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| CMS-EXO-17-029 ; CERN-EP-2018-136 | ||
| Search for a singly produced third-generation scalar leptoquark decaying to a $\tau$ lepton and a bottom quark in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 9 June 2018 | ||
| JHEP 07 (2018) 115 | ||
| Abstract: A search is presented for a singly produced third-generation scalar leptoquark decaying to a $\tau$ lepton and a bottom quark. Associated production of a leptoquark and a $\tau$ lepton is considered, leading to a final state with a bottom quark and two $\tau$ leptons. The search uses proton-proton collision data at a center-of-mass energy of 13 TeV recorded with the CMS detector, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Upper limits are set at 95% confidence level on the production cross section of the third-generation scalar leptoquarks as a function of their mass. From a comparison of the results with the theoretical predictions, a third-generation scalar leptoquark decaying to a $\tau$ lepton and a bottom quark, assuming unit Yukawa coupling ($\lambda$), is excluded for masses below 740 GeV. Limits are also set on $\lambda$ of the hypothesized leptoquark as a function of its mass. Above $\lambda = $ 1.4, this result provides the best upper limit on the mass of a third-generation scalar leptoquark decaying to a $\tau$ lepton and a bottom quark. | ||
| Links: e-print arXiv:1806.03472 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading order Feynman diagrams for the single production of third-generation LQs subsequently decaying to a $\tau $ lepton and a bottom quark, for the $s$-channel (left) and $t$-channel (right) processes. |
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Figure 1-a:
Leading order Feynman diagrams for the single production of third-generation LQs subsequently decaying to a $\tau $ lepton and a bottom quark, for the $s$-channel (left) and $t$-channel (right) processes. |
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Figure 1-b:
Leading order Feynman diagrams for the single production of third-generation LQs subsequently decaying to a $\tau $ lepton and a bottom quark, for the $s$-channel (left) and $t$-channel (right) processes. |
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Figure 2:
The product of acceptance, efficiency, and branching fraction as a function of $m_\text {LQ}$ for the single production of LQs in each of the three channels considered: $ {{\tau} _\mathrm {h}} {{\tau} _\mathrm {h}} $ (black solid line), $ {{\mu}} {{\tau} _\mathrm {h}} $ (red dashed line), and $ {\mathrm {e}} {{\tau} _\mathrm {h}} $ (blue dotted line). |
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Figure 3:
Observed $S_\text {T}$ distribution in the $ {\mathrm {e}} {{\tau} _\mathrm {h}} $ (upper left), $ {{\mu}} {{\tau} _\mathrm {h}} $ (upper right), and $ {{\tau} _\mathrm {h}} {{\tau} _\mathrm {h}} $ (lower left) signal regions, as well as in the $ {\mathrm {e}} {{\mu}}$ (lower right) control region, compared to the expected SM background contributions. The distribution labeled "electroweak'' contains the contributions from W+jets, Z+jets, and diboson processes. The signal distributions for single-LQ production with mass 700 GeV are overlaid to illustrate the sensitivity. For the signal normalization, $\lambda =$ 1 and $\beta =$ 1 are assumed. The background uncertainty bands represent the sum in quadrature of statistical and systematic uncertainties obtained from the fit. The lower panels show the ratio between the observed and expected events in each bin. In all plots, the horizontal and vertical error bars on the data points represent the bin widths and the Poisson uncertainties, respectively. |
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Figure 3-a:
Observed $S_\text {T}$ distribution |
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Figure 3-b:
Observed $S_\text {T}$ distribution |
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Figure 3-c:
Observed $S_\text {T}$ distribution |
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Figure 3-d:
Observed $S_\text {T}$ distribution in the $ {\mathrm {e}} {{\mu}}$ control region, compared to the expected SM background contributions. The distribution labeled "electroweak'' contains the contributions from W+jets, Z+jets, and diboson processes. The signal distributions for single-LQ production with mass 700 GeV are overlaid to illustrate the sensitivity. For the signal normalization, $\lambda =$ 1 and $\beta =$ 1 are assumed. The background uncertainty bands represent the sum in quadrature of statistical and systematic uncertainties obtained from the fit. The lower panel shows the ratio between the observed and expected events in each bin. In all plots, the horizontal and vertical error bars on the data points represent the bin widths and the Poisson uncertainties, respectively. |
