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| CMS-PAS-EXO-19-003 | ||
| Search for dark matter produced in association with a Z boson in proton-proton collisions at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| June 2020 | ||
| Abstract: A search for dark matter (DM) particles is performed using events with a Z boson candidate and large missing transverse momentum. The analysis is based on proton-proton collision data at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC in 2016-2018, corresponding to an integrated luminosity of 137 fb$^{-1}$. The search uses the decay channels $\mathrm{Z}\to\mathrm{e}\mathrm{e}$ and $\mathrm{Z}\to\mu\mu$. No significant excess of events is observed over the background expected from standard model processes. Limits are set on DM production in the context of simplified models with vector, axial-vector, scalar, and pseudoscalar mediators, as well as a two-Higgs-doublet model with an additional pseudoscalar mediator. The limits are also provided for spin-dependent and spin-independent scattering cross sections and are compared to those from direct-detection experiments. The results are also interpreted in the context of models of invisible Higgs boson decays, unparticles, and large extra dimensions. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, EPJC 81 (2021) 13. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Feynman diagrams illustrative of the BSM processes that produce a final state of a Z boson and missing transverse momentum: (upper left) simplified dark matter model for a spin-1 mediator, (upper right) 2HDM+a model, (lower left) invisible Higgs boson decays, and (lower right) unparticle / large extra dimensions. Here $\chi$ represents a DM particle, while H and a represent the additional neutral Higgs boson and pseudoscalar respectively. The dotted line represents either an unparticle or a graviton. |
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Figure 1-a:
Feynman diagrams illustrative of the BSM processes that produce a final state of a Z boson and missing transverse momentum: (upper left) simplified dark matter model for a spin-1 mediator, (upper right) 2HDM+a model, (lower left) invisible Higgs boson decays, and (lower right) unparticle / large extra dimensions. Here $\chi$ represents a DM particle, while H and a represent the additional neutral Higgs boson and pseudoscalar respectively. The dotted line represents either an unparticle or a graviton. |
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Figure 1-b:
Feynman diagrams illustrative of the BSM processes that produce a final state of a Z boson and missing transverse momentum: (upper left) simplified dark matter model for a spin-1 mediator, (upper right) 2HDM+a model, (lower left) invisible Higgs boson decays, and (lower right) unparticle / large extra dimensions. Here $\chi$ represents a DM particle, while H and a represent the additional neutral Higgs boson and pseudoscalar respectively. The dotted line represents either an unparticle or a graviton. |
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Figure 1-c:
Feynman diagrams illustrative of the BSM processes that produce a final state of a Z boson and missing transverse momentum: (upper left) simplified dark matter model for a spin-1 mediator, (upper right) 2HDM+a model, (lower left) invisible Higgs boson decays, and (lower right) unparticle / large extra dimensions. Here $\chi$ represents a DM particle, while H and a represent the additional neutral Higgs boson and pseudoscalar respectively. The dotted line represents either an unparticle or a graviton. |
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Figure 1-d:
Feynman diagrams illustrative of the BSM processes that produce a final state of a Z boson and missing transverse momentum: (upper left) simplified dark matter model for a spin-1 mediator, (upper right) 2HDM+a model, (lower left) invisible Higgs boson decays, and (lower right) unparticle / large extra dimensions. Here $\chi$ represents a DM particle, while H and a represent the additional neutral Higgs boson and pseudoscalar respectively. The dotted line represents either an unparticle or a graviton. |
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Figure 2:
Emulated ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution in data and simulation for the 3$\ell $ (left) and 4$\ell $ (right) CRs. The last bin also includes any events with emulated $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 800 GeV Uncertainty bands correspond to the postfit combined statistical and systematic components. |
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Figure 2-a:
Emulated ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution in data and simulation for the 3$\ell $ (left) and 4$\ell $ (right) CRs. The last bin also includes any events with emulated $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 800 GeV Uncertainty bands correspond to the postfit combined statistical and systematic components. |
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Figure 2-b:
Emulated ${{p_{\mathrm {T}}} ^\text {miss}}$ distribution in data and simulation for the 3$\ell $ (left) and 4$\ell $ (right) CRs. The last bin also includes any events with emulated $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 800 GeV Uncertainty bands correspond to the postfit combined statistical and systematic components. |
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Figure 3:
The ${{p_{\mathrm {T}}} ^\text {miss}}$ distributions for events in the signal region in the 0-jet (left) and 1-jet (right) categories. The last bin also includes any events with $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 800 GeV. The uncertainty band includes both statistical and systematic components. The $\mathrm{Z} \mathrm{h} $ (invisible) signal normalization assumes SM production rates and the branching fraction $\mathcal {B}(\mathrm{h} \to $ invisible) $= $ 1. |
