⚛️ phenomenology

Probing the existence of a new charged vector boson decaying into heavy neutral leptons using ultra-peripheral heavy ion collisions at ATLAS

This paper demonstrates that Ultra-peripheral Collisions at the LHC, specifically using lead ions at ATLAS and proton ions at the HL-LHC, offer a powerful avenue to discover or exclude new charged vector bosons decaying into heavy neutral leptons across a broad mass range (5–350 GeV) within the Vector Scotogenic Model, achieving high statistical significance for various mass scenarios.

Original authors: Y. M. Oviedo-Torres, Sebastian Tapia, J. Zamora-Saa

Published 2026-07-14

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CC BY 4.0

Original authors: Y. M. Oviedo-Torres, Sebastian Tapia, J. Zamora-Saa

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

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle smash-up machine. Usually, it fires two beams of protons (tiny, positively charged particles) directly at each other. It's like smashing two bags of marbles together; you get a massive explosion of debris, making it incredibly hard to spot a single, shiny new marble hiding in the pile. This "debris" is called background noise, and it often hides the rare, exciting signals of new physics.

But what if, instead of smashing the bags, you let them fly past each other so closely that their invisible electric fields brush against one another? This is called an Ultra-Peripheral Collision (UPC). In this paper, the authors suggest using this "brush-by" technique to hunt for a ghostly new particle: a charged vector boson that decays into a heavy neutral lepton (a heavy, invisible cousin of the neutrino).

The "Clean Room" Strategy

The authors propose a clever trick to avoid the messy debris. Instead of using protons, they suggest using lead ions (heavy atoms with a massive electric charge) for the current LHC runs. Because lead ions are so heavy and charged, their electric fields are like super-strong magnets. When two lead ions fly past each other without hitting, these fields can collide to create new particles, but without the messy "marble bag" explosion. It's like creating a new particle in a pristine, quiet room instead of a chaotic construction site.

The paper focuses on a specific theoretical model called the Vector Scotogenic Model. This model predicts a new, heavy, charged particle (let's call it $V^\pm$ ) that is unstable. It doesn't last long; it immediately splits apart into two muons (heavy electrons) and a heavy neutral lepton ($NL$), which is invisible to detectors. The detector would see two muons and a "missing" amount of energy (called MET or Missing Transverse Energy) because the invisible lepton ran away.

The Current Hunt: Lead Ions at ATLAS

The authors ran simulations to see if the ATLAS experiment (one of the giant detectors at the LHC) could spot this using lead-ion UPCs. They used data equivalent to 3.48 nb $^{-1}$ of integrated luminosity (a measure of how many collisions were observed).

Here is what their simulations suggest:

The Sweet Spot: They found that for very light versions of this new particle, specifically where the charged boson ( $V^\pm$ ) weighs between 5 GeV and 50 GeV, and the heavy neutral lepton ($NL$) is also light, the signal stands out clearly against the background.
The Exclusions: In these simulations, the ATLAS experiment could confidently say "No, this particle doesn't exist" (at a 95% confidence level) for specific mass combinations, such as a 30 GeV boson with a 20 GeV lepton, or a 30 GeV boson with a 10 GeV lepton.
The Discovery: If the particle does exist with a mass of 20 GeV for the boson and 10 GeV for the lepton, the simulations suggest ATLAS could find it with a statistical significance of 5 $\sigma$ (the gold standard for a discovery in physics).

The authors note that previous experiments like LEP-II (a collider that ran before the LHC) only looked for similar particles indirectly by searching for different types of particles (scalars). They didn't look directly for these specific spin-1 charged bosons in this low-mass range. So, this study suggests that ATLAS is now the first to directly probe this "blind spot" using lead ions.

The Future Hunt: Protons at the High-Luminosity LHC

The paper then looks ahead to the High-Luminosity LHC (HL-LHC), a future upgrade that will smash protons together with much higher energy (14 TeV) and much more data (150 fb $^{-1}$ ).

