The ATLAS Z + MET Excess in the MSSM

TL;DR

This paper explains the ATLAS Z + MET excess within the MSSM framework, identifying specific sparticle mass ranges and decay chains that fit the observed data while satisfying other experimental constraints.

Contribution

It introduces a detailed MSSM-based scenario that accounts for the excess, highlighting specific squark and neutralino mass configurations consistent with multiple search results.

Findings

Successful explanation of the excess with squark masses 500-750 GeV.

Bino-like neutralino near 350 GeV decaying into Higgsino-like LSP.

Predicted signals in future Run II searches in 0l + jets and 1l + jets channels.

Abstract

We demonstrate that the $3\sigma$ excess observed by ATLAS in the Z + MET channel can be explained within the context of the MSSM. Using the freedom inherent in the pMSSM, we perform a detailed analysis of the parameter space and find a scenario that describes the excess while simultaneously complying with all other search constraints from the Run I data at 7 and 8 TeV, including the Z + MET analysis by CMS. We generate a small sample of simplified models, using promising models from our existing pMSSM sample as seeds, and study their properties. The successful region is described by the production of 1st/2nd generation squark pairs, followed by their decay into a bino-like neutralino which in turn decays into a Higgsino-like LSP triplet by emitting a Z boson, i.e., $\tilde q\to\tilde B\to\tilde h$ with $\tilde q = \tilde Q_L,\tilde u_R,$ or $\tilde d_R$. The sweet spot for the sparticle spectrum is found to have squark masses in the 500-750 GeV range, with bino masses near 350 GeV with a mass splitting of 150-200 GeV with the Higgsino LSP. If this excess holds, then this scenario predicts that a signal will be observed in the 0l + jets and/or 1l + jets searches in the early operations of Run II.