Revisiting the exclusion limits from direct chargino-neutralino production at the LHC

We revisit the existing limits on the gaugino masses in various supersymmetric (SUSY) scenarios derived from Run-I data of the LHC. These limits obtained from the various final states rely heavily on the simplified assumptions regarding the masses, compositions, and decay branching ratios of the gauginos. The most severe exclusion limits on the gaugino masses are obtained from trilepton final states, while the second lightest neutralino (${\tilde{\chi }}_2^0$) decaying into the SM-like Higgs and lightest SUSY particle results in the weakest bounds. Our aim is to assess the extent of deviation of these exclusion limits in more realistic scenarios. After a brief discussion on the various decay modes of the ${\tilde{\chi }}_2^0$ and the lightest chargino (${\tilde{\chi }}_1^{\pm }$), we proceed to validate the ATLAS exclusion limits obtained from trilepton, $l\gamma \gamma$, and $lb\overline{b}$ final states associated with missing energy. We then consider different combinations of the relevant branching ratios to study their impact on the existing bounds. The results are presented alongside the existing exclusion limits to showcase the extent of the obtained deviation. We also observe that the three-body decay modes of ${\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^{\pm }$ via off-shell slepton decays resulting in trilepton final states provide bounds that are far more severe in some parts of the available parameter space than those obtained from the off-shell gauge boson decays.

Figure 1

Distributions of the BRs corresponding to the two two-body decay modes of ${\tilde{\chi }}_2^0$ shown as a function of $m_{{\tilde{\chi }}_2^0}$. (a) Distributions corresponding to $\mathrm{BR}({\tilde{\chi }}_2^0\to h{\tilde{\chi }}_1^0)$. (b) Distributions corresponding to $\mathrm{BR}({\tilde{\chi }}_2^0\to Z{\tilde{\chi }}_1^0)$. The different colored points in the plots correspond to the different $\mathrm{\Delta }{m}_{\tilde{\chi }}^0=m_{{\tilde{\chi }}_2^0}-m_{{\tilde{\chi }}_1^0}$ as indicated in the plot.

Figure 2

Distributions of the BRs corresponding to the three leptonic three-body decay modes of ${\tilde{\chi }}_2^0$ shown as a function of $m_{{\tilde{\chi }}_2^0}$. The plots show the distributions corresponding to (a) $\mathrm{BR}({\tilde{\chi }}_2^0\to l\overline{l}{\tilde{\chi }}_1^0)$, (b) $\mathrm{BR}({\tilde{\chi }}_2^0\to \nu \overline{\nu }{\tilde{\chi }}_1^0)$, and (c) $\mathrm{BR}({\tilde{\chi }}_2^0\to \tau \overline{\tau }{\tilde{\chi }}_1^0)$. Note that $\mathrm{BR}({\tilde{\chi }}_2^0\to l\overline{l}{\tilde{\chi }}_1^0)$ contains contributions from both the electron- and muon-associated final states while $\mathrm{BR}({\tilde{\chi }}_2^0\to \nu \overline{\nu }{\tilde{\chi }}_1^0)$ contains all three neutrino flavor contributions. The different colored (shaped) points correspond to the different $\mathrm{\Delta }{m}_{{\tilde{l}}_{L/R}/{\tilde{\nu }}_L/{\tilde{\tau }}_1}=m_{{\tilde{l}}_{L/R}/{\tilde{\nu }}_L/{\tilde{\tau }}_1}-m_{{\tilde{\chi }}_2^0}$ (see text for details).

Figure 3

Distributions of the BRs corresponding to the two two-body decay modes of ${\tilde{\chi }}_2^0$ shown as a function of $m_{{\tilde{\chi }}_2^0}$ for different signs of $\mu$ at a fixed LSP mass and two different $\tan \beta$ values. (a) Distributions corresponding to $\mathrm{BR}({\tilde{\chi }}_2^0\to h{\tilde{\chi }}_1^0)$. (b) Distributions corresponding to $\mathrm{BR}({\tilde{\chi }}_2^0\to Z{\tilde{\chi }}_1^0)$. $M_1$ is kept fixed at 100 GeV while all the sleptons and the squarks in the theory are decoupled.

Figure 4

Distributions of the BRs corresponding to the leptonic three-body decay modes of ${{\tilde{\chi }}_1}^{\pm }$ shown as a function of $m_{{{\tilde{\chi }}_1}^{\pm }}$. (a) Distributions corresponding to $\mathrm{BR}({{\tilde{\chi }}_1}^{\pm }\to l{\nu }_{l}l{\tilde{\chi }}_1^0)$. (b) Distributions corresponding to $\mathrm{BR}({{\tilde{\chi }}_1}^{\pm }\to \tau {\nu }_{\tau }{\tilde{\chi }}_1^0)$. Note that $\mathrm{BR}({{\tilde{\chi }}_1}^{\pm }\to l{\nu }_{l}l{\tilde{\chi }}_1^0)$ contains contributions from both the electron- and muon-associated final states. The different colored (shaped) points correspond to the different $\mathrm{\Delta }{m}_{{\tilde{l}}_L/{\tilde{\tau }}_1}^c=m_{{\tilde{l}}_L/{\tilde{\tau }}_1}-m_{{{\tilde{\chi }}_1}^{\pm }}$.

