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)

Links to documentation

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)

Material in the paper

Figures

ggchichi_v1.png Figure 1: Dominant diagram contributing to the production of DM particles in association with top quarks at the LHC.

MT2W.png 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).

n-1_met.png 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.

n-1_mt.png 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.

n-1_mt2w.png 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.

n-1_mindphij1j2met.png 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.

CR1_mt.png 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.

CR1_mt2w.png 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.

CR1_mindphij1j2met.png 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.

CR1_mindphij1j2met.png 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.

CR2_met.png 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.

CR2_mt.png 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.

CR2_mt2w.png 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.

CR2_mindphij1j2met.png 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.

cls_1l.png 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.

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

d1_xs.png 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

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

FinalYields.png 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.

SignalEff_CrossSectionLimits.png 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
PDFpdf CR1_met.pdf r1 manage 16.2 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng CR1_met.png r1 manage 79.1 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf CR1_mindphij1j2met.pdf r1 manage 18.0 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng CR1_mindphij1j2met.png r1 manage 81.7 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf CR1_mt.pdf r1 manage 16.7 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng CR1_mt.png r1 manage 80.2 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf CR1_mt2w.pdf r1 manage 17.0 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng CR1_mt2w.png r1 manage 85.2 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf CR2_met.pdf r1 manage 16.2 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng CR2_met.png r1 manage 79.2 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf CR2_mindphij1j2met.pdf r1 manage 18.1 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng CR2_mindphij1j2met.png r1 manage 82.3 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf CR2_mt.pdf r1 manage 16.7 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng CR2_mt.png r1 manage 80.0 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf CR2_mt2w.pdf r1 manage 17.0 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng CR2_mt2w.png r1 manage 84.1 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf FinalYields.pdf r1 manage 49.4 K 2015-04-26 - 13:53 DeborahPinna  
PNGpng FinalYields.png r1 manage 58.6 K 2015-04-26 - 13:41 DeborahPinna  
PDFpdf MT2W.pdf r1 manage 108.2 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng MT2W.png r1 manage 77.3 K 2015-04-26 - 13:35 DeborahPinna  
PDFpdf Mchichi.pdf r1 manage 14.2 K 2015-04-26 - 13:52 DeborahPinna  
PDFpdf SignalEff_CrossSectionLimits.pdf r1 manage 71.6 K 2015-04-26 - 13:53 DeborahPinna  
PNGpng SignalEff_CrossSectionLimits.png r1 manage 78.0 K 2015-04-26 - 13:41 DeborahPinna  
PDFpdf SystematicUncertainties.pdf r1 manage 96.8 K 2015-04-26 - 13:53 DeborahPinna  
PNGpng SystematicUncertainties.png r1 manage 110.3 K 2015-04-26 - 13:41 DeborahPinna  
PDFpdf cls_1l.pdf r1 manage 21.7 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng cls_1l.png r1 manage 129.7 K 2015-04-26 - 13:41 DeborahPinna  
PDFpdf d1_xs.pdf r1 manage 17.1 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng d1_xs.png r1 manage 119.9 K 2015-04-26 - 13:41 DeborahPinna  
PDFpdf ggchichi_v1.pdf r1 manage 171.4 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng ggchichi_v1.png r1 manage 61.2 K 2015-04-26 - 13:35 DeborahPinna  
PNGpng invMass.png r1 manage 106.2 K 2015-04-26 - 13:41 DeborahPinna  
PDFpdf n-1_met.pdf r1 manage 15.1 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng n-1_met.png r1 manage 66.3 K 2015-04-26 - 13:35 DeborahPinna  
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PNGpng n-1_mindphij1j2met.png r1 manage 63.3 K 2015-04-26 - 13:32 DeborahPinna  
PDFpdf n-1_mt.pdf r1 manage 15.2 K 2015-04-26 - 13:52 DeborahPinna  
PNGpng n-1_mt.png r1 manage 66.0 K 2015-04-26 - 13:34 DeborahPinna  
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