CMS-SUS-23-004

CMS-SUS-23-004 ; CERN-EP-2025-025
Search for dark matter production in association with a single top quark in proton-proton collisions at $\sqrt{s} =$ 13 TeV
CMS Collaboration
25 March 2025
Submitted to J. High Energy Phys.
Abstract: A search for the production of a single top quark in association with invisible particles is performed using proton-proton collision data collected with the CMS detector at the LHC at $\sqrt{s} =$ 13 TeV, corresponding to an integrated luminosity of 138 fb $^{-1}$ . In this search, a flavor-changing neutral current produces a single top quark or antiquark and an invisible state nonresonantly. The invisible state consists of a hypothetical spin-1 particle acting as a new mediator and decaying to two spin-1/2 dark matter candidates. The analysis searches for events in which the top quark or antiquark decays hadronically. No significant excess of events compatible with that signature is observed. Exclusion limits at 95% confidence level are placed on the masses of the spin-1 mediator and the dark matter candidates, and are compared to constraints from the dark matter relic density measurements. In a vector (axial-vector) coupling scenario, masses of the spin-1 mediator are excluded up to 1.85 (1.85) TeV with an expectation of 2.0 (2.0) TeV, whereas masses of the dark matter candidates are excluded up to 0.75 (0.55) TeV with an expectation of 0.85 (0.65) TeV.
Links: e-print arXiv:2503.20033 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagram of nonresonant mono-top production at tree level via a flavor-changing neutral current mediated by the spin-1 boson \mathrm{M}. The off-shell up quark (u) decays into an on-shell top quark (t) and an \mathrm{M} boson. The \mathrm{M} boson decays directly to a pair of DM candidates $\chi$ and $\overline{\chi}$ .
png pdf	Figure 2: Prefit distribution of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the SR. The last bin of the distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The distributions of background processes stem from simulation and are stacked together. A representative mono-top signal (vector coupling scenario) with a mediator mass of 1 TeV, a DM candidate mass of 150 GeV, and a cross section of 1 pb is overlaid as an orange line. The gray band represents the statistical and unconstrained systematic uncertainties in the simulated event yields.
png pdf	Figure 3: Categorization of events into SRs and CRs, which are sensitive to specific processes, namely the mono-top signal, V+jets ( $\mathrm{V} = \mathrm{Z}, \mathrm{W}, \gamma$ ) processes, and $\mathrm{t} \overline{\mathrm{t}}$ production. Each column contains categories that target the same process. For CRs with leptons in the final state, a version with electrons and a version with muons in the final states exist. Finally, in each category a split is performed based on whether the leading AK15 jet is t tagged or not.
png pdf	Figure 4: Prefit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the SR (t-pass) and SR (t-fail). The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The distributions of background processes stem from simulation and are stacked together. A representative mono-top signal (vector coupling scenario) with a mediator mass of 1 TeV, a DM candidate mass of 150 GeV, and a cross section of 1 pb is overlaid as an orange line. The gray band represents the statistical and unconstrained systematic uncertainties in the simulated event yields.
png pdf	Figure 4-a: Prefit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the SR (t-pass) and SR (t-fail). The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The distributions of background processes stem from simulation and are stacked together. A representative mono-top signal (vector coupling scenario) with a mediator mass of 1 TeV, a DM candidate mass of 150 GeV, and a cross section of 1 pb is overlaid as an orange line. The gray band represents the statistical and unconstrained systematic uncertainties in the simulated event yields.
png pdf	Figure 4-b: Prefit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the SR (t-pass) and SR (t-fail). The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The distributions of background processes stem from simulation and are stacked together. A representative mono-top signal (vector coupling scenario) with a mediator mass of 1 TeV, a DM candidate mass of 150 GeV, and a cross section of 1 pb is overlaid as an orange line. The gray band represents the statistical and unconstrained systematic uncertainties in the simulated event yields.
