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CMS-HIG-17-014 ; CERN-EP-2019-035
Search for a low-mass $\tau^{-}\tau^{+}$ resonance in association with a bottom quark in proton-proton collisions at $\sqrt{s} = $ 13 TeV
JHEP 05 (2019) 210
Abstract: A general search is presented for a low-mass $\tau^{-}\tau^{+}$ resonance produced in association with a bottom quark. The search is based on proton-proton collision data at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb$^{-1}$ . The data are consistent with the standard model expectation. Upper limits at 95% confidence level on the cross section times branching fraction are determined for two signal models: a light pseudoscalar Higgs boson decaying to a pair of $\tau$ leptons produced in association with bottom quarks, and a low-mass boson A decaying to a $\tau$-lepton pair that is produced in the decay of a bottom-like quark B such that B $\to$ bA. Masses between 25 and 70 GeV are probed for the light pseudoscalar boson with upper limits ranging from 250 to 44 pb. Upper limits from 20 to 0.3 pb are set on B masses between 170 and 450 GeV for A boson masses between 20 and 70 GeV.
Figures & Tables Summary Additional Figures & Tables References CMS Publications
Figures

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Figure 1:
Feynman diagrams of (left) a low-mass pseudoscalar Higgs boson (A) produced in association with bottom quarks, and (right) a bottom-like quark produced in $t$ channel, which decays into X and a bottom quark. The particle A decays into a $ {\tau}$-lepton pair.

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Figure 1-a:
Feynman diagram of a low-mass pseudoscalar Higgs boson (A) produced in association with bottom quarks. The particle A decays into a $ {\tau}$-lepton pair.

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Figure 1-b:
Feynman diagram of a bottom-like quark produced in $t$ channel, which decays into X and a bottom quark. The particle X decays into a $ {\tau}$-lepton pair.

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Figure 2:
Measured ${{m_{{\tau} {\tau}}}} $ distribution in the ${{{\mathrm {e}} {\tau}_\text {h}}} $ (left), and ${{{{\mu}} {\tau}_\text {h}}} $ (right) channel, compared to the expected SM background contributions. The signal distributions for ${{{{\mathrm {b}} {\overline {\mathrm {b}}}} \text {A}}} $ with a pseudoscalar mass of 40 and 60 GeV are overlaid to illustrate the sensitivity. They are normalized to the cross section times branching fraction of 800 pb. The 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.

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Figure 2-a:
Measured ${{m_{{\tau} {\tau}}}} $ distribution in the ${{{\mathrm {e}} {\tau}_\text {h}}} $ channel, compared to the expected SM background contributions. The signal distributions for ${{{{\mathrm {b}} {\overline {\mathrm {b}}}} \text {A}}} $ with a pseudoscalar mass of 40 and 60 GeV are overlaid to illustrate the sensitivity. They are normalized to the cross section times branching fraction of 800 pb. The 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.

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Figure 2-b:
Measured ${{m_{{\tau} {\tau}}}} $ distribution in the ${{{{\mu}} {\tau}_\text {h}}} $ channel, compared to the expected SM background contributions. The signal distributions for ${{{{\mathrm {b}} {\overline {\mathrm {b}}}} \text {A}}} $ with a pseudoscalar mass of 40 and 60 GeV are overlaid to illustrate the sensitivity. They are normalized to the cross section times branching fraction of 800 pb. The 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.

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Figure 3:
Measured ${{m_{{\tau} {\tau}}}} $ distribution in the ${{{\mathrm {e}} {\tau}_\text {h}}} $ (left), and ${{{{\mu}} {\tau}_\text {h}}} $ (right) final states, for the 1b1f (upper) and 1b1c (lower) categories, compared to the expected SM background contributions. The signal distributions for the VLQ model with ${{\mathrm {A}}} $ boson masses of 40 and 60 GeV are overlaid to illustrate the sensitivity. They are normalized to the cross section times branching fraction of 20 pb. The 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.

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Figure 3-a:
Measured ${{m_{{\tau} {\tau}}}} $ distribution in the ${{{\mathrm {e}} {\tau}_\text {h}}} $ final state, for the 1b1f category, compared to the expected SM background contributions. The signal distributions for the VLQ model with ${{\mathrm {A}}} $ boson masses of 40 and 60 GeV are overlaid to illustrate the sensitivity. They are normalized to the cross section times branching fraction of 20 pb. The 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.

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Figure 3-b:
Measured ${{m_{{\tau} {\tau}}}} $ distribution in the ${{{{\mu}} {\tau}_\text {h}}} $ final state, for the 1b1c category, compared to the expected SM background contributions. The signal distributions for the VLQ model with ${{\mathrm {A}}} $ boson masses of 40 and 60 GeV are overlaid to illustrate the sensitivity. They are normalized to the cross section times branching fraction of 20 pb. The 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.

