Lake- and Surface-Based Detectors for Forward Neutrino Physics

The paper proposes two cost-effective, medium-baseline neutrino experiments—SINE (a surface scintillator detector) and UNDINE (a submerged water Cherenkov detector)—to study LHC-produced neutrinos and constrain neutrino cross sections and cosmic-ray physics.

The Gist

The Cosmic Spotlight: Catching Ghost Particles with Lakes and Land

Imagine you are standing in a dark forest at night. Suddenly, a massive, high-powered searchlight sweeps past you from a distance. You can’t see the light beam itself because it’s invisible in the clear air, but you can see the tiny dust motes dancing in the beam, and you can see the way the light hits the trees.

In this paper, scientists are proposing a way to build "dust mote detectors" for the most elusive particles in the universe: neutrinos.

The Setup: The LHC as a Giant Flashlight

At the Large Hadron Collider (LHC) in Switzerland, scientists smash protons together at nearly the speed of light. These collisions act like a massive, invisible flashlight. They don't shoot out visible light, but they do shoot out a concentrated "beam" of neutrinos.

Neutrinos are often called "ghost particles." They are so tiny and antisocial that they can fly through a lead wall a light-year thick without hitting a single atom. Because they are so hard to catch, scientists usually have to build massive, expensive underground caverns to try and stop them.

The Big Idea: Using Nature as a Shield

The authors of this paper say, "Why dig expensive holes in the ground when we can use the geography we already have?" They propose two clever, cost-effective "traps" to catch these ghosts:

1. SINE: The "Upward-Looking" Surface Trap
Imagine a beam of light shining through a mountain. Even if you can't see the beam, if a particle in that beam hits a rock inside the mountain, it might kick out a "muon" (a heavier cousin of the neutrino) that flies out of the mountain and up toward the surface.

• The Plan: Place large, modular "scintillator panels" (think of them as high-tech, glowing solar panels) on the ground.
• The Trick: Instead of looking for particles coming down from space (like cosmic rays), SINE looks for particles coming up out of the Earth. It’s like looking for a flashlight beam by watching the shadows move upward from the grass.

2. UNDINE: The "Underwater" Net
The LHC is located near Lake Geneva. The scientists realized that the neutrino beam from one part of the LHC actually passes right through the lake.

• The Plan: Instead of building a building, why not just sink a detector into the lake? They propose using "water Cherenkov detectors"—essentially giant, glowing underwater eyes—submerged about 50 meters deep.
• The Trick: The lake acts as a natural shield, protecting the detector from the "noise" of cosmic rays from space, much like how a thick blanket muffles the sound of a noisy street.

Why Does This Matter? (The "So What?")

If we can catch millions of these "ghosts," we can answer some of the biggest mysteries in science:

• The Recipe of the Universe: We can learn exactly how many "flavors" of neutrinos exist and how they interact with matter.
• The Cosmic Mystery: There is a "muon puzzle" in space—we see more muons in cosmic rays than our math says we should. These detectors could tell us if the "recipe" for making particles in collisions needs to be adjusted.
• Hunting for "Heavy Ghosts": They are looking for Heavy Neutral Leptons—hypothetical particles that could explain why the universe exists at all and what "Dark Matter" might be.

Summary

Instead of building a multi-billion dollar underground fortress, these scientists want to use shipping containers on the grass and sensors in a lake to catch the invisible leftovers of the world's most powerful machine. It’s a clever, "low-cost, high-reward" way to study the fundamental building blocks of reality.

Technical Summary

Technical Summary: Lake- and Surface-Based Detectors for Forward Neutrino Physics

1. Problem Statement

The Large Hadron Collider (LHC) produces a highly collimated beam of neutrinos in the "forward direction" (along the collision axis) resulting from the decay of hadrons (pions, kaons, charm, and B mesons). While the proposed Forward Physics Facility (FPF) aims to study these neutrinos using underground detectors, there are logistical and financial constraints associated with excavating large caverns. Furthermore, existing short-baseline experiments (like FASER) have limited detector volumes. There is a need for medium-baseline (MBF$\nu$) experiments that can leverage the high intensity of the High-Luminosity LHC (HL-LHC) era to achieve kiloton-scale volumes without the prohibitive costs of deep underground excavation.

