# Search for the Production of Dark Matter in Association with Top Quark Pairs in the Single-lepton Final State in pp collisions at $\sqrt{s} = 8$ TeV (B2G-14-004)

## Abstract

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.

# Approved Plots and Tables (click on plot to get larger version)

## Figures

 Figure 1: Dominant diagram contributing to the production of DM particles in association with top quarks at the LHC.

 Figure 2: Schematic of a dileptonic $\mathrm{t}\bar{\mathrm{t}}$ event where only one lepton is reconstructed~\cite{MT2W}. This represents the dominant type of $\mathrm{t}\bar{\mathrm{t}}$ background to this search. The momentum of the W boson that decays to an unreconstructed lepton is indicated by $p_2$, and the momentum of the neutrino from the decay of the other W boson is indicated by $p_1$. The same notation is used in Eq. (3).

 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 6: Observed exclusion limits in the plane of DM particle mass and interaction scale, with the region below the solid curve excluded at a 90\% CL. The background-only expectations are represented by their median (dashed line) and by the 68\% and 95\% CL bands. A lower bound on the validity of the EFT is indicated by the upper edge of the hatched area. The four curves, corresponding to different g and R values, represent the lower bound on $M_{*}$ for which 50\% and 80\% of signal events have a pair of DM particles with an invariant mass less than $g\sqrt{M^3_{*}/m_{\mathrm{t}}}$, where $g=4\pi$ and $g=2\pi$ respectively.

 Figure 7: Invariant mass M$_{\chi\bar{\chi}}$ of two DM particles in selected signal events, for several DM mass hypotheses.

 Figure 8: The 90\% CL upper limits on the DM-nucleon spin-independent scattering cross sections as a function of the DM particle mass for the scalar operator considered in this paper. Also shown are 90\% CL limits from various direct DM search experiments [50-54]

## Tables

 Table 1: Systematic uncertainties from various sources and their impact on the total background prediction.

 Table 2: Expected number of background events in the SR, expected number of signal events for a DM particle with the mass $M_{\chi}= 1~\GeV$, assuming an interaction scale $M_{*}= 100~\GeV$, and observed data. The statistical and systematic uncertainties are given on the expected yields.

 Table 3: Signal efficiencies, and observed and expected limits at 90\% CL on production cross sections for $\mathrm{pp}\rightarrow \mathrm{t}\bar{\mathrm{t}}+\chi\bar{\chi}$, for various DM particle masses.

-- DeborahPinna - 06 Aug 2014

Topic attachments
I Attachment History Action Size Date Who Comment
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png CR1_met.png r1 manage 79.1 K 2015-04-26 - 13:32 DeborahPinna
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pdf CR1_mt.pdf r1 manage 16.7 K 2015-04-26 - 13:52 DeborahPinna
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pdf CR1_mt2w.pdf r1 manage 17.0 K 2015-04-26 - 13:52 DeborahPinna
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pdf CR2_met.pdf r1 manage 16.2 K 2015-04-26 - 13:52 DeborahPinna
png CR2_met.png r1 manage 79.2 K 2015-04-26 - 13:32 DeborahPinna
pdf CR2_mindphij1j2met.pdf r1 manage 18.1 K 2015-04-26 - 13:52 DeborahPinna
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pdf CR2_mt.pdf r1 manage 16.7 K 2015-04-26 - 13:52 DeborahPinna
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pdf CR2_mt2w.pdf r1 manage 17.0 K 2015-04-26 - 13:52 DeborahPinna
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pdf FinalYields.pdf r1 manage 49.4 K 2015-04-26 - 13:53 DeborahPinna
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pdf MT2W.pdf r1 manage 108.2 K 2015-04-26 - 13:52 DeborahPinna
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pdf Mchichi.pdf r1 manage 14.2 K 2015-04-26 - 13:52 DeborahPinna
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pdf SystematicUncertainties.pdf r1 manage 96.8 K 2015-04-26 - 13:53 DeborahPinna
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pdf cls_1l.pdf r1 manage 21.7 K 2015-04-26 - 13:52 DeborahPinna
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pdf d1_xs.pdf r1 manage 17.1 K 2015-04-26 - 13:52 DeborahPinna
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pdf ggchichi_v1.pdf r1 manage 171.4 K 2015-04-26 - 13:52 DeborahPinna
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pdf n-1_mindphij1j2met.pdf r1 manage 15.0 K 2015-04-26 - 13:52 DeborahPinna
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pdf n-1_mt.pdf r1 manage 15.2 K 2015-04-26 - 13:52 DeborahPinna
png n-1_mt.png r1 manage 66.0 K 2015-04-26 - 13:34 DeborahPinna
pdf n-1_mt2w.pdf r1 manage 14.9 K 2015-04-26 - 13:52 DeborahPinna
png n-1_mt2w.png r1 manage 67.3 K 2015-04-26 - 13:32 DeborahPinna
Topic revision: r12 - 2015-04-26 - DeborahPinna

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