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Search for supersymmetric particle pair production in final states with two oppositely charged leptons and large missing transverse momentum in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Using 138 fb $^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the CMS detector, this study searches for supersymmetric particle pair production in final states with two oppositely charged leptons and large missing transverse momentum, finding no excess over Standard Model predictions and setting new exclusion limits on chargino, top squark, and slepton masses within simplified models with $R$ -parity conservation.

Original author: CMS Collaboration

Published 2026-11-03

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CC BY 4.0

Original author: CMS Collaboration

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 author. For technical accuracy, refer to the original paper. Read full disclaimer

The universe we can see—stars, planets, and the atoms that make up our own bodies—accounts for only a tiny fraction of what exists. The rest is a mysterious substance called dark matter, which does not emit light but exerts a gravitational pull that holds galaxies together. Physicists have long searched for a particle that could explain this invisible mass, but the standard model of particle physics, our best map of the subatomic world, offers no candidate for it. This model also struggles to explain why the Higgs boson, a particle that gives other particles mass, is so much lighter than the forces of nature suggest it should be. To fix these gaps, scientists proposed a theory called supersymmetry. This idea suggests that every known particle has a heavier, invisible partner. If these partners exist, the lightest one would be stable and could be the dark matter that fills the cosmos.

In a recent study, researchers at the Large Hadron Collider in Switzerland put this idea to the test. They smashed protons together at incredibly high speeds, recreating the conditions just after the Big Bang, and looked for signs of these hypothetical partners. Specifically, they searched for pairs of particles called charginos, top squarks, and sleptons. If these particles were created, they would decay almost instantly into lighter particles, including electrons or muons (which are like heavy electrons) and a missing particle that carries away energy without being detected. The researchers analyzed a massive amount of collision data, looking for events where two oppositely charged leptons appeared alongside a significant imbalance in momentum, which would signal the escape of invisible particles.

The team examined 138 units of collision data, a volume that represents the most comprehensive look at this specific type of event to date. They sorted the events into different categories based on the types of particles produced and the amount of missing energy. By comparing what they saw against what the standard model predicts should happen without any new physics, they found no evidence of an excess. The data matched the predictions of known physics perfectly. This lack of a signal allowed the scientists to rule out a wide range of possibilities for where these supersymmetric particles might hide. They determined that if charginos exist, they must be heavier than 1120 GeV, and if top squarks exist, they must be heavier than 500 to 540 GeV, depending on the specific scenario. Similarly, they excluded the existence of sleptons with masses up to 700 GeV.

These results are significant because they push the boundaries of where scientists must look next. The search was particularly focused on "compressed" scenarios, where the new particles are only slightly heavier than the invisible particles they decay into. In such cases, the visible debris from the collision is very quiet and difficult to spot, making these events easy to miss. By refining their methods to catch these subtle signals, the researchers were able to extend the known limits of particle mass by hundreds of GeV. While the study did not find the new particles it sought, it successfully narrowed the search space, telling the scientific community that if supersymmetry exists, it is hiding in a much heavier, more elusive region than previously hoped. The work stands as a rigorous confirmation that the standard model holds firm in this energy range, leaving the mystery of dark matter and the hierarchy of particle masses to be solved by even more powerful experiments in the future.

Technical Summary: Search for Supersymmetric Particle Pair Production in Final States with Two Oppositely Charged Leptons and Large Missing Transverse Momentum

Problem and Motivation
The Standard Model (SM) of particle physics, while successful, lacks a viable dark matter candidate and fails to resolve the hierarchy between the Higgs boson mass and the Planck scale. Supersymmetry (SUSY) addresses these limitations by introducing superpartners for SM fields. In R-parity conserving models, the lightest neutralino ( $\tilde{\chi}^0_1$ ) is stable and serves as a dark matter candidate. This paper presents a search for the direct pair production of supersymmetric particles—specifically charginos ( $\tilde{\chi}^\pm_1$ ), top squarks ( $\tilde{t}_1$ ), and sleptons ( $\tilde{\ell}$ )—in proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV. The analysis targets final states characterized by two oppositely charged leptons (electrons or muons) and large missing transverse momentum ( $p^{\text{miss}}_T$ ), signatures arising from the decay of these particles into the stable neutralino.

Methodology
The analysis utilizes 138 fb $^{-1}$ of data collected by the CMS detector between 2016 and 2018. The event selection requires two isolated, oppositely charged leptons with $p_T > 25$ (leading) and $20$ (trailing) GeV, and $p^{\text{miss}}_T > 160$ GeV. To suppress SM backgrounds, events with a third lepton or an invariant mass of the lepton pair ( $m_{\ell\ell}$ ) near the Z boson mass are rejected.

