SND@LHC Upgrade for the High-Luminosity LHC: Physics Reach and Installation Scenarios

This paper presents the physics reach and design of the approved SND@LHC upgrade for the High-Luminosity LHC's Run 4, demonstrating that a specific installation scenario involving a 40 cm vertical and 30 cm horizontal shift increases the total neutrino interaction rate by a factor of five compared to alternative configurations.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle smasher. When it smashes protons together, it creates a chaotic explosion of particles shooting out in all directions. Most of our scientific "cameras" (detectors) are huge, round rooms that catch the debris flying sideways. But there's a special, narrow tunnel where particles shoot straight ahead, like a high-speed train leaving a station. This is the "forward region," and it's been a blind spot for scientists until now.

Enter SND@LHC (Scattering and Neutrino Detector at the LHC). Think of it as a specialized "mailroom" installed in that narrow tunnel, designed to catch the ghostly, hard-to-detect particles called neutrinos that zoom past the main detectors.

This paper is about upgrading that mailroom to handle a massive influx of mail coming in the next few years (the High-Luminosity LHC era). Here is the breakdown in simple terms:

1. The Problem: Too Much Mail, Wrong Angle

The LHC is about to get 10 times more powerful, meaning it will produce 10 times more neutrinos. The current detector is doing a great job, but it's like trying to catch rain with a small cup while standing under a firehose. It's also positioned slightly off-center, missing a lot of the "heavy" neutrinos that carry the most interesting information.

2. The Solution: A Bigger, Smarter Net

The scientists are building an upgraded version called SND@HL-LHC.

• The Old Way: The current detector uses photographic film (emulsions) to catch particles. It's great, but slow. If too many particles hit it at once, it gets "clogged."
• The New Way: The upgrade swaps the film for electronic sensors (like a super-fast digital camera). This allows it to handle the "firehose" of particles without getting overwhelmed.
• The New Feature: They are adding a magnet. Imagine the detector as a river. The magnet acts like a bend in the river that forces charged particles (like muons) to curve. By seeing which way they curve, scientists can tell if they are matter or antimatter, which was impossible before.

3. The Big Decision: Two Installation Scenarios

The team has to decide where to put this new detector in the narrow tunnel. They compared two options:

• Option A: The "Easy" Install (Baseline)

  • The Plan: Just slide the detector onto the existing floor of the tunnel. No digging, no breaking concrete.
  • The Catch: It sits a bit high up and off to the side. It's like trying to catch rain with a cup held slightly above your head. You'll catch some, but a lot will splash over the edge.
  • Result: Good data, but not the best possible data.

• Option B: The "Optimized" Install (Extended)

  • The Plan: Dig a hole in the floor, lower the detector by about 40 cm (16 inches), and slide it 30 cm (12 inches) closer to the center of the beam.
  • The Catch: It requires some construction work (removing concrete), which costs more time and effort.
  • The Payoff: This moves the detector right into the "sweet spot" of the particle stream.
  • Result: This is the game-changer. By moving it closer to the center, the detector catches five times more neutrinos than the "Easy" option. It's like moving your cup from the edge of the roof to right under the downspout.

4. Why Does This Matter? (The Physics)

Why do we care about catching 5x more neutrinos?

• The "Ghost" Hunters: Neutrinos are famous for being "ghosts" because they rarely interact with anything. Catching more of them lets us study how they behave at energies we've never seen before.
• The "Antimatter" Mystery: With the new magnet, they can finally spot tau antineutrinos. These are the "missing cousins" of a particle we've seen before. Finding them is like finding the last piece of a cosmic puzzle.
• The "Dark Matter" Search: The detector isn't just looking for neutrinos; it's also a net for "feebly interacting particles" (FIPs). These are hypothetical particles that could explain Dark Matter. The "Optimized" position makes the net much bigger, increasing the chances of catching a dark matter particle if it exists.
• The "Time Machine" Effect: By timing exactly when a neutrino hits the detector and matching it with a collision in the main ATLAS detector (down the hall), they can link the "forward" view with the "main" view, giving a 3D picture of the collision.

The Verdict

The paper concludes that while the "Easy" install is cheaper and faster, the "Optimized" install is scientifically superior. It turns a good experiment into a world-class one, increasing the discovery potential by a factor of five.

In a nutshell: The scientists are upgrading their detector to handle a super-charged particle beam. They have a choice: install it quickly in a slightly awkward spot, or do a little construction to move it into the perfect spot. The math says moving it is worth the extra effort because it will let them see five times more of the universe's most elusive particles.

Technical Summary

1. Problem Statement

The Large Hadron Collider (LHC) forward region (pseudorapidity $\eta \sim 7-8$) offers a unique environment to study high-energy neutrinos from heavy-flavor decays and feebly interacting particles (FIPs). While the current SND@LHC detector (operating since 2022) has proven the feasibility of detecting these particles, the upcoming High-Luminosity LHC (HL-LHC) will deliver an integrated luminosity an order of magnitude higher ($3000 \text{ fb}^{-1}$).

The current detector faces three main limitations for the HL-LHC era:

1. Rate Handling: The emulsion-based technology used in Run 3 cannot sustain the significantly higher muon rates expected at the HL-LHC.
2. Physics Reach: The current geometric acceptance limits the total neutrino interaction rate and the ability to perform precision measurements at the TeV scale.
3. Particle Identification: The lack of a magnetic field prevents the distinction between neutrinos and antineutrinos and limits momentum resolution for muons.

The paper addresses the need for an upgraded detector (SND@HL-LHC) and evaluates two installation scenarios to maximize physics output within the constraints of the TI18 tunnel.

