CMS-SUS-24-012

CMS-SUS-24-012 ; CERN-EP-2026-103
Search for electroweakinos in compressed-spectrum scenarios with low-momentum isolated tracks in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
28 April 2026
Submitted to Physical Review Letters
Abstract: A search for supersymmetric electroweakinos is performed using events with a low-momentum (soft) isolated track and large missing transverse momentum, targeting nearly mass-degenerate higgsino-like charginos and neutralinos. For mass splittings of 0.3--1 GeV, the chargino decays to the lightest neutralino and a low-momentum pion, which can produce a soft, potentially displaced track. A parameterized neural network separates signal from background using kinematic and impact parameter information. The analysis uses 138 fb$ ^{-1} $ of proton--proton collision data at a center-of-mass energy of 13 TeV recorded with the CMS detector. No significant excess above the standard model expectation is observed. At 95% confidence level, the considered higgsino model is excluded for mass splittings in the range 0.28--1.15 GeV and for chargino masses up to 185 GeV, setting stringent constraints on natural supersymmetry scenarios.
Links: e-print arXiv:2604.25604 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative diagrams for the production of signal events. The signature considered in the analysis also involves one or more jets from initial state radiation with significant $ p_{\mathrm{T}} $ in the final state, which are not shown in the figure. All processes are included in the interpretation of the search.
png pdf	Figure 1-a: Representative diagrams for the production of signal events. The signature considered in the analysis also involves one or more jets from initial state radiation with significant $ p_{\mathrm{T}} $ in the final state, which are not shown in the figure. All processes are included in the interpretation of the search.
png pdf	Figure 1-b: Representative diagrams for the production of signal events. The signature considered in the analysis also involves one or more jets from initial state radiation with significant $ p_{\mathrm{T}} $ in the final state, which are not shown in the figure. All processes are included in the interpretation of the search.
png pdf	Figure 1-c: Representative diagrams for the production of signal events. The signature considered in the analysis also involves one or more jets from initial state radiation with significant $ p_{\mathrm{T}} $ in the final state, which are not shown in the figure. All processes are included in the interpretation of the search.
png pdf	Figure 2: Distributions of example kinematic track observables for data and simulation. The filled histograms show tracks in background events, with negligible statistical uncertainty. Simulated tracks matched to charginos from selected benchmark models, indicated by the chargino mass labels $ m^{\pm} $ and $ \Delta m^{\pm} $ in GeVns, are shown as colored open histograms normalized to the total background yield. The green line in the lower panel of the left plot indicates the ratio of the data to the background simulation before the refinement procedure is applied.
png pdf	Figure 2-a: Distributions of example kinematic track observables for data and simulation. The filled histograms show tracks in background events, with negligible statistical uncertainty. Simulated tracks matched to charginos from selected benchmark models, indicated by the chargino mass labels $ m^{\pm} $ and $ \Delta m^{\pm} $ in GeVns, are shown as colored open histograms normalized to the total background yield. The green line in the lower panel of the left plot indicates the ratio of the data to the background simulation before the refinement procedure is applied.
png pdf	Figure 2-b: Distributions of example kinematic track observables for data and simulation. The filled histograms show tracks in background events, with negligible statistical uncertainty. Simulated tracks matched to charginos from selected benchmark models, indicated by the chargino mass labels $ m^{\pm} $ and $ \Delta m^{\pm} $ in GeVns, are shown as colored open histograms normalized to the total background yield. The green line in the lower panel of the left plot indicates the ratio of the data to the background simulation before the refinement procedure is applied.
png pdf	Figure 3: The expected and observed yields as a function of the 0.3 GeV (upper left), 0.6 GeV (upper right), and 1.0 GeV (lower left) NN signal node, where the Roman numerals indicate the SR bin. In the upper panels, uncertainties shown are statistical only, while in the lower panels, shaded regions indicate the relative systematic uncertainty in the background yields and black vertical bars show the total statistical uncertainty in the ratio. The lower-right plot shows the distributions in the $ \tau $-lepton node of tracks after a selection of $ {\widetilde{P}(\text{Signal}\,\vert\,\Delta m_{in}^{\pm}=0.3 GeV)} > - $ 2, where the green line in the lower panel shows the ratio of the data to background simulation before the refinement procedure. The colored open histograms are as in Fig. 2, except normalized to the integrated luminosity.
png pdf	Figure 3-a: The expected and observed yields as a function of the 0.3 GeV (upper left), 0.6 GeV (upper right), and 1.0 GeV (lower left) NN signal node, where the Roman numerals indicate the SR bin. In the upper panels, uncertainties shown are statistical only, while in the lower panels, shaded regions indicate the relative systematic uncertainty in the background yields and black vertical bars show the total statistical uncertainty in the ratio. The lower-right plot shows the distributions in the $ \tau $-lepton node of tracks after a selection of $ {\widetilde{P}(\text{Signal}\,\vert\,\Delta m_{in}^{\pm}=0.3 GeV)} > - $ 2, where the green line in the lower panel shows the ratio of the data to background simulation before the refinement procedure. The colored open histograms are as in Fig. 2, except normalized to the integrated luminosity.