Figure 5

Distribution of $\mathrm{BR}({{\tilde{\chi }}_1}^{\pm }\to {\tilde{\chi }}_1^0{e}^{\pm }{\nu }_e)$ shown as a function of $m_{{{\tilde{\chi }}_1}^{\pm }}$. The cyan, blue, red, and black points indicate $\mathrm{BR}({\tilde{\chi }}_2^0\to {\tilde{\chi }}_1^0{e}^{\pm }{e}^{\mp })$ $<10\%$, 10%–20%, 20%–30%, and 30%–40%, respectively.

Figure 6

Excess in $\mathrm{\Delta }{a}_{\mu }$ obtained at $1\sigma$, $2\sigma$, and $3\sigma$ level shown in the $m_{{{\tilde{\chi }}_1}^{\pm }}–m_{{\tilde{\chi }}_1^0}$ and $m_{{{\tilde{\chi }}_1}^{\pm }}–m_{{\tilde{\chi }}_1^0}$ plane. The cyan, blue, and red points represent $3\sigma$-, $2\sigma$-, and $1\sigma$-allowed points, respectively.

Figure 7

Validation of ATLAS $\text{trilepton}+{\cancel{\mathrm{E}}}_{\mathrm{T}}$ [16] analysis for slepton-mediated simplified models. The blue dotted line corresponds to 95% C.L. exclusion limits obtained by ATLAS and the solid red line corresponds to our validated results.

Figure 8

Validation of ATLAS $\text{trilepton}+{\cancel{\mathrm{E}}}_{\mathrm{T}}$ [16] analysis for $WZ$-mediated simplified models. The blue dotted line corresponds to 95% C.L. exclusion limits obtained by ATLAS and the solid red line corresponds to our validated results.

Figure 9

Validation of ATLAS $lbb+{\cancel{\mathrm{E}}}_{\mathrm{T}}$ and $l\gamma \gamma +{\cancel{\mathrm{E}}}_{\mathrm{T}}$ [16] analyses for $Wh$-mediated simplified models. The blue dotted and solid lines correspond to the 95% C.L. exclusion limits obtained by ATLAS and our validated results, respectively, corresponding to the $l\gamma \gamma +{\cancel{\mathrm{E}}}_{\mathrm{T}}$ final state. Similarly, the red dotted and solid lines represent the experimental bound and our validated results corresponding to the $lbb+{\cancel{\mathrm{E}}}_{\mathrm{T}}$ final state.

Figure 10

The exclusion lines shown in $m_{{\tilde{\chi }}_1^0}-m_{{{\tilde{\chi }}_1}^{\pm }}$ mass plane for trilepton final state with different choices of $\mathrm{BR}({\tilde{\chi }}_2^0\to Z{\tilde{\chi }}_1^0)$. The red line represents the present experimental bound, whereas the blue, green, and magenta dotted lines present our results obtained assuming $\mathrm{BR}({\tilde{\chi }}_2^0\to Z{\tilde{\chi }}_1^0)=75\%$, 50%, and 25%, respectively. The black dotted lines separate various kinematic regions where one particular decay mode ceases to exist and another opens up. The yellow shaded region represents the parameter space where either ${\tilde{\chi }}_2^0\to h{\tilde{\chi }}_1^0$ or ${\tilde{\chi }}_2^0\to Z{\tilde{\chi }}_1^0$ or both these decay modes are forbidden.

Figure 11

The exclusion lines shown in the $m_{{\tilde{\chi }}_1^0}–m_{{{\tilde{\chi }}_1}^{\pm }}$ mass plane for $l\gamma \gamma$ and trilepton final states with different choices of $\mathrm{BR}({\tilde{\chi }}_2^0\to h{\tilde{\chi }}_1^0)$. The solid black line represents the present experimental bound for $l\gamma \gamma$ final state, whereas the solid magenta and solid green lines present our results obtained assuming $\mathrm{BR}({\tilde{\chi }}_2^0\to h{\tilde{\chi }}_1^0)=75\%$ and 50%, respectively. The solid red, dotted blue, dotted green, and dotted magenta lines are the same as shown in Fig. 10.

Figure 12

The exclusion lines shown in the $m_{{\tilde{\chi }}_1^0}–m_{{{\tilde{\chi }}_1}^{\pm }}$ mass plane for the trilepton final state assuming $\mathrm{BR}({\tilde{\chi }}_2^0\to l\overline{l}{\tilde{\chi }}_1^0)=100\%$ and $\mathrm{BR}({{\tilde{\chi }}_1}^{\pm }\to {\tilde{\chi }}_1^0{l}^{\pm }{\nu }_l)=30\%$ where $l=e$, $\mu$, $\tau$. The solid black line represents the present experimental bound for the trilepton final state, whereas the yellow shaded region below the red line shows the obtained exclusion region from trilepton data for such off-shell decays of the electroweakinos.

Figure 13

The exclusion lines shown in the $M_1–m_{{\tilde{l}}_R}$ mass plane for the trilepton final state assuming a varying branching ratio. The orange region represents the experimental limit on $m_{{\tilde{l}}_R}$ derived from direct production of the sleptons, provided the $m_{{\tilde{l}}_L}$ are decoupled from the rest of the spectrum. The red and black solid lines correspond to the exclusion lines obtained for $M_2=M_1+25\mathrm{GeV}$ and $M_2=M_1+60\mathrm{GeV}$, respectively. The dotted red and black lines correspond to $m_{{\tilde{l}}_R}<m_{{{\tilde{\chi }}_1}^{\pm }}$ for the two scenarios $M_2=M_1+25\mathrm{GeV}$ and $M_2=M_1+60\mathrm{GeV}$.