Supersymmetry in the shadow of photini

TL;DR

This paper explores how the addition of photini, neutral fermions from string theory, alters supersymmetry collider signatures by reducing missing energy signals and affecting mass limits, with implications for LHC searches.

Contribution

It demonstrates the impact of photini on SUSY signatures at the LHC, showing reduced missing energy and altered mass bounds, extending understanding of neutral fermion effects in supersymmetric models.

Findings

Photini mixing reduces missing energy in SUSY events.

Limits on squark masses can decrease by 400 GeV due to photini.

Multilepton searches remain promising for SUSY discovery with photini.

Abstract

Additional neutral gauge fermions -- "photini" -- arise in string compactifications as superpartners of U(1) gauge fields. Unlike their vector counterparts, the photini can acquire weak-scale masses from soft SUSY breaking and lead to observable signatures at the LHC through mass mixing with the bino. In this work we investigate the collider consequences of adding photini to the neutralino sector of the MSSM. Relatively large mixing of one or more photini with the bino can lead to prompt decays of the lightest ordinary supersymmetric particle; these extra cascades transfer most of the energy of SUSY decay chains into Standard Model particles, diminishing the power of missing energy as an experimental handle for signal discrimination. We demonstrate that the missing energy in SUSY events with photini is reduced dramatically for supersymmetric spectra with MSSM neutralinos near the weak scale, and study the effects on limits set by the leading hadronic SUSY searches at ATLAS and CMS. We find that in the presence of even one light photino the limits on squark masses from hadronic searches can be reduced by 400 GeV, with comparable (though more modest) reduction of gluino mass limits. We also consider potential discovery channels such as dilepton and multilepton searches, which remain sensitive to SUSY spectra with photini and can provide an unexpected route to the discovery of supersymmetry. Although presented in the context of photini, our results apply in general to theories in which additional light neutral fermions mix with MSSM gauginos.