Axial-vector neutral-current measurements in coherent elastic neutrino-nucleus scattering experiments

This paper identifies fluorine-based compounds, particularly octafluoropropane, as optimal targets for measuring subleading axial-vector contributions in coherent elastic neutrino-nucleus scattering, demonstrating that such experiments could determine the axial coupling at the $\sim 10\%$ level and probe spin-dependent new physics.

The Gist

Imagine a giant, invisible dance floor where tiny particles called neutrinos are constantly bumping into heavy nuclei (the cores of atoms). For decades, scientists have been watching this dance, but they've only been paying attention to the main dancers: the Vector interactions. These are the loud, energetic moves that happen when any nucleus gets hit, regardless of its internal structure. Because these moves are so strong and common, they drown out everything else.

But there's a quieter, more subtle dancer on the floor: the Axial interaction. This move is special because it only happens if the nucleus is "spinning" (has a non-zero ground-state spin), kind of like a spinning top.

The Problem: The Quiet Dancer is Hard to Hear

Until now, scientists have mostly used heavy, slow-moving nuclei (like Xenon or Germanium) as their dance floors. In these heavy atoms, the "Vector" dance is so overwhelmingly loud that the "Axial" spin-dance is completely drowned out. It's like trying to hear a whisper in the middle of a rock concert. Even if you build a massive detector (a huge dance floor), the whisper remains inaudible because the heavy atoms just don't spin fast enough to make the Axial move noticeable.

The Solution: Find the Right Partner

The authors of this paper asked a simple question: "What if we change the dance partner?"

Instead of using heavy, sluggish atoms, they looked for lighter atoms that spin naturally and vigorously. They found the perfect candidate: Fluorine.

Think of Fluorine-based compounds (specifically a gas called Octafluoropropane, or $C_3F_8$) as the "spin artists" of the atomic world. They are light, they spin easily, and they are already being used by dark matter hunters (who are looking for invisible particles) in bubble-chamber experiments.

The Experiment: Tuning the Microphone

The researchers simulated a new experiment using these Fluorine targets. They imagined two different sources of neutrinos:

1. Spallation Sources: Like a high-energy particle accelerator (the ESS in Europe) that shoots protons to create neutrinos.
2. Nuclear Reactors: The steady, lower-energy neutrinos coming from power plants.

They found that by switching to Fluorine:

• The "whisper" of the Axial interaction became much louder relative to the "rock concert" of the Vector interaction.
• In heavy atoms, the Axial signal was suppressed by a factor of 100 or more. In Fluorine, the suppression dropped to about 10 or 25. It's still quieter than the Vector signal, but now it's loud enough to be heard with a good microphone (a sensitive detector).

The Payoff: Why Do We Care?

Why go through all this trouble to hear a whisper?

1. Measuring the "Spin" of the Universe: By isolating this Axial signal, scientists can measure a fundamental property of nature called the axial-vector coupling ($g_A$). Currently, we measure this using cold neutrons, but doing it with neutrinos in a new way would be a powerful cross-check, like verifying a math problem with two different methods.
2. Finding New Physics: If there are hidden, "spin-dependent" new forces or particles (New Physics) lurking in the universe, they might only talk to the spinning nuclei. If we only listen to the heavy, non-spinning atoms, we might miss these new secrets entirely. The Fluorine detector acts like a specialized radio tuned to a frequency only new physics can broadcast.

The Bottom Line

This paper is a proposal to stop shouting over the quiet dancer and instead invite the spin artist to the center stage. By using Fluorine-based detectors (specifically $C_3F_8$) and carefully tuning the sensitivity of the equipment, scientists believe they can measure these elusive Axial interactions with about 10% precision.

It's a shift from looking at the "big picture" (Vector interactions) to zooming in on the "fine details" (Axial interactions), potentially unlocking new secrets about how the universe works and what lies beyond our current understanding of physics.

Technical Summary

Here is a detailed technical summary of the paper "Axial-vector neutral-current measurements in coherent elastic neutrino-nucleus scattering experiments."

1. Problem Statement

Coherent Elastic Neutrino-Nucleus Scattering (CE$\nu$NS) is a process dominated by vector neutral-current interactions, which benefit from a coherent enhancement proportional to the square of the number of neutrons ($N^2$). While the vector component has been successfully measured by collaborations like COHERENT and dark matter (DM) experiments (XENONnT, LZ, PandaX), the axial-vector neutral-current contribution has remained experimentally unobserved.

The core problem is that the axial contribution depends on the nuclear spin of the target nucleus. In heavy nuclei (e.g., Cs, I, Xe, Ge) commonly used in current experiments, the axial contribution is strongly suppressed relative to the vector component due to:

1. Spin Suppression: Heavy nuclei often have zero ground-state spin or small net spin expectation values.
2. Coherence Dominance: The vector term scales as $N^2$, while the axial term scales with the spin expectation values of protons ($S_p$) and neutrons ($S_n$), making the axial signal negligible in standard heavy-target setups.

Consequently, extracting the axial coupling ($g_A$) or probing spin-dependent new physics has been considered infeasible with current target materials. The paper addresses the need to identify target materials and experimental configurations where the axial contribution is large enough to be statistically significant.

