Searches for electroweak neutralino and chargino production in channels with Higgs, $Z$, and $W$ bosons in $pp$ collisions at 8 TeV

Searches for supersymmetry (SUSY) are presented based on the electroweak pair production of neutralinos and charginos, leading to decay channels with Higgs, $Z$, and $W$ bosons and undetected lightest SUSY particles (LSPs). The data sample corresponds to an integrated luminosity of about $19.5{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV collected in 2012 with the CMS detector at the LHC. The main emphasis is neutralino pair production in which each neutralino decays either to a Higgs boson ($h$) and an LSP or to a $Z$ boson and an LSP, leading to $hh$, $hZ$, and $ZZ$ states with missing transverse energy ($E_{\mathrm{T}}^{\text{miss}}$). A second aspect is chargino-neutralino pair production, leading to $hW$ states with $E_{\mathrm{T}}^{\text{miss}}$. The decays of a Higgs boson to a bottom-quark pair, to a photon pair, and to final states with leptons are considered in conjunction with hadronic and leptonic decay modes of the $Z$ and $W$ bosons. No evidence is found for supersymmetric particles, and 95% confidence level upper limits are evaluated for the respective pair production cross sections and for neutralino and chargino mass values.

Figure 1

Event diagrams for the SUSY scenarios considered in this analysis. (Left) and (center) $hh$ and $hZ$ production in a GMSB model [28, 34], where $h$ is the Higgs boson, ${\tilde{\chi }}_1^0$ is the lightest neutralino NLSP, and $\tilde{G}$ is the nearly massless gravitino LSP. The ${\tilde{\chi }}_1^0{\tilde{\chi }}_1^0$ state is created through ${\tilde{\chi }}_1^0{\tilde{\chi }}_2^0$, ${\tilde{\chi }}_1^0{\tilde{\chi }}_1^{\pm }$, ${\tilde{\chi }}_2^0{\tilde{\chi }}_1^{\pm }$, and ${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_1^{\mp }$ production followed by the decay of the ${\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^{\pm }$ states to the ${\tilde{\chi }}_1^0$ and undetected SM particles, with ${\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^{\pm }$ the second-lightest neutralino and the lightest chargino, respectively. (Right) $hW$ production through chargino-neutralino ${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_2^0$ pair creation, with ${\tilde{\chi }}_1^0$ a massive neutralino LSP.

Figure 2

Distributions of events in the $4b$ sample of the $hh\to b\overline{b}b\overline{b}$ analysis, after all signal region requirements are applied except for that on the displayed variable, in comparison with simulations of background and signal events: (top) $|\mathrm{\Delta }{m}_{b\overline{b}}|$, (middle) $\mathrm{\Delta }{R}_{\max }$, and (bottom) $⟨m_{b\overline{b}}⟩$. For the signal events, results are shown for higgsino (${\tilde{\chi }}_1^0$) mass values of 250 and 400 GeV, with an LSP (gravitino) mass of 1 GeV. The background distributions are stacked while the signal distributions are not. The hatched bands indicate the statistical uncertainty of the total SM simulated prediction.

Figure 3

(Top left) Illustration of the signal (SIG) and sideband (SB) regions in the $|\mathrm{\Delta }{m}_{b\overline{b}}|$ versus $⟨m_{b\overline{b}}⟩$ plane of the $hh\to b\overline{b}b\overline{b}$ analysis. (Top right and bottom right) Distributions of simulated $t\overline{t}$ events in the $2b$ and $4b$ samples. (Bottom left) Distribution of simulated signal events in the $4b$ sample for a higgsino (${\tilde{\chi }}_1^0$) mass of 250 GeV and an LSP (gravitino) mass of 1 GeV. The plots employ an arbitrary integrated luminosity. The size of a box is proportional to the relative number of events.

Figure 4

Ratio of the number of events in the signal (SIG) region to that in the sideband (SB) region as a function of ${\mathcal{S}}_{\mathrm{MET}}$ bin (see Table 1), for the $2b$, $3b$, and $4b$ event samples of the $hh\to b\overline{b}b\overline{b}$ analysis. The simulated results account for the various expected SM processes. The results of a likelihood fit to data, in which the SIG/SB ratio is determined separately for each bin, are also shown.