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Figure 3-a:
The ${{p_{\mathrm {T}}} ^\text {miss}}$ distributions for events in the signal region in the 0-jet (left) and 1-jet (right) categories. The last bin also includes any events with $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 800 GeV. The uncertainty band includes both statistical and systematic components. The $\mathrm{Z} \mathrm{h} $ (invisible) signal normalization assumes SM production rates and the branching fraction $\mathcal {B}(\mathrm{h} \to $ invisible) $= $ 1. |
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Figure 3-b:
The ${{p_{\mathrm {T}}} ^\text {miss}}$ distributions for events in the signal region in the 0-jet (left) and 1-jet (right) categories. The last bin also includes any events with $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 800 GeV. The uncertainty band includes both statistical and systematic components. The $\mathrm{Z} \mathrm{h} $ (invisible) signal normalization assumes SM production rates and the branching fraction $\mathcal {B}(\mathrm{h} \to $ invisible) $= $ 1. |
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Figure 4:
The $m_{\text T}$ distributions for events in the signal region in the 0-jet (left) and 1-jet (right) categories. The last bin also includes any events with $m_{\text T} > $ 1500 GeV. The uncertainty band includes both statistical and systematic components. The signal normalization assumes the expected values for $(m_\mathrm{H},m_{\text {A}}) = $ (1200,300) GeV within the 2HDM+a framework. |
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Figure 4-a:
The $m_{\text T}$ distributions for events in the signal region in the 0-jet (left) and 1-jet (right) categories. The last bin also includes any events with $m_{\text T} > $ 1500 GeV. The uncertainty band includes both statistical and systematic components. The signal normalization assumes the expected values for $(m_\mathrm{H},m_{\text {A}}) = $ (1200,300) GeV within the 2HDM+a framework. |
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Figure 4-b:
The $m_{\text T}$ distributions for events in the signal region in the 0-jet (left) and 1-jet (right) categories. The last bin also includes any events with $m_{\text T} > $ 1500 GeV. The uncertainty band includes both statistical and systematic components. The signal normalization assumes the expected values for $(m_\mathrm{H},m_{\text {A}}) = $ (1200,300) GeV within the 2HDM+a framework. |
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Figure 5:
The 95% CL exclusion limits for the vector (left) and the axial-vector (right) simplified models. The limits are made as a function of both the mediator and DM particle masses. The coupling to quarks is fixed to $g_q=$ 0.25 and the coupling to DM is set to $g_\chi =$ 1. |
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Figure 5-a:
The 95% CL exclusion limits for the vector (left) and the axial-vector (right) simplified models. The limits are made as a function of both the mediator and DM particle masses. The coupling to quarks is fixed to $g_q=$ 0.25 and the coupling to DM is set to $g_\chi =$ 1. |
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Figure 5-b:
The 95% CL exclusion limits for the vector (left) and the axial-vector (right) simplified models. The limits are made as a function of both the mediator and DM particle masses. The coupling to quarks is fixed to $g_q=$ 0.25 and the coupling to DM is set to $g_\chi =$ 1. |
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Figure 6:
The 90% CL DM nucleon cross section limits for simplified DM in the spin-independent (left) and spin-dependent (right) cases. Here, the coupling to quarks is set to $g_q=$ 0.25 and the coupling to DM is set to $g_\chi =$ 1. Limits from the XENON1T [89], LUX [90], PandaX-ll [91], CDMSLite [92], and DarkSide-50 [93] experiments are shown for the spin-independent case with vector couplings. Limits from the PICO-60 [94], PICO-2L [95], IceCube [96], and Super Kamiokande [97] experiments are shown for the spin-dependent case with axial-vector couplings. |
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Figure 6-a:
The 90% CL DM nucleon cross section limits for simplified DM in the spin-independent (left) and spin-dependent (right) cases. Here, the coupling to quarks is set to $g_q=$ 0.25 and the coupling to DM is set to $g_\chi =$ 1. Limits from the XENON1T [89], LUX [90], PandaX-ll [91], CDMSLite [92], and DarkSide-50 [93] experiments are shown for the spin-independent case with vector couplings. Limits from the PICO-60 [94], PICO-2L [95], IceCube [96], and Super Kamiokande [97] experiments are shown for the spin-dependent case with axial-vector couplings. |
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Figure 6-b:
The 90% CL DM nucleon cross section limits for simplified DM in the spin-independent (left) and spin-dependent (right) cases. Here, the coupling to quarks is set to $g_q=$ 0.25 and the coupling to DM is set to $g_\chi =$ 1. Limits from the XENON1T [89], LUX [90], PandaX-ll [91], CDMSLite [92], and DarkSide-50 [93] experiments are shown for the spin-independent case with vector couplings. Limits from the PICO-60 [94], PICO-2L [95], IceCube [96], and Super Kamiokande [97] experiments are shown for the spin-dependent case with axial-vector couplings. |
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Figure 7:
The 95% CL cross section limits for simplified DM with scalar (left) and pseudoscalar (right) mediators. Here, the coupling to quarks is set to $g_q=$ 1, the coupling to DM is set to $g_\chi =$ 1 and the DM mass is $m_\chi = $ 1 GeV. |
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Figure 7-a:
The 95% CL cross section limits for simplified DM with scalar (left) and pseudoscalar (right) mediators. Here, the coupling to quarks is set to $g_q=$ 1, the coupling to DM is set to $g_\chi =$ 1 and the DM mass is $m_\chi = $ 1 GeV. |
