Hyperon-Nucleon Spectrometer

This whitepaper proposes the Hyperon-Nucleon Spectrometer (H-NS) at the High-Intensity heavy-ion Accelerator Facility (HIAF) to systematically investigate the unresolved $\Lambda$ polarization puzzle through high-precision measurements of hyperon and proton spin observables across a wide range of collision energies and systems.

The Gist

Imagine the universe is built out of tiny, invisible Lego bricks called quarks. These bricks snap together to form larger structures called protons and neutrons, which make up the nuclei of every atom in your body. But here's the mystery: we don't fully understand how these bricks spin or why they stick together the way they do. It's like trying to figure out how a complex clock works just by looking at the hands, without seeing the gears inside.

This paper proposes building a massive, high-tech microscope called the Hyperon-Nucleon Spectrometer (H-NS) to solve one of the biggest puzzles in physics: Why do some particles spin on their own?

The Mystery: The "Self-Polarizing" Particle

In the 1970s, scientists discovered something weird. When they smashed protons together (like two fast cars crashing), they created a particle called a Lambda (Λ) hyperon. Even though the crash was random and the cars weren't spinning, the resulting Lambda particles started spinning in a specific direction, as if they had a mind of their own.

Scientists have been trying to figure out why this happens for 50 years. It's like watching a coin land on its edge every single time you flip it, even though you didn't try to make it do that. This "self-polarization" is a clue to a hidden rulebook of nature (Quantum Chromodynamics, or QCD) that we haven't cracked yet.

The Solution: The H-NS "Super-Microscope"

To solve this, the paper proposes building the H-NS at a giant machine in China called HIAF (High-Intensity heavy-ion Accelerator Facility). Think of HIAF as a super-powered slingshot that can fire protons and heavy atoms at targets with incredible speed and precision.

The H-NS is designed to be the ultimate catcher's mitt for these collisions. Here is how it works, using simple analogies:

• The Magnet (The Giant Spoon): Inside the spectrometer is a massive superconducting magnet. Imagine a giant spoon curving the path of everything that flies through it. This helps scientists measure exactly how fast and in what direction the particles are moving.
• The Tracker (The High-Speed Camera): The core of the machine is made of layers of ultra-thin silicon sensors (called MAPS). Think of these as a stack of high-speed cameras taking millions of pictures per second. They are so sensitive they can see the tiny "ghost trails" left by particles as they decay. This is crucial because the Lambda particle is unstable; it breaks apart almost instantly. The tracker catches the pieces before they vanish.
• The Time-of-Flight (The Stopwatch): Some particles are hard to tell apart (like a proton vs. a kaon). The H-NS uses special sensors (LGADs) that act like super-accurate stopwatches. By measuring exactly how long it takes a particle to travel a short distance, the machine can identify what the particle is, just like you can tell a sprinter from a jogger by their time.
• The Polarimeter (The Spin Detector): This is a unique feature. The machine has a thin carbon foil that acts like a "spin checker." When a proton hits it, the way it bounces off tells the scientists exactly how much the proton was spinning. This allows them to measure the spin of protons directly, not just the Lambda particles.

What Will They Do?

The H-NS will run experiments in three different ways:

1. Proton vs. Proton: Smashing two protons together to see how they create spinning particles.
2. Proton vs. Nucleus: Shooting a proton into a heavy atom to see how the "crowd" of particles inside the atom affects the spin.
3. Nucleus vs. Nucleus: Smashing two heavy atoms together to create a tiny, hot soup of particles (like the early universe) to see if the whole "soup" spins.

They will do this across a wide range of speeds, from slow crashes to very fast ones, to see how the "self-spinning" effect changes.

Why Does It Matter?

The paper claims that by mapping out exactly how and why these particles spin, the H-NS will finally help us understand the origin of spin in the visible universe. It's like finding the missing instruction manual for the Lego bricks.

Furthermore, the technology built for H-NS isn't just for this one experiment. The paper states it will serve as a "training ground" and technology testbed for a future, even bigger machine called the Electron-ion Collider in China (EicC). The sensors and software developed here will help build the next generation of physics tools.

