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| CMS-PAS-EXO-16-049 | ||
| Search for dark matter produced in association with a top quark pair at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| April 2018 | ||
| Abstract: A search is performed for dark matter produced in association with a top quark pair in proton-proton collisions at $\sqrt{s} = $ 13 TeV recorded by the CMS detector at the LHC in 2016. The data correspond to an integrated luminosity of 35.9 fb$^{-1}$. Combined results of final states involving zero, one, or two leptons (electrons or muons) are presented. No significant excess is observed, and the results are interpreted in the context of simplified models of dark matter production via spin-0 mediators. Scalar (pseudoscalar) mediators with masses below 165 (223) GeV are excluded at 95% confidence level. These are the most stringent limits at the LHC on dark matter models involving a scalar mediator. Constraints on the coupling strength between dark matter and standard model quarks are also presented. | ||
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These preliminary results are superseded in this paper, PRL 122 (2019) 011803. The superseded preliminary plots can be found here. |
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| Figures | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Selected $ {{p_{\text {T}}^{\text {miss}}}}$ distributions in SRs: 2RTT SR for the all-hadronic (left), the $\ell +$jets (middle), and the different flavor, $ {M^{\ell \ell}_{{\text {T2}}}} > $ 110 GeV SR in dileptonic channel (right). The background distributions (filled histograms) are obtained after a background-only fit. The expected $ {{p_{\text {T}}^{\text {miss}}}}$ distribution for a signal corresponding to a 100 GeV pseudoscalar is overlaid (solid red line). The last bin contains overflow events. The lower panel shows the ratio of the observed data to the fitted distribution (points), and the ratio of the pre-fit background expectation to the fitted distribution (dashed magenta line). The horizontal bars indicate the bin width. |
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Figure 1-a:
Selected $ {{p_{\text {T}}^{\text {miss}}}}$ distribution in the 2RTT SR for the all-hadronic. The background distributions (filled histograms) are obtained after a background-only fit. The expected $ {{p_{\text {T}}^{\text {miss}}}}$ distribution for a signal corresponding to a 100 GeV pseudoscalar is overlaid (solid red line). The last bin contains overflow events. The lower panel shows the ratio of the observed data to the fitted distribution (points), and the ratio of the pre-fit background expectation to the fitted distribution (dashed magenta line). The horizontal bars indicate the bin width. |
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Figure 1-b:
Selected $ {{p_{\text {T}}^{\text {miss}}}}$ distribution in the $\ell +$jets SR. The background distributions (filled histograms) are obtained after a background-only fit. The expected $ {{p_{\text {T}}^{\text {miss}}}}$ distribution for a signal corresponding to a 100 GeV pseudoscalar is overlaid (solid red line). The last bin contains overflow events. The lower panel shows the ratio of the observed data to the fitted distribution (points), and the ratio of the pre-fit background expectation to the fitted distribution (dashed magenta line). The horizontal bars indicate the bin width. |
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Figure 1-c:
Selected $ {{p_{\text {T}}^{\text {miss}}}}$ distribution in the different flavor, $ {M^{\ell \ell}_{{\text {T2}}}} > $ 110 GeV SR in dileptonic channel. The background distributions (filled histograms) are obtained after a background-only fit. The expected $ {{p_{\text {T}}^{\text {miss}}}}$ distribution for a signal corresponding to a 100 GeV pseudoscalar is overlaid (solid red line). The last bin contains overflow events. The lower panel shows the ratio of the observed data to the fitted distribution (points), and the ratio of the pre-fit background expectation to the fitted distribution (dashed magenta line). The horizontal bars indicate the bin width. |
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Figure 2:
The exclusion limits at 95% CL on the signal strength $\mu $ computed as a function of the mediator and DM mass, assuming a scalar (left) and pseudoscalar (right) mediator. The mediator couplings are assumed to be $ {g_{\text {q}}}=g_{\chi}= $ 1. |
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Figure 2-a:
The exclusion limits at 95% CL on the signal strength $\mu $ computed as a function of the mediator and DM mass, assuming a scalar mediator. The mediator couplings are assumed to be $ {g_{\text {q}}}=g_{\chi}= $ 1. |
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Figure 2-b:
The exclusion limits at 95% CL on the signal strength $\mu $ computed as a function of the mediator and DM mass, assuming a pseudoscalar mediator. The mediator couplings are assumed to be $ {g_{\text {q}}}=g_{\chi}= $ 1. |
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Figure 3:
The 95% CL upper limits on the coupling strength of the mediator to the SM quarks under the assumption that $g_{\chi}= $ 1. A DM particle with mass of 1 GeV is assumed. |
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Figure 3-a:
The 95% CL upper limits on the coupling strength of the (scalar) mediator to the SM quarks under the assumption that $g_{\chi}= $ 1. A DM particle with mass of 1 GeV is assumed. |
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Figure 3-b:
The 95% CL upper limits on the coupling strength of the (pseudoscalar) mediator to the SM quarks under the assumption that $g_{\chi}= $ 1. A DM particle with mass of 1 GeV is assumed. |
| Summary |
| In summary, a comprehensive search for dark matter produced in association with a top quark pair yields no significant excess over the predicted background. The results presented in this article significantly extend the reach of the previous CMS result [15], and provide 30-60% better sensitivity compared to earlier searches targeting the same signature [51,52,53]. |
