Search for signatures of electroweakinos with photons, jets, and large missing transverse momentum in $\sqrt{s}=13$ TeV pp collisions with the ATLAS detector

Using 140 fb$^{-1}$ of 13 TeV proton-proton collision data, the ATLAS collaboration searched for electroweakino signatures in final states containing photons, jets, and missing transverse momentum, finding no significant excess over Standard Model predictions and setting 95% confidence level exclusion limits on gaugino masses up to 1.2 TeV.

The Gist

The Big Picture: A Cosmic Treasure Hunt

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle accelerator, essentially a giant, high-speed racetrack where protons (tiny subatomic particles) are smashed together at nearly the speed of light. When they crash, they create a shower of debris, much like smashing two complex watches together and seeing what gears, springs, and screws fly out.

The ATLAS experiment is one of the giant "cameras" (detectors) watching these crashes. This paper describes a specific search the ATLAS team conducted using data from 2015 to 2018. They were looking for a very specific, rare type of debris that shouldn't exist according to our current understanding of physics (the Standard Model).

The Theory: The "Invisible Ghost" and the "Shiny Flash"

The scientists were hunting for evidence of Supersymmetry (SUSY). Think of the Standard Model as a completed puzzle of the universe. SUSY suggests there is a hidden, larger puzzle where every piece we know has a "shadow twin" that is heavier and harder to find.

In this specific search, they were looking for a scenario involving:

1. Neutralinos: These are the "shadow twins" of particles like the photon and the Z boson. Imagine them as heavy, invisible ghosts that are created in pairs during the crash.
2. Gravitinos: These are the lightest, most elusive particles in the theory. They are like "ghosts of ghosts"—so light and weak that they pass right through the detector without leaving a trace. In this theory, they are the ultimate "missing" piece.
3. The Decay: When a heavy Neutralino ghost decays, it might turn into a Gravitino (which vanishes) and a Photon (a particle of light) or a Z boson (which quickly breaks into other particles).

The Signature: What They Were Looking For

If this theory is true, a collision should produce a very specific "fingerprint" in the detector:

• A Bright Flash: At least one high-energy photon (a particle of light).
• A Jet Stream: A spray of particles (jets) created by the debris.
• The Great Disappearance: A huge amount of "missing" energy. Since the Gravitino ghosts escape the detector unseen, the math of the collision won't add up. The energy going in won't equal the energy coming out. This "missing momentum" is the smoking gun.

The Investigation: Sifting Through the Noise

The team analyzed a massive amount of data (140 "inverse femtobarns," which is a fancy way of saying they looked at trillions of collisions).

To find their signal, they had to filter out the "noise." Imagine trying to hear a specific whisper in a crowded stadium. Most of the time, the "missing energy" is just a measurement error or a particle that got lost in the detector walls. The team built three different "search zones" (Signal Regions) based on how much energy was missing:

• Low Mass Zone: Looking for lighter ghosts.
• Medium Mass Zone: Looking for medium-weight ghosts.
• High Mass Zone: Looking for very heavy ghosts.

They also had to be careful not to confuse real signals with "fake" ones, like a jet of particles that accidentally looked like a photon, or a measurement glitch that made it look like energy disappeared. They used advanced statistical tricks and "control rooms" (where they knew the physics was standard) to calibrate their expectations.

The Results: The Silence of the Ghosts

After crunching the numbers, the result was clear: They found nothing.

• No Excess: The number of events they saw with a photon, jets, and missing energy matched exactly what the Standard Model predicted. There was no "extra" whisper in the stadium.
• No New Physics: They did not find evidence of these specific Supersymmetric particles.

What This Means (According to the Paper)

Since they didn't find the ghosts, they had to set boundaries on where they could be hiding.

• The Exclusion Limit: They can now say with 95% confidence that if these specific "bino-higgsino" ghosts exist, they must be heavier than 1.2 TeV (a unit of mass).
• The Map: They created a map showing that for certain combinations of how these particles decay, masses up to 1.2 TeV are ruled out. If they exist, they are heavier than the heaviest particles we have found so far.

