The SHiP/NA67 experiment at the ECN3 high-intensity beam facility at the CERN SPS

This paper outlines the physics motivation, experimental design, and expected sensitivity of the SHiP/NA67 experiment, a high-intensity beam dump facility at CERN approved in 2024 to search for feebly interacting particles and study neutrino phenomenology over a 15-year operational period.

The Gist

Imagine the universe is a giant, bustling city. For decades, scientists have been looking for new residents (new particles) by building massive, high-speed highways (like the Large Hadron Collider) to crash cars together at incredible speeds. They hope that if they smash things hard enough, new, heavy residents will pop out.

But there's a problem: some residents are so shy and weak that they don't show up in these high-speed crashes, even if they exist. They are the "feeble interactors"—particles that are light but barely talk to anything else. To find them, you don't need a bigger hammer; you need a massive crowd.

This paper introduces SHiP (Search for Hidden Particles), a new experiment approved in 2024 to be built at CERN in Switzerland. Think of SHiP not as a sledgehammer, but as a giant net cast into a river of particles.

The Setup: A Particle Factory

The experiment uses a powerful beam of protons (the "river") and smashes it into a thick block of tungsten (the "net").

• The Goal: This crash creates a massive shower of heavy particles, some of which might decay into the "shy" hidden particles we are looking for.
• The Volume: Over 15 years, they plan to fire enough protons to create 600 billion billion (6×10²⁰) hits. This is an unprecedented amount of data.

The Challenge: The "Noise" Problem

When you smash protons into a target, you get a lot of "noise." The biggest troublemaker is a flood of muons (a type of particle) and neutrinos. It's like trying to hear a whisper in a stadium full of screaming fans.

• The Solution: SHiP uses a giant magnetic shield (like a force field) to deflect the screaming muons away.
• The "Zero-Background" Room: Behind the shield, there is a long, empty tunnel (the decay volume). The experiment is designed so that if any particle enters this room that shouldn't be there, sensors instantly flag it. This creates a "zero-background" environment where even a single hidden particle decaying would be a clear, undeniable signal.

The Two Main Jobs of the Experiment

1. The "Ghost Hunter" (Finding Hidden Particles)
The experiment looks for particles that are produced in the crash, fly through the shield, and then decay (break apart) inside the long tunnel.

• What they hunt: Heavy Neutral Leptons (HNLs), Dark Photons, Dark Scalars, and Axion-like particles.
• The Analogy: Imagine a secret agent (the hidden particle) sneaking through a security checkpoint. They are invisible to the guards, but once they reach a safe room (the tunnel), they take off their disguise and reveal themselves. SHiP's cameras are so sensitive they can spot that disguise coming off.
• Why it matters: These particles could explain why the universe has more matter than antimatter, what dark matter is, and why neutrinos have mass.

2. The "Neutrino Observatory" (Studying the Ghosts)
While the main goal is finding new particles, the crash also produces a massive flood of neutrinos (ghostly particles that pass through everything).

• The Special Catch: SHiP will catch about 1,000 tau neutrinos per year. This is a huge number compared to previous experiments.
• The Analogy: Previous experiments were like trying to study a rare bird by spotting it once a decade. SHiP will be like a birdwatching tower that sees thousands of these rare birds every year.
• The Goal: This allows scientists to study how these neutrinos interact with matter in ways never seen before, specifically looking at how they behave when they turn into "tau" particles.

The Timeline and Future

• Current Status: The project is in the "Technical Design" phase (finalizing the blueprints).
• Construction: The facility is being built now.
• Launch: They expect to start shooting the beam in 2033.
• Early Wins: Even before the full 15-year run is complete, the data collected in the first few years will likely set the world's best limits on where these hidden particles aren't, effectively narrowing the search for the rest of the physics community.

In Summary

The SHiP experiment is a shift in strategy. Instead of trying to smash things harder to find heavy new physics, it is trying to look at a massive volume of data to find the light, shy particles that have been hiding in plain sight. It combines a "muon-deflecting shield," a "silent tunnel," and "super-sensitive cameras" to listen for the faint whispers of the universe's hidden secrets.

Technical Summary

Technical Summary of the SHiP/NA67 Experiment Paper

Problem and Motivation
The Standard Model (SM) remains consistent with current data but fails to explain several established observations, including the origin and smallness of neutrino masses, the baryon asymmetry of the Universe, and the nature of dark matter. These gaps suggest the existence of Physics Beyond the Standard Model (BSM). While high-energy frontier facilities (e.g., HL-LHC, FCC) search for heavy new states, many potential BSM candidates may be too weakly coupled to be produced directly at existing machines. These "feebly interacting particles" (FIPs), such as heavy neutral leptons (HNLs), dark photons, dark scalars, axion-like particles (ALPs), and light dark matter, interact with the SM with strengths too low for traditional detection but possess masses in the O(100 MeV) to few-GeV range. The paper argues that high-intensity searches, where large production rates compensate for small coupling strengths, are the necessary strategy to explore this parameter space.

