Opening up the compressed region of stop searches at 13 TeV LHC

Light top superpartners play a key role in stabilizing the electroweak scale in supersymmetric theories. For $R$-parity conserved supersymmetric models, traditional searches are not sensitive to the compressed regions. In this paper, we propose a new method targeting this region, with stop and neutralino mass splitting ranging from $m_{\tilde t} - m_\chi \gtrsim m_t$ to about 20 GeV. In particular, we focus on the signal process in which a pair of stops are produced in association with a hard jet, and define a new observable $R_M$ whose distribution has a peak in this compressed region. The position of the peak is closely correlated with $m_{\tilde t}$. We show that for 13 TeV LHC with a luminosity of 3000 fb$^{-1}$, this analysis can extend the reach of stop in the compressed region to $m_{\tilde t}$ around 800 GeV.