Forward neutrino production and event rates at the Future Circular Collider for hadron collisions

This study estimates the production yields and event rates of high-energy neutrinos from 100 TeV proton-proton collisions at the Future Circular Collider and evaluates the feasibility of detecting direct $W^{\pm}$ boson production from neutrino-nucleus interactions.

The Gist

The Cosmic Searchlight: Finding Tiny Particles in a Giant Particle Storm

Imagine you are standing in the middle of a massive, high-speed highway during a torrential thunderstorm. Thousands of cars are zooming past you at incredible speeds, creating a chaotic spray of water and wind. Now, imagine that among all those cars, there are a few "ghost cars"—vehicles that are almost invisible, passing through walls and objects without leaving a trace.

This paper is about how we might build a specialized "sensor" to catch those ghost cars at a future, ultra-powerful particle collider called the Future Circular Collider (FCC).

Here is the breakdown of what the scientists are proposing:

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1. The "Super-Highway" (The FCC)

Right now, we have the Large Hadron Collider (LHC), which is like a very fast highway. But scientists want to build something much bigger and faster: the FCC. This machine will smash protons together at energies so high they make our current technology look like a tricycle.

When these protons smash together, they create a massive "storm" of particles. Most of these particles fly off in all directions, but a specific group called neutrinos—the "ghost particles"—fly straight ahead, right along the path of the beam.

2. The "Ghost Hunters" (Neutrino Detection)

Neutrinos are notoriously difficult to catch. They are like tiny, invisible needles flying through a haystack of lead. Because they rarely interact with anything, you usually need a massive detector to see even one.

The researchers looked at a hypothetical setup where we place a detector (similar to one called FASER) a short distance (0.5 km or 2 km) downstream from the crash site. They used computer simulations to predict how many of these "ghost neutrinos" would fly into our detector.

The finding: Even though they are hard to catch, the FCC will be so powerful that it will produce a "flood" of these neutrinos, giving us enough data to actually study them.

3. The "Golden Ticket": Catching a W Boson

This is the most exciting part of the paper. Usually, when a neutrino hits something, it just leaves a tiny, faint mark. But the researchers discovered that at these extreme energies, something special might happen: Direct W Boson production.

Think of it like this:

• Normal Neutrino Interaction: A ghost car bumps into a wall, leaving a tiny scratch.
• W Boson Production: The ghost car hits the wall so hard that it suddenly transforms into a bright, exploding firework.

The W boson is a heavy, powerful particle. Seeing it created directly from a neutrino hitting an atom would be like seeing a ghost suddenly turn into a solid, glowing object. This has never been proven in an experiment before, and the FCC could be the place where we finally see it happen.

4. Why does this matter? (The "New Physics" Map)

Why go through all this trouble? Because the "Standard Model" (our current rulebook for how the universe works) is incomplete. It’s like having a map of the world that shows all the continents but leaves out the oceans.

By catching these neutrinos and watching for these "firework" W bosons, scientists can look for "glitches" in the rules. These glitches might reveal:

• Dark Matter: The invisible stuff that makes up most of the universe.
• Sterile Neutrinos: Even more mysterious "ghosts" that we haven't found yet.

Summary in a Nutshell

The scientists are saying: "If we build this massive new particle collider, it won't just be a tool for smashing things; it will act like a giant, high-energy searchlight. This light will allow us to catch 'ghost particles' and watch them transform into heavy particles, helping us rewrite the rulebook of the universe."

Technical Summary

Technical Summary: Forward Neutrino Production and Event Rates at the Future Circular Collider

1. Problem Statement

As the particle physics community looks beyond the Large Hadron Collider (LHC), the proposed Future Circular Collider (FCC-hh) aims to operate at center-of-mass energies of $\mathcal{O}(100)$ TeV. While primarily designed for hadron-hadron collisions, such high-energy, high-luminosity environments are expected to produce intense, collimated neutrino beams via the weak decay of hadrons.

