CMS-PAS-EXO-17-029

CMS-PAS-EXO-17-029
Search for singly produced third-generation leptoquarks decaying to a $\tau$ lepton and a b quark in proton-proton collisions at $\sqrt{s}= $ 13 TeV
CMS Collaboration
March 2018

Abstract: A search is presented for singly produced third-generation 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 proton-proton collision data at a center-of-mass 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 third-generation 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, third-generation 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Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 07 (2018) 115. The superseded preliminary plots can be found here.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 & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagrams of singly produced third-generation LQs decaying to a $\tau $ lepton and a b quark.
png pdf	Figure 1-a: Feynman diagrams of singly produced third-generation LQs decaying to a $\tau $ lepton and a b quark.
png pdf	Figure 1-b: Feynman diagrams of singly produced third-generation LQs decaying to a $\tau $ lepton and a b quark.
png pdf	Figure 2: Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (top-left), $ {{\mu}} {{\tau}_{\rm h}} $ (top-right), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottom-left) and $ {\mathrm {e}} {{\mu}}$ (bottom-right) 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 2-a: Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (top-left), $ {{\mu}} {{\tau}_{\rm h}} $ (top-right), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottom-left) and $ {\mathrm {e}} {{\mu}}$ (bottom-right) 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 2-b: Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (top-left), $ {{\mu}} {{\tau}_{\rm h}} $ (top-right), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottom-left) and $ {\mathrm {e}} {{\mu}}$ (bottom-right) 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 2-c: Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (top-left), $ {{\mu}} {{\tau}_{\rm h}} $ (top-right), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottom-left) and $ {\mathrm {e}} {{\mu}}$ (bottom-right) 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 2-d: Measured $S_\text {T}$ distribution in $ {{\tau}_{\rm h}} {{\tau}_{\rm h}} $ (top-left), $ {{\mu}} {{\tau}_{\rm h}} $ (top-right), $ {\mathrm {e}} {{\tau}_{\rm h}} $ (bottom-left) and $ {\mathrm {e}} {{\mu}}$ (bottom-right) 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.
png pdf	Figure 4: The 95% confidence level expected and observed exclusion limits on the Yukawa coupling $\lambda $ at the LQ-lepton-quark 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 pair-produced 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 B-factory experiments [23].
png pdf	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
png pdf	Table 1: List of systematic uncertainties considered in this analysis. Those uncertainties marked with a * are treated as correlated among channels.
png pdf	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 third-generation scalar leptoquarks decaying to a $\tau$ lepton and a b quark. The final states of an electron or muon plus one hadronically-decaying $\tau$ lepton or two hadronically-decaying $\tau$ 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 proton-proton collisions at a center-of-mass 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 third-generation 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, third-generation 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 B-factory experiments.

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