In Summary

The ATLAS collaboration looked for a specific type of "invisible ghost" particle that would leave a bright flash and a trail of missing energy. They looked through 140 trillion collisions and found no evidence of it. While they didn't find the new physics they were hoping for, they successfully narrowed the search, telling future physicists: "If these particles exist, they are heavier than 1.2 TeV, so look harder in that direction."

Technical Summary

1. Problem and Physics Motivation

The paper addresses the search for physics beyond the Standard Model (SM), specifically focusing on Supersymmetry (SUSY) within the framework of General Gauge Mediation (GGM).

• Theoretical Context: In GGM models, the gravitino ($\tilde{G}$) is the Lightest Supersymmetric Particle (LSP) and is stable due to R-parity conservation, making it a dark matter candidate. The Next-to-Lightest Supersymmetric Particle (NLSP) is assumed to be the lightest neutralino ($\tilde{\chi}^0_1$), which is a mixture of bino and higgsino states.
• Decay Signature: The $\tilde{\chi}^0_1$ decays into a gravitino and a Standard Model boson: a photon ($\gamma$), a $Z$ boson, or a Higgs boson ($h$). The branching ratios depend on the bino/higgsino composition.
• Target Process: The analysis targets the pair production of electroweakinos (specifically $\tilde{\chi}^0_1\tilde{\chi}^0_2$, $\tilde{\chi}^0_1\tilde{\chi}^\pm_1$, etc.) where the heavier states decay to the $\tilde{\chi}^0_1$, which subsequently decays.
• Final State: The search looks for events containing at least one isolated high-$p_T$ photon, jets, and large missing transverse momentum ($E^{\text{miss}}_T$) arising from the two escaping gravitinos. This signature is distinct from previous searches that focused exclusively on diphoton final states or strong production channels.

2. Methodology

Data and Simulation

• Dataset: The analysis uses the full Run-2 dataset collected by the ATLAS detector at the LHC, corresponding to an integrated luminosity of 140 fb$^{-1}$ at a center-of-mass energy of $\sqrt{s}=13$ TeV (2015–2018).
• Signal Simulation: Signal samples were generated using MadGraph 5 interfaced with Pythia 8. The simulation covered $\tilde{\chi}^0_1$ masses from 150 to 1450 GeV. A simplified model was used where the $\tilde{\chi}^0_1$ branching ratios to $\gamma\tilde{G}$, $Z\tilde{G}$, and $h\tilde{G}$ were set equal initially, allowing for re-weighting to test various mixing scenarios. Cross-sections were calculated at NLO+NLL precision using Resummino.
• Background Simulation: SM backgrounds ($Z(\to\nu\nu)\gamma$, $W\gamma$, $t\bar{t}\gamma$, $\gamma$+jets, etc.) were simulated using Sherpa, MadGraph5_aMC@NLO, and Powheg.

Event Reconstruction and Selection

• Object Definition:
  • Photons: Required to have $p_T > 50$ GeV (offline) and pass "tight" identification and isolation criteria.
  • Jets: Reconstructed with the anti-$k_t$ algorithm ($R=0.4$).
  • $E^{\text{miss}}_T$: Calculated using an object-based algorithm including a soft term.
• Trigger: Events were selected using a single-photon trigger with an offline requirement of $p_T > 145$ GeV.
• Signal Regions (SRs): Three discovery regions were defined based on the $\tilde{\chi}^0_1$ mass hypothesis (Low, Medium, High), distinguished primarily by $E^{\text{miss}}_T$ thresholds:
  • SRL: $E^{\text{miss}}_T > 200$ GeV.
  • SRM: $E^{\text{miss}}_T > 300$ GeV.
  • SRH: $E^{\text{miss}}_T > 400$ GeV.
  • Additional orthogonal regions (SRLe, SRMe) were defined with tighter $E^{\text{miss}}_T$ significance requirements to optimize exclusion limits.
• Discriminants: To suppress backgrounds, the analysis utilized the ratio $E^{\text{miss}}_T / m_{\text{eff}}$ (where $m_{\text{eff}}$ is the scalar sum of $E^{\text{miss}}_T$, leading photon $p_T$, and jet $p_T$) and angular separations $\Delta\phi(\gamma, E^{\text{miss}}_T)$ and $\Delta\phi(\text{jet}, E^{\text{miss}}_T)$.

