The Single Photon Signature of a Light Long-lived Neutralino at Remote Detectors at the LHC

This paper investigates the sensitivity of various proposed remote LHC detectors to light long-lived neutralinos decaying into a photon and neutrino in R-parity violating supersymmetric models, finding that the ANUBIS detector offers the best projected sensitivity among the considered experiments.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle smasher. Scientists smash protons together to see what tiny pieces fly out. Usually, they look for heavy, short-lived particles that vanish instantly. But this paper asks a different question: What if a ghostly, invisible particle is created, flies a long way, and then suddenly flashes a single photon (a particle of light) before disappearing?

Here is the story of that search, broken down into simple concepts.

The Invisible Ghost: The Neutralino

In the world of physics, there is a theory called Supersymmetry (SUSY). It suggests that for every known particle, there is a heavier "superpartner." One of these superpartners is called the neutralino.

Usually, scientists think the neutralino is heavy and stable (it never dies). But this paper explores a "light" version. Imagine a ghost that is so light it weighs less than a grain of sand, but it has a special trick: it can live for a surprisingly long time. Because it interacts so weakly with normal matter, it can slip right through the walls of the main detectors at the LHC without anyone noticing.

The Magic Trick: The Single Photon

This ghostly neutralino doesn't just vanish; it eventually decays. In the specific scenarios the authors studied, the neutralino performs a magic trick: it turns into a neutrino (another invisible ghost) and a photon (a single flash of light).

• The Problem: If this happens inside the main detector, the flash of light is lost in the noise of billions of other collisions.
• The Solution: Since the neutralino is "long-lived," it travels far away from the collision point—perhaps hundreds of meters—before it decides to flash its light. This is like a firefly that flies out of a crowded stadium and only lights up in a quiet, empty field far away.

The Remote Detectors: Watching the Field

To catch this specific flash, the paper looks at several proposed "remote detectors" (like ANUBIS, FASER, CODEX-b, MATHUSLA, etc.). Think of these as specialized cameras placed in tunnels or shafts far away from the main collision point. They are designed to ignore the chaos of the stadium and only look for that one lonely flash of light in the dark.

The authors simulated what would happen if these cameras were turned on, testing six different "scenarios" (different rules for how the ghost is made and how it decays).

The New Simulation: The "Long Walk"

A key improvement in this paper is how they calculated the path of the ghost.

• Old Way: Previous studies assumed the ghost was born exactly at the center of the collision point and then walked straight to the detector.
• New Way: The authors realized that the "parent" particles (mesons) that create the ghost are also long-lived. They might take a few steps away from the center before they give birth to the ghost.
• The Analogy: Imagine a parent walking down a hallway before handing a note to a child. If the parent walks 10 meters down the hall before handing the note, the child starts their journey 10 meters closer to the destination. The authors found that accounting for this "parent's walk" changes the results significantly, making some detectors much better at catching the ghost than previously thought.

The Results: Who Wins the Race?

The authors compared the sensitivity of all these remote detectors. They asked: "Which camera can see the faintest flash?"

• The Winner: ANUBIS came out on top. It is like having the most sensitive night-vision goggles placed in the perfect spot. It can detect the ghost even if the "flash" is very rare or the ghost is very hard to catch.
• The Runner-up: MATHUSLA was also very strong.
• The Loser: FASER (which has already taken data) was found to be the least sensitive of the group for these specific scenarios. This doesn't mean FASER is bad; it just means that for this specific type of ghost, the other detectors are better positioned or have better coverage.

The Bottom Line

The paper concludes that there is a whole new window of discovery we haven't fully explored yet. If these light, long-lived neutralinos exist, the remote detectors (especially ANUBIS) have a real chance of seeing them. By improving the simulation to account for the "long walk" of the parent particles, the authors showed that our chances of finding this "single photon signature" are better than we thought.

In short: We are looking for a ghost that flies far away and flashes a light. We built better maps to track its path, and we found that the ANUBIS detector is the best place to catch it.

Technical Summary

Technical Summary: The Single Photon Signature of a Light Long-lived Neutralino at Remote Detectors at the LHC

Problem Statement
This paper investigates the phenomenology of light, long-lived neutralinos ($\tilde{\chi}^0_1$) within R-parity violating (RPV) supersymmetric models. While R-parity conserving scenarios typically predict missing transverse momentum signatures, RPV models allow the Lightest Supersymmetric Particle (LSP) to decay. Specifically, the authors focus on a light neutralino (mass $m_{\tilde{\chi}^0_1} \lesssim 1.5$ GeV) that decays via a radiative, one-loop process into a photon and a neutrino ($\tilde{\chi}^0_1 \to \gamma + \nu$). This signature is motivated by the potential for "displaced vertices" or delayed decays that evade standard collider searches. The study aims to quantify the discovery potential for this specific signature across a broad set of proposed and existing remote detectors at the Large Hadron Collider (LHC), extending beyond previous work that focused primarily on FASER and FASER2.

