scalar leptoquark (LQ$_{S}$, left), vector leptoquark (LQ$_{V}$) with k = 1 (center), and LQ$_{V}$ with k = 0 (right). The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. |
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Additional Figure 2-a:
Single leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search, for scalar leptoquark (LQ$_{S}$). The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. |
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Additional Figure 2-b:
Single leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search, for vector leptoquark (LQ$_{V}$) with k = 1. The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. |
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Additional Figure 2-c:
Single leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search, for vector leptoquark (LQ$_{V}$) with k = 0. The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. |
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Additional Figure 3:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 0.8 TeV, for scalar leptoquark (left), vector leptoquark with k = 1 (center), and vector leptoquark with k = 0 (right). |
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Additional Figure 3-a:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 0.8 TeV, for scalar leptoquark. |
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Additional Figure 3-b:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 0.8 TeV, for vector leptoquark with k = 1. |
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Additional Figure 3-c:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 0.8 TeV, for vector leptoquark with k = 0. |
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Additional Figure 4:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 1.7 TeV, for scalar leptoquark (left), vector leptoquark with k = 1 (center), and vector leptoquark with k = 0 (right). |
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Additional Figure 4-a:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 1.7 TeV, for scalar leptoquark. |
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Additional Figure 4-b:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 1.7 TeV, for vector leptoquark with k = 1. |
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Additional Figure 4-c:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 1.7 TeV, for vector leptoquark with k = 0. |
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Additional Figure 5:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 0.8 TeV, for scalar leptoquark (left), vector leptoquark with k = 1 (center), and vector leptoquark with k = 0 (right). The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. |
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Additional Figure 5-a:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 0.8 TeV, for scalar leptoquark. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. |
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Additional Figure 5-b:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 0.8 TeV, for vector leptoquark with k = 1. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. |
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Additional Figure 5-c:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 0.8 TeV, for vector leptoquark with k = 0. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. |
png pdf |
Additional Figure 6:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 1.7 TeV, for scalar leptoquark (left), vector leptoquark with k = 1 (center), and vector leptoquark with k = 0 (right). The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. |
png pdf |
Additional Figure 6-a:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 1.7 TeV, for scalar leptoquark. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. |
png pdf |
Additional Figure 6-b:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 1.7 TeV, for vector leptoquark with k = 1. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. |
png pdf |
Additional Figure 6-c:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 1.7 TeV, for vector leptoquark with k = 0. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. |
| Additional Tables | |
png pdf |
Additional Table 1:
Pair leptoquark signal selection efficiency relative to each requirement sought in the analysis, including all the events that enter the top and b jet categories defined in the search. The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton, having the leptoquark decay a branching fraction of 0.5 to each fermion pair. |
png pdf |
Additional Table 2:
Single leptoquark signal selection efficiency relative to each requirement sought in the analysis, including all the events that enter the top and b jet categories defined in the search, for a leptoquark mass of 0.8 TeV and different hypotheses: scalar leptoquark (LQ$_{S}$, upper), vector leptoquark (LQ$_{V}$) with k = 1 (center), and LQ$_{V}$ with k = 0 (lower). The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. |
png pdf |
Additional Table 3:
Single leptoquark signal selection efficiency relative to each requirement sought in the analysis, including all the events that enter the top and b jet categories defined in the search, for a leptoquark mass of 1.7 TeV and different hypotheses: scalar leptoquark (LQ$_{S}$, upper), vector leptoquark (LQ$_{V}$) with k = 1 (center), and LQ$_{V}$ with k = 0 (lower). The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. |
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