The light stop window

TL;DR

The paper argues for a light-stop window in supersymmetry, with a predominantly right-handed stop of 200–400 GeV nearly degenerate with the neutralino LSP and a gluino possibly below 1.5 TeV. It shows that RG running naturally creates a large left–right stop mass split under modest gluino masses and that maximal stop mixing helps accommodate the observed Higgs mass while mitigating fine-tuning. In compressed spectra, four-body stop decays can dominate and provide experimental handles via leptons, while an additional jet aids triggering; the authors demonstrate that CMS razor analyses already set bounds stronger than published ATLAS/CMS results, extending sensitivity to Stop masses up to ~250 GeV even with small mass gaps. The work highlights the complementarity of flavor, dark matter, and collider constraints in shaping this window and emphasizes the role of upcoming LHC13/14 data in fully probing the scenario.

Abstract

We show that a right-handed stop in the 200-400 GeV mass range, together with a nearly degenerate neutralino and, possibly, a gluino below 1.5 TeV, follows from reasonable assumptions, is consistent with present data, and offers interesting discovery prospects at the LHC. Triggering on an extra jet produced in association with stops allows the experimental search for stops even when their mass difference with neutralinos is very small and the decay products are too soft for direct observation. Using a razor analysis, we are able to set stop bounds that are stronger than those published by ATLAS and CMS.

Figures (8)

• Figure 1: The Higgs mass in low-energy supersymmetry for large $\tan\beta\approx 20$. The shaded region in the $(X_t, m_{S})$ plane corresponds to the observed value of $m_h$. Higher-order corrections and the uncertainty in the top mass amount to an error of a few $\,\mathrm{GeV}$ in $m_h$.
• Figure 3: Illustrative example of renormalization group evolution from the unification scale to the weak scale of gaugino masses $M_1$, $M_2$, $M_3$ (green curves), of the stop mass parameters $m_{{\tilde{t}}_L}$ and $m_{{\tilde{t}}_R}$ (full and dashed blue curves, respectively), $y_t A_t$ (red dashed curve), $m_{H_u}$ (black curve), in a configuration leading to $m_{{\tilde{t}}_R} \ll m_{{\tilde{t}}_L}$ at the weak scale. All masses are in GeV units and we assumed the MSSM.
• Figure 5: Correlation between ${\rm BR}(B\to X_s \gamma)$ and $\epsilon_K$. The two ellipses denote the $68\%$ and $90\%$ CL experimental range. All points reproduce the observed Higgs mass. The two black curves are obtained varying $m_{{\tilde{t}}_R}$ between $200\,\mathrm{GeV}$ and $400\,\mathrm{GeV}$ (from left to right) for $m_{S}=500\,\mathrm{GeV}$, $\mu = 250\,\mathrm{GeV}$, and $\tan\beta=20$ (dashed curve) or $\tan\beta = 10$ (full curve). The points are obtained varying the parameters in the range $\mu = [150-400]\,\mathrm{GeV}$ and $m_{{\tilde{t}}_R}=[200-400]\,\mathrm{GeV}$, with $m_{S} < 700 \,\mathrm{GeV}$ and $\tan\beta=20$ (red) or $\tan\beta = 10$ (blue).
• Figure 6: Points in the supersymmetric parameter space that lead to the correct DM abundance.
• Figure 7: Distribution of $M_R$ (left), $R^2$ (center), and box-by-box event fraction (right) for pair-produced stop events as a function of the stop mass, for $\tilde{t} \to \ell \nu_\ell b N$ decays and $m_{\tilde{t}} - M_{\rm DM}= 30$ GeV. Even if this case is the most favorable for the selection of leptonic final states, the hadronic box is the most populated due to the small value of $m_{\tilde{t}} - M_{\rm DM}$.