CMSPASEXO17029  
Search for singly produced thirdgeneration leptoquarks decaying to a $\tau$ lepton and a b quark in protonproton collisions at $\sqrt{s}= $ 13 TeV  
CMS Collaboration  
March 2018  
Abstract: A search is presented for singly produced thirdgeneration scalar leptoquarks decaying to a $\tau$ lepton and a b quark. Events containing two $\tau$ leptons and one jet originating from a b quark are considered. The search is based on protonproton collision data at a centerofmass energy of 13 TeV recorded with the CMS detector and corresponding to an integrated luminosity of 35.9 fb$^{1}$. Upper limits are set on the thirdgeneration scalar leptoquark production cross section as a function of the leptoquark mass. Results are compared with theoretical predictions to obtain lower limits on the leptoquark mass. At 95% confidence level, thirdgeneration scalar leptoquarks decaying to a $\tau$ lepton and a b quark with unit coupling are excluded for masses below 744 GeV. Limits are also set on the couplings of such leptoquarks as a function of their mass.  
Links:
CDS record (PDF) ;
inSPIRE record ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, JHEP 07 (2018) 115. The superseded preliminary plots can be found here. 
Figures  
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Figure 1:
Feynman diagrams of singly produced thirdgeneration LQs decaying to a $\tau $ lepton and a b quark. 
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Figure 1a:
Feynman diagrams of singly produced thirdgeneration LQs decaying to a $\tau $ lepton and a b quark. 
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Figure 1b:
Feynman diagrams of singly produced thirdgeneration LQs decaying to a $\tau $ lepton and a b quark. 
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Figure 2:
Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (topleft), $ {{\mu}} {{\tau}_{\rm h}} $ (topright), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottomleft) and $ {\mathrm {e}} {{\mu}}$ (bottomright) channels, compared to expected Standard Model background contributions. The distribution called Electroweak contains the contributions from $ {\mathrm {W}}+ \text {jets}$, $ {\mathrm {Z}} + \text {jets}$, and diboson processes. The signal distributions for the 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 uncertainty bands represent the sum in quadrature of statistical and systematic uncertainties obtained from the fit. The lower panels in all plots compare the observed and expected events in each bin. 
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Figure 2a:
Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (topleft), $ {{\mu}} {{\tau}_{\rm h}} $ (topright), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottomleft) and $ {\mathrm {e}} {{\mu}}$ (bottomright) channels, compared to expected Standard Model background contributions. The distribution called Electroweak contains the contributions from $ {\mathrm {W}}+ \text {jets}$, $ {\mathrm {Z}} + \text {jets}$, and diboson processes. The signal distributions for the 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 uncertainty bands represent the sum in quadrature of statistical and systematic uncertainties obtained from the fit. The lower panels in all plots compare the observed and expected events in each bin. 
png pdf 
Figure 2b:
Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (topleft), $ {{\mu}} {{\tau}_{\rm h}} $ (topright), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottomleft) and $ {\mathrm {e}} {{\mu}}$ (bottomright) channels, compared to expected Standard Model background contributions. The distribution called Electroweak contains the contributions from $ {\mathrm {W}}+ \text {jets}$, $ {\mathrm {Z}} + \text {jets}$, and diboson processes. The signal distributions for the 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 uncertainty bands represent the sum in quadrature of statistical and systematic uncertainties obtained from the fit. The lower panels in all plots compare the observed and expected events in each bin. 
png pdf 
Figure 2c:
Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (topleft), $ {{\mu}} {{\tau}_{\rm h}} $ (topright), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottomleft) and $ {\mathrm {e}} {{\mu}}$ (bottomright) channels, compared to expected Standard Model background contributions. The distribution called Electroweak contains the contributions from $ {\mathrm {W}}+ \text {jets}$, $ {\mathrm {Z}} + \text {jets}$, and diboson processes. The signal distributions for the 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 uncertainty bands represent the sum in quadrature of statistical and systematic uncertainties obtained from the fit. The lower panels in all plots compare the observed and expected events in each bin. 
