New approach for detecting a compressed bino/wino at the LHC

In some supersymmetric models like split supersymmetry or models with nonuniversal gaugino mass, ‘bino [lightest sparticle (LSP)] and wino [next-to-lightest sparticle (NLSP)] may have rather small mass splitting in order to provide the correct dark matter relic density through bino/wino coannihilation. Such a scenario with the compressed bino/wino is difficult to explore at the LHC. In this work we propose to probe this scenario from $pp\to j{\tilde{\chi }}_2^0{\tilde{\chi }}_1^{\pm }$ followed by ${\tilde{\chi }}_2^0\to \gamma {\tilde{\chi }}_1^0$ and ${\tilde{\chi }}_1^{\pm }\to W^{*}{\tilde{\chi }}_1^0\to {\ell }^{\pm }\nu {\tilde{\chi }}_1^0$ (this method is also applicable to the compressed bino/Higgsino scenario). Through a detailed Monte Carlo simulation for both the signal and the backgrounds, we find that for a mass splitting $\mathrm{\Delta }M\sim 10–15\mathrm{GeV}$ between bino (LSP) and wino (NLSP), the 14 TeV LHC with a luminosity of $1000{\mathrm{fb}}^{-1}$ can probe the wino up to 150 GeV (the sensitivity can reach $3.4\sigma$ for $\mathrm{\Delta }M=10\mathrm{GeV}$ and $2\sigma$ for $\mathrm{\Delta }M=15\mathrm{GeV}$). We also investigate the dark matter detection sensitivities for this scenario and find that the planned XENON-1T (2017) cannot fully cover the parameter space with a wino below 150 GeV allowed by relic density and the LUX limits.

Figure 1

The relevant Feynman diagrams for the loop decay ${\tilde{\chi }}_2^0\to \gamma {\tilde{\chi }}_0^1$ in our study. Here $k=1,2$.

Figure 2

The dependence of the branching ratio of ${\tilde{\chi }}_2^0\to \gamma {\tilde{\chi }}_0^1$ on the masses of ${\tilde{\chi }}_{1,2}^0$. We use susy-hit [20] to calculate the branching ratio. The gluino and Higgsino mass parameters $M_3$ and $\mu$ are assumed at 2 TeV and 1 TeV, respectively. We vary parameter $\tan \beta$ from 3 to 50 and the gaugino masses $M_{1,2}$ from 100 to 300 GeV. All of the samples are required to satisfy the Higgs mass $125\pm 2\mathrm{GeV}$. The slepton and the first two generation squark sectors are set with a common mass $M_{\mathrm{SUSY}}=2\mathrm{TeV}$.

Figure 3

The dependence of the cross section of ${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_2^{0}j$ on the $m_{{\tilde{\chi }}_2^0}$ at 14 TeV LHC.

Figure 4

The scatter plots of the samples allowed by the $2\sigma$ dark matter relic density and various collider constraints listed in the context. The left panel shows the plots in the plane of $m_{{\tilde{\chi }}_1^0}$ versus $m_{{\tilde{\chi }}_2^0}$ while the right panel shows the neutralino dark matter-nucleon scattering cross section versus the neutralino mass.