The search strategy employs a simultaneous maximum likelihood (ML) fit to the $m_{T2}$ distribution (a transverse mass variable generalized for two invisible particles) across multiple Search Regions (SRs) and Control Regions (CRs). The SRs are categorized based on:

Lepton Flavor: Same-flavor (SF) and different-flavor (DF) channels.
$p^{\text{miss}}_T$ Binning: Four bins ranging from 160 GeV to $>380$ GeV.
Jet Multiplicity: Specifically tailored for top squark searches to distinguish between compressed mass spectra (low jet activity) and scenarios with larger mass splittings.
- Chargino/Slepton Searches: Veto $b$ -tagged jets to reject $t\bar{t}$ and $tW$ backgrounds.
- Top Squark Searches: Define regions with zero $b$ -tagged jets (sensitive to compressed spectra where $b$ -jets are soft) and regions with at least one $b$ -tagged jet. A specific "hard ISR" selection is applied for high $p^{\text{miss}}_T$ to exploit recoil against initial-state radiation jets.

Background estimation relies on data-driven methods where possible. The normalizations of dominant backgrounds ( $t\bar{t}$ , $tW$ , $WW$ ) are constrained by the ML fit in low- $m_{T2}$ regions. Subleading backgrounds ( $WZ$ , $ZZ$ , $t\bar{t}Z$ , Drell-Yan) are constrained using dedicated CRs. Systematic uncertainties, including those related to detector performance, luminosity, and theoretical modeling (e.g., $m_{T2}$ tail modeling), are incorporated via nuisance parameters.

Key Contributions

Extended Mass Reach: This analysis extends previous CMS constraints by optimizing event selection for specific topologies, particularly in the compressed mass region for top squarks and the high-mass region for charginos.
Comprehensive Signal Scenarios: The paper interprets results for three distinct production mechanisms:
1. Chargino pair production via intermediate sleptons/sneutrinos and via direct decay to $W\tilde{\chi}^0_1$ .
2. Top squark pair production decaying to $t\tilde{\chi}^0_1$ (compressed region $m_W < \Delta m \lesssim m_t$ ) and via $\tilde{t}_1 \to b\tilde{\chi}^\pm_1 \to bW\tilde{\chi}^0_1$ .
3. Slepton pair production decaying to $\ell\tilde{\chi}^0_1$ .
Improved Background Modeling: The analysis introduces specific corrections for the modeling of the $m_{T2}$ tail in $t\bar{t}$ and $WW$ events, addressing mismodeling observed in data at high $p^{\text{miss}}_T$ and refining the treatment of non-prompt leptons.

Results
No significant excess of events over the SM predictions was observed in any of the search regions. Consequently, 95% confidence level (CL) upper limits were set on the production cross sections for the considered models. The observed limits generally agree with expected limits within experimental uncertainties.

Charginos: In the scenario where charginos decay via intermediate sleptons, chargino masses up to 1120 GeV are excluded for a massless neutralino. For a chargino mass of $\sim 820$ GeV, neutralino masses up to 490 GeV are excluded. In the direct decay scenario ( $\tilde{\chi}^\pm_1 \to W^\pm \tilde{\chi}^0_1$ ), chargino masses between 260–380 GeV are excluded for neutralino masses below 25 GeV.
Top Squarks: In the compressed mass region ( $m_W < m_{\tilde{t}_1} - m_{\tilde{\chi}^0_1} \lesssim m_t$ ), top squark masses up to 500–540 GeV are excluded for the decay $\tilde{t}_1 \to t^*\tilde{\chi}^0_1$ . For the decay chain $\tilde{t}_1 \to b\tilde{\chi}^\pm_1 \to bW\tilde{\chi}^0_1$ , masses up to 540 GeV are excluded in the compressed region.
Sleptons: Slepton masses are excluded up to 700 GeV for a massless neutralino. For slepton masses below 150 GeV, mass splittings greater than 30 GeV are excluded.

Significance
The paper claims these results constitute the tightest constraints currently available for chargino pair production involving decay modes through intermediate sleptons. The analysis successfully complements previous searches in low-mass compressed spectra and hadronic final states, effectively covering the gap between them. By extending the exclusion reach for top squarks in the compressed region by approximately 100 GeV compared to previous dilepton searches, the work significantly narrows the parameter space for natural SUSY scenarios where the top squark mass is near the TeV scale. The results provide robust constraints on simplified SUSY models, reinforcing the absence of evidence for supersymmetry in the explored mass and decay configurations.

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