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CMS-B2G-16-027 ; CERN-EP-2018-233
Search for leptoquarks coupled to third-generation quarks in proton-proton collisions at $\sqrt{s} = $ 13 TeV
Phys. Rev. Lett. 121 (2018) 241802
Abstract: Three of the most significant measured deviations from standard model predictions, the enhanced decay rate for $\mathrm{B}\to{\mathrm{D}^{(*)}} \tau\nu$, hints of lepton universality violation in $\mathrm{B}\to{\mathrm{K}^{(*)}} \ell\ell$ decays, and the anomalous magnetic moment of the muon, can be explained by the existence of leptoquarks (LQs) with large couplings to third-generation quarks and masses at the TeV scale. The existence of these states can be probed at the LHC in high energy proton-proton collisions. A novel search is presented for pair production of LQs coupled to a top quark and a muon using data at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$, recorded by the CMS experiment. No deviation from the standard model prediction has been observed and scalar LQs decaying exclusively into $\mathrm{t}\mu$ are excluded up to masses of 1420 GeV. The results of this search are combined with those from previous searches for LQ decays into $\mathrm{t}\tau$ and $\mathrm{b}\nu$, which excluded scalar LQs below masses of 900 and 1080 GeV. Vector LQs are excluded up to masses of 1190 GeV for all possible combinations of branching fractions to $\mathrm{t}\mu$, $\mathrm{t}\tau$ and $\mathrm{b}\nu$. With this analysis, all relevant couplings of LQs with an electric charge of $-1/3$ to third-generation quarks are probed for the first time.
Figures Summary Additional Figures References CMS Publications
Figures

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Figure 1:
Distributions for ${M_{{\mathrm {LQ}}}^{\text {rec}}}$ (category A, left) and ${S_{\mathrm {T}}}$ (category B, right) after applying the full selection and estimating the ${{\mathrm {t}\overline {\mathrm {t}}}}$ and DY+jets background contributions from data in category B. All backgrounds are normalized according to the post-fit nuisance parameters based on the corresponding SM cross sections. In the upper panels, the hatched areas correspond to the total uncertainty. In the lower panels, the gray bands indicate the total uncertainty.

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Figure 1-a:
Distribution for ${M_{{\mathrm {LQ}}}^{\text {rec}}}$ (category A) after applying the full selection and estimating the ${{\mathrm {t}\overline {\mathrm {t}}}}$ and DY+jets background contributions from data in category B. All backgrounds are normalized according to the post-fit nuisance parameters based on the corresponding SM cross sections. In the upper panel, the hatched area corresponds to the total uncertainty. In the lower panel, the gray band indicates the total uncertainty.

png pdf
Figure 1-b:
Distribution for ${S_{\mathrm {T}}}$ (category B) after applying the full selection and estimating the ${{\mathrm {t}\overline {\mathrm {t}}}}$ and DY+jets background contributions from data. All backgrounds are normalized according to the post-fit nuisance parameters based on the corresponding SM cross sections. In the upper panel, the hatched area corresponds to the total uncertainty. In the lower panel, the gray band indicates the total uncertainty.

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Figure 2:
Observed upper limits on the production cross section for pair production of LQs decaying into a top quark and a muon or a $\tau $ lepton (left) and LQs decaying into a top quark and a muon or into a bottom quark and a neutrino (right) at 95% CL in the ${M_{{\mathrm {LQ}}}} $-$ {\mathcal {B}({{\mathrm {LQ}} \to {\mathrm {t}}\mu})}$ plane. The lines show the lower mass exclusion limits for scalar (black) and vector (colored) LQs. They are derived by using the prediction for the scalar and vector LQ signal calculated at NLO [48] and LO [49], respectively.

png pdf
Figure 2-a:
Observed upper limits on the production cross section for pair production of LQs decaying into a top quark and a muon or a $\tau $ lepton at 95% CL in the ${M_{{\mathrm {LQ}}}} $-$ {\mathcal {B}({{\mathrm {LQ}} \to {\mathrm {t}}\mu})}$ plane. The lines show the lower mass exclusion limits for scalar (black) and vector (colored) LQs. They are derived by using the prediction for the scalar and vector LQ signal calculated at NLO [48] and LO [49], respectively.

png pdf
Figure 2-b:
Observed upper limits on the production cross section for pair production of LQs decaying into a top quark and a muon or into a bottom quark and a neutrino at 95% CL in the ${M_{{\mathrm {LQ}}}} $-$ {\mathcal {B}({{\mathrm {LQ}} \to {\mathrm {t}}\mu})}$ plane. The lines show the lower mass exclusion limits for scalar (black) and vector (colored) LQs. They are derived by using the prediction for the scalar and vector LQ signal calculated at NLO [48] and LO [49], respectively.
Summary
In summary, this analysis represents the first search for leptoquarks (LQs) decaying to top quarks and muons, reaching LQ masses of $\mathcal{O}$(1 TeV) and placing direct constraints on the corresponding LQ coupling, thus probing the region of interest of models including LQs. With this result, all relevant couplings of LQs with an electric charge of $-1/3$ to third-generation quarks are examined for the first time.
Additional Figures

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Additional Figure 1:
Observed and expected upper limits on the production cross section for pair production of LQs decaying into a top quark and a muon at 95% CL as a function of the LQ mass. The theory predictions show the pair production cross sections for scalar (black) and vector (colored) LQs, where the acceptance for both types of LQs is similar.

png pdf
Additional Figure 2:
Observed upper limits on the production cross section for pair production of LQs decaying to a top quark and a muon or a tau lepton at 95% CL in the $M_{\mathrm {LQ}}$--$\mathcal {B}(\mathrm {LQ}\to {\mathrm {t}}+\mu)$ plane. The lines show the lower mass exclusion limits for scalar LQs from the $ {\mathrm {t}}$+$\tau $ channel alone (red) and the combination (black).

png pdf
Additional Figure 3:
Observed upper limits on the production cross section for pair production of LQs decaying into a top quark and a muon or into a bottom quark and a neutrino at 95% CL in the $M_{\mathrm {LQ}}$--$\mathcal {B}(\mathrm {LQ}\to {\mathrm {t}}+\mu)$ plane. The lines show the lower mass exclusion limits for scalar LQs from the b+$\nu $ channel alone (red) and the combination (black).
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