(Light) Stop Signs

TL;DR

The paper tackles the stealth-stop regime, where a light stop with $m_{ ilde{t}}$ near $m_t$ mimics SM $t\bar t$ production and challenges direct discovery. It proposes and evaluates robust discriminants—top-quark spin correlations, the rapidity gap $\Delta y(t,\bar t)$, and related kinematic proxies—to separate stops from tops, arguing that spin correlations are particularly resilient to higher-order uncertainties. Using dileptonic channels and matrix-element techniques, the authors show that spin-correlation measurements can exclude a $200$ GeV stop at 95% confidence with $20~\text{fb}^{-1}$ at $8~\text{TeV}$, and they explore the complementary role of rapidity-based observables at both parton and jet levels. The work also discusses how systematic uncertainties in top production impact rapidity-gap analyses and advocates a multivariate approach that blends several observables to maximize sensitivity in the stealthy region, with implications for guiding experimental searches and improving SM top modeling.

Abstract

Stop squarks with a mass just above the top's and which decay to a nearly massless LSP are difficult to probe because of the large SM di-top background. Here we discuss search strategies which could be used to set more stringent bounds in this difficult region. In particular, we note that both the rapidity difference Delta y(t,tbar) and spin correlations (inferred from, for example, Delta phi(l+,l-)) are sensitive to the presence of stops. We emphasize that systematic uncertainties in top quark production can confound analyses looking for stops, making theoretical and experimental progress on the understanding of Standard Model top production at high precision a very important task. We estimate that spin correlation alone, which is relatively robust against such systematic uncertainties, can exclude a 200 GeV stop at 95% confidence with 20 fb^-1 at the 8 TeV LHC.