Light stops and observation of supersymmetry at LHC run II

Light stops consistent with the Higgs boson mass of $\sim 126\mathrm{GeV}$ are investigated within the framework of minimal supergravity. It is shown that models with light stops which are also consistent with the thermal relic density constraints require stop coannihilation with the neutralino LSP. The analysis shows that the residual set of parameter points with light stops satisfying both the Higgs mass and the relic density constraints lie within a series of thin strips in the $m_0-m_{1/2}$ plane for different values of $A_0/m_0$. Consequently, this region of minimal supergravity parameter space makes a number of very precise predictions. It is found that light stops of mass down to 400 GeV or lower can exist consistent with all constraints. A signal analysis for this class of models at LHC run II is carried out and the dominant signals for their detection identified. Also computed is the minimum integrated luminosity for $5\sigma$ discovery of the models analyzed. If supersymmetry is realized in this manner, the stop masses can be as low as 400 GeV or lower, and the mass gap between the lightest neutralino and lightest stop will be approximately 30–40 GeV. We have optimized the ATLAS signal regions specifically for stop searches in the parameter space and find that a stop with mass $\sim 375\mathrm{GeV}$ can be discovered with as little as $\sim 60{\mathrm{fb}}^{-1}$ of integrated luminosity at run II of the LHC; the integrated luminosity needed for discovery could be further reduced with more efficient signature analyses. The direct detection of dark matter in this class of models is also discussed. It is found that dark matter cross sections lie close to, but above, coherent neutrino scattering and would require multiton detectors such as LZ to see a signal of dark matter for this class of models.

Figure 1

Left panel: A three-dimensional plot of the stop-neutralino coannihilation region satisfying Eq. (5) and the Higgs boson mass constraint with three axes chosen as $m_0,A_0/m_0$, and $m_{\frac{1}{2}}$ and with $\tan \beta =10$. The image is colored by $A_0/m_0$ values in the range $[-2.2,-2.3]$. Right panel: Two-dimensional projection of the left panel in the $m_{\frac{1}{2}}-m_0$ plane. The allowed regions are colored by the stop-neutralino mass gap $\mathrm{\Delta }m\equiv m_{\tilde{t}}-m_{{\tilde{\chi }}_1^0}$.

Figure 2

Top panels: Same as the two panels of Fig. 1 except that relic density constraint ${\mathrm{\Omega }}_{\mathrm{LSP}}<0.12$ is imposed. Bottom panels: Same as the two panels of Fig. 1 except that the strong relic density constraint $0.0946<{\mathrm{\Omega }}_{\mathrm{LSP}}<0.1306$ is imposed.

Figure 3

Left panel: A plot of the neutralino mass versus stop mass for the full coannihilation region defined by Eq. (3) and displayed in Fig. 1. The image is colored by the mass gap $\mathrm{\Delta }m$ between the stop and neutralino masses. All masses are in GeV. Right panel: Same as left panel but restricted to only those points which satisfy the strict relic density constraint (ii) and which are discoverable with a luminosity of $\mathcal{L}\le 3000{\mathrm{fb}}^{-1}$, according to the analysis in Sec. 3.

Figure 4

Top panels: Full SNOWMASS SM background after triggering cuts and a pre-cut of $E_T^{\mathrm{miss}}\ge 100\mathrm{GeV}$, broken into individual processes and scaled to $100{\mathrm{fb}}^{-1}$. Left panel gives $M_{\mathrm{Eff}}(\mathrm{incl})$ and right panel gives $E_T^{\mathrm{miss}}$. Individual data sets are labeled according to Eq. (4). Bottom panels: $M_{\mathrm{Eff}}(\mathrm{incl})$ (left) and $E_T^{\mathrm{miss}}$ (right) for the full SNOWMASS SM background, scaled to $100{\mathrm{fb}}^{-1}$, after cuts which define the 2jl-opt signal region (see Table 3 and Sec. 3b) are applied.

Figure 5

Left panel: Distribution in $M_{\mathrm{Eff}}$ for the M1 signal region for model i. Plotted is the number of counts for the SUSY signal and the square root of the total SM backgrounds. The analysis is done at $369{\mathrm{fb}}^{-1}$ of integrated luminosity, which gives a $5\sigma$ discovery in this signal region. Right panel: The same analysis as in the left panel but for $E_T^{\mathrm{miss}}$.

Figure 6

Left panel: Distribution in $M_{\mathrm{Eff}}$ for the 2jl signal region for model i. Plotted is the number of counts for the SUSY signal and the square root of the total SM backgrounds. The analysis is done at $88{\mathrm{fb}}^{-1}$ of integrated luminosity, which gives a $5\sigma$ discovery in this signal region. Right panel: The same analysis as in the left panel but for $E_T^{\mathrm{miss}}$.

Figure 7

Left panel: Distribution of $\mathrm{\Delta }\varphi (j_1,E_T^{\mathrm{miss}})$ against the square root of the full SM background at $100{\mathrm{fb}}^{-1}$ for model i before any cuts are applied. Right panel: The same as in the left panel but for $\mathrm{\Delta }\varphi (j_2,E_T^{\text{miss}})$.

Figure 8

Distribution in $M_{\mathrm{Eff}}$ (left panels) and $E_T^{\mathrm{miss}}$ (right panels) for the optimized two-jet signal region 2jl-opt. Top panels show model i normalized to $61{\mathrm{fb}}^{-1}$ of integrated luminosity, while the middle and bottom panels give models vii and xii, normalized to 367 and $2140{\mathrm{fb}}^{-1}$ of integrated luminosity, respectively. These numbers give a $5\sigma$ discovery in this signal region for the three model points. Plotted in all three cases is the number of counts for the SUSY signal and the square root of the total SM background.