2. Methodology and Detector Design

The authors propose a comprehensive upgrade of the SND@LHC detector, retaining its modular concept but replacing key technologies to handle HL-LHC conditions.

Key Technological Upgrades:

• Fully Electronic Readout: Replaces nuclear emulsions with silicon microstrip detectors (repurposed from the CMS Tracker) to cope with high muon rates and provide real-time data.
• Magnetised Spectrometer: Introduces a magnetic field to measure muon charge and momentum, enabling the separation of neutrinos ($\nu$) and antineutrinos ($\bar{\nu}$).
• Target Region: A compact stack of 58 tungsten plates (7 mm thick) interleaved with silicon layers, providing $\sim 116$ radiation lengths and 4 interaction lengths for neutrino interactions and vertex reconstruction.
• Magnetised Calorimeter: Located downstream, consisting of 50 mm iron slabs interleaved with silicon layers. It measures hadronic energy and provides muon tracking in a $\sim 1.7$ T magnetic field.
• Veto and Timing Systems:
  • Veto: Three planes of plastic scintillators with SiPMs to reject charged backgrounds (efficiency $\sim 10^{-9}$).
  • Timing: A dedicated system with $<100$ ps resolution to associate events with the correct LHC bunch crossing.

Installation Scenarios Evaluated:
The paper compares two configurations within the TI18 tunnel:

1. Baseline Configuration: The detector is installed on the existing tunnel floor with minimal civil engineering. It remains slightly off-axis ($\sim 57$ cm vertical, $-39$ cm horizontal from the collision axis).
2. Extended Configuration: The detector is lowered by 43 cm and shifted horizontally by 29 cm (requiring the removal of $4.5 \text{ m}^3$ of concrete). This brings the detector center much closer to the collision axis ($\sim 14$ cm vertical, $10$ cm horizontal) while maintaining an off-axis position to optimize charm decay neutrino flux.

Simulation Framework:
Physics performance was evaluated using Monte Carlo simulations (DPMJET/FLUKA, GENIE, POWHEG+PYTHIA8) assuming $3000 \text{ fb}^{-1}$ integrated luminosity and various beam crossing angles.

3. Key Contributions

• Design of SND@HL-LHC: A detailed technical specification for a fully electronic, magnetised detector capable of operating in the extreme radiation and rate environment of the HL-LHC.
• Comparative Installation Analysis: A rigorous comparison between a "low-impact" (Baseline) and a "high-performance" (Extended) installation, quantifying the trade-offs between civil engineering effort and physics gain.
• Physics Reach Projections: Comprehensive projections for neutrino cross-sections, Parton Distribution Functions (PDFs), Lepton Flavour Universality (LFU) tests, and searches for new physics (FIPs).
• Synergy with SHiP: The design serves as a prototype for the proposed SND detector at the SHiP experiment, validating technologies for future fixed-target experiments.

4. Results

The simulation results demonstrate a dramatic improvement in physics performance for the Extended Configuration compared to the Baseline.

Neutrino Interaction Rates:

• The Extended configuration increases the total neutrino interaction rate by a factor of five compared to the Baseline.
• Total Events (Target + HCAL):
  • Baseline: $\sim 2.7 \times 10^4$ events.
  • Extended: $\sim 1.1 \times 10^5$ events.
• This gain is driven by the increased azimuthal acceptance ($\Delta\phi \sim 250$ mrad vs. 66 mrad) and reduced distance to the beam axis.

Physics Precision:

• Cross-Sections: Statistical uncertainty on $\nu_\mu$ and $\bar{\nu}_\mu$ cross-sections above 350 GeV improves from 3% (Baseline) to <1% (Extended).
• PDF Constraints:
  • Small-$x$ gluon PDF uncertainty reduced from 10% to 5%.
  • Large-$x$ strange quark PDF uncertainty reduced from 6% to 3%.
• Lepton Flavour Universality (LFU):
  • $\nu_e/\nu_\tau$ ratio uncertainty improves from 6% (Baseline) to 2% (Extended).
  • $\nu_e/\nu_\mu$ ratio uncertainty improves from 2% to 1%.
• Tau Antineutrino Observation:
  • Baseline: Significance of $\sim 3\sigma$ (approx. 4 observable events).
  • Extended: Significance of $\sim 5\sigma$ (approx. 9 observable events), marking the first potential direct observation of tau antineutrinos.

New Physics (FIPs):

• The Extended configuration significantly improves sensitivity to Light Dark Matter (LDM) and other feebly interacting particles due to the higher flux of particles intersecting the detector.

5. Significance

This paper establishes the scientific and technical roadmap for the SND@LHC experiment during the HL-LHC era.

• Scientific Impact: The upgrade transforms the experiment from a discovery-phase detector into a precision instrument capable of measuring neutrino interactions at the TeV scale, constraining fundamental QCD parameters (PDFs), and testing the Standard Model's Lepton Flavour Universality with unprecedented precision.
• Discovery Potential: The Extended configuration is critical for the first direct observation of tau antineutrinos and for probing the parameter space of feebly interacting particles (like dark photons and heavy neutral leptons) that are inaccessible to other experiments.
• Strategic Value: The successful implementation of the Extended configuration (requiring civil engineering) offers a five-fold increase in physics output, justifying the investment. Furthermore, the detector serves as a vital prototype for the SHiP experiment, ensuring technology readiness for future fixed-target physics programs.

In conclusion, while the Baseline configuration is feasible, the Extended configuration is strongly recommended to fully exploit the HL-LHC potential, offering a factor of five increase in event rates and enabling 5$\sigma$ discoveries in neutrino physics and searches for new particles.