Proposal for the first measurement of antiproton polarization in proton-nucleus interactions

This paper proposes and evaluates the experimental feasibility of the first dedicated measurement of transverse antiproton polarization in proton-nucleus collisions at CERN, utilizing left-right asymmetry in elastic scattering to probe the spin structure of the antinucleon-nucleon interaction.

The Gist

The Big Idea: Catching a Spinning Antiproton

Imagine you are at a busy train station. Every day, thousands of people (protons) rush through the station. Occasionally, two people bump into each other, and out of the chaos, a new person appears: an antiproton.

For decades, physicists have known these "anti-people" exist. But there is a mystery: Do they spin?

In the world of particles, "spin" isn't like a spinning top; it's more like a tiny internal compass needle. We know that some particles (like hyperons) naturally pick up a spin direction when they are born from collisions, even if the people who bumped into them weren't spinning. This is a huge clue about how the universe works at a tiny level.

But for antiprotons, we have no idea. Do they just tumble out randomly, or do they also pick up a specific spin direction? This paper proposes a plan to find out.

The Problem: We Can't Just "Ask" Them

Currently, we don't have a machine that can take a bunch of random antiprotons and force them to spin in the same direction (polarize them). It's like trying to organize a crowd of people all facing North without giving them a compass or a command.

However, if the antiprotons naturally spin when they are created (like a coin landing on heads more often than tails), we could just catch the ones spinning the right way and use them for experiments. This would be a game-changer for physics.

The Plan: The "Left-Right" Test

The authors propose a clever experiment at CERN (the giant particle lab in Switzerland) to see if these antiprotons have a natural spin.

The Analogy: The Billiard Table
Imagine you have a pool table. You shoot a ball (the antiproton) at a stationary ball (a proton in a tank of liquid hydrogen).

• If the incoming ball has no spin, it will bounce off to the left or the right with equal probability. It's a 50/50 coin flip.
• If the incoming ball has a spin, the laws of physics say it will be slightly more likely to bounce to the left than the right (or vice versa).

The experiment is essentially setting up a super-sensitive pool table to count how many balls bounce left versus right. If there is a difference, even a tiny one, it proves the antiprotons were spinning when they were created.

The Setup: The "Spin Detector"

To do this, the team designed a specific machine (detailed in the paper) that looks like a high-tech tunnel:

1. The Gun: They shoot a beam of protons at a target to create antiprotons.
2. The Filter: They use special detectors (like a "Cherenkov" camera) to make sure they are only looking at antiprotons and not confusing them with other particles (like pions).
3. The Target: The antiprotons fly through a tank of liquid hydrogen.
4. The Trap: Detectors surround the tank to catch the antiprotons as they bounce off the hydrogen. They measure exactly where the antiproton went.

The Prediction: Will It Work?

The team ran millions of computer simulations (like playing the game a million times in a video game) to see if their machine is good enough.

• The Result: They found that if antiprotons have a spin of about 7% to 12% (which is a decent amount in the particle world), their machine can detect it with high confidence.
• The Time: It would take about 8 weeks of running the experiment to gather enough data to be sure.

Why Should We Care?

If they find that antiprotons do spin naturally, it changes our understanding of the "Strong Force" (the glue that holds the universe together).

1. New Physics: It would prove that the rules of how matter and antimatter interact are more complex than we thought.
2. Better Tools: It would give scientists a "free" way to create polarized antiproton beams. Instead of building a massive, expensive machine to force them to spin, we could just catch the ones that spin naturally. This would open the door to new experiments that were previously impossible.

The Bottom Line

This paper is a blueprint. It says, "We think antiprotons might spin when they are born. We have built a virtual machine to test this, and it looks like we can catch them in the act. Let's build the real machine at CERN and find out!"

It's a proposal to solve a mystery that has been sitting in the dark for decades, using a simple trick: watching which way the particles bounce.

Technical Summary

1. Problem Statement

Despite extensive research into spin-dependent phenomena in inclusive hadron production, the microscopic origin of transverse polarization remains poorly understood, particularly for antiprotons.

• The Gap: While large transverse polarizations (20–30%) are well-documented for hyperons (e.g., $\Lambda$) produced in unpolarized proton collisions, it is unknown if antiprotons produced in similar unpolarized $p+A$ collisions acquire a transverse polarization.
• Theoretical Motivation: If antiprotons exhibit polarization, it would provide direct empirical access to spin-orbit components and spin-flip amplitudes of the antinucleon-nucleon ($\bar{p}N$) interaction. This is crucial because the short-range dynamics of $\bar{p}N$ interactions differ significantly from nucleon-nucleon ($NN$) interactions due to strong annihilation channels.
• Experimental Challenge: Currently, there is no efficient method to produce a polarized antiproton beam without significant infrastructure modifications. Existing methods (e.g., spin filtering, hyperon decay) have limitations in intensity or energy range. This proposal investigates whether the production process itself generates polarization, which could serve as a source for polarized beams.