Figure 5

Distribution of simulated $t\overline{t}$ [“genuine $E_{\mathrm{T}}^{\text{miss}}$ (SM)”], signal [“genuine $E_{\mathrm{T}}^{\text{miss}}$ (SUSY)”], and QCD multijet (“spurious $E_{\mathrm{T}}^{\text{miss}}$”) events using loosened selection criteria (see text) in bins of ${\mathcal{S}}_{\mathrm{MET}}$ and $E_{\mathrm{T}}^{\text{miss}}$. The uncertainties are statistical. The bin edges for $E_{\mathrm{T}}^{\text{miss}}$ have been adjusted so that the number of $t\overline{t}$ events in each bin is about the same as for the corresponding ${\mathcal{S}}_{\mathrm{MET}}$ bin. The signal events correspond to the higgsino pair production scenario of Fig. 1 (left) with a higgsino (${\tilde{\chi }}_1^0$) mass of 250 GeV and an LSP (gravitino) mass of 1 GeV.

Figure 6

Observed numbers of events as a function of $E_{\mathrm{T}}^{\text{miss}}$ significance (${\mathcal{S}}_{\mathrm{MET}}$) bin for the $hh\to b\overline{b}b\overline{b}$ analysis, in comparison with the SM background estimate from the likelihood fit, for the (top) $3b$-SIG and (bottom) $4b$-SIG regions. The hatched bands show the total uncertainty of the background prediction, with statistical and systematic terms combined. The expected (unstacked) results for signal events, with higgsino (${\tilde{\chi }}_1^0$) mass values of 250 and 400 GeV and an LSP (gravitino) mass of 1 GeV, are also shown.

Figure 7

Distribution of diphoton invariant mass $m_{\gamma \gamma }$ after all selection criteria are applied except for that on $m_{\gamma \gamma }$, for the $h(\to \gamma \gamma )h(\to b\overline{b})$ search. The result of a fit to a power-law function using data in the sideband regions (see text) is indicated by the solid line. The dotted line shows an interpolation of the fitted function into the Higgs boson mass region excluded from the fit. The expected results for signal events, with a higgsino (${\tilde{\chi }}_1^0$) mass value of 130 GeV and an LSP (gravitino) mass of 1 GeV, are also shown.

Figure 8

Observed numbers of events as a function of the scalar sum of $p_{\mathrm{T}}$ values of the two Higgs boson candidates, $S_{\mathrm{T}}^h$, for the $hh\to \gamma \gamma b\overline{b}$ analysis, in comparison with the SM background estimate, (top) for a control sample with loose tagging requirements for $b$ jets, and (bottom) for the nominal selection. The hatched bands show the total uncertainty of the background prediction, with statistical and systematic terms combined. The (unstacked) results for signal events, with higgsino (${\tilde{\chi }}_1^0$) mass values of 130 and 200 GeV and an LSP (gravitino) mass of 1 GeV, are also shown.

Figure 9

Results for the $hZ$ and $hW$ analysis in the $\gamma \gamma +2$ jets final state after all selection criteria are applied except for that on the displayed variable. (Top) Distribution of diphoton invariant mass $m_{\gamma \gamma }$. The result of a fit to a power-law function using data in the sideband regions (see text) is indicated by the solid line. The dotted line shows an interpolation of the fitted function into the Higgs boson mass region excluded from the fit. The expected result for $hZ$ signal events with a higgsino (${\tilde{\chi }}_1^0$) mass of 130 GeV and an LSP (gravitino) mass of 1 GeV, multiplied by a factor of 30 for better visibility, is also shown. (Bottom) Observed numbers of events as a function of $E_{\mathrm{T}}^{\text{miss}}$ in comparison with the SM background estimate. The hatched bands show the total uncertainty of the background prediction, with statistical and systematic terms combined. The expected (unstacked) results for $hZ$ signal events, with the indicated values of the higgsino (${\tilde{\chi }}_1^0$) mass and an LSP (gravitino) mass of 1 GeV, are also shown.

