CMS-PAS-EXO-16-028

CMS-PAS-EXO-16-028
Search for dark matter in association with a top quark pair at $\sqrt{s}= $ 13 TeV in the dilepton channel
CMS Collaboration
October 2016

Abstract: A search is performed for dark matter production with top quark pairs decaying to dileptons in proton-proton collisions at centre of mass energy 13 TeV. Results are interpreted using simplified models for collider production of dark matter and constraints are placed on the parameter space. The data corresponds to an integrated luminosity of 2.2 fb$^{-1}$ collected by the CMS detector at the LHC in 2015. A combination is performed with previously reported analyses in the semileptonic and hadronic channels.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The dominant contribution to the $\mathrm{ t \bar{t} }$+DM process with a spin-0 mediator in the simplified model scheme.
png pdf	Figure 2: Background-only post-fit $ {E_{\mathrm {T}}^{\text {miss}}} $ distributions for each dilepton channel $\mathrm{ee}$, $\mathrm{e}\mu$ and $\mu\mu$, with the pre-fit SM background, and scalar $M_{\text{MED}}= $ 10 GeV, $M_{\text{DM}}= $ 1 GeV signal overlaid. The last bin includes overflow. The uncertainty band describes the post-fit uncertainty on the total SM background.
png pdf	Figure 2-a: Background-only post-fit $ {E_{\mathrm {T}}^{\text {miss}}} $ distribution for the $\mathrm{ee}$ channel, with the pre-fit SM background, and scalar $M_{\text{MED}}= $ 10 GeV, $M_{\text{DM}}= $ 1 GeV signal overlaid. The last bin includes overflow. The uncertainty band describes the post-fit uncertainty on the total SM background.
png pdf	Figure 2-b: Background-only post-fit $ {E_{\mathrm {T}}^{\text {miss}}} $ distribution for the $\mathrm{e}\mu$ channel, with the pre-fit SM background, and scalar $M_{\text{MED}}= $ 10 GeV, $M_{\text{DM}}= $ 1 GeV signal overlaid. The last bin includes overflow. The uncertainty band describes the post-fit uncertainty on the total SM background.
png pdf	Figure 2-c: Background-only post-fit $ {E_{\mathrm {T}}^{\text {miss}}} $ distribution for the $\mu\mu$ channel, with the pre-fit SM background, and scalar $M_{\text{MED}}= $ 10 GeV, $M_{\text{DM}}= $ 1 GeV signal overlaid. The last bin includes overflow. The uncertainty band describes the post-fit uncertainty on the total SM background.
png pdf	Figure 3: The observed and expected limits, expressed as the ratio of the 95% C.L. upper limit on DM production cross section to the cross section from simplified model expectations for scalar and pseudoscalar models with a dark matter mass of 1 GeV and $g_q = g_{\text{DM}} =$ 1.
png pdf	Figure 3-a: The observed and expected limits, expressed as the ratio of the 95% C.L. upper limit on DM production cross section to the cross section from simplified model expectations for the scalar model with a dark matter mass of 1 GeV and $g_q = g_{\text{DM}} =$ 1.
png pdf	Figure 3-b: The observed and expected limits, expressed as the ratio of the 95% C.L. upper limit on DM production cross section to the cross section from simplified model expectations for the pseudoscalar model with a dark matter mass of 1 GeV and $g_q = g_{\text{DM}} =$ 1.
png pdf	Figure 4: The observed and expected limits, expressed as the ratio of the 95% C.L. upper limit on DM production cross section to the cross section from simplified model expectations for scalar and pseudoscalar models in the combination of dileptonic, semileptonic, and hadronic channels.
png pdf	Figure 4-a: The observed and expected limits, expressed as the ratio of the 95% C.L. upper limit on DM production cross section to the cross section from simplified model expectations for the scalar model in the combination of dileptonic, semileptonic, and hadronic channels.
png pdf	Figure 4-b: The observed and expected limits, expressed as the ratio of the 95% C.L. upper limit on DM production cross section to the cross section from simplified model expectations for the pseudoscalar model in the combination of dileptonic, semileptonic, and hadronic channels.

Tables
png pdf	Table 1: Background-only post-fit yields and uncertainties. The pre-fit yields for the scalar $M_{\text{MED}}= $ 10 GeV, $M_{\text{DM}}= $ 1 GeV signal and SM expected backgrounds are listed with statistical uncertainties only.
png pdf	Table 2: The observed and expected limits, expressed as the ratio of the 95% C.L. upper limit on DM production cross section to the cross section from simplified model expectations for scalar (S) and pseudoscalar (PS) models from the search in the dilepton channel.
png pdf	Table 3: Listing of systematic uncertainties shown as a percentage of the expected yields for the dominant backgrounds in each channel. The normalisation uncertainties in the dilepton channel apply to the SM $\mathrm{ t \bar{t} }$($2\ell $) process and fall within the uncertainty ranges listed in Sec. {sec:syst}. The exceptions are $R_{\text {in/out}}$ and fakes normalisation which only impact the DY and fakes processes respectively, in the dilepton channel. The uncertainties listed for the $R_{\text {in/out}}$ and Fakes normalisation systematics are per dilepton channel. The uncertainties listed in the ``Semileptonic'' and ``Hadronic'' columns are described in ref [16] and apply to the dominant backgrounds in the respective channels. A ``-'' denotes that the systematic source is not applicable. The shape systematics are illustrative of the magnitude of the uncertainty relative to the expected yield. Sources common across channels are correlated in the channel combination fit with the exception of the lepton efficiency. The lepton efficiency and lepton trigger efficiency are inclusive in lepton flavour for the semileptonic and hadronic channels, however each systematic is split per flavour in the dilepton channel, and thus they are not correlated between the dilepton channel and the others. Uncertainties listed as ranges are across various control/signal regions in each channel.
png pdf	Table 4: The observed and expected limits, expressed as the ratio of the 95% C.L. upper limit on DM production cross section to the cross section from simplified model expectations for scalar (S) and pseudoscalar (PS) models in the combination of dileptonic, semileptonic, and hadronic channels.

Summary

A search for dark matter produced in association with top quark pairs decaying to dileptons has been presented. The data sample corresponds to proton-proton collisions at $\sqrt{s}= $ 13 TeV and integrated luminosity of 2.2 fb$^{-1}$ collected by the CMS detector at the LHC during 2015. No significant deviation from SM background expectation is observed in the $E_{\mathrm{T}}^{\text{miss}}$ spectrum for the three dilepton channels ($\mathrm{ee}$, $\mathrm{e}\mu$, $\mu\mu$). A combination with recent results in the semileptonic and hadronic channels is performed. The combination analysis obtains an expected exclusion of scalar mediators with masses up to 39 GeV at 95% CL, with the assumption of Dirac DM particles with $M_{\textrm{DM}}= $ 1 GeV and $g_q=g_{\mathrm{DM}}= $ 1. The observed limits at 95% C.L. fall just short of excluding any range of mediator masses.

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