png pdf 
Figure 2d:
Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (topleft), $ {{\mu}} {{\tau}_{\rm h}} $ (topright), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottomleft) and $ {\mathrm {e}} {{\mu}}$ (bottomright) channels, compared to expected Standard Model background contributions. The distribution called Electroweak contains the contributions from $ {\mathrm {W}}+ \text {jets}$, $ {\mathrm {Z}} + \text {jets}$, and diboson processes. The signal distributions for the 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 uncertainty bands represent the sum in quadrature of statistical and systematic uncertainties obtained from the fit. The lower panels in all plots compare the observed and expected events in each bin. 
png pdf 
Figure 3:
Observed and expected limits at 95% confidence level (CL) on the product of cross section and branching fraction, obtained from the combination of the $ {\mathrm {e}} {{\tau}_{\rm h}} $, $ {{\mu}} {{\tau}_{\rm h}} $ and $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ channels. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits. 
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Figure 4:
The 95% confidence level expected and observed exclusion limits on the Yukawa coupling $\lambda $ at the LQleptonquark vertex, as a function of the LQ mass. A branching fraction of the LQ to a $\tau $ lepton and a b quark $\beta = $ 1 is assumed. The red line corresponds to the limit obtained from a search for pairproduced LQs decaying to $\ell \tau _\text {h}\mathrm{bb} $ [24]. The vertically shaded region is the expected exclusion limit from this analysis. The diagonally shaded blue region shows the parameter space preferred by the anomalies reported by Bfactory experiments [23]. 
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Figure 5:
The acceptance for the singly produced LQ signals in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (black), $ {{\mu}} {{\tau}_{\rm h}} $ (red), and $ {\mathrm {e}} {{\tau}_{\rm h}} $ (blue) channels, as a function of the leptoquark mass. The acceptance includes the branching fraction of $\tau \tau \to {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (42%), $\tau \tau \to {{\mu}} {{\tau}_{\rm h}} $ (21%), and $\tau \tau \to {\mathrm {e}} {{\tau}_{\rm h}} $ (21%). 
Tables  
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Table 1:
List of systematic uncertainties considered in this analysis. Those uncertainties marked with a * are treated as correlated among channels. 
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Table 2:
Number of events observed in the $ {\mathrm {e}} {{\tau}_{\rm h}} $, $ {{\mu}} {{\tau}_{\rm h}} $, and $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ channels for $S_\text {T} > $ 500 GeV, compared to the background expectations and to the event yield expected for single LQ processes with LQ mass 700 GeV ($\lambda = $ 1 and $\beta = $ 1). "Electroweak'' contains the contributions from $ {\mathrm {W}}+ \text {jets}$, $ {\mathrm {Z}} + \text {jets}$, and diboson processes. The uncertainties represent the sum in quadrature of statistical and systematic uncertainties and are obtained by the binned maximum likelihood fit of the $S_\text {T}$ distribution. 
Summary 
A search has been made for singly produced thirdgeneration scalar leptoquarks decaying to a $\tau$ lepton and a b quark. The final states of an electron or muon plus one hadronicallydecaying $\tau$ lepton or two hadronicallydecaying $\tau$ leptons are explored. In both final states at least one energetic jet originating from a b quark is required. The search is based on a data sample of protonproton collisions at a centerofmass energy of 13 TeV recorded with the CMS detector and corresponding to an integrated luminosity of 35.9 fb$^{1}$. The data are found to be in good agreement with the standard model prediction and upper limits are set on the thirdgeneration scalar leptoquark production cross section as a function of the leptoquark mass. 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 b quark, thirdgeneration scalar leptoquarks with masses below 744 GeV are excluded at 95% confidence level. A limit is also placed on the Yukawa coupling at the LQ$\tau$b vertex, allowing for leptoquark masses up to 1052 GeV to be excluded for large Yukawa couplings of $\lambda > $ 2.5. This is the first result showing the limit in this parameter space plane, and with respect to the preferred region reported by Bfactory experiments. 
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