Figure 10

Distribution of the diphoton invariant mass $m_{\gamma \gamma }$ after all selection criteria are applied except for that on $m_{\gamma \gamma }$, for the $hh$, $hZ$, and $hW\to \gamma \gamma +\text{leptons}$ analysis, for the (top) muon and (bottom) electron samples. The result of a fit to a power-law function using data in the sideband regions (see text) is indicated by the solid line. The dotted line shows an interpolation of the fitted function into the Higgs boson mass region excluded from the fit. The expected results for $hh$ events, with a higgsino (${\tilde{\chi }}_1^0$) mass value of 130 GeV and an LSP (gravitino) mass of 1 GeV, are also shown.

Figure 11

Observed numbers of events as a function of transverse mass $M_{\mathrm{T}}$ for the $hh$, $hZ$, and $hW\to \gamma \gamma +\text{leptons}$ analysis, in comparison with the (stacked) SM background estimates, for the (top) muon and (bottom) electron samples. The hatched bands show the total uncertainty of the background prediction, with statistical and systematic terms combined. The (unstacked) results for various signal scenarios are also shown. For the $hh$ and $hZ$ scenarios, the higgsino (${\tilde{\chi }}_1^0$) mass is 130 GeV and the LSP (gravitino) mass is 1 GeV. For the $hW$ scenario, $m_{{\tilde{\chi }}_2^0}=m_{{\tilde{\chi }}_1^{\pm }}=130\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=1\mathrm{GeV}$ [see Fig. 1 (right)].

Figure 12

Observed numbers of events as a function of $E_{\mathrm{T}}^{\text{miss}}$ for the $hh$, $hZ$, and $hW\to \gamma \gamma +\text{leptons}$ analysis in comparison with the (stacked) SM background estimates, for the (top) muon and (bottom) electron samples. The hatched bands show the total uncertainty of the background prediction, with statistical and systematic terms combined. The (unstacked) results for various signal scenarios are also shown. For the $hh$ and $hZ$ scenarios, the higgsino (${\tilde{\chi }}_1^0$) mass is 130 GeV and the LSP (gravitino) mass is 1 GeV. For the $hW$ scenario, $m_{{\tilde{\chi }}_2^0}=m_{{\tilde{\chi }}_1^{\pm }}=130\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=1\mathrm{GeV}$.

Figure 13

Distribution of $M_{\mathrm{T}2}^{\mathrm{j}\ell }$ for the $h(\to b\overline{b})Z(\to {\ell }^{+}{\ell }^{-})$ analysis after all signal-region requirements are applied except for that on $M_{\mathrm{T}2}^{\mathrm{j}\ell }$, in comparison with (stacked) SM background estimates taken from simulation. For this result, $E_{\mathrm{T}}^{\text{miss}}>60\mathrm{GeV}$. The (unstacked) signal results for a higgsino (${\tilde{\chi }}_1^0$) mass of 200 GeV and an LSP (gravitino) mass of 1 GeV are also shown.

Figure 14

Distribution of $E_{\mathrm{T}}^{\text{miss}}$ in comparison with the (stacked) SM background estimates for the $h(\to b\overline{b})Z(\to {\ell }^{+}{\ell }^{-})$ analysis, for data control samples enriched in (top) SM $Z+\text{jets}$ events, and (bottom) $t\overline{t}$ events. The hatched bands in the ratio plots (lower panels) indicate the uncertainty of the total background prediction, with statistical and systematic terms combined.

Figure 15

Observed numbers of events as a function of $E_{\mathrm{T}}^{\text{miss}}$ for the $h(\to b\overline{b})Z(\to {\ell }^{+}{\ell }^{-})$ analysis, in comparison with the (stacked) SM background estimates. The (unstacked) results for a higgsino (${\tilde{\chi }}_1^0$) mass of 200 GeV and an LSP (gravitino) mass of 1 GeV are also shown. The hatched band in the ratio plot (lower panel) indicates the uncertainty of the total background prediction, with statistical and systematic terms combined.

Figure 16

Observed and expected 95% confidence level upper limits on the cross section for higgsino pair production in the $hh$ topology as a function of the higgsino mass for the combined $b\overline{b}b\overline{b}$, $\gamma \gamma b\overline{b}$, $\gamma \gamma +\text{leptons}$, and three-or-more-lepton channels. The dark (green), light (yellow), and medium-dark (orange) bands indicate the one-, two-, and three-standard-deviation uncertainty intervals, respectively, for the expected results. The theoretical cross section and the expected curves for the individual search channels are also shown.