png pdf	Figure 5: Statistical model used for the estimation of the major background processes for one bin of the $R_{\mathrm{T}}$ distribution. The contributions of $\mathrm{Z}(\nu\nu)$ +jets and $\mathrm{t} \overline{\mathrm{t}}$ production in the SR are estimated with freely floating parameters $r^{\mathrm{Z}(\nu\nu)}$ and $r^{{\mathrm{t}\overline{\mathrm{t}}} }$ . Constraints on $\mathrm{Z}(\nu\nu)$ +jets production are obtained by expressing similar processes in the SR and CRs as products of $r^{\mathrm{Z}(\nu\nu)}$ and a TF, obtained from simulation. Concerning $\mathrm{t} \overline{\mathrm{t}}$ production, the $\mathrm{t} \overline{\mathrm{t}}$ processes in the ${\mathrm{t}\overline{\mathrm{t}}} (\ell\nu)$ and $\mathrm{W}(\ell\nu)$ CRs are expressed in terms of $r^{{\mathrm{t}\overline{\mathrm{t}}} }$ and a TF. All processes not depicted in this illustration are estimated using simulated events. Regions containing charged leptons are included twice in this model, once for electrons and once for muons. The model is implemented for the t-pass and t-fail regions separately.
png pdf	Figure 6: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the SR (t-pass) and SR (t-fail) after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. A representative mono-top signal (vector coupling scenario) with a mediator mass of 1 TeV, a DM candidate mass of 150 GeV, and a cross section of 1 pb is overlaid as an orange line. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 6-a: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the SR (t-pass) and SR (t-fail) after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. A representative mono-top signal (vector coupling scenario) with a mediator mass of 1 TeV, a DM candidate mass of 150 GeV, and a cross section of 1 pb is overlaid as an orange line. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 6-b: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the SR (t-pass) and SR (t-fail) after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. A representative mono-top signal (vector coupling scenario) with a mediator mass of 1 TeV, a DM candidate mass of 150 GeV, and a cross section of 1 pb is overlaid as an orange line. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 7: Upper limits at 95% CL on $\sigma\mathcal{B}$ of mono-top production presented in the two-dimensional plane spanned by the mediator and DM candidate masses for a mediator mass between 200 and 2250 GeV and a DM candidate mass between 1 and 1125 GeV only considering on-shell decays of the mediator to the DM candidates. The mediator has vector couplings to quarks and DM candidates in the upper plot and axial-vector couplings in the lower plot. The median expected exclusion range is indicated by a black solid line, demonstrating the search sensitivity of the analysis. The 68% probability interval of the expected exclusion is shown in black dashed lines. Contours of theory predictions for constant values of $\sigma\mathcal{B}$ are shown in gray dashed lines. The observed exclusion contour of mediator and DM candidate masses is represented by the red solid line. The exclusion contour obtained from measurements of the DM relic density $\Omega_{\text{nbm}}h^{2}$ by the Planck Collaboration is shown in the gray solid line.
png pdf	Figure 7-a: Upper limits at 95% CL on $\sigma\mathcal{B}$ of mono-top production presented in the two-dimensional plane spanned by the mediator and DM candidate masses for a mediator mass between 200 and 2250 GeV and a DM candidate mass between 1 and 1125 GeV only considering on-shell decays of the mediator to the DM candidates. The mediator has vector couplings to quarks and DM candidates in the upper plot and axial-vector couplings in the lower plot. The median expected exclusion range is indicated by a black solid line, demonstrating the search sensitivity of the analysis. The 68% probability interval of the expected exclusion is shown in black dashed lines. Contours of theory predictions for constant values of $\sigma\mathcal{B}$ are shown in gray dashed lines. The observed exclusion contour of mediator and DM candidate masses is represented by the red solid line. The exclusion contour obtained from measurements of the DM relic density $\Omega_{\text{nbm}}h^{2}$ by the Planck Collaboration is shown in the gray solid line.
png pdf	Figure 7-b: Upper limits at 95% CL on $\sigma\mathcal{B}$ of mono-top production presented in the two-dimensional plane spanned by the mediator and DM candidate masses for a mediator mass between 200 and 2250 GeV and a DM candidate mass between 1 and 1125 GeV only considering on-shell decays of the mediator to the DM candidates. The mediator has vector couplings to quarks and DM candidates in the upper plot and axial-vector couplings in the lower plot. The median expected exclusion range is indicated by a black solid line, demonstrating the search sensitivity of the analysis. The 68% probability interval of the expected exclusion is shown in black dashed lines. Contours of theory predictions for constant values of $\sigma\mathcal{B}$ are shown in gray dashed lines. The observed exclusion contour of mediator and DM candidate masses is represented by the red solid line. The exclusion contour obtained from measurements of the DM relic density $\Omega_{\text{nbm}}h^{2}$ by the Planck Collaboration is shown in the gray solid line.
