Reinterpretation of searches for supersymmetry in models with variable R-parity-violating coupling strength using the full ATLAS Run 2 Dataset

Using the full ATLAS Run 2 dataset of 140 fb$^{-1}$ at 13 TeV, this study reinterprets thirteen supersymmetry searches to set comprehensive 95% confidence level mass limits on gluinos, top squarks, tau-sleptons, and charginos/neutralinos across a broad spectrum of R-parity-violating coupling strengths, thereby bridging the gap between prompt and long-lived particle signatures.

The Gist

The Great SUSY Hunt: A New Look at the Invisible

Imagine the universe as a giant, bustling city. The Standard Model is the city's official blueprint, describing all the known buildings (particles like electrons and quarks) and how they interact. But physicists suspect there's a hidden "underground" layer to this city—a secret society of particles called Supersymmetry (SUSY). In this secret society, every known particle has a "shadow twin" (a superpartner) that is heavier and harder to spot.

For decades, scientists at the Large Hadron Collider (LHC) have been smashing protons together to find these shadow twins. The big question was: Where are they?

This paper is a massive "re-examination" of the evidence. Instead of looking for the shadows in just one specific way, the ATLAS team decided to look at the entire city map again, using a new set of rules.

The Mystery of the "R-Parity" Switch

In the original blueprint, there was a rule called R-parity. Think of R-parity as a security guard at the exit of the secret society.

• If the guard is ON (Conserved): When a shadow twin is created, it must eventually turn into the "Lightest Supersymmetric Particle" (LSP). This LSP is invisible and escapes the city (the detector) without leaving a trace. This is like a ghost slipping out a back door. Most previous searches looked for this "ghost leaving a hole in the air" (missing energy).
• If the guard is OFF (Violated): The LSP isn't stable. It decays (breaks apart) into normal particles inside the city. But here's the twist: depending on how strong the "decay force" is, the LSP might break apart immediately, or it might linger for a while, traveling a few meters before exploding into normal matter.

The Problem: Previous searches were like looking for a ghost that runs away instantly, or a ghost that explodes the moment it's born. They missed the middle ground: the ghost that wanders around the city for a few seconds before disappearing.

The New Strategy: The "Variable Speed" Search

This paper is like a detective re-investigating a cold case with a new theory: "What if the ghost moves at different speeds?"

The team took 13 different search strategies (some looking for missing energy, some looking for strange particle tracks, some looking for jets of debris) and re-ran them against the full dataset from 2015–2018. They didn't just look for one specific speed; they looked for every possible speed of the LSP decay, from "instant" to "long-lived."

The Analogy of the Firecracker:
Imagine you are looking for a specific type of firecracker in a dark field.

1. Old Search: You only looked for firecrackers that explode the instant you light them (Prompt).
2. Old Search #2: You only looked for firecrackers that fly away and never explode (Stable/Long-lived).
3. This New Search: You looked for firecrackers that explode instantly, those that fly a few feet and explode, and those that fly a mile and explode. You checked every distance.

What Did They Find? (The Results)

The team didn't find the shadow twins (no new particles were discovered), but they did something very important: They drew a much tighter map of where the shadow twins cannot be.

Think of it like a "Wanted" poster. Before, the poster said, "The criminal might be anywhere between 100 and 1,000 miles away." Now, after this re-examination, the poster says, "We know for sure the criminal is not between 100 and 1,800 miles away."

Here are the specific "No-Go Zones" they established:

• The Heavy Gluinos: If these heavy shadow twins exist, they must be heavier than 1.8 TeV (a massive weight in particle physics), regardless of how fast they decay.
• The Top Squarks: If these exist, they are heavier than 2.4 TeV in certain scenarios.
• The Tau Sleptons: If these exist, they are heavier than 340 GeV in specific conditions.
• The Higgsinos: If these exist, they are heavier than 800 GeV to 1 TeV.

Why This Matters

This is a huge victory for physics, even without a discovery.

