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| CMS-EXO-17-015 ; CERN-EP-2018-278 | ||
| Search for dark matter in events with a leptoquark and missing transverse momentum in proton-proton collisions at 13 TeV | ||
| CMS Collaboration | ||
| 26 November 2018 | ||
| Phys. Lett. B 795 (2019) 76 | ||
| Abstract: A search is presented for dark matter 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 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 low-${p_{\mathrm{T}}}$ standard model particles. The signature for signal events would be significant missing transverse momentum from the dark matter in conjunction with 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 be consistent with the background predicted by the standard model. For the first benchmark scenario considered, dark matter masses up to 500 GeV are excluded for leptoquark masses ${{m_{\mathrm{LQ}}}} \approx$ 1400 GeV, and up to 300 GeV for ${{m_{\mathrm{LQ}}}} \approx$ 1500 GeV. For the second benchmark scenario, dark matter masses up to 600 GeV are excluded for ${{m_{\mathrm{LQ}}}} \approx$ 1400 GeV. | ||
| Links: e-print arXiv:1811.10151 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
An example Feynman diagram for the signal process considered in this study, where 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. |
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Figure 2:
The ${{m_{\mu \mathrm {j}}}}$ distributions in data and simulation for the (left) ${\mathrm{t} {}\mathrm{\bar{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 data points are statistical. The gray band shows the total uncertainty in the background prediction, including both statistical and systematic terms. |
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Figure 2-a:
The ${{m_{\mu \mathrm {j}}}}$ distributions in data and simulation for the ${\mathrm{t} {}\mathrm{\bar{t}}} $-enriched control sample for the combined 2016 and 2017 data sets. The data-to-simulation normalization scale factors have been applied to the simulated distribution. The lower panel shows the ratio of the observed to the simulated results. The vertical error bars on the data points are statistical. The gray band shows the total uncertainty in the background prediction, including both statistical and systematic terms. |
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Figure 2-b:
The ${{m_{\mu \mathrm {j}}}}$ distributions in data and simulation for the W+jets-enriched control sample for the combined 2016 and 2017 data sets. The data-to-simulation normalization scale factors have been applied to the simulated distribution. The lower panel shows the ratio of the observed to the simulated results. The vertical error bars on the data points are statistical. The gray band shows the total uncertainty in the background prediction, including both statistical and systematic terms. |
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Figure 3:
The observed distribution of ${{m_{\mu \mathrm {j}}}}$ in comparison to the post-fit SM background predictions for the combined 2016 and 2017 data sets. "Post-fit'' means that the constraints from the maximum likelihood fit are incorporated. The unstacked predictions for two signal models with $ {{m_{\mathrm {LQ}}}} = $ 1000 GeV and $ {m_{\mathrm {DM}}} = $ 400 GeV are also shown: one with $ {B} =$ 0.5 and the other with $ {B} =$ 0.1. The difference is just an overall relative normalization of about 2 for the latter compared to the former. The ratio of the observed results to the total SM prediction is shown in the lower panel. The vertical error bars on the data points are statistical. The gray band shows the total uncertainty in the background prediction, including both statistical and systematic terms. |
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Figure 4:
Observed 95% CL upper limits on the product of cross section and branching fraction for the signal model of Fig. 1 assuming $B = {\cal {B}}(\mathrm {LQ}\to {\mathrm {c}}\mu /\mathrm{s} \mu ) |_{{m_{\mathrm {DM}}} = {m_{{\mathrm {X}}}} =0} $ to be (left) 0.5 or (right) 0.1. The solid and dashed black curves show the observed and expected 95% CL exclusion curves, taking into account both upper and lower components of the LQ doublet. The solid blue curve shows the observed exclusion limit for the upper component of the LQ doublet, i.e. to a muon and a c quark. The dotted blue curve shows the corresponding observed limits from the recast of the results from a search for pair produced second-generation LQs [23]. |
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Figure 4-a:
Observed 95% CL upper limits on the product of cross section and branching fraction for the signal model of Fig. 1 assuming $B = {\cal {B}}(\mathrm {LQ}\to {\mathrm {c}}\mu /\mathrm{s} \mu ) |_{{m_{\mathrm {DM}}} = {m_{{\mathrm {X}}}} =0} $ to be 0.5. The solid and dashed black curves show the observed and expected 95% CL exclusion curves, taking into account both upper and lower components of the LQ doublet. The solid blue curve shows the observed exclusion limit for the upper component of the LQ doublet, i.e. to a muon and a c quark. The dotted blue curve shows the corresponding observed limits from the recast of the results from a search for pair produced second-generation LQs [23]. |
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Figure 4-b:
Observed 95% CL upper limits on the product of cross section and branching fraction for the signal model of Fig. 1 assuming $B = {\cal {B}}(\mathrm {LQ}\to {\mathrm {c}}\mu /\mathrm{s} \mu ) |_{{m_{\mathrm {DM}}} = {m_{{\mathrm {X}}}} =0} $ to be 0.1. The solid and dashed black curves show the observed and expected 95% CL exclusion curves, taking into account both upper and lower components of the LQ doublet. The solid blue curve shows the observed exclusion limit for the upper component of the LQ doublet, i.e. to a muon and a c quark. The dotted blue curve shows the corresponding observed limits from the recast of the results from a search for pair produced second-generation LQs [23]. |
| Tables | |
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Table 1:
Systematic uncertainties affecting the normalization of signal and background distributions. The PDF uncertainty affects the signal distribution only, while the other uncertainties affect both the signal and background distributions. |
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Table 2:
Observed number of events, post-fit SM background predictions and post-fit uncertainties 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 predictions for two signal models with $ {{m_{\mathrm {LQ}}}} = $ 1000 GeV and $ {m_{\mathrm {DM}}} = $ 400 GeV are also shown: one with $ {B} =$ 0.5 and the other with $ {B} =$ 0.1. The uncertainties represent the statistical and systematic terms added in quadrature. |
| Summary |
| A search has been performed for dark matter in events containing a muon, a jet, and significant missing transverse momentum. The study is conducted using proton-proton collision data at $\sqrt{s} = $ 13 TeV recorded with the CMS detector, corresponding to an integrated luminosity of 77.4 fb$^{-1}$. It is assumed that dark matter is produced through the production of a leptoquark pair, with one leptoquark decaying to a muon and a jet, and the other to dark matter and low-${p_{\mathrm{T}}}$ standard model particles. The analysis 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, with the requirement of significant missing transverse momentum, as is expected from the presence of dark matter. The observation of such a peak in this novel search would provide strong evidence for the existence of both dark matter particles and leptoquarks. The data are observed to agree with the standard model background predictions within the uncertainties. Upper limits on the product of the cross section and branching fraction are obtained at 95% confidence level as a function of the leptoquark and dark matter particle masses. For the first benchmark scenario considered, dark matter masses up to 500 GeV are excluded for leptoquark masses ${{m_{\mathrm{LQ}}}} \approx$ 1400 GeV, and up to 300 GeV for ${{m_{\mathrm{LQ}}}} \approx$ 1500 GeV. For the second benchmark scenario, dark matter masses up to 600 GeV are excluded for ${{m_{\mathrm{LQ}}}} \approx$ 1400 GeV. |
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