Background Estimation

• Dominant Backgrounds: $Z(\to\nu\nu)\gamma$, $W\gamma$, and $t\bar{t}\gamma$.
• Control Regions (CRs): Dedicated CRs were used to normalize the dominant backgrounds using data:
  • CRZ: Leptonic $Z$ decays ($Z \to ee/\mu\mu$) with a photon, used to normalize $Z(\to\nu\nu)\gamma$.
  • CRW: Single lepton + photon + $b$-jet veto for $W\gamma$.
  • CRT: Lepton + photon + $\ge 2$ $b$-jets for $t\bar{t}\gamma$.
• Fake Photons: Backgrounds from jets or electrons misidentified as photons were estimated using data-driven techniques (ABCD method in isolation/identification planes) and fake factors derived from $Z \to ee$ samples.
• Statistical Analysis: A simultaneous profile likelihood fit was performed across CRs, Validation Regions (VRs), and SRs to constrain background normalizations and set limits.

3. Key Contributions

1. Inclusive Search Strategy: Unlike previous ATLAS searches that focused on exclusive diphoton signatures or specific Higgs decays, this analysis employs a more inclusive selection targeting at least one photon plus jets and $E^{\text{miss}}_T$. This significantly broadens the sensitivity to various GGM scenarios.
2. Branching Ratio Scanning: A major innovation is the interpretation of results as a function of the $\tilde{\chi}^0_1$ branching ratios. Instead of assuming fixed ratios, the analysis scans the parameter space of $B(\tilde{\chi}^0_1 \to \gamma\tilde{G})$, $B(\tilde{\chi}^0_1 \to Z\tilde{G})$, and $B(\tilde{\chi}^0_1 \to h\tilde{G})$.
3. Extended Mass Reach: The analysis extends the exclusion reach for electroweakino masses up to 1.2 TeV, surpassing previous ATLAS electroweakino searches.
4. Benchmark Models: The results are interpreted in two specific benchmark scenarios:
   • $\gamma + Z$: 50% $\gamma\tilde{G}$ and 50% $Z\tilde{G}$.
   • $\gamma + h$: 50% $\gamma\tilde{G}$ and 50% $h\tilde{G}$.

4. Results

• Observation: No significant excess of events was observed in any signal region. The observed event yields were consistent with Standard Model predictions (e.g., in SRH, 8 observed events vs. 14.6 expected).
• Model-Independent Limits: Upper limits at 95% Confidence Level (CL) were set on the visible cross-section ($\langle A\epsilon\sigma \rangle_{95}^{\text{obs}}$) for new physics processes.
• Model-Dependent Exclusions:
  • In the $\gamma + Z$ benchmark, neutralino masses up to ~1.2 TeV are excluded for pure photon branching ratios.
  • In the $\gamma + h$ benchmark, similar exclusion limits are reached.
  • Branching Ratio Dependence: The exclusion limit on the $\tilde{\chi}^0_1$ mass decreases as the branching ratio to photons decreases. For $B(\tilde{\chi}^0_1 \to \gamma\tilde{G}) = 10\%$, the exclusion limit drops to approximately 450 GeV.
  • 2D Limits: The paper provides exclusion contours in the plane of branching ratios ($B(\gamma\tilde{G})$ vs. $B(Z\tilde{G})$ or $B(h\tilde{G})$) for fixed masses, showing that sensitivity is highest when the photon branching ratio is dominant.

5. Significance

This paper represents a comprehensive update to the search for electroweakinos in the GGM framework using the complete Run-2 dataset.

• Sensitivity: It achieves the most stringent limits to date for electroweakino production in the photon+jet+$E^{\text{miss}}_T$ channel, probing masses up to 1.2 TeV.
• Robustness: By scanning over branching ratios, the analysis provides robust constraints on a wide range of SUSY parameter spaces, rather than being limited to specific theoretical assumptions about the neutralino composition.
• Complementarity: The results complement searches for strong production (gluinos/squarks) and other electroweakino decay modes (e.g., trileptons), providing a complete picture of the SUSY search landscape at the LHC.
• Future Outlook: The absence of a signal places strong constraints on low-energy SUSY breaking scales and the nature of the NLSP, guiding future theoretical models and the design of Run-3 and High-Luminosity LHC searches.