1. Closing the Loopholes: By checking the "long-lived" middle ground, they closed a massive gap in our knowledge. We now know the shadow twins aren't hiding in the "slow decay" zone.
2. Better Tools: They proved that old search methods can be adapted to find new types of signals. It's like realizing your metal detector can also find gold if you just change the frequency.
3. Guiding the Future: By telling us exactly where the shadow twins aren't, they are telling the next generation of scientists exactly where to look next. If the shadow twins exist, they must be even heavier or even stranger than we thought.

The Bottom Line

The ATLAS collaboration didn't find the "Holy Grail" of supersymmetry in this paper. Instead, they did something just as valuable: They swept the floor clean. They checked every corner of the room, from the floorboards to the ceiling, using every possible flashlight setting. They found nothing, but now we know with absolute certainty that the "ghosts" of supersymmetry aren't hiding in the places we used to think they might be. The hunt continues, but the map is now much clearer.

Technical Summary

1. Problem Statement

Supersymmetry (SUSY) is a leading extension of the Standard Model (SM), but its discovery has been hindered by the lack of observed signals. Traditional searches often assume R-parity conservation (RPC), where the Lightest Supersymmetric Particle (LSP) is stable and escapes detection, resulting in large missing transverse momentum ($E_T^{miss}$). Alternatively, searches assume R-parity violation (RPV) with large couplings, leading to prompt decays of the LSP into SM particles.

However, a critical "blind spot" exists in the intermediate regime where RPV couplings are small but non-zero. In this regime, the LSP becomes long-lived (LLP), decaying within the detector volume but with a measurable displacement from the primary vertex. Standard prompt-search algorithms often fail to reconstruct these displaced objects efficiently, while dedicated LLP searches may lack sensitivity to specific topologies. Furthermore, previous reinterpretations were limited to Run-1 data or specific coupling scenarios. This paper addresses the need for a comprehensive coverage of the full parameter space of RPV coupling strengths ($\lambda, \lambda', \lambda''$) using the full ATLAS Run 2 dataset (140 fb$^{-1}$ at $\sqrt{s}=13$ TeV).

2. Methodology

Signal Models

The study reinterprets 13 existing ATLAS searches across six simplified SUSY models designed to cover strong and electroweak production modes with varying lifetimes:

1. Gqq+LQD: Gluino pair production decaying via $\lambda'_{111} = \lambda'_{211}$ to light quarks and leptons.
2. Gqq+UDD: Gluino pair production decaying via $\lambda''_{112}$ to light quarks.
3. Gtt: Gluino pair production decaying via $\lambda''_{323}$ to top quarks.
4. Stop: Top-squark pair production (and resonant single production) decaying via $\lambda''_{323}$.
5. Stau: $\tau$-slepton and $\tau$-sneutrino pair production decaying via $\lambda_{i33}$.
6. Higgsino: Nearly mass-degenerate higgsino-like charginos and neutralinos decaying via $\lambda_{i33}$.

In these models, the LSP lifetime ($\tau_{LSP}$) is a function of the RPV coupling strength and the masses of intermediate sfermions. The analysis scans the full range from RPC (stable LSP) to prompt RPV decays, covering the intermediate LLP regime.

Reinterpretation Framework

• Dataset: Full ATLAS Run 2 dataset (140 fb$^{-1}$).
• Simulation: Full ATLAS detector simulation (Geant4) is used for signal samples to accurately model displaced vertices, which cannot be reliably approximated by fast simulation.
• Tools: The RECAST framework is employed for most analyses to preserve the full event selection and background estimation workflows. For some analyses, original code is reused directly.
• Systematic Uncertainties: A major innovation is the development of dedicated systematic uncertainties for non-prompt objects. Standard reconstruction algorithms (optimized for prompt particles) lose efficiency for displaced jets, $b$-jets, and $\tau$-leptons. The authors quantify these losses as a function of displacement ($L_{xy}$) and impact parameters ($d_0$), applying corrections to signal acceptance.
  • Jet Energy Scale (JES): Uncertainties up to 50% for highly displaced jets.
  • $b$-tagging: Efficiency drops for displaced $b$-jets; dedicated uncertainties are derived.
  • $\tau$-leptons: Reconstruction efficiency drops significantly for displaced $\tau$-leptons.