In short: The paper is a blueprint for a new, high-tech machine designed to catch spinning particles in the act, solve a 50-year-old mystery about why they spin, and teach us the fundamental rules of how matter is built.

Technical Summary

Technical Summary: Hyperon-Nucleon Spectrometer (H-NS) White Paper

1. Problem Statement

The paper addresses the long-standing "Λ polarization puzzle" in non-perturbative Quantum Chromodynamics (QCD). Despite decades of study, the mechanism behind the large transverse polarization of Λ hyperons produced in unpolarized hadronic collisions remains theoretically unexplained. Key open questions include:

• Origin of Polarization: Why do Λ hyperons exhibit significant transverse polarization (up to 30–40%) in unpolarized proton-proton (pp) and proton-nucleus (pA) collisions, particularly at high Feynman-$x_F$ and transverse momentum ($p_T$)?
• Energy Dependence: Existing data suggests a "scaling" behavior where polarization appears independent of center-of-mass energy ($\sqrt{s}$), contradicting QCD factorization predictions that polarization should decrease at high energies due to helicity conservation.
• Hyperon-Nucleon Interactions: Experimental constraints on hyperon-nucleon (YN) scattering lengths and interaction potentials are insufficient, limiting the understanding of hypernuclei structure and neutron star properties.
• Global Polarization: The mechanism driving the global polarization of hyperons in heavy-ion collisions (AA) at low energies ($\sqrt{s_{NN}} < 3$ GeV) requires investigation to determine if vorticity-induced polarization models remain valid in the hadronic gas regime.
• Proton Spin: Direct measurement of final-state proton polarization in unpolarized collisions is challenging, hindering the comparison between hyperon and nucleon spin dynamics.

2. Methodology and Experimental Design

The paper proposes the construction of the Hyperon-Nucleon Spectrometer (H-NS) at the High-Intensity heavy-ion Accelerator Facility (HIAF) in Huizhou, China. The experiment utilizes a fixed-target mode with high-intensity proton and heavy-ion beams.

2.1 Beam and Collision Parameters

• Beam Energy: Proton beams from 3 GeV to 9.3 GeV (current HIAF) and up to 32 GeV (HIAF-Upgrade).
• Collision Systems: Systematic studies in $pp$, $pA$, and $AA$ collisions.
• Kinematic Coverage: Designed to cover the transition from the near-threshold hadronic regime (dominated by resonance excitations) to the intermediate-energy QCD regime, with a focus on high-$x_F$ production where polarization effects are maximal.

2.2 Detector Conceptual Design

The H-NS is a cylindrical spectrometer featuring a 1.5 T superconducting solenoid magnet and specialized subsystems optimized for precision tracking, particle identification (PID), and polarization measurement:

• Tracking System: Utilizes Monolithic Active Pixel Sensors (MAPS) with a material budget of <0.5% $X_0$ per layer. It consists of five concentric barrel layers (5–35 cm radius) and five forward disks (40–100 cm), providing high-granularity vertexing (<500 µm resolution) essential for reconstructing weak decay vertices of hyperons.
• Time-of-Flight (TOF) / PID: Employs Low Gain Avalanche Diodes (LGAD), specifically AC-LGAD technology, offering ~30 ps timing resolution and ~100 µm spatial resolution. This enables 3$\sigma$ separation of pions/kaons up to 2 GeV/c and protons/kaons up to 5 GeV/c.
• Calorimetry: An endcap Electromagnetic Calorimeter (ECAL) using PbWO$_4$ crystals (central) and lead-glass (outer) for neutral particle detection ($\gamma$, $\pi^0$, $n$) with energy resolution $\sigma_E/E \approx 3\%/\sqrt{E}$.
• Nucleon Polarimeter: A unique feature integrating a thin carbon foil target (1 mm) within the tracker. This allows for the measurement of final-state proton polarization via elastic $p$-C scattering, utilizing the spin-dependent analyzing power of the scattering process.
• Targets: Supports unpolarized targets (C, Ca, Ti foils, Liquid Hydrogen) and future polarized targets ($^3$He gas via MEOP, solid polarized protons/deuterons via DNP).