png pdf	Figure 8: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{W}(\mathrm{e}\nu)$ (t-pass) and $\mathrm{W}(\mathrm{e}\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 8-a: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{W}(\mathrm{e}\nu)$ (t-pass) and $\mathrm{W}(\mathrm{e}\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 8-b: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{W}(\mathrm{e}\nu)$ (t-pass) and $\mathrm{W}(\mathrm{e}\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 9: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{W}(\mu\nu)$ (t-pass) and $\mathrm{W}(\mu\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 9-a: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{W}(\mu\nu)$ (t-pass) and $\mathrm{W}(\mu\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 9-b: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{W}(\mu\nu)$ (t-pass) and $\mathrm{W}(\mu\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 10: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{Z}(\mathrm{e}\mathrm{e})$ (t-pass) and $\mathrm{Z}(\mathrm{e}\mathrm{e})$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 10-a: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{Z}(\mathrm{e}\mathrm{e})$ (t-pass) and $\mathrm{Z}(\mathrm{e}\mathrm{e})$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 10-b: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{Z}(\mathrm{e}\mathrm{e})$ (t-pass) and $\mathrm{Z}(\mathrm{e}\mathrm{e})$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 11: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{Z}(\mu\mu)$ (t-pass) and $\mathrm{Z}(\mu\mu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 11-a: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{Z}(\mu\mu)$ (t-pass) and $\mathrm{Z}(\mu\mu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 11-b: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\mathrm{Z}(\mu\mu)$ (t-pass) and $\mathrm{Z}(\mu\mu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 12: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the ${\mathrm{t}\overline{\mathrm{t}}} (\mathrm{e}\nu)$ (t-pass) and ${\mathrm{t}\overline{\mathrm{t}}} (\mathrm{e}\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 12-a: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the ${\mathrm{t}\overline{\mathrm{t}}} (\mathrm{e}\nu)$ (t-pass) and ${\mathrm{t}\overline{\mathrm{t}}} (\mathrm{e}\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 12-b: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the ${\mathrm{t}\overline{\mathrm{t}}} (\mathrm{e}\nu)$ (t-pass) and ${\mathrm{t}\overline{\mathrm{t}}} (\mathrm{e}\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 13: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the ${\mathrm{t}\overline{\mathrm{t}}} (\mu\nu)$ (t-pass) and ${\mathrm{t}\overline{\mathrm{t}}} (\mu\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 13-a: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the ${\mathrm{t}\overline{\mathrm{t}}} (\mu\nu)$ (t-pass) and ${\mathrm{t}\overline{\mathrm{t}}} (\mu\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 13-b: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the ${\mathrm{t}\overline{\mathrm{t}}} (\mu\nu)$ (t-pass) and ${\mathrm{t}\overline{\mathrm{t}}} (\mu\nu)$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 14: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\gamma$ (t-pass) and $\gamma$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 14-a: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\gamma$ (t-pass) and $\gamma$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 14-b: Postfit distributions of the magnitude of the hadronic recoil $R_{\mathrm{T}}$ in the $\gamma$ (t-pass) and $\gamma$ (t-fail) CRs after a fit of the background model to the data. The last bin of each distribution also contains events with $R_{\mathrm{T}} >$ 1000 GeV. The background processes are stacked together. The gray band represents the statistical and postfit systematic uncertainties in the predicted background yields after the fit.
png pdf	Figure 15: Upper limits at 95% CL on $\sigma\mathcal{B}$ of mono-top production presented in the two-dimensional plane spanned by the mediator and DM candidate masses for a mediator mass between 200 and 2250 GeV and a DM candidate mass between 1 and 1125 GeV only considering on-shell decays of the mediator to the DM candidates. The mediator has vector couplings to quarks and DM candidates in the upper plot and axial-vector couplings in the lower plot. The median expected exclusion range is indicated by a black solid line, demonstrating the search sensitivity of the analysis. The 68% probability interval of the expected exclusion is shown in black dashed lines. Contours of theory predictions for constant values of $\sigma\mathcal{B}$ are shown in gray dashed lines. The observed exclusion contour of mediator and DM candidate masses is represented by the red solid line. The exclusion contour obtained from measurements of the DM relic density $\Omega_{\text{nbm}}h^{2}$ by the Planck Collaboration is shown in the gray solid line.
