CMS-EXO-20-010

CMS-EXO-20-010 ; CERN-EP-2023-083
Search for inelastic dark matter in events with two displaced muons and missing transverse momentum in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
19 May 2023
Phys. Rev. Lett. 132 (2024) 041802
Abstract: A search for dark matter in events with a displaced nonresonant muon pair and missing transverse momentum is presented. The analysis is performed using an integrated luminosity of 138 fb$ ^{-1} $ of proton-proton (pp) collision data at a center-of-mass energy of 13 TeV produced by the LHC in 2016--2018. No significant excess over the predicted backgrounds is observed. Upper limits are set on the product of the inelastic dark matter production cross section $ \sigma(\mathrm{p}\mathrm{p}\to \mathrm{A}' \to \chi_1 \, \chi_2) $ and the decay branching fraction $ \mathcal{B}(\chi_2 \to \chi_1 \, \mu^{+} \, \mu^{-}) $, where A' is a dark photon and $ \chi_1 $ and $ \chi_2 $ are states in the dark sector with near mass degeneracy. This is the first dedicated collider search for inelastic dark matter.
Links: e-print arXiv:2305.11649 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of IDM production and decay in pp collisions. The heavier DM state $ \chi_2 $ can be long-lived, and decays to $ \chi_1 $ and to a muon pair via an off-shell dark photon A'.
png pdf	Figure 2: Simulated muon reconstruction efficiency of standard (blue squares) and displaced (red circles) reconstruction algorithms as a function of transverse vertex displacement $ v_{xy} $. The two dashed vertical gray lines denote the ends of the fiducial tracker and muon detector regions, respectively.
png pdf	Figure 3: Measured min-$d_{xy} $ distribution in the 2-match category, after requiring the min-$d_{xy} $ muon to pass the isolation requirement $ I_{\text{PF}}^{\text{rel}} < $ 0.25 (i.e.,, the B and D bins of the ABCD plane). Overlaid with a red histogram is the background predicted from the region of the ABCD plane failing the same requirement (the A and C bins), as well as three signal benchmark hypotheses (as defined in the legends), assuming $ \alpha_{\text{D}} = \alpha_{\text{EM}} $ (the fine-structure constant). The red hatched bands correspond to the background prediction uncertainty. The last bin includes the overflow.
png pdf	Figure 4: Two-dimensional observed limits on $ \sigma(\mathrm{p}\mathrm{p} \to \mathrm{A}' \to \chi_1 \chi_2) \, \mathcal{B}(\chi_2 \to \chi_1 \mu^{+} \mu^{-}) $, for $ \Delta = $ 0.1 $ m_1 $ (left) and 0.4 $ m_1 $ (right), as functions of the DM mass $ m_1 $ and the interaction strength $ y $, with $ m_{\mathrm{A}'} = $ 3 $ m_1 $. Solid (dashed) curves denote the observed (expected) exclusion limits at 95% CL, with 68% CL uncertainty bands around the expectation. Regions above the curves are excluded, depending on the $ \alpha_{\text{D}} $ hypothesis: $ \alpha_{\text{D}} = \alpha_{\text{EM}} $ (dark blue) or 0.1 (light magenta).
png pdf	Figure 4-a: Two-dimensional observed limits on $ \sigma(\mathrm{p}\mathrm{p} \to \mathrm{A}' \to \chi_1 \chi_2) \, \mathcal{B}(\chi_2 \to \chi_1 \mu^{+} \mu^{-}) $, for $ \Delta = $ 0.1 $ m_1 $, as functions of the DM mass $ m_1 $ and the interaction strength $ y $, with $ m_{\mathrm{A}'} = $ 3 $ m_1 $. Solid (dashed) curves denote the observed (expected) exclusion limits at 95% CL, with 68% CL uncertainty bands around the expectation. Regions above the curves are excluded, depending on the $ \alpha_{\text{D}} $ hypothesis: $ \alpha_{\text{D}} = \alpha_{\text{EM}} $ (dark blue) or 0.1 (light magenta).
png pdf	Figure 4-b: Two-dimensional observed limits on $ \sigma(\mathrm{p}\mathrm{p} \to \mathrm{A}' \to \chi_1 \chi_2) \, \mathcal{B}(\chi_2 \to \chi_1 \mu^{+} \mu^{-}) $, for $ \Delta = $ 0.4 $ m_1 $, as functions of the DM mass $ m_1 $ and the interaction strength $ y $, with $ m_{\mathrm{A}'} = $ 3 $ m_1 $. Solid (dashed) curves denote the observed (expected) exclusion limits at 95% CL, with 68% CL uncertainty bands around the expectation. Regions above the curves are excluded, depending on the $ \alpha_{\text{D}} $ hypothesis: $ \alpha_{\text{D}} = \alpha_{\text{EM}} $ (dark blue) or 0.1 (light magenta).

Tables
png pdf	Table 1: Definition of ABCD bins and yields in data, per match category. The predicted yield in the bin with the smallest backgrounds (bin D) is extracted from the simultaneous four-bin fit by assuming zero signal, which corresponds to (Obs. B) $\times$ (Obs. C) $ / $ (Obs. A) in this limit.
png pdf	Table 2: Systematic uncertainties in the analysis, in percent. The jet uncertainties are larger in 2017 because of noise issues with the ECAL endcap. The tracking inefficiency in 2016 is caused by the unexpected saturation of photodiode signals in the tracker.

Summary

In summary, a search has been presented for inelastically coupled dark matter (DM) with a unique final-state signature including a soft, displaced muon pair collimated with the missing transverse momentum vector. The analysis is performed using proton-proton collision data produced by the LHC at a center-of-mass energy of 13 TeV and collected with the CMS experiment in 2016-2018. The data sample corresponds to an integrated luminosity of 138 fb$ ^{-1} $. Control samples in data are used to predict the background, and no significant excess is observed over standard model expectations. Upper limits are set on the product of the DM production cross section and decay branching fraction into muons as a function of DM mass $ m_1 $ and interaction strength. This is the first dedicated collider search for inelastic dark matter and it significantly expands the sensitivity to $ m_1 $ above the GeV scale.

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