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Figure 7-b:
The 95% CL cross section limits for simplified DM with scalar (left) and pseudoscalar (right) mediators. Here, the coupling to quarks is set to $g_q=$ 1, the coupling to DM is set to $g_\chi =$ 1 and the DM mass is $m_\chi = $ 1 GeV. |
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Figure 8:
The 95% CL upper limits on the 2HDM+a model with the mixing angles set to $\tan(\beta)=$ 1 and $\sin(\theta)=$ 0.35 and with a DM particle mass of $m_{\chi} = $ 10 GeV. The limits are shown as a function of both mediator masses. |
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Figure 9:
The 95% CL upper limits on unparticle+Z production cross section as a function of the scaling dimension $d_{\textsf U}$. These limits apply to a fixed value of the effective cutoff scale $\Lambda _{\textsf U} = $ 15 TeV and a fixed coupling $\lambda =$ 1. |
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Figure 10:
The 95% CL cross section limits in the ADD scenario as a function of $M_{\text D}$ for different values of $n$. |
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Figure 10-a:
The 95% CL cross section limits in the ADD scenario as a function of $M_{\text D}$ for different values of $n$. |
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Figure 10-b:
The 95% CL cross section limits in the ADD scenario as a function of $M_{\text D}$ for different values of $n$. |
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Figure 10-c:
The 95% CL cross section limits in the ADD scenario as a function of $M_{\text D}$ for different values of $n$. |
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Figure 10-d:
The 95% CL cross section limits in the ADD scenario as a function of $M_{\text D}$ for different values of $n$. |
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Figure 10-e:
The 95% CL cross section limits in the ADD scenario as a function of $M_{\text D}$ for different values of $n$. |
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Figure 10-f:
The 95% CL cross section limits in the ADD scenario as a function of $M_{\text D}$ for different values of $n$. |
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Figure 11:
The 95% CL expected and observed exclusion limits on $M_{\text D}$ as a function of the number of extra dimensions $n$. |
| Tables | |
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Table 1:
Summary of the kinematic selections for the signal region. |
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Table 2:
Summary of impact of the systematic uncertainties considered in the $\mathrm{Z} \mathrm{h} $ (invisible) model assuming $\mathcal {B}(\mathrm{h} \to $ invisible) $= $ 1 (signal) and $\mathcal {B}(\mathrm{h} \to $ invisible) $= $ 0 (no signal). Here, lepton measurement refers to the combined trigger, lepton reconstruction and identification efficiencies, and the lepton momentum and electron energy scale systematics. |
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Table 3:
Observed number of events, post-fit background estimates in the signal regions. The reported uncertainty represents the sum in quadrature of the statistical and systematic uncertainties. |
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Table 4:
Expected yields and the product of acceptance and efficiency for several models used in the analysis. The quoted values correspond to the $\mathrm{Z} $ to $\ell \ell $ decays. The reported uncertainty represents the sum in quadrature of the statistical and systematic uncertainties. |
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Table 5:
Observed and expected 95% CL limits on parameters for the simplified DM model with both vector and axial vector mediators, invisible decays of the Higgs boson, two-Higgs doublet model, large extra dimensions in the ADD scenario, and unparticle model. For the scalar/pseudoscalar the limits are dependent on the mediator mass so the lowest values for the ratio of observed to theoretical cross sections are presented. |
| Summary |
| Events with a Z boson recoiling against missing transverse momentum in proton-proton collisions at the LHC were used to search for physics beyond the standard model. The results are interpreted in the context of several different models of the coupling mechanism between dark matter and ordinary matter: simplified models of dark matter with vector, axial-vector, scalar, and pseudoscalar mediators, invisible decays of a standard-model-like Higgs boson, and a two-Higgs double model with an extra pseudoscalar. Outside of the context of dark matter, models that invoke large extra dimensions or propose the production of unparticles could contribute to the same final signature and are also considered. The observed limits on the production cross sections are used to constrain parameters of each of these models. The search utilizes the entire Run 2 dataset of the CMS experiment, corresponding to an integrated luminosity of 137 fb$^{-1}$ at $ \sqrt{s} = $ 13 TeV. No evidence of physics beyond the Standard Model is observed. Comparing to the previous results in this channel with 35.9 fb$^{-1}$ for CMS [3] and for ATLAS [4], the exclusion limits for simplified dark matter mediators, gravitons and unparticles are significantly extended. For the case of a standard-model-like Higgs Boson, an upper limit of 29% is set for the branching fraction to fully invisible decays at 95% confidence level. Results for the 2HDM+a model are presented in this final state for the first time by CMS and probe masses of the pseudoscalar mediator up to 440 GeV and of the heavy Higgs scalar up to 1200 GeV when the other model parameters are set to specific benchmark values. |
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