CMS-PAS-EXO-17-015

CMS-PAS-EXO-17-015
Search for dark matter in final states with a leptoquark and missing transverse momentum in proton-proton collisions at 13 TeV
CMS Collaboration
July 2018

Abstract: A search for dark matter is presented in proton-proton collisions at a center-of-mass energy of $\sqrt{s}= $ 13 TeV using events with at least one high transverse momentum ($p_{\mathrm{T}}$) muon, at least one high $p_{\mathrm{T}}$ jet, and large missing transverse momentum. The search is performed in the context of the so-called coannihilation codex, a new paradigm that introduces novel scenarios for dark matter production. The data were collected with the CMS detector at the CERN LHC in 2016 and 2017, and correspond to an integrated luminosity of 77.4 fb$^{-1}$. In the examined scenario, a pair of scalar leptoquarks is assumed to be produced. One leptoquark decays to a muon and a jet while the other decays to dark matter and soft standard model particles. The dark matter signature is given by a peak at the leptoquark mass in the invariant mass distribution of the highest $p_{\mathrm{T}}$ muon and jet. The data are observed to agree with the predictions from the standard model. Leptoquarks with masses up to 1160 GeV are excluded.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 795 (2019) 76. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Example Feynman diagram for the signal process considered in this study, where ${\mathrm {g}}$ is a gluon, LQ a leptoquark, DM a dark matter particle, and X a new Dirac fermion. The superscript "*'' indicates an off-shell particle.
png pdf	Figure 2: The ${{m_{{\mu} \mathrm {j}}}}$ distributions in data and simulation for the (left) ${{\mathrm {t}\overline {\mathrm {t}}}}$- and (right) W+jets-enriched control samples for the combined 2016 and 2017 data sets. The respective data-to-simulation normalization scale factors have been applied to the simulated distributions. The lower panels show the ratio of the observed to the simulated results. The vertical error bars on the observed points indicate the statistical uncertainties. The gray band shows the total uncertainties, including both statistical and systematic terms.
png pdf	Figure 2-a: The ${{m_{{\mu} \mathrm {j}}}}$ distributions in data and simulation for the (left) ${{\mathrm {t}\overline {\mathrm {t}}}}$- and (right) W+jets-enriched control samples for the combined 2016 and 2017 data sets. The respective data-to-simulation normalization scale factors have been applied to the simulated distributions. The lower panels show the ratio of the observed to the simulated results. The vertical error bars on the observed points indicate the statistical uncertainties. The gray band shows the total uncertainties, including both statistical and systematic terms.
png pdf	Figure 2-b: The ${{m_{{\mu} \mathrm {j}}}}$ distributions in data and simulation for the (left) ${{\mathrm {t}\overline {\mathrm {t}}}}$- and (right) W+jets-enriched control samples for the combined 2016 and 2017 data sets. The respective data-to-simulation normalization scale factors have been applied to the simulated distributions. The lower panels show the ratio of the observed to the simulated results. The vertical error bars on the observed points indicate the statistical uncertainties. The gray band shows the total uncertainties, including both statistical and systematic terms.
png pdf	Figure 3: The observed distribution of ${{m_{{\mu} \mathrm {j}}}}$ in comparison to the postfit SM background predictions for the combined 2016 and 2017 data sets. "Postfit'' means that the constraints from the maximum likelihood fit are incorporated. The unstacked prediction for a signal model with $ {{m_{\mathrm {LQ}}}} = $ 1000 GeV and $ {m_{\mathrm {DM}}} = $ 400 GeV is also shown. The ratio of the observed results to the total SM prediction is shown in the lower panel. The vertical error bars on the observed points indicate the statistical uncertainties. The gray band shows the total uncertainties, including both statistical and systematic terms.
png pdf	Figure 4: Observed upper limits at 95% CL on the product of cross section and branching fraction, for the signal model of Fig. xxxxx. The hatched (blue) band represents the region where the dark matter relic density requirement can be satisfied within 3 standard deviations ($\sigma $), including the APV bounds. The solid and dashed (black) curves show the observed and expected 95% CL exclusion curves.

Tables
png pdf	Table 1: Systematic uncertainties in the yield of signal events assuming leptoquark and dark matter masses of 1000 and 400 GeV, respectively.
png pdf	Table 2: Observed number of events and postfit SM background predictions for the combined 2016 and 2017 data sets. "Electroweak" refers to the sum of expected events from the single top quark, Z boson, and diboson background processes. The prediction for a signal model with $ {{m_{\mathrm {LQ}}}} = $ 1000 GeV and $ {m_{\mathrm {DM}}} = $ 400 GeV is also shown. The uncertainties represent the statistical and systematic terms added in quadrature.

Summary

A search is performed for dark matter in events containing a muon, a jet, and significant missing transverse momentum. The study is performed using proton-proton collision data at $\sqrt{s} = $ 13 TeV recorded with the CMS detector at the LHC. The data correspond to an integrated luminosity of 77.4 fb$^{-1}$.

We assume the production of dark matter through the pair production of a leptoquark pair, in which one leptoquark decays to a muon and a jet and the other to DM and soft standard model particles. The study is performed by searching for a peak in the leptoquark candidate invariant mass $ m_{\mu\mathrm{j}} $ distribution formed from the highest ${p_{\mathrm{T}}}$ muon and jet in an event, after requiring significant missing transverse momentum as is expected from the presence of DM. The search for a peak in $ m_{\mu\mathrm{j}} $ provides a novel means to search for DM at the LHC. The data are observed to agree with the standard model predictions within the uncertainties. Upper limits on the product of cross section and branching fraction, divided by the theoretical cross section, are determined at 95% confidence level as a function of the leptoquark and dark matter particle masses. Leptoquarks with masses up to 1160 GeV are excluded for dark matter mass ${m_{\mathrm{DM}}} \approx $ 300 GeV, and up to 1000 GeV for ${m_{\mathrm{DM}}} \approx $ 425 GeV.

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