Since protons are lighter than lead ions, they don't have as strong an electric field, so the "brush-by" collisions are rarer. However, the sheer amount of data and the higher energy make up for it.

Higher Masses: The simulations suggest that with proton UPCs, the HL-LHC could explore much heavier particles, specifically in the range of 100 GeV to 350 GeV.
Beating the Competition: The authors compared their results to searches for supersymmetry (SUSY), a popular theory that predicts similar-looking particles (like sleptons). They found that their proposed method could cover a larger area of the "mass map" than previous SUSY searches, especially in "compressed" regions where the new particles are very close in mass to each other (making them hard to spot in normal collisions).
Confidence: In these simulations, the HL-LHC could exclude these heavier mass scenarios with 95% confidence, and many of them could reach the 5 $\sigma$ discovery threshold.

The Verdict

This paper doesn't claim to have found the particle yet. Instead, it acts as a detailed treasure map. It suggests that by using the "clean room" technique of ultra-peripheral collisions, the ATLAS experiment has the tools to find a new charged vector boson in mass ranges that were previously invisible or too messy to study.

For the current LHC (Lead ions): It suggests we can find or rule out these particles if they are light (between 5 GeV and 50 GeV).
For the future HL-LHC (Protons): It suggests we can hunt for heavier versions (between 100 GeV and 350 GeV) with high confidence.

The authors emphasize that while the background in these collisions is much cleaner than in normal smash-ups, the challenge is that the signals are subtle. But their simulations show that with the right "filters" (cuts on the angle and speed of the muons), the signal can be separated from the noise, offering a promising new window into the secrets of the universe.

Technical Summary: Probing the Existence of a New Charged Vector Boson Decaying into Heavy Neutral Leptons using Ultra-Peripheral Heavy Ion Collisions at ATLAS

Problem and Motivation
The Standard Model (SM) fails to address fundamental issues such as the origin of neutrino masses, the nature of dark matter, and matter-antimatter asymmetry. While hadron colliders like the Large Hadron Collider (LHC) are primary tools for exploring Beyond the Standard Model (BSM) physics, conventional proton-proton ($pp$) collisions suffer from significant irreducible hadronic (QCD) backgrounds. This background obscures signals of new physics, particularly in low-mass regions or compressed mass scenarios where soft leptons and low missing transverse energy (MET) are produced.

This paper investigates the potential of Ultra-Peripheral Collisions (UPCs) to mitigate these challenges. In UPCs, beam components (such as lead ions or protons) pass close enough to interact via intense electromagnetic fields without fragmenting. This mechanism generates a "clean" environment dominated by photon-photon ( $\gamma\gamma$ ) interactions, significantly reducing QCD backgrounds. The study focuses on the Vector Scotogenic Model (VSM), which predicts the existence of a new charged vector boson doublet ( $V^\pm$ ) and Heavy Neutral Leptons (HNLs, $N_L$ ). The specific signal channel analyzed is the production of $V^\pm$ pairs decaying into dimuons and missing transverse energy ( $\mu^+\mu^- + \text{MET}$ ).

Methodology
The authors employ a two-pronged analysis strategy targeting different mass ranges and collider configurations:

Current LHC (Pb-Pb UPCs):
- Setup: Analysis of Lead-Lead collisions at $\sqrt{s_{NN}} = 5.02$ TeV with an integrated luminosity of $L_{int} = 3.48 \text{ nb}^{-1}$ .
- Model: The VSM Lagrangian includes a vector doublet $V_\mu$ and HNLs. Coupling constants are set to $\beta_1=0$ and $\beta_2=\beta_3=0.5$ to ensure consistency with perturbativity and dark matter constraints.
- Simulation: Signal and background events are generated using MadGraph5_aMC@NLO with FeynRules UFO files. The process $^{208}_{82}\text{Pb} + ^{208}_{82}\text{Pb} \to \mu^+\mu^- + \text{MET} + ^{208}_{82}\text{Pb}^* + ^{208}_{82}\text{Pb}^*$ is simulated.
- Analysis: A random search algorithm optimizes kinematic and angular cuts (transverse momentum $p_T$ , pseudorapidity $\eta$ , acoplanarity $\alpha$ , angular separation $\Delta R$ , invariant mass $M$ , and $\cos\theta$ ) to maximize statistical significance ( $S$ ) against the background.
High-Luminosity LHC (pp-UPCs):
- Setup: Analysis of proton-proton collisions at $\sqrt{s} = 14$ TeV with $L_{int} = 150 \text{ fb}^{-1}$ .
- Strategy: Utilizes tagging techniques (Forward Proton Detectors or Large Rapidity Gaps) to identify exclusive $\gamma\gamma$ events.
- Comparison: Results are compared against existing exclusion limits from Supersymmetry (SUSY) searches for sleptons and neutralinos, noting the topological similarity but distinct spin-1 nature of the $V^\pm$ boson.

Key Contributions and Results

Low-Mass Sensitivity (Pb-Pb UPCs):
The study demonstrates that the ATLAS experiment can directly probe the mass range $5 \text{ GeV} < M_{V^\pm}, M_{N_L} < 50 \text{ GeV}$ , a region where LEP-II only provided indirect constraints via slepton searches and where conventional $pp$ searches are hindered by high QCD backgrounds.
- Exclusion: With $3.48 \text{ nb}^{-1}$ , ATLAS can exclude at 95% Confidence Level (C.L.) specific scenarios: $(M_{V^\pm}, M_{N_L}) = (30, 20)$ GeV, $(30, 10)$ GeV, and $(20, 10)$ GeV.
- Discovery: A discovery significance of $5\sigma$ is achievable for the $(20, 10)$ GeV scenario.
- Kinematics: The signal is distinguished by softer muon $p_T$ (peaking below 20 GeV), lower invariant mass, and softer MET compared to the background, which is dominated by $W$ boson production.
High-Mass Sensitivity (HL-LHC pp-UPCs):
The analysis extends the reach to the mass range $100 \text{ GeV} < M_{V^\pm}, M_{N_L} < 350$ GeV.
- Exclusion and Discovery: The HL-LHC can exclude scenarios in the $100-350$ GeV range at 95% C.L. Most scenarios in the $5-250$ GeV range can reach $5\sigma$ significance.
- Compressed Mass Regions: The clean environment of pp-UPCs allows for sensitivity to compressed mass scenarios (where $\Delta m = M_{V^\pm} - M_{N_L}$ is small) that are typically challenging for conventional $pp$ analyses due to soft lepton triggers.
- Comparison with SUSY: The projected exclusion limits cover parameter space beyond current direct slepton limits from ATLAS, CMS, and LEP. The authors emphasize that while the final state topology mimics slepton-neutralino production, the spin-1 nature of $V^\pm$ results in distinct angular distributions (acoplanarity, $\Delta R$ , $\cos\theta$ ) that offer additional discrimination power.

Significance
The paper claims that Ultra-Peripheral Collisions offer a powerful, complementary avenue for BSM physics searches. By leveraging the electromagnetic dominance of UPCs, the study provides a direct experimental test for new charged vector bosons in mass regions previously inaccessible to direct searches (below the LEP-II indirect limit) and in high-mass compressed regions where standard hadronic backgrounds are prohibitive. The results suggest that existing Pb-Pb data at ATLAS is sufficient to test light vector boson models, while future HL-LHC pp-UPC data can significantly extend the discovery reach for the Vector Scotogenic Model, covering parameter space not accessible to current SUSY searches.

The "Clean Room" Strategy

The Current Hunt: Lead Ions at ATLAS

The Future Hunt: Protons at the High-Luminosity LHC

The Verdict

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