NO LESS: Novel Opportunities for Light Exotic Searches at the SPS

This paper demonstrates that a minimally reconfigured version of CERN's existing NA62 experiment, operating in a future ECN3 beam-dump facility with an optimized geometric setup, can achieve highly competitive sensitivity for detecting feebly interacting particles in the MeV to GeV mass range immediately upon beam availability.

The Gist

Imagine the world of particle physics as a massive, high-speed highway. For decades, scientists have been building bigger and bigger highways (like the Large Hadron Collider) to smash particles together at incredible speeds, hoping to find new, exotic creatures hiding in the debris.

But there's a problem: some of these creatures are so shy and weak that they slip right past the giant detectors at the highway's end. They are the "feeble interactors"—particles that barely touch anything.

This paper, titled "NO LESS," proposes a clever, low-cost solution to catch these shy particles using a facility at CERN (the European particle physics lab) that is already being built.

Here is the story in simple terms:

1. The Setup: The "Wall" and the "Shy Ghosts"

Imagine a giant wall (the Beam Dump) built to stop a high-speed train of protons. When the protons hit the wall, they create a shower of particles. Most of these are heavy and crash into the wall, but some might be the "shy ghosts" (new exotic particles) we are looking for.

Because these ghosts are so weak, they don't stop at the wall. They pass right through it and travel a long distance before they might finally "decay" (turn into something we can see, like light or electrons). To catch them, we need a long, empty tunnel (a decay volume) after the wall, followed by a giant camera (the detector) to snap a picture of what they turn into.

2. The Problem: The "Perfect" Camera is Late

CERN is building a brand-new, state-of-the-art facility called BDF (Beam Dump Facility) with a massive, custom-built camera system called SHiP. This camera is designed to be the ultimate ghost hunter.

However, SHiP is a huge project. It will take years to build and won't be ready until after 2026.
Meanwhile, the BDF facility will be ready to shoot protons at the wall much sooner. If we wait for SHiP, we lose years of data.

3. The Solution: The "Swiss Army Knife" Detectors

Enter the NA62 experiment. Right now, NA62 is a working detector sitting in the same building (ECN3 hall). It's currently looking for different things, but it has all the necessary parts: magnets, trackers, and cameras.

The authors of this paper asked a simple question: "What if we just moved the NA62 detectors to the new BDF wall and pointed them at the ghosts?"

They didn't just guess; they ran computer simulations to test three different ways to do this:

• The "Minimalist" Approach (BDF 0): Just take the NA62 detectors, move them to the new spot, and leave them exactly as they are. It's like moving your living room furniture to a new house without rearranging a single chair.
• The "Tweaked" Approach (BDF 3a): Move the detectors closer together and remove a few bulky parts (like a giant mirror) to make the tunnel longer. This is like rearranging the furniture to make the hallway wider so more ghosts can fit through.
• The "Dream" Approach (BDF 4): This is the original, massive SHiP design. It's the "Ferrari" of detectors, but it takes years to build.

4. The Big Surprise

The paper's main finding is a happy one: You don't need the Ferrari to win the race.

Even the "Minimalist" approach (just moving the existing NA62 detectors) is surprisingly powerful.

• The Analogy: Imagine you are trying to catch a rare bird. The "Dream" approach is building a massive, high-tech aviary. The "Minimalist" approach is just taking your existing bird net and moving it to the right tree.
• The authors found that the "Minimalist" net catches almost as many birds as the massive aviary for many types of particles.

5. Why This Matters

• Speed: We could start hunting for these new particles immediately after the new facility is ready, rather than waiting years for the big detector to be built.
• Cost: It uses equipment we already have, saving millions of dollars.
• Competition: There are other labs around the world (like in the US) building similar detectors. If CERN waits, they might miss the discovery. By using the "Minimalist" setup, CERN can stay in the race right now.

The Bottom Line

The paper argues that while the "perfect" detector (SHiP) is great, we shouldn't wait for it to start our search. By simply re-arranging the "old" detectors (NA62) at the new site, we can start hunting for the universe's shyest particles today.

It's a reminder that sometimes, you don't need a brand-new, expensive tool to make a breakthrough; you just need to look at the tools you already have in a new way.

Technical Summary

1. Problem Statement

The search for Feebly Interacting Particles (FIPs) in the MeV–GeV mass range is a priority in particle physics, as these particles could serve as portals to Dark Matter or explain other Beyond the Standard Model (BSM) phenomena.

• The Context: The CERN SPS (Super Proton Synchrotron) is undergoing upgrades. The Beam Dump Facility (BDF) in the ECN3 hall is being prepared for the SHiP experiment, a dedicated facility designed to search for FIPs with high sensitivity.
• The Gap: The full SHiP detector is not expected to be operational immediately after the Long Shutdown 3 (LS3) of the LHC (scheduled for ~2026). Conversely, the NA62 experiment, which is currently operational in ECN3 and has already demonstrated background-free performance in beam-dump mode, is scheduled to be dismantled in 2026.
• The Question: Can the existing NA62 detector infrastructure be reconfigured for the new BDF target to provide competitive sensitivity to FIPs immediately after LS3, before the full SHiP detector is ready?

