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| CMS-EXO-19-015 ; CERN-EP-2020-216 | ||
| Search for singly and pair-produced leptoquarks coupling to third-generation fermions in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 7 December 2020 | ||
| Phys. Lett. B 819 (2021) 136446 | ||
| Abstract: A search for leptoquarks produced singly and in pairs in proton-proton collisions is presented. The leptoquark (LQ) may be a scalar particle of charge -1/3e coupling to a top quark plus a tau lepton ($\mathrm{t}\tau$) or a bottom quark plus a neutrino (b$\nu$), or a vector particle of charge +2/3e, coupling to t$\nu$ or b$\tau$. In this analysis the signatures t$\tau\nu$b and t$\tau\nu$ are probed, using data recorded by the CMS experiment at the CERN LHC at $\sqrt{s} = $ 13 TeV and that correspond to an integrated luminosity of 137 fb$^{-1}$. These signatures have not been previously explored in a dedicated search. The data are found to be in agreement with the standard model prediction. Lower limits at 95% confidence level are set on the LQ mass in the range 0.98-1.73 TeV, depending on the LQ spin and its coupling $\lambda$ to a lepton and a quark, and assuming equal branching fractions for the two LQ decay modes considered. These are the most stringent constraints to date on the existence of leptoquarks in this scenario. | ||
| Links: e-print arXiv:2012.04178 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Main Feynman diagrams for leptoquark production and decay: pairwise (left), and in combination with a lepton (right). The leptoquark LQ$_{\text {S}}$ may couple to t$ \tau $ or b$ \nu $, while LQ$_{\text {V}}$ may couple to t$ \nu $ or b$ \tau $. |
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Figure 2:
Distribution of the variable ${S_{\mathrm {T}}}$ for events passing the signal selection for the SM background estimation (stacked filled histograms), data (black points), and different hypotheses of LQ signals (lines). Upper left: boosted top quark candidate (hadronically decaying top quark reconstructed in the fully or partially merged topology) and exactly one b jets; lower left: boosted top quark candidate and at least two b jets; upper right: resolved top quark candidate (hadronically decaying top quark reconstructed in the resolved topology) and exactly one b jets; lower-right: resolved top quark candidate and at least two b jets. The cross-hatched band in the upper panels represents the total uncertainty (statistical+systematic). The lower panel of each distribution shows the ratio, and its uncertainty, between the observation and the SM expectation. |
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Figure 2-a:
Distribution of the variable ${S_{\mathrm {T}}}$ for events passing the signal selection for the SM background estimation (stacked filled histograms), data (black points), and different hypotheses of LQ signals (lines), for a boosted top quark candidate (hadronically decaying top quark reconstructed in the fully or partially merged topology) and exactly one b jets. The cross-hatched band in the upper panel represents the total uncertainty (statistical+systematic). The lower panel of shows the ratio, and its uncertainty, between the observation and the SM expectation. |
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Figure 2-b:
Distribution of the variable ${S_{\mathrm {T}}}$ for events passing the signal selection for the SM background estimation (stacked filled histograms), data (black points), and different hypotheses of LQ signals (lines), for a boosted top quark candidate and at least two b jets. The cross-hatched band in the upper panel represents the total uncertainty (statistical+systematic). The lower panel of shows the ratio, and its uncertainty, between the observation and the SM expectation. |
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Figure 2-c:
Distribution of the variable ${S_{\mathrm {T}}}$ for events passing the signal selection for the SM background estimation (stacked filled histograms), data (black points), and different hypotheses of LQ signals (lines), for a resolved top quark candidate (hadronically decaying top quark reconstructed in the resolved topology) and exactly one b jets. The cross-hatched band in the upper panel represents the total uncertainty (statistical+systematic). The lower panel of shows the ratio, and its uncertainty, between the observation and the SM expectation. |
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Figure 2-d:
Distribution of the variable ${S_{\mathrm {T}}}$ for events passing the signal selection for the SM background estimation (stacked filled histograms), data (black points), and different hypotheses of LQ signals (lines), in the topology for a resolved top quark candidate and at least two b jets. The cross-hatched band in the upper panel represents the total uncertainty (statistical+systematic). The lower panel of shows the ratio, and its uncertainty, between the observation and the SM expectation. |
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Figure 3:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {S}}} {\overline {{\text {LQ}}}_{\text {S}}})$ (upper), $\sigma ({\mathrm{p}} \to \nu {\text {LQ}_{\text {S}}})$ with $\lambda = $ 1.5 and 2.5 (middle left and right), and $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {S}}} {\overline {{\text {LQ}}}_{\text {S}}})+\sigma ({\mathrm{p}} \to \nu {\text {LQ}_{\text {S}}})$ with $\lambda = $ 1.5 and 2.5 (lower left and right), as a function of the mass of the LQ$_{\text {S}}$. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO, except for pair-produced LQ$_{\text {S}}$, 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 $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {S}}} {\overline {{\text {LQ}}}_{\text {S}}})$, as a function of the mass of the LQ$_{\text {S}}$. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at NLO [44]. |
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Figure 3-b:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to \nu {\text {LQ}_{\text {S}}})$ with $\lambda = $ 1.5, as a function of the mass of the LQ$_{\text {S}}$. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
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Figure 3-c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to \nu {\text {LQ}_{\text {S}}})$ with $\lambda = $ 2.5, as a function of the mass of the LQ$_{\text {S}}$. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
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Figure 3-d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {S}}} {\overline {{\text {LQ}}}_{\text {S}}})+\sigma ({\mathrm{p}} \to \nu {\text {LQ}_{\text {S}}})$ with $\lambda = $ 1.5, as a function of the mass of the LQ$_{\text {S}}$. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
