⚛️ phenomenology

What has the LHC told us about the electroweakino sector of the Minimal Supersymmetric Standard Model?

This paper presents the most comprehensive global fit of the MSSM electroweakino sector to date, utilizing extensive LEP and LHC Run 2 data to establish stringent mass limits (up to ~1 TeV) and demonstrate that no single scenario can simultaneously explain observed ATLAS and CMS excesses in compressed spectra.

Original authors: Peter Athron, Csaba Balázs, Andy Buckley, Jon Butterworth, Christopher Chang, Andrew Fowlie, Tomás E. Gonzalo, Vinay Hegde, Ida-Marie Fauske Johansson, Adil Jueid, Tore Klungland, Anders Kvellestad, F

Published 2026-09-14

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Peter Athron, Csaba Balázs, Andy Buckley, Jon Butterworth, Christopher Chang, Andrew Fowlie, Tomás E. Gonzalo, Vinay Hegde, Ida-Marie Fauske Johansson, Adil Jueid, Tore Klungland, Anders Kvellestad, Farvah Mahmoudi, Gregory D. Martinez, Holly Pacey, Tomasz Procter, Are Raklev, Roberto Ruiz de Austri, Pat Scott, Martin White, Yang Zhang, Pengxuan Zhu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

For decades, physicists have been searching for a deeper layer of reality beneath the known particles of the universe. The Standard Model, our best current map of the subatomic world, works remarkably well but leaves major questions unanswered, such as why the universe contains more matter than antimatter or what dark matter is made of. One of the most popular ideas to solve these puzzles is supersymmetry, a theory suggesting that every known particle has a heavier, invisible partner. Among these hypothetical partners are the electroweakinos, a family of particles that are electrically neutral or carry a single electric charge. If they exist, they could explain the nature of dark matter and stabilize the fundamental forces of nature. However, despite years of searching at the Large Hadron Collider, these particles have remained elusive, leaving scientists to wonder if they are simply too heavy to be found or if they hide in ways that current experiments have missed.

A large international team of researchers has now performed the most comprehensive analysis to date to determine exactly what the Large Hadron Collider has told us about these elusive particles. Using a sophisticated computer framework called GAMBIT, the team simulated millions of possible scenarios for how these particles could behave, combining data from dozens of different searches and precision measurements taken during the collider's second major run. They focused on two main possibilities: one where the lightest supersymmetric particle is a stable neutral particle, and another where it is an even lighter, nearly massless particle called a gravitino. By systematically testing these scenarios against real data, the team mapped out which combinations of particle masses and properties are still allowed by nature and which have been definitively ruled out.

The results paint a picture of a universe where the lightest versions of these particles are not hiding in the low-mass regions that were once considered the most likely. For a specific type of particle known as a bino, which is a neutral partner to a force-carrying particle, the team found that if it is light, the next-heavier partner in the family must be quite massive, weighing at least 760 gigaelectronvolts. This is a significant increase in the lower mass limit compared to previous studies, driven by the sheer volume of data collected and the improved sensitivity of the detectors. While the researchers found that the complex interactions of these particles could explain some small, unexpected bumps in the data seen by the ATLAS and CMS experiments, they discovered that no single scenario could explain the bumps seen by both experiments at the same time. This suggests that the anomalies are likely statistical fluctuations rather than a clear signal of new physics.

When the team included the possibility of a light gravitino, the constraints became even tighter. In this scenario, the lightest neutral particle is forced to be heavier, with masses below roughly 1,000 gigaelectronvolts being excluded for most configurations. Interestingly, if the lightest particle is dominated by a specific type of composition known as a Higgsino, the mass limit is slightly lower, around 650 gigaelectronvolts, but still significantly higher than what was preferred in earlier studies. The analysis also revealed that newer searches and measurements have closed off a low-mass region that was previously thought to be a promising hiding spot for these particles.

The study highlights how the search for new physics has evolved from looking for simple, isolated signals to performing a global, statistical combination of hundreds of different data points. The team had to carefully account for how different searches might overlap and how the particles decay into other known particles, such as photons or W and Z bosons, before vanishing into the dark sector. They found that while some specific mass combinations still survive the current data, the window for finding these particles at lower energies is closing rapidly. The researchers noted that future experiments with even higher energy and luminosity will be needed to probe the remaining possibilities, particularly for scenarios where the particles decay slowly or produce very subtle signals. This work serves as a definitive baseline, showing that if these supersymmetric partners exist, they are likely heavier and more complex than the simplest versions of the theory predicted, requiring more powerful tools to uncover them.

Problem
The Minimal Supersymmetric Standard Model (MSSM) electroweakino sector, comprising four neutralinos and two charginos, remains a primary target for Beyond the Standard Model (BSM) physics searches at the Large Hadron Collider (LHC). While strongly interacting sparticles have been constrained to multi-TeV scales, weakly interacting electroweakinos are harder to constrain due to lower production cross-sections and model-dependent interpretations. Previous global fits, such as those in Refs. [14] and [30], indicated that the LHC data available at the time did not impose general constraints on the lightest chargino ( $\tilde{\chi}^\pm_1$ ) and lightest neutralino ( $\tilde{\chi}^0_1$ ) mass plane when all four electroweakino parameters were allowed to vary. Furthermore, recent LHC Run 2 searches have reported excesses in compressed spectra scenarios (deviations from Standard Model predictions at the $2\sigma$ level), raising questions about whether these anomalies can be simultaneously explained within a consistent MSSM framework. There is a need for a rigorous, statistically combined assessment of the full Run 2 dataset to determine the true exclusion limits and the viability of these excesses.