2. Methodology

The authors employ a phenomenological analysis combining nuclear physics theory with experimental forecasting:

• Theoretical Framework:

  • They utilize the differential cross-section formula separating vector and axial terms.
  • Unlike the vector form factor (often modeled phenomenologically via Helm or Klein-Nystrand models), the axial form factor is calculated using ab initio methods based on chiral effective field theory. This involves shell-model calculations to determine nuclear spin structure functions ($S_{ij}$) and multipole decompositions.
  • The analysis assumes isospin symmetry and focuses on leading-order spin-dependent one-body currents, neglecting sub-leading two-body currents as their impact is expected to be smaller than experimental uncertainties.

• Target Material Screening:

  • The authors evaluate event rates for various target materials, comparing "heavy" targets (CsI, NaI, Xe, Ge) against "medium-light" nuclei with non-zero ground-state spin (e.g., $^3$He, $^{19}$F, $^{29}$Si, $^{27}$Al, Deuterium).
  • They calculate the ratio of axial-to-vector event rates to identify the most promising candidates.

• Experimental Configuration:

  • Neutrino Sources: Two fluxes are considered:
    1. Spallation Neutron Sources (ESS-like): Pion decay-at-rest ($\pi^+$) producing $\nu_\mu, \bar{\nu}_\mu, \nu_e$.
    2. Nuclear Reactors: Electron antineutrinos ($\bar{\nu}_e$) from fission products.
  • Detector Parameters: The study simulates detectors with varying fiducial masses (50 kg to 350 kg) and recoil energy thresholds (2 keV and 3 keV).
  • Statistical Analysis: A $\chi^2$ function is constructed to project the sensitivity to the axial-vector coupling constant ($g_A$), varying $g_A$ while keeping the strange axial coupling ($g_A^s$) fixed. Systematic uncertainties include neutrino flux normalization ($\sigma_{flux}$).

3. Key Contributions

• Identification of Optimal Targets: The paper definitively identifies fluorine-based compounds as the superior targets for axial-current measurements. Specifically, Octafluoropropane ($C_3F_8$) is highlighted as the most promising candidate due to the large spin expectation value of the $^{19}$F nucleus and its established use in dark matter searches (PICO collaboration).
• Quantification of Suppression: The authors demonstrate that in heavy targets (CsI, Xe), the axial contribution is suppressed by orders of magnitude (e.g., ~5 axial events vs. ~39,000 vector events in CsI for a 50 kg detector). In contrast, for $C_3F_8$, the suppression is significantly reduced (factor of ~25 for spallation sources and ~10 for reactor neutrinos).
• Flux Dependence: The study reveals that reactor neutrinos provide a relatively enhanced axial contribution compared to spallation sources. This is attributed to the lower energy of reactor neutrinos, where the momentum-transfer dependence of the nuclear spin structure functions is less suppressive than at the higher energies typical of spallation sources.
• Feasibility of Precision Measurement: The paper provides a roadmap for measuring $g_A$ with $\sim10\%$ precision, a feat previously thought impossible in CE$\nu$NS.

4. Results

• Event Rates:
  • Heavy Targets: Even with tonne-scale detectors, the axial signal remains overwhelmed by the vector background. For example, in a 50 kg CsI detector, the axial yield is negligible (5 events) compared to the vector yield (39,176 events).
  • Fluorine Targets ($C_3F_8$): For a 50 kg $C_3F_8$ detector exposed to a spallation source for 3 years, the axial contribution becomes statistically accessible. The ratio of axial to total events is significantly higher than in heavy targets.
• Sensitivity to $g_A$:
  • Using a benchmark configuration of a 350 kg $C_3F_8$ detector with a 2 keV recoil threshold and 1% flux uncertainty, the authors project a measurement of the axial-vector coupling $g_A$ with $\sim10\%$ precision.
  • Threshold Sensitivity: Lowering the recoil threshold from 3 keV to 2 keV significantly improves sensitivity, particularly when flux uncertainties are high.
  • Mass Scaling: Increasing the fiducial mass from 50 kg to 350 kg yields a more substantial improvement in precision than reducing flux uncertainty alone.
• New Physics Probes: The analysis confirms that measuring the axial component allows for the direct probing of spin-dependent new physics, which would otherwise be hidden by the dominant vector interactions in standard analyses.

5. Significance

• Completing the Standard Model Picture: Measuring the axial component in CE$\nu$NS provides an independent determination of the axial-vector coupling ($g_A$), complementing measurements from cold neutron decay experiments. It also offers potential sensitivity to the strange quark contribution ($g_A^s$).
• Bridging Neutrino and Dark Matter Physics: The paper leverages the infrastructure and expertise of the Dark Matter community (specifically the PICO collaboration's experience with $C_3F_8$ bubble chambers) to advance neutrino physics. It validates the use of DM detectors for precision neutrino measurements.
• Future Experimental Roadmap: The study provides concrete justification for upcoming experiments at the European Spallation Source (ESS) and J-PARC to utilize fluorine-based targets. It argues that moving away from heavy noble gases (Xe, Ar) to fluorine compounds is essential for the next generation of precision CE$\nu$NS physics.
• New Physics Window: By isolating the axial current, these measurements open a unique window to detect spin-dependent interactions from Beyond Standard Model (BSM) physics, which are currently inaccessible in vector-dominated analyses.

In conclusion, this work shifts the paradigm of CE$\nu$NS experiments from purely vector-dominated measurements to a dual-vector/axial approach, identifying $C_3F_8$ as the critical material to unlock the subleading axial physics sector.