Cornering Light Stops with Dileptonic mT2

TL;DR

This paper investigates the effectiveness of dileptonic mT2 in detecting light stops in supersymmetric models, especially in challenging compressed spectra, and maps out the exclusion and discovery potential with 2012 collider data.

Contribution

It introduces an optimized dileptonic mT2 search strategy for difficult stop spectra and accounts for effects of stop chirality and LSP type, extending previous analyses.

Findings

Right-handed stops with bino-like LSP are excludable at m(stop) ~ m(top) with 2012 data.

Higgsino-like LSP scenarios remain largely unobservable with the same data.

The study maps out parameter regions for exclusion and potential discovery.

Abstract

Supersymmetric spectra with a stop NLSP and a neutralino or gravitino LSP present a special challenge for collider searches. For stop pairs directly produced from QCD, the visible final-state particles are identical to those of top quark pair production, giving very similar kinematics but often with much smaller rates. The situation is exacerbated for compressed spectra with m(stop) ~ m(top) + m(LSP), as well as for lighter stops which can suffer from low acceptance efficiencies. In this note, we explore the power of a direct stop search using dileptonic mT2, similar to the one recently performed by ATLAS, but more optimized to cover these difficult regions of the (m(stop),m(LSP)) plane. Our study accounts for the effects of stop chirality and LSP identity, which can be significant. In particular, our estimates suggest that m(stop) ~ m(top) with a massless LSP is excludable for right-handed stops with bino-like (gravitino) LSP with 2012 (2011) data, but remains largely unobservable in the case of a higgsino-like singlino LSP. For each of these cases we map out the regions of parameter space that can be excluded with 2012 data, as well as currently allowed regions that would yield discovery-level significance. We also comment on the prospects of a precision mT2 shape measurement, and consider the potential of ATLAS's dileptonic stop -> b chi^+ searches when re-interpreted for light stops decaying directly to the LSP.