CMS-PAS-SUS-24-003

CMS-PAS-SUS-24-003
Search for Higgsinos in final states with low-momentum lepton-track pairs at 13 TeV
CMS Collaboration
15 May 2025

Abstract: A search for the pair production of Higgsinos in final states with large missing transverse momentum and either two reconstructed muons or a reconstructed lepton (muon or electron) and an isolated track is presented. The analyzed data are proton-proton collisions with an integrated luminosity of 138 fb$ ^{-1} $ collected by the CMS experiment in proton-proton collisions at $ \sqrt{s} = $ 13 TeV. The signal scenario considers two neutralino states differing in mass by small values of approximately 0.5--5 GeV, in which the heavier neutralino decays into the lighter neutralino and two same-flavor leptons. The selection focuses on cases in which either the lepton $ p_{\mathrm{T}} $ or the opening angle between the leptons is smaller than that required by previous searches. Multivariate discriminants are used to enhance the sensitivity by efficiently rejecting backgrounds from SM processes or fake tracks and leptons. The search explores a unique phase space and probes a previously unexplored region in the signal model parameter space. Mass differences between the lightest and next-to-lightest neutralinos are probed as low as 1.5 GeV, assuming a 100 GeV Higgsino, as well as Higgsino masses up to 145 GeV for a mass difference of 4 GeV.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagrams illustrating the production and decay of electroweakinos in the Higgsino simplified model, through the $ \tilde{\chi}_{2}^{0} \tilde{\chi}_{1}^{0} $ (left) and $ \tilde{\chi}_{2}^{0} \tilde{\chi}_{1}^{\pm} $ (right) processes.
png pdf	Figure 1-a: Feynman diagrams illustrating the production and decay of electroweakinos in the Higgsino simplified model, through the $ \tilde{\chi}_{2}^{0} \tilde{\chi}_{1}^{0} $ (left) and $ \tilde{\chi}_{2}^{0} \tilde{\chi}_{1}^{\pm} $ (right) processes.
png pdf	Figure 1-b: Feynman diagrams illustrating the production and decay of electroweakinos in the Higgsino simplified model, through the $ \tilde{\chi}_{2}^{0} \tilde{\chi}_{1}^{0} $ (left) and $ \tilde{\chi}_{2}^{0} \tilde{\chi}_{1}^{\pm} $ (right) processes.
png pdf	Figure 2: Unweighted distributions of event-level BDT scores for events drawn from the signal and background training samples in the dimuon category (left) and muon+track category (right), based on Phase-1 conditions.
png pdf	Figure 2-a: Unweighted distributions of event-level BDT scores for events drawn from the signal and background training samples in the dimuon category (left) and muon+track category (right), based on Phase-1 conditions.
png pdf	Figure 2-b: Unweighted distributions of event-level BDT scores for events drawn from the signal and background training samples in the dimuon category (left) and muon+track category (right), based on Phase-1 conditions.
png pdf	Figure 3: Distributions of the reconstructed ditau invariant mass ($ m_{\tau\tau} $) in the BDT sideband control region, shown for Phase-0 (left) and Phase-1 (right). The non-$ \tau\tau $ background is estimated using the data-driven jetty background method described in the text.
png pdf	Figure 3-a: Distributions of the reconstructed ditau invariant mass ($ m_{\tau\tau} $) in the BDT sideband control region, shown for Phase-0 (left) and Phase-1 (right). The non-$ \tau\tau $ background is estimated using the data-driven jetty background method described in the text.
png pdf	Figure 3-b: Distributions of the reconstructed ditau invariant mass ($ m_{\tau\tau} $) in the BDT sideband control region, shown for Phase-0 (left) and Phase-1 (right). The non-$ \tau\tau $ background is estimated using the data-driven jetty background method described in the text.
png pdf	Figure 4: Prefit expected and observed distributions of the event BDT output score in the signal regions for the dimuon category (top) and the dimuon invariant mass in the signal region for events with event classifier scores greater than 0.1 (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The gray hatching shows the statistical uncertainty in the background prediction, while the green band indicates the relative systematic uncertainty in the predicted background. The vertical black bars represent the total uncertainty, including both statistical and systematic components. Two example signal scenarios are also shown as colored lines.
png pdf	Figure 4-a: Prefit expected and observed distributions of the event BDT output score in the signal regions for the dimuon category (top) and the dimuon invariant mass in the signal region for events with event classifier scores greater than 0.1 (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The gray hatching shows the statistical uncertainty in the background prediction, while the green band indicates the relative systematic uncertainty in the predicted background. The vertical black bars represent the total uncertainty, including both statistical and systematic components. Two example signal scenarios are also shown as colored lines.
png pdf	Figure 4-b: Prefit expected and observed distributions of the event BDT output score in the signal regions for the dimuon category (top) and the dimuon invariant mass in the signal region for events with event classifier scores greater than 0.1 (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The gray hatching shows the statistical uncertainty in the background prediction, while the green band indicates the relative systematic uncertainty in the predicted background. The vertical black bars represent the total uncertainty, including both statistical and systematic components. Two example signal scenarios are also shown as colored lines.
png pdf	Figure 4-c: Prefit expected and observed distributions of the event BDT output score in the signal regions for the dimuon category (top) and the dimuon invariant mass in the signal region for events with event classifier scores greater than 0.1 (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The gray hatching shows the statistical uncertainty in the background prediction, while the green band indicates the relative systematic uncertainty in the predicted background. The vertical black bars represent the total uncertainty, including both statistical and systematic components. Two example signal scenarios are also shown as colored lines.
png pdf	Figure 4-d: Prefit expected and observed distributions of the event BDT output score in the signal regions for the dimuon category (top) and the dimuon invariant mass in the signal region for events with event classifier scores greater than 0.1 (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The gray hatching shows the statistical uncertainty in the background prediction, while the green band indicates the relative systematic uncertainty in the predicted background. The vertical black bars represent the total uncertainty, including both statistical and systematic components. Two example signal scenarios are also shown as colored lines.
png pdf	Figure 5: Prefit expected and observed distributions of the event BDT output score in the signal regions for the muon + exclusive track category (top), and the electron + exclusive track category (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The vertical black bars represent the total uncertainty, including both statistical and systematic components. Example signal benchmark scenarios are also shown as colored lines.
png pdf	Figure 5-a: Prefit expected and observed distributions of the event BDT output score in the signal regions for the muon + exclusive track category (top), and the electron + exclusive track category (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The vertical black bars represent the total uncertainty, including both statistical and systematic components. Example signal benchmark scenarios are also shown as colored lines.
png pdf	Figure 5-b: Prefit expected and observed distributions of the event BDT output score in the signal regions for the muon + exclusive track category (top), and the electron + exclusive track category (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The vertical black bars represent the total uncertainty, including both statistical and systematic components. Example signal benchmark scenarios are also shown as colored lines.
png pdf	Figure 5-c: Prefit expected and observed distributions of the event BDT output score in the signal regions for the muon + exclusive track category (top), and the electron + exclusive track category (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The vertical black bars represent the total uncertainty, including both statistical and systematic components. Example signal benchmark scenarios are also shown as colored lines.
png pdf	Figure 5-d: Prefit expected and observed distributions of the event BDT output score in the signal regions for the muon + exclusive track category (top), and the electron + exclusive track category (bottom), shown separately for Phase-0 (left) and Phase-1 (right). The vertical black bars represent the total uncertainty, including both statistical and systematic components. Example signal benchmark scenarios are also shown as colored lines.
png pdf	Figure 6: The 95% confidence level (CL) upper limits on the fully-degenerate Higgsino production cross section, calculated at NLO+NLL precision [65,66], are shown in color in the plane of $ \Delta m^\pm $ versus the chargino mass. All relevant production modes are simulated at leading order, and the Z* boson is set to decay into either two electrons or two muons with a branching fraction of 5%. The expected (red) and observed (black) exclusion contours are shown assuming the theoretical cross section. Dashed red lines indicate the expected limits with $ \pm $1 and $ \pm$2$ \sigma) $ experimental uncertainty. Dashed black lines indicate the observed limit when varying the theoretical cross section by its uncertainty. The green line represents the minimum $ \Delta m^\pm $ allowed by the theoretical calculation accounting for radiative corrections, as described in [23].
png pdf	Figure 7: Comparison of limits with analyses featuring final states with disappearing tracks [34], a soft isolated track [38], and soft opposite-sign electron pairs [67].