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Figure 4:
Observed (black solid) and expected (black dotted) limits at 95% confidence level on the product of cross section $\sigma $ and branching fraction $\beta $, obtained from the combination of the $ {\mathrm {e}} {{\tau} _\mathrm {h}} $, $ {{\mu}} {{\tau} _\mathrm {h}} $, and $ {{\tau} _\mathrm {h}} {{\tau} _\mathrm {h}} $ signal regions, as well as from the $ {\mathrm {e}} {{\mu}}$ control region, as a function of the LQ mass. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits. The theory prediction is indicated by the blue solid line, together with systematic uncertainties due to the choice of PDF and renormalization and factorization scales [45], indicated by the blue band. |
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Figure 5:
Expected and observed exclusion limits at 95% confidence level 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. The red vertical line indicates the limit obtained from a search for pair-produced LQs decaying to $\ell {{\tau} _\mathrm {h}} {\mathrm {b}} {\mathrm {b}} $ [37]. The area with vertical shading shows the expected exclusion region for the present analysis. The region with diagonal blue shading shows the parameter space preferred by one of the models proposed to explain anomalies observed in B physics [36]. |
| Tables | |
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Table 1:
Summary of systematic uncertainties in the signal acceptance and background estimate. The uncertainties have been grouped into those affecting the normalization of distributions and those affecting the shape, and uncertainties marked with a * are treated as correlated among channels. |
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Table 2:
Numbers of events observed in the $ {\mathrm {e}} {{\tau} _\mathrm {h}} $, $ {{\mu}} {{\tau} _\mathrm {h}} $, and $ {{\tau} _\mathrm {h}} {{\tau} _\mathrm {h}} $ channels for $S_\text {T} > $ 500 GeV, compared to the background expectations and to the event yield expected for single-LQ processes with $m_\text {LQ} = $ 700 GeV ($\lambda =$ 1 and $\beta =$ 1). The "electroweak'' background contains the contributions from W+jets, Z+jets, and diboson processes. The uncertainties represent the sum in quadrature of statistical and systematic contributions, and are obtained using the binned maximum likelihood fit of the $S_\text {T}$ distribution. |
| Summary |
| A search for singly produced third-generation scalar leptoquarks, each decaying to a $\tau$ lepton and a bottom quark has been presented. The final state of an electron or muon plus one hadronically decaying $\tau$ lepton and the final state with two hadronically decaying $\tau$ leptons are explored. In all final states at least one energetic jet identified as originating from a bottom quark is required. The search is based on a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the CMS detector, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The data are found to be in agreement with the standard model predictions. Upper limits as a function of the leptoquark mass are set on the third-generation scalar leptoquark production cross section. Results are compared with theoretical predictions to obtain lower limits on the leptoquark mass. Assuming the leptoquark always decays to a $\tau$ lepton and a bottom quark with unit Yukawa coupling $\lambda = $ 1, third-generation scalar leptoquarks with mass below 740 GeV are excluded at 95% confidence level. Mass limits are also placed as a function of $\lambda$. For values of $\lambda > $ 1.4, the mass limit obtained by this analysis exceeds that of the search considering pair production and provides the best upper limit. For $\lambda = $ 2.5, leptoquarks are excluded in the mass range up to 1050 GeV. This is the first time that limits have been presented in the $\lambda$ versus mass plane, allowing the results to be considered in the preferred parameter space of models that invoke third-generation leptoquarks to explain anomalies observed in B hadron decays. These results thus demonstrate the important potential of single leptoquark production studies to complement pair production constraints on such models, as additional data become available. |
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