2. Methodology

The authors utilized a comparative simulation framework to evaluate various detector configurations against the proposed SHiP design.

• Simulation Tool: The study employed Alpinist, a public simulation framework that implements unified theoretical assumptions for various FIP benchmark cases (BCs). This ensures a fair comparison of production cross-sections, kinematics, and decay modes across different geometries.
• Beam Parameters:
  • Source: 400 GeV protons from the SPS.
  • Target: A fixed BDF target design (titanium-zirconium-doped molybdenum alloy) located 22 m from the ECN3 entrance.
  • Statistics: All scenarios assumed an integrated luminosity of $8 \times 10^{19}$ Protons on Target (PoT), corresponding to roughly two years of data taking (approx. 80 times the current NA62 statistics).
• Detector Configurations: The study evaluated eight hypothetical scenarios, categorized into three main groups:
  1. BDF 0 (Minimal): The current NA62 detectors are kept in their existing positions downstream of the decay volume entrance (33.5 m). Upstream detectors are removed to make space for the target. Includes both on-axis and off-axis beam configurations.
  2. BDF 1–3 (Rearranged): Configurations involving the removal of the NA62 RICH detector and the relocation of the STRAW spectrometer stations closer to the calorimeter to extend the decay volume and optimize angular acceptance. BDF 3a represents the most sensitive rearrangement.
  3. BDF 4 (Full SHiP): The originally proposed SHiP configuration with a 50 m decay volume and a large-acceptance spectrometer.
• Background Assessment: The authors extrapolated background rates from NA62's current performance. They argued that a "zero-background" regime is achievable at BDF by installing an enhanced muon sweeping system (using additional MTR and MBPL magnets) to compensate for the shorter distance between the dump and the detector compared to the original NA62 setup.

3. Key Contributions

• Feasibility of "Bridge" Experiments: The paper demonstrates that a "minimalist" reconfiguration of existing NA62 hardware can yield physics results immediately post-LS3, bridging the gap before the full SHiP detector is operational.
• Geometric Sensitivity Analysis: The study quantifies how different production mechanisms (e.g., Bremsstrahlung vs. Meson Decays) respond to detector geometry. It highlights that while extending the decay volume is crucial for long-lived particles, the solid angle acceptance is the dominant factor for specific models (like Heavy Neutral Leptons).
• Benchmarking: It provides a direct, apples-to-apples comparison of sensitivity limits for Dark Photons, Higgs-like scalars, Heavy Neutral Leptons (HNLs), and Axion-like Particles (ALPs) across minimal, optimized, and full SHiP configurations.

4. Results

The sensitivity was expressed as 90% Confidence Level (CL) exclusion bounds on coupling parameters vs. mass.

• Dark Photons (BC1):
  • Production is dominated by forward Bremsstrahlung.
  • Result: The difference in sensitivity between the minimal (BDF 0), rearranged (BDF 3a), and full SHiP (BDF 4) configurations is marginal. The decay volume length is the primary driver, and even the minimal setup performs competitively.
• Higgs-like Scalars (BC4/BC5):
  • Similar to Dark Photons, production is forward-peaked.
  • Result: Sensitivity differences between configurations are small. The off-axis configuration (BDF 0) shows slightly reduced sensitivity compared to on-axis, but remains competitive.
• Heavy Neutral Leptons (HNLs) (BC6–BC8):
  • Produced primarily via $D$ and $B$ meson decays, which have broader angular distributions.
  • Result: The solid angle acceptance is critical here. The full SHiP configuration (BDF 4) offers significantly better sensitivity, particularly above the $D$-meson mass threshold, compared to the NA62-based setups. However, the rearranged BDF 3a configuration still offers substantial reach compared to current limits.
• Axion-like Particles (ALPs) (BC9–BC11):
  • Photon-coupled and gluon-coupled ALPs are forward-emitted (similar to Dark Photons), showing minimal geometry dependence.
  • Fermion-coupled ALPs (BC10) rely more on $B$-meson decays, favoring larger solid angles, but the minimal setup still provides competitive exclusion limits.

Key Finding: Even the BDF 0 (Minimal) configuration, which requires no major reconstruction of the detector hall, can probe a significant portion of the FIP parameter space, often reaching within a factor of 2–3 of the full SHiP sensitivity for forward-peaked models.

5. Significance

• Strategic Value: The paper argues that waiting for the full SHiP detector might result in a loss of discovery potential. A "minimal" NA62-based experiment could start collecting data immediately after LS3, potentially discovering new physics years earlier.
• Cost-Effectiveness: Utilizing existing, proven detectors (NA62) reduces the risk and cost associated with building a new facility from scratch.
• Background Validation: The study reinforces the feasibility of zero-background searches in beam-dump experiments by demonstrating that the NA62 background performance can be maintained or improved at the BDF with standard magnetic sweeping techniques.
• Community Impact: By using the public Alpinist framework, the authors provide a transparent and reproducible baseline for the community to evaluate future FIP search strategies at CERN and elsewhere.

In conclusion, the paper establishes that NO LESS (Novel Opportunities for Light Exotic Searches) is a viable and highly competitive strategy. It suggests that the physics community should seriously consider deploying a reconfigured NA62 detector at the BDF immediately after LS3 to maximize the scientific output of the SPS beam.