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Figure 3-c:
Measured ${{m_{{\tau} {\tau}}}} $ distribution in the ${{{\mathrm {e}} {\tau}_\text {h}}} $ final state, for the 1b1f category, compared to the expected SM background contributions. The signal distributions for the VLQ model with ${{\mathrm {A}}} $ boson masses of 40 and 60 GeV are overlaid to illustrate the sensitivity. They are normalized to the cross section times branching fraction of 20 pb. The 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.

png pdf
Figure 3-d:
Measured ${{m_{{\tau} {\tau}}}} $ distribution in the ${{{{\mu}} {\tau}_\text {h}}} $ final state, for the 1b1c category, compared to the expected SM background contributions. The signal distributions for the VLQ model with ${{\mathrm {A}}} $ boson masses of 40 and 60 GeV are overlaid to illustrate the sensitivity. They are normalized to the cross section times branching fraction of 20 pb. The 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.

png pdf
Figure 4:
Observed (solid) and expected (dashed) limits at 95% confidence level on the product of cross section for the production of the ${{{{\mathrm {b}} {\overline {\mathrm {b}}}} \text {A}}}$ signal and branching fraction ${{\text {A}}} \to {{\tau} {\tau}} $, obtained from the combination of the ${{{\mathrm {e}} {\tau}_\text {h}}} $ and ${{{{\mu}} {\tau}_\text {h}}} $ channels. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits. Representative 2HDMs with varied sets of the $\tan\beta $ and ${{m_\text {A}}} $ parameters are overlaid for two types of Yukawa couplings to the down-type fermions: one which is SM-like, and one in which the Yukawa coupling is negative ("wrong-sign'').

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Figure 5:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the ${{{\mathrm {q}} {\mathrm {b}}\text {A}}}$ signal and branching fraction ${{\mathrm {A}}} \to {{\tau} {\tau}} $, obtained from the combination of the ${{{\mathrm {e}} {\tau}_\text {h}}} $ and ${{{{\mu}} {\tau}_\text {h}}} $ channels. The ${{m_\text {B}}} $ values of 170 (upper left), 300 (upper right), and 450 GeV are considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

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Figure 5-a:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the ${{{\mathrm {q}} {\mathrm {b}}\text {A}}}$ signal and branching fraction ${{\mathrm {A}}} \to {{\tau} {\tau}} $, obtained from the combination of the ${{{\mathrm {e}} {\tau}_\text {h}}} $ and ${{{{\mu}} {\tau}_\text {h}}} $ channels. The ${{m_\text {B}}} $ value of 170 GeV is considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

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Figure 5-b:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the ${{{\mathrm {q}} {\mathrm {b}}\text {A}}}$ signal and branching fraction ${{\mathrm {A}}} \to {{\tau} {\tau}} $, obtained from the combination of the ${{{\mathrm {e}} {\tau}_\text {h}}} $ and ${{{{\mu}} {\tau}_\text {h}}} $ channels. The ${{m_\text {B}}} $ value of 300 GeV is considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

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Figure 5-c:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the ${{{\mathrm {q}} {\mathrm {b}}\text {A}}}$ signal and branching fraction ${{\mathrm {A}}} \to {{\tau} {\tau}} $, obtained from the combination of the ${{{\mathrm {e}} {\tau}_\text {h}}} $ and ${{{{\mu}} {\tau}_\text {h}}} $ channels. The ${{m_\text {B}}} $ value of 450 GeV is considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.
Tables

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Table 1:
Sources of systematic uncertainties and their effects on the acceptance or shape resulting from a variation of the nuisance parameter equivalent to one standard deviation.
Summary
This paper presents a general search for a low-mass $\tau^{-}\tau^{+}$ resonance produced in association with a bottom quark. After defining the signal region by the presence of an electron or muon consistent with the decay of a $\tau$ lepton, a hadronically decaying $\tau$ lepton, and a jet originating from a bottom quark, an excess over standard model background is searched for in the reconstructed invariant mass distribution of the inferred $\tau^{-}\tau^{+}$ system. The data are consistent with the standard model background. We set upper limits at 95% confidence level on the cross section times branching fraction for two signal models: a light pseudoscalar Higgs boson decaying to a pair of $\tau$ leptons produced in association with a bottom quark, and a low-mass boson A decaying to a $\tau$ lepton pair that is produced in the decay of a bottom-like quark B as $\mathrm{B}\to\mathrm{b}\mathrm{A}$. For both scenarios, A boson masses between 20 and 70 GeV are probed. Upper limits at 95% confidence level ranging from 250 to 44 pb are set on the light pseudoscalar, and from 20 to 0.3 pb on B masses between $170$ and 450 GeV. This is the first search for an A resonance in this final state using the center-of-mass energy of 13 TeV. Since many extensions of the standard model have similar event kinematics as this analysis, these results could also be applied to put constraints on other low-mass $\tau^{-}\tau^{+}$ resonances. If there were a Yukawa-type enhancement between the signal and the $\tau$ leptons, then the constraints on the signal production cross section by this analysis would improve by a factor of $m_\tau^2/m_\mu^2$.