2. Methodology

The authors propose a dedicated experiment (P371) at the CERN T11 beamline to measure the transverse polarization of antiprotons produced in proton-nucleus interactions.

Experimental Setup

• Beam: A primary proton beam (24 GeV/c) strikes a target, producing secondary antiprotons. The experiment focuses on a secondary antiproton beam with a nominal momentum of 3.5 GeV/c.
• Detection System: The setup includes:
  • Trigger/Start: Scintillator plates.
  • Particle Identification (PID): A threshold Aerogel Cherenkov detector ($n \approx 1.03$) to veto pions (which have $\beta \approx 1$) while allowing antiprotons ($\beta \approx 0.966$) to pass. A DIRC detector is used for offline PID.
  • Tracking: Scintillating fiber hodoscopes and straw tube detectors to reconstruct tracks of primary and scattered particles with $\sim 100\text{--}150 \, \mu\text{m}$ resolution.
  • Analyzer Target: A 120 mm long, 60 mm diameter liquid hydrogen ($LH_2$) target to induce elastic $\bar{p}p$ scattering.
  • Stop/Trigger: Scintillator paddles behind the DIRC.

Measurement Principle

• Observable: The polarization ($P$) is determined via the left-right asymmetry ($\epsilon$) in elastic $\bar{p}p$ scattering within the Coulomb-Nuclear Interference (CNI) region.
• Analyzing Power ($A_y$): In the CNI region, the interference between electromagnetic and nuclear amplitudes creates a non-zero analyzing power.
  • Theoretical models (One-Boson Exchange) predict $A_y \approx 4.5\%$ at 3.5 GeV/c for specific momentum transfers ($|t| \approx 0.0025 \, (\text{GeV}/c)^2$).
  • The asymmetry is defined as: $\epsilon = \frac{N_L - N_R}{N_L + N_R} = A_y P \cos \phi$.
• Monte Carlo Simulation:
  • Simulations were performed using GEANT4.
  • Events were weighted to simulate elastic scattering based on experimental differential cross-section data ($\bar{p}p$ at 3.7 GeV/c, assumed similar to 3.5 GeV/c).
  • A scaling factor was applied to correct the simulation for the CNI region where standard models deviate from data.
  • A polarization-dependent weight ($\omega_p$) was applied to generate asymmetries for various assumed polarization degrees ($P$).

3. Key Contributions

• Feasibility Demonstration: The paper provides the first comprehensive feasibility study for measuring transverse antiproton polarization directly from production.
• Optimization of Kinematic Regions: The authors identified the optimal scattering angular range to maximize the figure of merit (FoM). They determined that a polar angle range of $\theta = 6.7$ to $35$ mrad yields the best sensitivity, where the analyzing power is significant ($|A_y| > 1.5\%$) and background is manageable.
• Systematic Uncertainty Analysis: The study outlines dominant systematic error sources, including:
  • Detector acceptance asymmetries (mitigated by symmetry checks and pion control channels).
  • Antiproton identification purity (estimated at ~95%, leading to <1% systematic error).
  • Uncertainties in the theoretical analyzing power ($A_y$).
  • Target/detector alignment.

4. Results

Based on the Monte Carlo simulations with an integrated luminosity of 1.18 nb$^{-1}$ (corresponding to approximately 8 weeks of data taking and a yield of $1.6 \times 10^6$ events):

• Statistical Sensitivity:
  • A polarization of 12% can be distinguished from zero with a 5$\sigma$ significance.
  • A polarization of 7% can be distinguished with 3$\sigma$ significance.
• Precision: The statistical uncertainty decreases as the number of events increases, following the expected $1/\sqrt{N}$ behavior.
• Angular Dependence: The analysis confirms that restricting the angular range to $6.7 < \theta < 35$ mrad maximizes the Figure of Merit (FoM), balancing the analyzing power magnitude against the available cross-section.

5. Significance

• Fundamental Physics: A non-zero measurement would provide the first direct evidence of spin-dependent dynamics in the baryon-antibaryon sector, constraining the spin structure of the $\bar{p}N$ interaction and testing non-perturbative QCD models (e.g., string fragmentation, recombination models).
• Beam Technology: If the production process generates sufficient polarization, it could serve as a primary source for polarized antiproton beams. This would eliminate the need for complex, low-efficiency spin-filtering or spin-flipping techniques, enabling a new class of experiments (e.g., Drell-Yan processes with polarized beams/targets to study transversity distributions).
• Future Outlook: The results validate the proposal to proceed with the P371 experiment at CERN, establishing a concrete experimental basis for exploring antiproton spin phenomena at intermediate energies.