png pdf	Figure 3-b: The expected and observed yields as a function of the 0.3 GeV (upper left), 0.6 GeV (upper right), and 1.0 GeV (lower left) NN signal node, where the Roman numerals indicate the SR bin. In the upper panels, uncertainties shown are statistical only, while in the lower panels, shaded regions indicate the relative systematic uncertainty in the background yields and black vertical bars show the total statistical uncertainty in the ratio. The lower-right plot shows the distributions in the $ \tau $-lepton node of tracks after a selection of $ {\widetilde{P}(\text{Signal}\,\vert\,\Delta m_{in}^{\pm}=0.3 GeV)} > - $ 2, where the green line in the lower panel shows the ratio of the data to background simulation before the refinement procedure. The colored open histograms are as in Fig. 2, except normalized to the integrated luminosity.
png pdf	Figure 3-c: The expected and observed yields as a function of the 0.3 GeV (upper left), 0.6 GeV (upper right), and 1.0 GeV (lower left) NN signal node, where the Roman numerals indicate the SR bin. In the upper panels, uncertainties shown are statistical only, while in the lower panels, shaded regions indicate the relative systematic uncertainty in the background yields and black vertical bars show the total statistical uncertainty in the ratio. The lower-right plot shows the distributions in the $ \tau $-lepton node of tracks after a selection of $ {\widetilde{P}(\text{Signal}\,\vert\,\Delta m_{in}^{\pm}=0.3 GeV)} > - $ 2, where the green line in the lower panel shows the ratio of the data to background simulation before the refinement procedure. The colored open histograms are as in Fig. 2, except normalized to the integrated luminosity.
png pdf	Figure 3-d: The expected and observed yields as a function of the 0.3 GeV (upper left), 0.6 GeV (upper right), and 1.0 GeV (lower left) NN signal node, where the Roman numerals indicate the SR bin. In the upper panels, uncertainties shown are statistical only, while in the lower panels, shaded regions indicate the relative systematic uncertainty in the background yields and black vertical bars show the total statistical uncertainty in the ratio. The lower-right plot shows the distributions in the $ \tau $-lepton node of tracks after a selection of $ {\widetilde{P}(\text{Signal}\,\vert\,\Delta m_{in}^{\pm}=0.3 GeV)} > - $ 2, where the green line in the lower panel shows the ratio of the data to background simulation before the refinement procedure. The colored open histograms are as in Fig. 2, except normalized to the integrated luminosity.
png pdf	Figure 4: The expected and observed data yields in the 12 SRs (left), where the uncertainties in the upper panel are statistical only, while in the lower panel the shaded region indicates the relative systematic uncertainty in the background yields and the black vertical bars show the total statistical uncertainty in the ratio. The colored open histograms are as in Fig. 3. The right plot shows the observed 95% CL upper limit on the cross section, accounting for all production modes of the higgsino model. The expected (red) and observed (black) solid contours indicate the region excluded to the left of the curves, assuming NLO+NLL cross sections [58,59]. Dashed lines show the variation in the expected (observed) limits arising from experimental (theoretical) uncertainties. The chargino branching fractions and the minimum $ \Delta m^{\pm} $ allowed by radiative corrections (green line) are taken from Ref. [60].
png pdf	Figure 4-a: The expected and observed data yields in the 12 SRs (left), where the uncertainties in the upper panel are statistical only, while in the lower panel the shaded region indicates the relative systematic uncertainty in the background yields and the black vertical bars show the total statistical uncertainty in the ratio. The colored open histograms are as in Fig. 3. The right plot shows the observed 95% CL upper limit on the cross section, accounting for all production modes of the higgsino model. The expected (red) and observed (black) solid contours indicate the region excluded to the left of the curves, assuming NLO+NLL cross sections [58,59]. Dashed lines show the variation in the expected (observed) limits arising from experimental (theoretical) uncertainties. The chargino branching fractions and the minimum $ \Delta m^{\pm} $ allowed by radiative corrections (green line) are taken from Ref. [60].
png pdf	Figure 4-b: The expected and observed data yields in the 12 SRs (left), where the uncertainties in the upper panel are statistical only, while in the lower panel the shaded region indicates the relative systematic uncertainty in the background yields and the black vertical bars show the total statistical uncertainty in the ratio. The colored open histograms are as in Fig. 3. The right plot shows the observed 95% CL upper limit on the cross section, accounting for all production modes of the higgsino model. The expected (red) and observed (black) solid contours indicate the region excluded to the left of the curves, assuming NLO+NLL cross sections [58,59]. Dashed lines show the variation in the expected (observed) limits arising from experimental (theoretical) uncertainties. The chargino branching fractions and the minimum $ \Delta m^{\pm} $ allowed by radiative corrections (green line) are taken from Ref. [60].

Summary

In summary, a search for electroweakinos in compressed-spectrum scenarios has been performed using events containing a soft, isolated track and large missing transverse momentum. The results of the analysis are tabulated in [63]. A parameterized neural network was employed to optimize sensitivity across a range of models not previously probed. No significant excess above the standard model prediction was observed. The resulting 95% confidence level limits exclude chargino masses up to 185 GeV for a mass splitting between the chargino and lightest neutralino of 0.55 GeV, and mass splittings between 0.28 and 1.15 GeV for a chargino mass of 100 GeV, placing stringent constraints on natural supersymmetry scenarios.

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