The optimized selection of this analysis targets previously unexplored decays of heavy bottom-like quarks, providing new sensitivity to vector-like quarks.
Additional Figures

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Additional Figure 1:
Observed (solid) and expected (dashed) limits at 95% confidence level on the product of cross section for the production of the ${{{{\mathrm {b}} {\overline {\mathrm {b}}}} \, {\mathrm {A}}}}$ signal and branching fraction ${{{\mathrm {A}}}} \to {{{\tau}\, {\tau}}} $, obtained for the ${{{\mathrm {e}}\, {\tau}_\mathrm {h}}} $ (left) and ${{{{\mu}}\, {\tau}_\mathrm {h}}} $ (right) channels. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

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Additional Figure 1-a:
Observed (solid) and expected (dashed) limits at 95% confidence level on the product of cross section for the production of the ${{{{\mathrm {b}} {\overline {\mathrm {b}}}} \, {\mathrm {A}}}}$ signal and branching fraction ${{{\mathrm {A}}}} \to {{{\tau}\, {\tau}}} $, obtained for the ${{{\mathrm {e}}\, {\tau}_\mathrm {h}}} $ channel. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

png pdf
Additional Figure 1-b:
Observed (solid) and expected (dashed) limits at 95% confidence level on the product of cross section for the production of the ${{{{\mathrm {b}} {\overline {\mathrm {b}}}} \, {\mathrm {A}}}}$ signal and branching fraction ${{{\mathrm {A}}}} \to {{{\tau}\, {\tau}}} $, obtained for the ${{{{\mu}}\, {\tau}_\mathrm {h}}} $ channel. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

png pdf
Additional Figure 2:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the signal and branching fraction ${{\mathrm {X}}} \to {{{\tau}\, {\tau}}} $, obtained for the ${{{\mathrm {e}}\, {\tau}_\mathrm {h}}} $ (left) and ${{{{\mu}}\, {\tau}_\mathrm {h}}} $ (right) channels in the 1b1f (top) and 1b1c (bottom) categories. The scenario with ${{m_{{{\mathrm {B}}}}}} = $ 170 GeV is considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

png pdf
Additional Figure 2-a:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the signal and branching fraction ${{\mathrm {X}}} \to {{{\tau}\, {\tau}}} $, obtained for the ${{{\mathrm {e}}\, {\tau}_\mathrm {h}}} $ channel in the 1b1f category. The scenario with ${{m_{{{\mathrm {B}}}}}} = $ 170 GeV is considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

png pdf
Additional Figure 2-b:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the signal and branching fraction ${{\mathrm {X}}} \to {{{\tau}\, {\tau}}} $, obtained for the ${{{{\mu}}\, {\tau}_\mathrm {h}}} $ channel in the 1b1f category. The scenario with ${{m_{{{\mathrm {B}}}}}} = $ 170 GeV is considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

png pdf
Additional Figure 2-c:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the signal and branching fraction ${{\mathrm {X}}} \to {{{\tau}\, {\tau}}} $, obtained for the ${{{\mathrm {e}}\, {\tau}_\mathrm {h}}} $ channel in the 1b1c category. The scenario with ${{m_{{{\mathrm {B}}}}}} = $ 170 GeV is considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.

png pdf
Additional Figure 2-d:
Observed (solid) and expected (dotted) limits at 95% confidence level on the product of cross section for the production of the signal and branching fraction ${{\mathrm {X}}} \to {{{\tau}\, {\tau}}} $, obtained for the ${{{{\mu}}\, {\tau}_\mathrm {h}}} $ channel in the 1b1c category. The scenario with ${{m_{{{\mathrm {B}}}}}} = $ 170 GeV is considered. The green and yellow bands represent the one and two standard deviation uncertainties in the expected limits.
Additional Tables

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Additional Table 1:
The product of acceptance, efficiency, and branching fraction of the ${{{{\mathrm {b}} {\overline {\mathrm {b}}}} \, {\mathrm {A}}}} $ signal with ${{{\mathrm {A}}}} \to {{{\tau}\, {\tau}}} $ in the ${{{{\mu}}\, {\tau}_\mathrm {h}}} $ and ${{{\mathrm {e}}\, {\tau}_\mathrm {h}}} $ channels of the 1 b tag category, for different A boson mass values. The selections are as described in Section 5. The uncertainty refers to the statistical uncertainty only.

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Additional Table 2:
The product of acceptance, efficiency, and branching fraction of the signal with ${{\mathrm {X}} }\to {{{\tau}\, {\tau}}} $ in the ${{{{\mu}}\, {\tau}_\mathrm {h}}} $ and ${{{\mathrm {e}}\, {\tau}_\mathrm {h}}} $ channels of the 1b1f and 1b1f categories, for different X boson mass values. The selections are as described in Section 5. The uncertainty refers to the statistical uncertainty only.
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Compact Muon Solenoid
LHC, CERN