Methodology
The authors employ a Monte Carlo simulation framework to estimate the number of observable events ($N^{obs}_{\tilde{\chi}^0_1}$) for various detectors. The procedure involves:

1. Theoretical Framework: Utilizing the RPV-Minimal Supersymmetric Standard Model (RPV-MSSM) where the neutralino is produced via rare decays of scalar mesons (pions, kaons, D-mesons, B-mesons) induced by RPV couplings ($\lambda'$ and $\lambda$). The dominant decay mode is assumed to be the radiative channel $\tilde{\chi}^0_1 \to \gamma + \nu$, mediated by loops involving sfermions and fermions.
2. Event Generation: Using Pythia 8.313 to simulate $10^6$ proton-proton collisions at $\sqrt{s} = 14$ TeV. Unlike previous studies (e.g., Ref. [70]) that used the FORESEE tool, this work simulates the actual decay vertices of the parent mesons.
3. Improved Simulation of Flight Paths: A key methodological improvement is the explicit calculation of the parent meson's decay vertex. If a meson is long-lived, it may decay macroscopically away from the interaction point (IP). The authors calculate the neutralino's flight path ($L_{T,i}$) from this displaced vertex to the detector and its path length within the fiducial volume ($L_{I,i}$).
4. Veto Conditions: The simulation incorporates specific veto conditions based on the detector geometries and surrounding infrastructure (e.g., TAN/TAS absorbers, cavern walls) to reject events where the parent meson decays too early or in a region that blocks the neutralino's path.
5. Benchmark Scenarios: Six distinct benchmark scenarios are defined, varying the non-zero RPV couplings and the dominant meson production channel (pions, kaons, charm mesons, or B-mesons).

Key Contributions

• Expanded Detector Scope: The paper extends the sensitivity analysis of the single-photon signature to a comprehensive list of LHC remote detectors: ANUBIS, CODEX-b, FACET, FASER, FASER2, MAPP, MAPP2, and MATHUSLA.
• Refined Simulation Technique: The authors improve upon existing simulation methods (specifically Ref. [70]) by accounting for the extended flight path of the parent meson. They demonstrate that neglecting the displaced decay vertices of long-lived mesons (like charged pions and kaons) can lead to an underestimation of the effective geometric acceptance and the average decay probability of the neutralino.
• Comparative Sensitivity Study: The work provides a direct comparison of the projected search sensitivities across all listed detectors under well-motivated benchmark scenarios, identifying which experimental configurations are most optimal for specific mass and coupling ranges.

Results
The study presents 3-event isocurves (corresponding to a 95% C.L. exclusion limit) in the plane of the RPV coupling strength ($\lambda/m^2_{SUSY}$) versus the neutralino mass ($m_{\tilde{\chi}^0_1}$) for all six benchmarks.

• Detector Performance: Across all benchmarks, ANUBIS consistently demonstrates the highest sensitivity, capable of probing coupling strengths down to $\sim 10^{-9} - 10^{-10} \text{ GeV}^{-2}$ depending on the scenario. MATHUSLA also shows high sensitivity. Conversely, FASER and MAPP exhibit the lowest sensitivity among the considered experiments, often remaining below existing coupling constraints.
• Impact of Displaced Decays: The improved simulation reveals that for scenarios dominated by long-lived parent mesons (e.g., Benchmark 1 with pions and Benchmark 2 with kaons), the sensitivity of detectors in the far-forward region (like FASER2) is enhanced compared to previous estimates. This is because mesons decaying outside the geometric acceptance of the detector can still produce neutralinos that travel into the detector volume.
• Mass Dependence: The sensitivity varies with the neutralino mass. For light neutralinos ($m_{\tilde{\chi}^0_1} < 35$ MeV), charged pion decays dominate, and the ordering of detector sensitivity is influenced by the angular acceptance and the kinematics of displaced decays. For heavier neutralinos produced via charm or bottom mesons (which decay promptly), the sensitivity ordering remains consistent, with ANUBIS leading.
• Excluded Regions: The paper identifies unexplored sensitivity ranges for all detectors, indicating that significant discovery potential remains even after accounting for current low-energy constraints on RPV couplings.

Significance and Claims
The authors claim that their work provides a more robust and comprehensive assessment of the single-photon signature of light long-lived neutralinos than previously available. By incorporating the finite decay length of parent mesons, they argue that previous simulations may have underestimated the sensitivity of certain detectors, particularly in regions dominated by long-lived meson decays. The primary significance of the paper lies in establishing ANUBIS as the most sensitive detector concept for this specific signature among the proposed LHC experiments, while clarifying the relative capabilities of the entire suite of remote detectors. The authors modestly note that while FASER has already taken data, the other experiments represent future opportunities for discovery, and their analysis defines the projected reach for these upcoming facilities. They explicitly restrict their scope to collider-based experiments, leaving fixed-target and neutrino reactor studies for future work.