png pdf	Figure 15-a: Upper limits at 95% CL on $\sigma\mathcal{B}$ of mono-top production presented in the two-dimensional plane spanned by the mediator and DM candidate masses for a mediator mass between 200 and 2250 GeV and a DM candidate mass between 1 and 1125 GeV only considering on-shell decays of the mediator to the DM candidates. The mediator has vector couplings to quarks and DM candidates in the upper plot and axial-vector couplings in the lower plot. The median expected exclusion range is indicated by a black solid line, demonstrating the search sensitivity of the analysis. The 68% probability interval of the expected exclusion is shown in black dashed lines. Contours of theory predictions for constant values of $\sigma\mathcal{B}$ are shown in gray dashed lines. The observed exclusion contour of mediator and DM candidate masses is represented by the red solid line. The exclusion contour obtained from measurements of the DM relic density $\Omega_{\text{nbm}}h^{2}$ by the Planck Collaboration is shown in the gray solid line.
png pdf	Figure 15-b: Upper limits at 95% CL on $\sigma\mathcal{B}$ of mono-top production presented in the two-dimensional plane spanned by the mediator and DM candidate masses for a mediator mass between 200 and 2250 GeV and a DM candidate mass between 1 and 1125 GeV only considering on-shell decays of the mediator to the DM candidates. The mediator has vector couplings to quarks and DM candidates in the upper plot and axial-vector couplings in the lower plot. The median expected exclusion range is indicated by a black solid line, demonstrating the search sensitivity of the analysis. The 68% probability interval of the expected exclusion is shown in black dashed lines. Contours of theory predictions for constant values of $\sigma\mathcal{B}$ are shown in gray dashed lines. The observed exclusion contour of mediator and DM candidate masses is represented by the red solid line. The exclusion contour obtained from measurements of the DM relic density $\Omega_{\text{nbm}}h^{2}$ by the Planck Collaboration is shown in the gray solid line.

Tables
png pdf	Table 1: Requirements on electrons, photons, and muons that pass the loose or tight selection. For all objects, the minimal $p_{\mathrm{T}}$ , the maximal $\|\eta\|$ , and the efficiency of the object identification (ID) are provided. For muons, the requirements on the relative isolation $I_{\text{rel}}^{\mu}$ are also listed. A more detailed discussion is given in the text.
png pdf	Table 2: Overview of the selections for the SR and the CRs, including the preselection. In the $\mathrm{Z}(\ell\ell)$ CRs, loose leptons are used for the selection of additional objects in the final state. In all other CRs, leptons and photons from the tight collections are used to determine the number of additional objects. CRs with final state leptons are defined separately for electrons and muons.

Summary

A search for dark matter (DM) produced in association with a single top quark via a flavor changing neutral current, referred to as nonresonant mono-top production, was presented. The analysis was performed using data collected by the CMS experiment in 2016, 2017, and 2018 at the LHC at a center-of-mass-energy of 13 TeV, and corresponding to an integrated luminosity of 138 fb

$^{-1}$ . The Lorentz boost of the top quark is exploited to cluster the products of the hadronic top quark decay into a large-radius jet. Furthermore, a machine-learning-based discriminator is used to distinguish large-radius jets originating from hadronic top quark decays and large-radius jets produced purely through quantum chromodynamics processes. A robust statistical model was built to determine the main backgrounds in the signal regions using data in dedicated control regions. The distribution of the hadronic recoil in the signal and control regions is used to perform the statistical fit to the data. The data are consistent with the background-only hypothesis, and no evidence for DM produced in association with a single top quark was found. Limits at 95% confidence level are calculated for the product of the signal production cross section and the branching fraction of the mediator decaying into DM candidates. Limits were obtained for both a purely vector and a purely axial-vector mediator that couples to two DM candidates and to two standard model quarks: one from the first generation and another from the third. The analysis excludes mediators with masses up to 1.85 TeV, where 2.0 TeV is expected, for both the vector and the axial-vector coupling scenarios. Dark matter candidate masses below 750 (550) GeV, where 850 (650) GeV is expected, are excluded for the vector (axial-vector) coupling scenario. In both cases, the exclusion limits are calculated for mediator masses greater than 200 GeV and DM candidate masses greater than 1 GeV. The exclusion limit on the spin-1 mediator mass obtained in this search exceeds the previous CMS result on mono-top production [27] using the 2016 data set by 100 GeV.

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