Mixed Stops and the ATLAS on-Z Excess

TL;DR

This paper explores a minimal supersymmetric model with mixed stops to explain the ATLAS Z+jets+ETmiss excess, showing it can fit the data without conflicting with other searches and highlighting unique decay topologies.

Contribution

It introduces a novel decay topology involving mixed stops and demonstrates its compatibility with observed excesses while remaining consistent with existing constraints.

Findings

The excess can be explained by a compressed stop spectrum with mixed stops.

The proposed topology is not excluded by current searches.

CMS background estimates may be affected by these new topologies.

Abstract

The ATLAS experiment has recently observed a 3-sigma excess in a channel with a leptonically decaying Z, jets, and ETmiss. It is tantalizing to interpret the signal as the first sign of a natural supersymmetric spectrum. We study such a possibility in a minimal model containing light stops and a neutralino LSP. The signal is characterized by a novel topology (compared to previous attempts) where the Z is emitted from a colored particle in the first step of a decay chain, namely t2 -> t1 Z which is characteristic of mixed stops. We show that the excess is compatible with a compressed stop spectrum and is not excluded by any other relevant search, finding some regions of parameter space with signal strength within 1-sigma of that measured by the ATLAS collaboration. In addition, we notice that the corresponding CMS search could be prone to background contamination in unexpected topologies of this kind.