Statistical Analysis

Limits are set at 95% Confidence Level (CL) using the $CL_s$ prescription. Individual limits are reported for each of the 13 analyses (e.g., RPC 0-lepton, RPV DV+jets, RPC multi-$b$, etc.). No combination of analyses is performed due to overlapping signal regions, but the collective sensitivity is presented.

3. Key Contributions

• Unified Coverage: This is the first reinterpretation to cover the entire spectrum of RPV coupling strengths (from $\sim 10^{-6}$ to $\sim 1$) using the full Run 2 dataset, bridging the gap between RPC and prompt RPV searches.
• LLP Sensitivity in Prompt Searches: The study demonstrates that standard prompt searches (optimized for RPC or prompt RPV) retain significant sensitivity to LLP signatures, particularly when the LSP decays within the inner detector but outside the prompt reconstruction window.
• Systematic Uncertainty Framework: The paper establishes a rigorous methodology for evaluating systematic uncertainties associated with displaced object reconstruction (jets, $b$-jets, $\tau$s), a critical step for future LLP searches in general.
• Model Independence: By varying coupling strengths and lifetimes, the results are presented in a way that allows interpretation for a broad class of RPV scenarios, not just specific benchmark points.

4. Key Results

Gluino Models

• Gtt (Top-enriched): Gluino masses are excluded up to 1.8 TeV regardless of the RPV coupling value. This is a robust limit covering the prompt, LLP, and RPC regimes.
• Gqq+LQD ($\lambda'$): Gluino masses are excluded up to 2.2 TeV for high and low coupling values. The RPC 0-lepton search dominates the low-coupling (long-lived) region, while RPV same-sign/3-lepton searches dominate the high-coupling region.
• Gqq+UDD ($\lambda''$): Gluino masses are excluded up to 2.6 TeV for prompt decays ($\tau_{LSP} \approx 0.01$ ns) and 2.3 TeV for longer lifetimes ($\tau_{LSP} \approx 100$ ns). The DV+jets analysis provides the strongest limits in the intermediate LLP regime ($\lambda'' \approx 10^{-3}$).

Stop Models

• Stop (via $\lambda''_{323}$): Exclusion limits vary significantly with coupling.
  • At high $\lambda''$ (prompt decays to $bs$), top-squark masses up to 2.4 TeV are excluded.
  • At low/intermediate $\lambda''$, limits range from 1.0 to 1.7 TeV.
  • The weakest sensitivity occurs around $\lambda'' \approx 0.1$, where the decay topology is challenging for both prompt and dedicated LLP searches.

Slepton and Higgsino Models

• Stau (via $\lambda_{i33}$): $\tau$-slepton masses between 180 GeV and 340 GeV are excluded for couplings $\lambda < 10^{-4}$. Sensitivity drops significantly in the intermediate range ($10^{-3} \le \lambda \le 10^{-1}$) due to the failure of large $E_T^{miss}$ and opposite-sign charge requirements.
• Higgsino (via $\lambda_{i33}$): Higgsino masses up to 800 GeV – 1.0 TeV are excluded for large couplings where decays are prompt. For very small couplings ($\lambda < 3 \times 10^{-6}$), no exclusion limits can be established as the LSP becomes effectively stable (RPC-like) but with low mass splitting, evading soft-lepton and disappearing track searches not included in this reinterpretation.

5. Significance

This work significantly expands the discovery potential of the ATLAS experiment for Supersymmetry. By reinterpreting existing searches with a variable coupling framework, the collaboration has:

1. Closed the "LLP Gap": Provided robust constraints on the previously unconstrained intermediate lifetime regime.
2. Validated Search Strategies: Demonstrated that a diverse set of analyses (from multi-jet to multi-lepton) are complementary and necessary to cover the full RPV parameter space.
3. Set a Precedent: Established a standard for handling systematic uncertainties in displaced object reconstruction, which is essential for future searches for Long-Lived Particles (LLPs) in the High-Luminosity LHC era.

The results indicate that while many regions of the RPV parameter space are now excluded, specific "blind spots" remain (particularly in intermediate coupling regimes for stop and stau models), motivating the development of dedicated LLP triggers and reconstruction algorithms for future data taking.