Rare and Experimentally Challenging Supersymmetry Signatures

TL;DR

This review surveys rare and experimentally challenging supersymmetry signatures at the LHC, emphasizing how detector limitations, trigger strategies, and analysis innovations shape the search program. It covers MSSM-inspired targets (notably semi-compressed spectra and electroweakinos) and extended-MSSM scenarios (gravitinos, Stealth SUSY, axions, and mini-split) that yield LLPs, soft final states, or low-MET events, often requiring novel reconstruction and interpretation techniques. The authors discuss methodological advances (ISR boosting, RJR, ML-based selections) and underscore gaps in current coverage, arguing for broad, signature-driven searches and de-simplified model explorations to maximize sensitivity in the HL-LHC era. They also situate collider results within a wider physics context, including DM searches, EDMs, and flavor constraints, to motivate complementary approaches and future collider prospects. Overall, the paper argues that despite extensive exclusions, SUSY remains a compelling framework that motivates diverse, innovative experimental strategies and continued exploration of rare signatures.

Abstract

Supersymmetry has long played a central role in the search for physics beyond the Standard Model at colliders, providing a comprehensive and internally consistent framework for generating well-motivated experimental signatures. For more than fifteen years of LHC operation, the CMS and ATLAS collaborations have achieved remarkable sensitivity to a wide range of supersymmetric signatures. Despite this unprecedented reach, no conclusive evidence for supersymmetry has emerged. If supersymmetry is nature's solution to outstanding questions in particle physics, it is necessarily challenging to find. In this article, we review supersymmetric signatures that are particularly rare or otherwise challenging, with a focus on searches at the Large Hadron Collider. We highlight experimental challenges relating to detector constraints and analysis difficulties, in addition to model challenges in the interpretation and optimization of searches. We also identify regions of signature space that remain comparatively unconstrained and therefore represent promising targets for future exploration.

Figures (7)

• Figure 1: A wide array of conventional SUSY search results from the ATLAS and CMS experiments is shown for strongly produced sparticles (top) and those produced via EW processes (bottom), as a function of the mass of the produced sparticle, $m_X$, and the mass of the neutralino LSP, $m_{\tilde{\chi}_{1}^0}$. Different colors illustrate searches with varying assumptions about the sparticle couplings and decays ATLAS:2024ldaSekmen:2025bxvATL-PHYS-PUB-2024-014CMSSUSYSummaryPlots.
• Figure 2: An abstract representation of the challenges faced by modern searches for supersymmetry. The challenges inherent to searches for a particular SUSY model are distinct from the challenges inherent to a particular experimental signature. The overlaps between challenges are intended to be illustrative of actual overlapping challenges, but they are neither exhaustive nor prescriptive.
• Figure 3: The observed limits at 95% CL for simplified-model Higgsino production, for $\Delta m_{\pm}^{}= \Delta m(\tilde{\chi}_{1}^{\pm},\tilde{\chi}_{1}^0)$ as a function of $m_{\tilde{\chi}_{1}^0}$. Results from LEP, ATLAS, and CMS are shown in solid colors for different signatures, while the dashed lines illustrate conservative projections for the HL-LHC dataset ATLAS:2019lngATLAS:2021moaATLAS:2019lngATLAS:2022rmeCMS:2023mnyCMS-PAS-EXO-23-017CMS-PAS-SUS-24-003ATLAS:2018jjfATLAS:2024umcATLAS:2025lhc.
• Figure 4: The observed limits at 95% CL for simplified-model $\tilde{\tau}_{}\tilde{\tau}_{}$ production, as a function of $m_{\tilde{\tau}_{}}$. On the left, the limits for the MSSM $\tilde{\tau}_{}\rightarrow\tau\tilde{\chi}_{1}^0$ process are shown with varying $\Delta m_{\tilde{\tau}_{}} = m_{\tilde{\tau}_{}} - m_{\tilde{\chi}_{1}^0}$. On the right, limits for GGM $\tilde{\tau}_{}\rightarrow\tau\tilde{G}$ decays with a massless $\tilde{G}$ are shown as a function of $\tilde{\tau}_{}$ lifetime, and are discussed in Section \ref{['sec:gravitinos']}. Hashed lines indicate exclusion to solo $\tilde{\tau}_{R}\tilde{\tau}_{R}$ production, while filled areas exclude the degenerate case where $m_{\tilde{\tau}_{L}} = m_{\tilde{\tau}_{R}}$ and both are produced LEP_sleptonsLEP_sleptons_gmsbATLAS:2019gtiCMS:2012wcgATLAS:2024fubCMS:2024nhnCMS:2022sykATLAS:2020wjhCMS:2021kdmATLAS:2022pibATLAS:2025fdmATLAS:2024vnc.
• Figure 5: Excluded cross-section results are shown from a collection of Run-2 analyses as a function of the $A\times\varepsilon$ of the search. Lines represent similar signatures or model parameters, and arrows denote the evolution of an analysis. The shaded region shows the minimum possible cross-section exclusion for a given integrated luminosity and $A\times\varepsilon$. CMS:2019gwfATLAS:2023otiATLAS:2017tnydvmetthesisCMS:2024nhnCMS:2021kdmATLAS:2020wjhATLAS:2023iosATLAS:2019lffATLAS:2025evxATLAS:2017oalATLAS:2022rme