Methodology and Experimental Concept
The SHiP/NA67 experiment is designed as a general-purpose, high-intensity beam dump experiment located in the ECN3 hall at the CERN Super-Proton-Synchrotron (SPS), utilizing the new Beam Dump Facility (BDF). The methodology relies on three core components to detect FIPs:

1. High-Intensity Production: A 400 GeV/c proton beam is directed onto a tungsten target to maximize the production of heavy-flavour hadrons (charm and beauty), which serve as sources for FIPs. The experiment aims to accumulate $6 \times 10^{20}$ protons on target (PoT) over approximately 15 years.
2. Background Suppression: The primary challenge is the intense flux of muons and neutrinos emerging from the dump. To achieve "zero-background" conditions, the experiment employs a magnetized active muon shield to deflect residual muons by six orders of magnitude.
3. Decay and Detection: A long, instrumented decay volume allows FIPs to decay into visible SM particles. The experiment utilizes a multi-layered detector system to reconstruct these rare decays while suppressing backgrounds to negligible levels.

Detector Subsystems
The experimental setup includes several specialized subsystems:

• Target and Muon Shield: A tungsten target with active cooling and a magnetized hadron absorber. A magnetic active muon shield reduces the muon flux by six orders of magnitude.
• Scattering and Neutrino Detector (SND@SHiP): Located downstream of the target, this system uses a silicon-tungsten target and an iron-scintillator magnetized tracking calorimeter. It is designed to identify neutrino interactions of all flavors, specifically targeting $\mathcal{O}(10^4)$ $\nu_\tau$ interactions per year. It aims to measure structure functions $F_4$ and $F_5$ (accessible only via tau neutrinos), study parton distribution functions at low $x$, and measure strange-quark nucleon content. It also enables the detection of light dark matter scattering off electrons.
• Background Taggers: To ensure a zero-background environment, the experiment uses an Upstream Background Tagger (UBT) with straw trackers and scintillator tiles, and a Surrounding Background Tagger (SBT). The SBT encloses the helium-filled decay volume with approximately 780 cells containing ~145,000 L of liquid scintillator, read out by wavelength-shifting optical modules. This system achieves >99% efficiency and sub-1 ns timing resolution to tag residual muons and neutrino interactions in the vessel walls.
• Hidden Sector Decay Spectrometer: This system reconstructs visible decay products within the evacuated/helium-filled decay volume. It comprises:
  • A spectrometer straw tracker (four stations) coupled to a large dipole magnet (0.6–0.8 T m), providing 120 $\mu$m spatial resolution.
  • A timing detector using scintillator bars with $\le$50 ps resolution to suppress combinatorial background.
  • A calorimeter system (SplitCal concept) with electromagnetic and hadronic sections, including dedicated high-precision layers to measure electromagnetic shower directions and identify neutral final states (e.g., $X \to \gamma\gamma$).

Key Contributions and Expected Results
The paper outlines the expected sensitivity of SHiP across a broad range of FIP models.

• FIP Sensitivity: For masses in the O(100 MeV) to few-GeV range, SHiP is projected to provide world-leading or record sensitivity to couplings well beyond the reach of existing and proposed experiments. It targets HNLs, dark photons, dark scalars, ALPs, elastic and inelastic light dark matter, and millicharged particles.
• Neutrino Physics: The experiment will produce an intense flux of neutrinos, notably $\mathcal{O}(10^3)$ $\nu_\tau$ per year. This will enable the first determination of structure functions $F_4$ and $F_5$ and provide high-statistics samples for studying neutrino interactions across all flavors.
• Timeline: The collaboration is currently in the Technical Design Report (TDR) phase, with outputs expected within two years. The BDF is under construction, with beam commissioning foreseen by 2033. The paper notes that the data collected before the subsequent long shutdown will already allow SHiP to set world-leading limits over much of its parameter space.

Significance
The paper claims that the energy frontier has thus far revealed no new physics beyond the SM, strongly motivating a complementary exploration of the intensity frontier. SHiP/NA67 is presented as a facility with world-leading sensitivity to directly explore the feebly interacting particle frontier and study all-flavor neutrino interactions. By operating in an essentially background-free regime, the experiment aims to resolve fundamental questions regarding neutrino masses, dark matter, and baryon asymmetry that have escaped detection by previous methods. The experiment is approved for 2024 and is scheduled to begin data taking in 2033 at the CERN SPS.