Figure 17

(Top) Observed and expected 95% confidence level upper limits on the cross section for higgsino pair production in the $ZZ$ topology as a function of the higgsino mass for the combined three-or-more-lepton and ${\ell }^{+}{\ell }^{-}+2$ jets channels. The dark (green) and light (yellow) bands indicate the one- and two-standard-deviation uncertainty intervals, respectively, for the expected results. The theoretical cross section and the expected curves for the individual search channels are also shown. (Bottom) Corresponding results for the $hZ$ topology, assuming the ${\tilde{\chi }}_1^0\to h\tilde{G}$ and ${\tilde{\chi }}_1^0\to Z\tilde{G}$ branching fractions each to be 0.5, ignoring contributions from $hh$ and $ZZ$ events, for the individual and combined $\gamma \gamma +\text{leptons}$, $b\overline{b}{\ell }^{+}{\ell }^{-}$, and three-or-more-lepton channels.

Figure 18

Observed and expected 95% confidence level exclusion regions for higgsino pair production, with all channels combined, in the plane of the higgsino branching fraction to a Higgs boson and LSP, versus the higgsino mass. The dark (green) and light (yellow) bands indicate the one- and two-standard-deviation uncertainty intervals, respectively. The excluded regions correspond to the area below the contours.

Figure 19

Observed and expected 95% confidence level upper limits on the cross section for higgsino pair production as a function of the higgsino mass assuming the ${\tilde{\chi }}_1^0\to h\tilde{G}$ and ${\tilde{\chi }}_1^0\to Z\tilde{G}$ branching fractions each to be 0.5, including contributions from $hh$ and $ZZ$ events, for the combined $b\overline{b}b\overline{b}$, $\gamma \gamma b\overline{b}$, $\gamma \gamma +\text{leptons}$, $b\overline{b}{\ell }^{+}{\ell }^{-}$, three-or-more-lepton, and ${\ell }^{+}{\ell }^{-}+2$ jets channels. The dark (green) and light (yellow) bands indicate the one- and two-standard-deviation uncertainty intervals, respectively, for the expected results. The theoretical cross section and the expected curves for the individual search channels are also shown.

Figure 20

(Top) Observed and (bottom) expected 95% confidence level exclusion regions for higgsino pair production in the plane of the higgsino branching fraction to a Higgs boson and the LSP, versus the higgsino mass, with each principal search channel group removed in turn from the combination. The excluded regions correspond to the area below the contours.

Figure 21

The search channel that provides the most stringent 95% confidence level upper limit on ${\tilde{\chi }}_1^0$ higgsino pair production in the plane of the higgsino branching fraction to a Higgs boson and the LSP, versus the higgsino mass.

Figure 22

(Top) Observed and expected 95% confidence level upper limits on the cross section for electroweak chargino-neutralino ${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_2^0$ pair production (with $m_{{\tilde{\chi }}_1^{\pm }}=m_{{\tilde{\chi }}_2^0}$) as a function of the LSP and ${\tilde{\chi }}_2^0$ masses for the combined results on single-lepton, same-sign dilepton, and multilepton data from Ref. [36] with the diphoton data presented here. (Bottom) Corresponding results as a function of the ${\tilde{\chi }}_2^0$ mass for an LSP mass of 1 GeV. The dark (green) band indicates the one-standard-deviation interval. The theoretical cross section is also shown.

Figure 23

Observed and expected 95% confidence level upper limits on the cross section for chargino-neutralino ${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_2^0$ pair production (with $m_{{\tilde{\chi }}_1^{\pm }}=m_{{\tilde{\chi }}_2^0}$) as a function of the ${\tilde{\chi }}_2^0$ mass assuming an LSP mass of 1 GeV, for (top) the $\gamma \gamma +2$ jets study of Sec. 6b, and (middle and bottom), the $\gamma \gamma +\text{leptons}$ studies (for the muon and electron samples, respectively) of Sec. 6c. The dark (green) and light (yellow) bands indicate the one- and two-standard-deviation uncertainty intervals, respectively. The theoretical cross section is also shown.