2. Methodology

The authors propose two novel, cost-effective detector concepts that utilize the natural geography surrounding the LHC to achieve kiloton-scale masses:

• SINE (Surface-based Integrated Neutrino Experiment):

  • Concept: A surface-based detector designed to observe upward-going muons.
  • Mechanism: Neutrinos from the CMS interaction point travel through ~18 km of bedrock. When they undergo charged-current (CC) interactions in the rock, they produce high-energy muons that travel upward and exit the Earth's surface.
  • Design: A modular array of shipping containers instrumented with scintillator panels.
  • Background Mitigation: To separate signal muons from cosmic-ray muons, SINE employs a multi-layered strategy: beam timing (leveraging the 25 ns LHC bunch spacing), panel timing (using the $\sim$ns resolution of scintillators to ensure perpendicular travel), and spatial segmentation (using 1D/2D cuts on transverse displacement).

• UNDINE (UNDerwater Integrated Neutrino Experiment):

  • Concept: A water Cherenkov detector submerged in Lake Geneva.
  • Mechanism: Neutrinos from the LHCb interaction point pass through the lake. The detector observes all-flavor neutrino interactions within its volume.
  • Design: A 30 kt detector consisting of five cylindrical modules (12.5 m radius/height) instrumented with photomultiplier tubes (PMTs).
  • Background Mitigation: Uses the natural water overburden of the lake ($\sim$50 m) to shield against cosmic-ray muons.

The researchers used the SIREN simulation package and various hadron-production models (e.g., EPOS-LHC, Pythia 8.2) to estimate interaction rates and physics sensitivities.

3. Key Contributions

• New Experimental Paradigms: Introduced the concept of using "upstream bedrock" as a target for surface detectors (SINE) and "natural water bodies" as overburden for submerged detectors (UNDINE).
• Cost-Efficiency Analysis: Demonstrated that these detectors could be deployed for significantly less than the estimated $100M cost of the FPF by avoiding massive excavation.
• Comprehensive Physics Reach: Provided a roadmap for using these detectors to solve long-standing problems in particle and cosmic-ray physics.

4. Results

• Event Rates: During the HL-LHC era, SINE is expected to record $\sim 10^6$ neutrino interactions, while UNDINE is expected to record $\sim 10^4$ to $10^5$ interactions.
• Cross-Section Measurements: Both detectors can perform competitive measurements of TeV-scale neutrino DIS cross-sections. UNDINE specifically offers the ability to distinguish between $\nu_e$ and $\nu_\mu$ flavors.
• Hadron Production Models: By measuring ratios (e.g., SINE total/UNDINE total), the experiments can distinguish between different charm-production models at a $\gtrsim 3\sigma$ confidence level, potentially resolving current tensions in FASER data.
• Cosmic-Ray Muon Puzzle: The detectors can constrain "strangeness enhancement" ($f_s$), a proposed solution to the excess of muons observed in cosmic-ray air showers.
• Heavy Neutral Leptons (HNLs): SINE provides a unique "broadband" sensitivity to HNLs across nearly three orders of magnitude in mass ($m_N \lesssim 2$ GeV via meson decay and $m_N \approx 20$ GeV via neutrino upscattering).

5. Significance

This paper presents a highly viable, low-cost alternative/complement to the FPF. By moving the detectors to the surface or into a lake, the proposal bypasses the most expensive part of forward physics infrastructure (cavern excavation) while actually increasing the effective target mass through the use of bedrock and lake water. This enables a high-statistics program for studying TeV neutrino interactions, heavy neutral leptons, and hadronic production, which are critical for understanding both the Standard Model and potential New Physics.