| Additional Figures | |
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Additional Figure 1:
The $p_{\text {T}}^{\text {miss}}$ distribution for the 0-lepton, 0-b-tag control region in the all-hadronic channel 2RTT category. This control region is enriched in $\text {W} +$jets and $\text {Z}(\nu \bar{\nu}) +$jets. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
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Additional Figure 2:
The $p_{\text {T}}^{\text {miss}}$ distribution for the 1-lepton, $\geq $1-b-tag control region in the all-hadronic channel 2RTT category. This control region is enriched in $\ell +$jets $\mathrm{t\bar{t}}$. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
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Additional Figure 3:
The $p_{\text {T}}^{\text {miss}}$ distribution for the 1-lepton, 0-b-tag control region in the all-hadronic channel 2RTT category. This control region is enriched in $\text {W} +$jets. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
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Additional Figure 4:
The $p_{\text {T}}^{\text {miss}}$ distribution for the signal region in the all-hadronic channel 01RTT category. The background distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The prefit $p_{\text {T}}^{\text {miss}}$ distribution of pseudoscalar $m_a = $ 100 GeV is overlayed. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
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Additional Figure 5:
The $p_{\text {T}}^{\text {miss}}$ distribution for the 0-lepton, 0-b-tag control region in the all-hadronic channel 01RTT category. This control region is enriched in $\text {W} +$jets and $\text {Z}(\nu \bar{\nu}) +$jets. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
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Additional Figure 6:
The $p_{\text {T}}^{\text {miss}}$ distribution for the 1-lepton, $\geq $2-b-tag control region in the all-hadronic channel 01RTT category. This control region is enriched in $\ell +$jets $\mathrm{t\bar{t}}$. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
png pdf |
Additional Figure 7:
The $p_{\text {T}}^{\text {miss}}$ distribution for the 1-lepton, 0-b-tag control region in the all-hadronic channel 01RTT category. This control region is enriched in $\text {W} +$jets. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
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Additional Figure 8:
Hadronic recoil distribution for the 2-lepton, 0-b-tag control region in the all-hadronic channel. This control region uses $\text {Z}(\ell \ell) +$jets to constrain the estimate of $\text {Z}(\nu \bar{\nu}) +$jets in the signal region. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
png pdf |
Additional Figure 9:
The $p_{\text {T}}^{\text {miss}}$ distribution for the 0-b-tag control region in the $\ell +$jets channel. This control region is enriched in $\text {W} +$jets. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
png pdf |
Additional Figure 10:
The $p_{\text {T}}^{\text {miss}}$ distribution for the 2-lepton control region in the $\ell +$jets channel. This control region is enriched in dileptonic $\mathrm{t\bar{t}}$. The distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. The horizontal error bars indicate the bin width. |
png pdf |
Additional Figure 11:
The $p_{\text {T}}^{\text {miss}}$ distribution for same-flavor events in the dileptonic channel $M_{T2}^{\ell \ell} > $ 110 GeV category. The background distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The prefit $p_{\text {T}}^{\text {miss}}$ distribution of pseudoscalar $m_a = $ 100 GeV is overlayed. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. The horizontal error bars indicate the bin width. |
png pdf |
Additional Figure 12:
The $p_{\text {T}}^{\text {miss}}$ distribution for same-flavor events in the dileptonic channel $M_{T2}^{\ell \ell} < $ 110 GeV category. The background distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The prefit MET distribution of pseudoscalar $m_a = $ 100 GeV is overlayed. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. The horizontal error bars indicate the bin width. |
png pdf |
Additional Figure 13:
The $p_{\text {T}}^{\text {miss}}$ distribution for opposite-flavor events in the dileptonic channel $M_{T2}^{\ell \ell} < $ 110 GeV category. The background distributions correspond to the background-only fit, while the magenta histogram is the pre-fit estimate of the overall background. The prefit MET distribution of pseudoscalar $m_a = $ 100 GeV is overlayed. The last bin contains overflow events. The uncertainty band shown in the observed data/fitted background ratio panel are the fit values, while the magenta lines correspond to the ratio of prefit expectations to fitted background. |
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Additional Figure 14:
The 95% CL upper limits on production rate of a scalar mediator decaying to dark matter pair. A DM particle with mass of 1 GeV is assumed. The theory cross section is computed at NLO in QCD. |
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Additional Figure 15:
The 95% CL upper limits on production rate of a pseudoscalar mediator decaying to dark matter pair. A DM particle with mass of 1 GeV is assumed. The theory cross section is computed at NLO in QCD. |
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Additional Figure 16:
Exclusion Limits at 90% CL for pseudoscalar mediators are compared to the velocity averaged DM annihilation cross section upper limits from Fermi-LAT. |
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Additional Figure 17:
Exclusion limits at 90% CL in the $m_{DM}$ vs $\sigma _{SI}$ plane for the scalar mediator hypothesis. The observed exclusion in this search is compare to the direct detection (DD) results from the CRESST-II, CDMSLite, PandaX-II, LUX, XENON1T, and CDX-10 experiments. |
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