Methodology
The authors perform global fits using the GAMBIT 2.7 framework, focusing on two specific models:

EWMSSM: The electroweak sector of the MSSM with all other sparticles decoupled. This model is defined by four parameters: the bino mass $M_1$ , wino mass $M_2$ , Higgsino mass parameter $\mu$ , and $\tan\beta$ .
$\tilde{G}$ -EWMSSM: An extension of the EWMSSM including a light gravitino ( $\tilde{G}$ ) as the lightest supersymmetric particle (LSP), motivated by gauge-mediated supersymmetry breaking. The gravitino mass is fixed at 1 eV for the main scan, with auxiliary scans at 1 meV and 1 keV to test phenomenological dependence.

The analysis scans the parameter space ( $M_1, M_2, \mu, \tan\beta$ ) using the differential evolution sampler Diver. The likelihood function is constructed from three components:

Direct LHC Searches: 34 searches from ATLAS and CMS (Run 2, $\sqrt{s}=13$ TeV) targeting new particles. The study utilizes the full likelihood framework where available (e.g., specific ATLAS analyses) and the simplified likelihood framework with covariance matrices where provided (CMS). For analyses without correlation data, the most sensitive signal region is selected.
Model-Independent LHC Measurements: 63 Standard Model precision measurements (differential cross-sections) processed via the Contur package to assess contamination from BSM physics.
LEP Constraints: Cross-section limits from the Large Electron-Positron collider for low-mass regions.

The study employs a frequentist approach, profiling the likelihood over nuisance parameters. A critical technical component involves handling the statistical overlap between different LHC searches; the authors simulate benchmark points to identify correlated signal regions and select the combination of analyses that provides the greatest expected exclusion without double-counting events.

Key Contributions

Comprehensive Dataset: This work represents the most complete summary of collider constraints on the MSSM electroweakino sector to date, incorporating 34 LHC searches and 63 SM measurements from Run 2.
Updated Global Fits: It updates and extends previous GAMBIT studies [14, 30] by utilizing the full Run 2 dataset, significantly enhancing statistical power.
Excess Analysis: The paper rigorously tests whether the EWMSSM and $\tilde{G}$ -EWMSSM can simultaneously explain the excesses observed by ATLAS and CMS in compressed spectra searches.
Gravitino Mass Dependence: It explicitly investigates how the exclusion limits in the $\tilde{G}$ -EWMSSM scenario depend on the assumed gravitino mass, a factor often fixed or ignored in simplified model interpretations.
Technical Validation: The paper details updates to the ColliderBit module, including interfaces to Pythia 8, validation of soft lepton searches, and handling of multiple jet collections and signal region overlaps.

Results

EWMSSM Constraints: For a light bino-like $\tilde{\chi}^0_1$ , the mass of the next-to-lightest neutralino ( $\tilde{\chi}^0_2$ ) is excluded up to approximately 760 GeV. This is a significant improvement over previous studies which found no general exclusion in the $(m_{\tilde{\chi}^\pm_1}, m_{\tilde{\chi}^0_1})$ plane.
Excesses: While the MSSM electroweakinos can explain individual excesses observed by ATLAS and CMS in searches for compressed spectra, the authors find no scenarios that fit these excesses simultaneously. The preferred regions for ATLAS and CMS excesses are orthogonal in the parameter space.
$\tilde{G}$ -EWMSSM Constraints: When a light gravitino is included, the study excludes scenarios up to $(m_{\tilde{\chi}^0_1}, m_{\tilde{\chi}^0_2}) \sim (900, 1000)$ GeV. For Higgsino-dominated $\tilde{\chi}^0_1$ , the constraint is tighter, requiring $m_{\tilde{\chi}^0_1} \approx m_{\tilde{\chi}^0_2} \gtrsim 650$ GeV. This low-mass Higgsino region, previously preferred in earlier studies, is now disfavoured by newer, higher-luminosity searches.
Best-Fit Regions: The best-fit region for the $\tilde{G}$ -EWMSSM corresponds to $|M_1| \ll |\mu|, M_2$ , with $m_{\tilde{\chi}^0_1} \lesssim 650$ GeV and $m_{\tilde{\chi}^0_2} \sim 900\text{--}1300$ GeV. This preference arises from fitting small excesses in a CMS search for final states with two photons and missing energy. The authors note that these scenarios predict complex final states (e.g., $4\ell/4b/2\ell2b + 2\gamma + \text{MET}$ ) that are not fully targeted by current standard searches.
Gravitino Mass Impact: Lowering the gravitino mass to 1 meV weakens constraints from photon+MET searches because direct decays of heavier electroweakinos to the gravitino become competitive, reducing the photon signal from $\tilde{\chi}^0_1 \to \gamma \tilde{G}$ . Conversely, higher gravitino masses (1 keV) introduce long-lived particle constraints not covered by the prompt decay assumption of the main analysis.

Significance
The paper claims to provide the "most complete summary of collider constraints on the electroweakino sector of the MSSM performed to date." It demonstrates that while the LHC has not yet discovered electroweakinos, the parameter space has been significantly narrowed, particularly for light bino and Higgsino scenarios. The study highlights that the "holes" in exclusion limits identified in earlier runs are largely closed by the full Run 2 dataset. Furthermore, it clarifies that the observed experimental excesses in compressed spectra cannot be reconciled within a single MSSM scenario under the current analysis framework. The authors position this work as a baseline for future comparisons with Run 3 data and emphasize that the inclusion of long-lived particle searches will be the next critical step in probing the remaining viable parameter space.

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