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Figure 3-e:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {S}}} {\overline {{\text {LQ}}}_{\text {S}}})+\sigma ({\mathrm{p}} \to \nu {\text {LQ}_{\text {S}}})$ with $\lambda = $ 2.5, as a function of the mass of the LQ$_{\text {S}}$. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
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Figure 4:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})$ (upper), $\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 1.5 and 2.5 (middle left and right), and $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})+\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 1.5 and 2.5 (lower left and right), as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 0. The bands represent the one- and two-standard deviation variations of the expected limit. 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 $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})$, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 0. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 4-b:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 1.5, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 0. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 4-c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 2.5, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 0. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 4-d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})+\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 1.5, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 0. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 4-e:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})+\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 2.5, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 0. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 5:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})$ (upper), $\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 1.5 and 2.5 (middle left and right), and $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})+\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 1.5 and 2.5 (lower left and right), as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 1. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 5-a:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})$, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 1. The bands represent the one- and two-standard deviation variations of the expected limit. 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 $\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 1.5, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 1. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 5-c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 2.5, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 1. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 5-d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})+\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 1.5, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 1. The bands represent the one- and two-standard deviation variations of the expected limit. 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 $\sigma ({\mathrm{p}} \to {\text {LQ}_{\text {V}}} {\overline {{\text {LQ}}}_{\text {V}}})+\sigma ({\mathrm{p}} \to \tau {\text {LQ}_{\text {V}}})$ with $\lambda = $ 2.5, as a function of the mass of the LQ$_{\text {V}}$, with $k = $ 1. The bands represent the one- and two-standard deviation variations of the expected limit. The solid blue curve indicates the theoretical predictions at LO. |
png pdf |
Figure 6:
The observed and expected (solid and dashed lines) 95% CL LQ exclusion limits in the plane of the LQ-lepton-quark 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$_{\text {S}}$ with equal couplings to t$ \tau $, b$ \nu $, while the lower plots are for an LQ$_{\text {V}}$ assuming $k = $ 0 (left) and 1 (right) and equal couplings to t$ \nu $, b$ \tau $. For LQ$_{\text {V}}$, the gray area shows the band preferred (95% CL) by the B physics anomalies: $\lambda = C m_{\mathrm {LQ}}$, where $C = \sqrt {0.7 \pm 0.2 }$ TeV$ ^{-1}$ and $m_{\mathrm {LQ}}$ is expressed in TeV [42]. |
png pdf |
Figure 6-a:
The observed and expected (solid and dashed lines) 95% CL LQ exclusion limits in the plane of the LQ-lepton-quark 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$_{\text {S}}$ with equal couplings to t$ \tau $, b$ \nu $. |
png pdf |
Figure 6-b:
The observed and expected (solid and dashed lines) 95% CL LQ exclusion limits in the plane of the LQ-lepton-quark coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The plot is for an LQ$_{\text {V}}$ assuming $k = $ 0 and equal couplings to t$ \nu $, b$ \tau $. The gray area shows the band preferred (95% CL) by the B physics anomalies: $\lambda = C m_{\mathrm {LQ}}$, where $C = \sqrt {0.7 \pm 0.2 }$ TeV$ ^{-1}$ and $m_{\mathrm {LQ}}$ is expressed in TeV [42]. |
png pdf |
Figure 6-c:
The observed and expected (solid and dashed lines) 95% CL LQ exclusion limits in the plane of the LQ-lepton-quark 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 is for an LQ$_{\text {V}}$ assuming $k = $ 1 and equal couplings to t$ \nu $, b$ \tau $. The gray area shows the band preferred (95% CL) by the B physics anomalies: $\lambda = C m_{\mathrm {LQ}}$, where $C = \sqrt {0.7 \pm 0.2 }$ TeV$ ^{-1}$ and $m_{\mathrm {LQ}}$ is expressed in TeV [42]. |
| Tables | |
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Table 1:
Yields from the SM background estimation, data, and expected signal, for the selected events. |
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Table 2:
Lower limits on the mass in TeV of the leptoquarks LQ$_{\text {S}}$, LQ$_{\text {V}}$ $k = $ 0, and LQ$_{\text {V}}$ $k = $ 1, based on the pair- and single-production mechanisms taken either separately or together. The results of 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 coupled 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 and a $\tau$ lepton ($\mathrm{t}\tau$) or a bottom quark and a neutrino ($\mathrm{b}\nu$, scalar LQ) or else to $\mathrm{t}\nu$ and $\mathrm{b}\tau$ (vector LQ), resulting in the $\mathrm{t}\tau\nu\mathrm{b}$ and $\mathrm{t}\tau\nu$ signatures. 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 Lorentz-boosted top quark, whose decay daughters may not be resolved as individual jets. 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 CERN LHC in proton-proton collisions at $\sqrt{s} = $ 13 TeV. The observations are found to be in agreement with the standard model predictions. 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% confidence level, is 0.98-1.73 TeV, depending on $\lambda$ and the leptoquark spin. These results represent the most stringent limits to date on the existence of such leptoquarks for the case of a decay branching fraction of 0.5 to each lepton-quark pair. They allow a relevant portion of the parameter space preferred by the B-physics anomalies in several models [41,42] to be excluded. |
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