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| CMS-PAS-SUS-23-006 | ||
| Search for Z' bosons decaying into charginos in final states with two oppositely charged leptons and missing transverse momentum | ||
| CMS Collaboration | ||
| 10 April 2025 | ||
| Abstract: A search for massive leptophobic Z' bosons decaying into a pair of charginos at the CERN LHC with the CMS detector is presented. The final state consists of two oppositely charged leptons and missing transverse momentum. The analysis is based on proton-proton collision data collected at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. A parametrized neural network is employed for signal extraction. The data are found to be consistent with the standard model, and upper limits are set on the Z' boson production cross section versus Z' and chargino masses. The analysis is sensitive to Z' boson masses up to about 3.5 TeV, assuming a 100% branching fraction to charginos. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
A simplified diagram showing the signal process in this analysis: a leptophobic $ \mathrm{Z}^{'} $ boson decaying into two charginos, each subsequently decaying into a lepton and a neutralino. |
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Figure 2:
A few important kinematical variables are shown for the data set from the $ \mathrm{e}^\pm\mu^\mp $ channel using 2018 data. Leading lepton $ p_{\mathrm{T}} $ (top left), dilepton invariant mass (top right), transverse mass (bottom left) and stransverse mass (bottom right) distributions are shown. Several benchmark signal distributions are superimposed (colored lines), demonstrating good separation power between background and signal. The bottom panel of each plot displays the data-to-MC ratio (before fitting with systematics, i.e., prefit), along with the corresponding total uncertainty band (gray), which includes all systematic and statistical uncertainties except for normalization factor uncertainty of certain background processes. |
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Figure 2-a:
A few important kinematical variables are shown for the data set from the $ \mathrm{e}^\pm\mu^\mp $ channel using 2018 data. Leading lepton $ p_{\mathrm{T}} $ (top left), dilepton invariant mass (top right), transverse mass (bottom left) and stransverse mass (bottom right) distributions are shown. Several benchmark signal distributions are superimposed (colored lines), demonstrating good separation power between background and signal. The bottom panel of each plot displays the data-to-MC ratio (before fitting with systematics, i.e., prefit), along with the corresponding total uncertainty band (gray), which includes all systematic and statistical uncertainties except for normalization factor uncertainty of certain background processes. |
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Figure 2-b:
A few important kinematical variables are shown for the data set from the $ \mathrm{e}^\pm\mu^\mp $ channel using 2018 data. Leading lepton $ p_{\mathrm{T}} $ (top left), dilepton invariant mass (top right), transverse mass (bottom left) and stransverse mass (bottom right) distributions are shown. Several benchmark signal distributions are superimposed (colored lines), demonstrating good separation power between background and signal. The bottom panel of each plot displays the data-to-MC ratio (before fitting with systematics, i.e., prefit), along with the corresponding total uncertainty band (gray), which includes all systematic and statistical uncertainties except for normalization factor uncertainty of certain background processes. |
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Figure 2-c:
A few important kinematical variables are shown for the data set from the $ \mathrm{e}^\pm\mu^\mp $ channel using 2018 data. Leading lepton $ p_{\mathrm{T}} $ (top left), dilepton invariant mass (top right), transverse mass (bottom left) and stransverse mass (bottom right) distributions are shown. Several benchmark signal distributions are superimposed (colored lines), demonstrating good separation power between background and signal. The bottom panel of each plot displays the data-to-MC ratio (before fitting with systematics, i.e., prefit), along with the corresponding total uncertainty band (gray), which includes all systematic and statistical uncertainties except for normalization factor uncertainty of certain background processes. |
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Figure 2-d:
A few important kinematical variables are shown for the data set from the $ \mathrm{e}^\pm\mu^\mp $ channel using 2018 data. Leading lepton $ p_{\mathrm{T}} $ (top left), dilepton invariant mass (top right), transverse mass (bottom left) and stransverse mass (bottom right) distributions are shown. Several benchmark signal distributions are superimposed (colored lines), demonstrating good separation power between background and signal. The bottom panel of each plot displays the data-to-MC ratio (before fitting with systematics, i.e., prefit), along with the corresponding total uncertainty band (gray), which includes all systematic and statistical uncertainties except for normalization factor uncertainty of certain background processes. |
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Figure 3:
Asimov significance as a function of $ m_{\mathrm{Z}^{'}}$, where the significances were obtained using the PNN model (dot markers with solid lines) and an NN model trained on a specific signal point ($ m_{\mathrm{Z}^{'}}= $ 2500 GeV and $ m_{\tilde{\chi}_{1}^{+}}= $ 345 GeV) (triangle markers with dashed lines). The blue (PNN) and orange (NN) curves correspond to $ m_{\tilde{\chi}_{1}^{+}}= $ 345 GeV, while the green (PNN) and red (NN) curves correspond to $ m_{\tilde{\chi}_{1}^{+}}= $ 845 GeV. The significance values coincide at $ m_{\mathrm{Z}^{'}}= $ 2.5 TeV and $ m_{\tilde{\chi}_{1}^{+}}= $ 345 GeV (blue and orange), and the PNN significance progressively exceeds that of the dedicated NN as the mass points deviate further from this reference point. For this plot, MC simulations in the $ e\mu $ channel with 2017 luminosity were used. |
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Figure 4:
DNN score distributions in control regions (for which selections are defined in Table 3 for the main background processes $ \mathrm{t} \overline{\mathrm{t}} $ for the $ \mathrm{e}^\pm\mu^\mp $ channel (top left), WW for the $ \mathrm{e}^\pm\mu^\mp $ channel (top right), and DY for the $ \mathrm{e}^+\mathrm{e}^- $+$ \mu^+\mu^- $ channel (bottom). The bottom panel of each plot shows data-to-MC ratios with background-only fit MC, along with the corresponding uncertainty band (gray) and with prefit MC, along with the corresponding uncertainty band (green). |
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Figure 4-a:
DNN score distributions in control regions (for which selections are defined in Table 3 for the main background processes $ \mathrm{t} \overline{\mathrm{t}} $ for the $ \mathrm{e}^\pm\mu^\mp $ channel (top left), WW for the $ \mathrm{e}^\pm\mu^\mp $ channel (top right), and DY for the $ \mathrm{e}^+\mathrm{e}^- $+$ \mu^+\mu^- $ channel (bottom). The bottom panel of each plot shows data-to-MC ratios with background-only fit MC, along with the corresponding uncertainty band (gray) and with prefit MC, along with the corresponding uncertainty band (green). |
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Figure 4-b:
DNN score distributions in control regions (for which selections are defined in Table 3 for the main background processes $ \mathrm{t} \overline{\mathrm{t}} $ for the $ \mathrm{e}^\pm\mu^\mp $ channel (top left), WW for the $ \mathrm{e}^\pm\mu^\mp $ channel (top right), and DY for the $ \mathrm{e}^+\mathrm{e}^- $+$ \mu^+\mu^- $ channel (bottom). The bottom panel of each plot shows data-to-MC ratios with background-only fit MC, along with the corresponding uncertainty band (gray) and with prefit MC, along with the corresponding uncertainty band (green). |
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Figure 4-c:
DNN score distributions in control regions (for which selections are defined in Table 3 for the main background processes $ \mathrm{t} \overline{\mathrm{t}} $ for the $ \mathrm{e}^\pm\mu^\mp $ channel (top left), WW for the $ \mathrm{e}^\pm\mu^\mp $ channel (top right), and DY for the $ \mathrm{e}^+\mathrm{e}^- $+$ \mu^+\mu^- $ channel (bottom). The bottom panel of each plot shows data-to-MC ratios with background-only fit MC, along with the corresponding uncertainty band (gray) and with prefit MC, along with the corresponding uncertainty band (green). |
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Figure 5:
DNN score distributions obtained after the fit for the signal regions in the $ \mathrm{e}^+\mathrm{e}^- $ (top left), mm (top right), and $ \mathrm{e}^\pm \mu^\mp $ (bottom) channels. Signal distributions for a benchmark point with with $ m_{\mathrm{Z}^{'}}= $ 2500 GeV and $ m_{\tilde{\chi}_{1}^{+}}= $ 345 GeV are superimposed on the plots. The bottom panel of each plot shows data-to-MC ratios with background-only fit MC, along with the corresponding uncertainty band (gray) and with prefit MC, along with the corresponding uncertainty band (green). |
png pdf |
Figure 5-a:
DNN score distributions obtained after the fit for the signal regions in the $ \mathrm{e}^+\mathrm{e}^- $ (top left), mm (top right), and $ \mathrm{e}^\pm \mu^\mp $ (bottom) channels. Signal distributions for a benchmark point with with $ m_{\mathrm{Z}^{'}}= $ 2500 GeV and $ m_{\tilde{\chi}_{1}^{+}}= $ 345 GeV are superimposed on the plots. The bottom panel of each plot shows data-to-MC ratios with background-only fit MC, along with the corresponding uncertainty band (gray) and with prefit MC, along with the corresponding uncertainty band (green). |
png pdf |
Figure 5-b:
DNN score distributions obtained after the fit for the signal regions in the $ \mathrm{e}^+\mathrm{e}^- $ (top left), mm (top right), and $ \mathrm{e}^\pm \mu^\mp $ (bottom) channels. Signal distributions for a benchmark point with with $ m_{\mathrm{Z}^{'}}= $ 2500 GeV and $ m_{\tilde{\chi}_{1}^{+}}= $ 345 GeV are superimposed on the plots. The bottom panel of each plot shows data-to-MC ratios with background-only fit MC, along with the corresponding uncertainty band (gray) and with prefit MC, along with the corresponding uncertainty band (green). |
png pdf |
Figure 5-c:
DNN score distributions obtained after the fit for the signal regions in the $ \mathrm{e}^+\mathrm{e}^- $ (top left), mm (top right), and $ \mathrm{e}^\pm \mu^\mp $ (bottom) channels. Signal distributions for a benchmark point with with $ m_{\mathrm{Z}^{'}}= $ 2500 GeV and $ m_{\tilde{\chi}_{1}^{+}}= $ 345 GeV are superimposed on the plots. The bottom panel of each plot shows data-to-MC ratios with background-only fit MC, along with the corresponding uncertainty band (gray) and with prefit MC, along with the corresponding uncertainty band (green). |
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Figure 6:
The expected upper limits at 95% CL on the $ \mathrm{p}\mathrm{p} \rightarrow \mathrm{Z}^{'} $ cross section, along with the exclusion lines for the observed and expected limits, are presented. The cross-section limits are shown in the plane of $ \mathrm{Z}^{'} $ boson and $ \tilde{\chi}_{1}^{\pm} $ masses, combining the $ \mathrm{e}^+\mathrm{e}^- $, mm, and $ \mathrm{e}^\pm \mu^\mp $ channels. The color grid shows median expected upper limits. The black solid line denotes the median exclusion and the black dashed lines denote 68% quantiles. The red solid line denotes exclusion line of observed data. The region to the left of the exclusion curves is excluded. |
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Figure 7:
Observed and expected upper limits at 95% CL on the $ \mathrm{p}\mathrm{p} \rightarrow \mathrm{Z}^{'} $ cross section versus $ \mathrm{Z}^{'} $ boson mass for different values of $ \tilde{\chi}_{1}^{\pm} $ masses are presented for the combination of $ \mathrm{e}^+\mathrm{e}^- $, mm and $ \mathrm{e}^\pm \mu^\mp $ channels. Limits are scaled for the different values of the $ \tilde{\chi}_{1}^{\pm} $ mass. The black dot with solid line represent the observed upper limits, while the dashed lines with the green and yellow bands show the expected upper limits with 68% and 95% quantiles. |
| Tables | |
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Table 1:
Summary of object definitions and selections. |
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Table 2:
Signal event selection and search channels. |
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Table 3:
Comparison of selection criteria for the signal, and control regions used in this analysis, following the requirement of two oppositely-charged leptons and $ p_{\mathrm{T}} $ thresholds. |
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Table 4:
Normalization factors and their uncertainties for the $ \mathrm{t} \overline{\mathrm{t}} $, WW, and DY processes are listed in this table. |
| Summary |
| A search was conducted for a leptophobic $ \mathrm{Z}^{'} $ boson decaying into two charginos, which subsequently decay into W bosons and neutralinos. A data set of proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $ was analyzed. This is the first search for this process using LHC data. An analysis was designed in the dilepton plus missing transverse momentum final state, in the $ \mathrm{e}^+\mathrm{e}^- $, mm, and $ \mathrm{e}^\pm \mu^\mp $ channels. A parametrized neural network was employed to enhance the signal sensitivity. The analysis was interpreted using simplified model spectra featuring the production and decay process of the leptophobic $ \mathrm{Z}^{'} $ boson derived from the U(1)' extension of the minimal supersymmetric standard model. Upper limits on the $ \mathrm{Z}^{'} $ boson production cross section have been presented in the $ \mathrm{Z}^{'} $ boson mass versus chargino mass plane, with $ m_{\tilde{\chi}_{1}^{\pm}} = 2m_{{\tilde{\chi}_{1}^{0}}} $. The analysis could exclude $ \mathrm{Z}^{'} $ boson masses up to about 3.5 TeV for the specific case of $ \mathrm{Z}^{'} $ boson decaying exclusively to charginos, and charginos exclusively decaying to W bosons and neutralinos. |
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