Tables
png pdf	Table 1: Input variables to the BDT used for selecting in-signal tracks in the exclusive track category, ranked by their importance as determined by the TMVA algorithm.
png pdf	Table 2: Transfer factors and their associated statistical and total relative uncertainties, used to extrapolate background predictions from control regions to the signal region.

Summary

A search for Higgsino pair production in compressed mass spectra scenarios is performed using low-momentum lepton-track pairs in proton-proton collisions at $ \sqrt{s} = $ 13 TeV, based on a data sample corresponding to an integrated luminosity of 137 fb$ ^{-1} $ [68,69,70,71] collected with the CMS detector. The results are interpreted in a simplified model featuring a dark matter candidate neutralino that is nearly mass-degenerate with a slightly heavier neutralino and two charginos. The search targets a region of parameter space where sensitivity was limited in previous analyses. This region, characterized by low-mass Higgsinos, is of particular theoretical interest because of its relevance for naturalness and fine-tuning arguments, offering possible resolutions to both the Large and Small Hierarchy problems. The observed yields are statistically consistent with the background-only hypothesis, though a modest excess is observed in the most sensitive signal regions, more pronounced in Phase-1 than in Phase-0. The local significance of the excess reaches approximately 3 standard deviations. These results place additional constraints on natural supersymmetry and other models predicting electroweak multiplet dark matter.

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