There is currently a lack of theoretical and simulated data regarding neutrino-nucleus interaction cross-sections at these extreme energy scales (up to 50 TeV). Furthermore, while the Glashow resonance ($\bar{\nu}_e + e^- \to W^-$) is a known mechanism for $W$ boson production, it occurs at 6.3 PeV—well beyond the FCC-hh energy range. There is a need to investigate alternative mechanisms, specifically the direct production of $W^\pm$ bosons via neutrino-nucleus interactions mediated by virtual photons, which could serve as a new probe for the Standard Model (SM) and Beyond the Standard Model (BSM) physics.

2. Methodology

The authors employed a simulation-based approach to estimate neutrino fluxes and interaction rates:

• Simulation Framework: Used Pythia8 to simulate $10^8$ inelastic proton-proton ($pp$) collisions at $\sqrt{s} = 100$ TeV with an integrated luminosity of $1\text{ ab}^{-1}$.
• Tuning and Validation: To ensure accuracy, the authors tuned Pythia8 (specifically using Tune:pp=19) and adjusted the charm quark mass ($m_c = 1.0\text{ GeV}/c^2$) to achieve agreement with measured inelastic and heavy-flavor cross-sections at $\sqrt{s} = 13$ TeV. The results were further validated against the EPOS.LHC-R generator and existing experimental data from the FASER collaboration.
• Detector Configuration: They modeled a hypothetical detector similar to FASER$\nu$ (target mass $\approx 128$ kg or the full $1100$ kg) positioned either 0.5 km or 2 km downstream from the interaction point.
• Calculations:
  • Neutrino Flux: Estimated by tracking the decay of unstable particles (muons, pions, kaons, etc.) produced within the ATLAS beam pipe geometry.
  • Event Rates: Calculated using the charged current (CC) neutrino scattering cross-section per nucleon ($\sigma \approx 1.0 \times 10^{-38} \text{ cm}^2\text{GeV}^{-1}$) and accounting for detection efficiencies ($\epsilon$).
  • $W^\pm$ Production: Evaluated the direct production of $W$ bosons from neutrino-nucleus interactions.

3. Key Contributions

• First-time Evaluation of Direct $W^\pm$ Production: The study provides the first feasibility assessment of observing on-shell $W^\pm$ boson production in a collider environment via neutrino-nucleus scattering.
• Energy Scale Extension: It extends neutrino physics predictions from the TeV scale (LHC) to the much higher energy regimes of the FCC-hh.
• Comparative Analysis: The paper compares different detector placements (0.5 km vs. 2 km) and different detector scales (subset vs. full FASER$\nu$ target), providing a roadmap for future experimental design.

4. Results

• Neutrino Flux and Energy: The simulation predicts neutrino energies reaching up to approximately 50 TeV. While higher energy neutrinos are produced further from the interaction point, the flux at the detector decreases with distance due to the requirement for higher collimation to hit the fixed target area.
• Event Rates:
  • Charged current $\nu_\mu$ interactions are expected to be the most abundant and can be observed at energies approaching 50 TeV.
  • For a 128 kg target, significant event rates are predicted for $\nu_e$, $\nu_\mu$, and $\nu_\tau$.
• $W^\pm$ Production:
  • The study finds that direct $W^\pm$ production is feasible.
  • Unlike the Glashow resonance (which only produces $W^-$), this mechanism allows for the production of both $W^-$ and $W^+$.
  • Using the full FASER$\nu$ target ($1100$ kg), the production of $W$ bosons becomes substantial, providing a clear signal that could be separated from standard CC interactions by identifying an additional high-energy charged lepton.

5. Significance

This research demonstrates that the FCC-hh will not only be a "discovery machine" for high-energy hadrons but also a powerful neutrino factory. The ability to observe direct $W^\pm$ production and high-energy neutrino-nucleus scattering provides a unique laboratory to:

1. Test the Standard Model at unprecedented energy scales.
2. Probe BSM physics, including dark matter candidates, sterile neutrinos, and non-standard neutrino interactions (NSI).
3. Complement existing observations (like those from IceCube) by providing a controlled collider environment to study neutrino physics from a different angle.