A search is presented for dark matter produced in association with a pair of top quarks in pp collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 19.7 fb$^{-1}$. This search requires the presence of one lepton, multiple jets, and large missing transverse energy. No excess of events is found above the standard model expectation, and upper limits are derived on the production cross section. Interpreting the findings in the context of a scalar contact interaction between fermionic dark matter particles and top quarks, lower limits on the interaction scale are set. These limits are also interpreted in terms of the dark matter-nucleon scattering cross sections for the spin-independent scalar operator, which are particularly strong for low-mass dark matter particles.
Figure 1: Dominant diagram contributing to the production of DM particles in association with top quarks at the LHC. |
Figure 3a: Distribution of $\met$. The distribution is plotted after applying all other selections, showing the discriminating power between signal and background. Two simulated DM signals with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The last bin of the \met distribution includes the overflow. |
Figure 3b: Distribution of $\mt$. The distribution is plotted after applying all other selections, showing the discriminating power between signal and background. Two simulated DM signals with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The last bin of the \met distribution includes the overflow. |
Figure 3c: Distribution of $M_{\mathrm{T2}}^{\mathrm{W}}$. The distribution is plotted after applying all other selections, showing the discriminating power between signal and background. Two simulated DM signals with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The last bin of the \met distribution includes the overflow. |
Figure 3d: Distribution of min $\Delta\phi(j_{1,2},\vec{E}_{\mathrm{T}}^{\mathrm{miss}})$The distribution is plotted after applying all other selections, showing the discriminating power between signal and background. Two simulated DM signals with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. |
Figure 4a: Distribution of $\met$ in CR1 after applying the SFs. Two simulated DM signals with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The error bars on the data-to-background ratio take into account both the statistical uncertainty in data and the total uncertainty in the background prediction. The last bin of the \met distribution includes the overflow. |
Figure 4b: Distribution of $\mt$ in CR1 after applying the SFs. Two simulated DM signals with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The error bars on the data-to-background ratio take into account both the statistical uncertainty in data and the total uncertainty in the background prediction. The last bin of the \mt distribution includes the overflow. |
Figure 4c: Distribution of $M_{\mathrm{T2}}^{\mathrm{W}}$ in CR1 after applying the SFs. Two simulated DM signals with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The error bars on the data-to-background ratio take into account both the statistical uncertainty in data and the total uncertainty in the background prediction. The last bin of the $M_{\mathrm{T2}}^{\mathrm{W}}$ distribution includes the overflow. |
Figure 4d: Distribution of min $\Delta\phi(j_{1,2},\vec{E}_{\mathrm{T}}^{\mathrm{miss}})$ in CR1 after applying the SFs. Two simulated DM signals with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The error bars on the data-to-background ratio take into account both the statistical uncertainty in data and the total uncertainty in the background prediction. |
Figure 5a: Distribution of $\met$ in CR2 after applying the SFs. Two simulated DM signals with with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The error bars on the data-to-background ratio take into account both the statistical uncertainty in data and the total uncertainty in the background prediction. The last bin of the \met distribution includes the overflow. |
Figure 5b: Distribution of $\mt$ in CR2 after applying the SFs. Two simulated DM signals with with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The error bars on the data-to-background ratio take into account both the statistical uncertainty in data and the total uncertainty in the background prediction. The last bin of the \mt distribution includes the overflow. |
Figure 5c: Distribution of $M_{\mathrm{T2}}^{\mathrm{W}}$ in CR2 after applying the SFs. Two simulated DM signals with with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The error bars on the data-to-background ratio take into account both the statistical uncertainty in data and the total uncertainty in the background prediction. The last bin of the $M_{\mathrm{T2}}^{\mathrm{W}}$ distribution includes the overflow. |
Figure 5d: Distribution of min $\Delta\phi(j_{1,2},\vec{E}_{\mathrm{T}}^{\mathrm{miss}})$ in CR2 after applying the SFs. Two simulated DM signals with with mass $M_{\chi}$ of 1 and 600 GeV and an interaction scale $M_{*}$ of 100 GeV are included for comparison. The hatched region represents the total uncertainty in the background prediction. The error bars on the data-to-background ratio take into account both the statistical uncertainty in data and the total uncertainty in the background prediction. |
Figure 7: Invariant mass M$_{\chi\bar{\chi}}$ of two DM particles in selected signal events, for several DM mass hypotheses. |
Table 1: Systematic uncertainties from various sources and their impact on the total background prediction. |
-- DeborahPinna - 06 Aug 2014
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CR1_met.pdf | r1 | manage | 16.2 K | 2015-04-26 - 13:52 | DeborahPinna | ||
png | CR1_met.png | r1 | manage | 79.1 K | 2015-04-26 - 13:32 | DeborahPinna | |
CR1_mindphij1j2met.pdf | r1 | manage | 18.0 K | 2015-04-26 - 13:52 | DeborahPinna | ||
png | CR1_mindphij1j2met.png | r1 | manage | 81.7 K | 2015-04-26 - 13:32 | DeborahPinna | |
CR1_mt.pdf | r1 | manage | 16.7 K | 2015-04-26 - 13:52 | DeborahPinna | ||
png | CR1_mt.png | r1 | manage | 80.2 K | 2015-04-26 - 13:32 | DeborahPinna | |
CR1_mt2w.pdf | r1 | manage | 17.0 K | 2015-04-26 - 13:52 | DeborahPinna | ||
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CR2_met.pdf | r1 | manage | 16.2 K | 2015-04-26 - 13:52 | DeborahPinna | ||
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CR2_mindphij1j2met.pdf | r1 | manage | 18.1 K | 2015-04-26 - 13:52 | DeborahPinna | ||
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CR2_mt.pdf | r1 | manage | 16.7 K | 2015-04-26 - 13:52 | DeborahPinna | ||
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SignalEff_CrossSectionLimits.pdf | r1 | manage | 71.6 K | 2015-04-26 - 13:53 | DeborahPinna | ||
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