2.3 Simulation and Optimization

The design was optimized using the HnsRoot software framework (based on FairRoot/Geant4).

• Geometry Optimization: Systematic scans of barrel radius ($R_o$), detector length ($L$), and barrel-to-total length ratio ($R_b$) were performed. The selected configuration ($R_i=5$ cm, $R_o=50$ cm, $L=150$ cm, $R_b=0.33$) balances tracking efficiency, mass resolution, and technical feasibility.
• Magnet Choice: A solenoid configuration was selected over a dipole to ensure axial symmetry and simplified engineering, despite a slight trade-off in small-angle mass resolution.
• Performance Metrics: Simulations indicate momentum resolution of $\sim$2% at 1 GeV/c, vertex resolution <500 µm, and high tracking efficiency across the acceptance ($5^\circ$ to $100^\circ$).

3. Key Contributions and Physics Projections

The paper outlines specific physics goals and projects the statistical precision achievable with the H-NS:

3.1 Hyperon Polarization in $pp$ and $pA$

• Goal: Map the energy dependence of $\Lambda$ polarization from threshold to 32 GeV to resolve the tension between "scaling" observations and QCD predictions.
• Projection: With $\sim 10^{13}$ $pp$ events (accumulated over a few months), the statistical uncertainty on $\Lambda$ polarization is projected to reach 0.002%. For the exclusive channel $pp \to pK^+\Lambda$ ($\sim 10^9$ events), the uncertainty is projected at 0.06%. This will allow precise extraction of polarizing fragmentation functions and twist-3 correlations.

3.2 Proton Polarization Measurement

• Goal: Simultaneously measure proton polarization to disentangle hyperon-specific mechanisms from general spin dynamics.
• Projection: Using the integrated nucleon polarimeter, the uncertainty on proton polarization is projected to reach 0.02% with $10^{13}$ events. This capability is unique to H-NS and enables direct comparison between $\Lambda$ and proton spin observables.

3.3 Global Polarization in Heavy-Ion Collisions

• Goal: Investigate the global polarization of $\Lambda$ and light (hyper-)nuclei (e.g., hypertriton $^3_\Lambda$H) in non-central $AA$ collisions at low energies ($\sqrt{s_{NN}} \approx 2.8$ GeV).
• Projection: With $10^{11}$ Au+Au events, the uncertainty on global polarization is projected to reach 0.1%. This will test vorticity models in the hadronic gas phase and probe the spin structure of light nuclei.

3.4 Hyperon-Nucleon Interactions and Rare Processes

• Goal: Extract $\Lambda N$ scattering lengths via final-state interactions in $pp \to pK^+\Lambda$ and search for rare processes (e.g., parity violation, CP violation, lepton-number violation, and H-dibaryon searches).
• Capability: The high statistics and precise vertexing allow for the reconstruction of Dalitz plots and invariant mass spectra necessary to constrain YN interaction models and search for forbidden decays.

4. Significance and Claims

The paper positions the H-NS as a critical facility for advancing the understanding of non-perturbative QCD and the origin of hadron spin. Its significance is claimed to be threefold:

1. Resolving the Polarization Puzzle: By providing a systematic, high-precision energy scan across the transition from hadronic to partonic regimes, H-NS aims to finally elucidate the mechanism of spontaneous hyperon polarization, a phenomenon unexplained by leading-order perturbative QCD.
2. Unique Experimental Capabilities: The simultaneous measurement of hyperon and proton polarization in the same experimental setup, combined with the ability to study global polarization in $AA$ collisions, offers a comprehensive view of spin dynamics in strongly interacting matter that is inaccessible to existing facilities (e.g., NICA, J-PARC, FAIR).
3. Technological Precedent for EicC: The H-NS serves as a technology verification platform for the future Electron-ion Collider in China (EicC). The development and deployment of MAPS and AC-LGAD technologies for H-NS will directly benefit the detector design and physics program of EicC, ensuring a smooth transition and sustainable research capability in China's nuclear and particle physics landscape.

The document concludes that the H-NS program, supported by a collaboration of over 21 Chinese institutes, is ready for implementation pending funding, with the potential to deliver unprecedented constraints on the dynamics of spin in strongly interacting matter.