First Measurement of Sub-GeV $\nu_{\mu}$ Charged-Current Coherent Pion Production on Argon in MicroBooNE

MicroBooNE reports the first measurement of the charged-current coherent pion production cross section on argon at sub-GeV neutrino energies, yielding a flux-averaged value of $(9.1 \pm 1.2_{\text{stat}} \pm 1.2_\text{syst}) \times 10^{-40}\,\text{cm}^2/\text{Ar}$ that offers a valuable tool for constraining neutrino flux uncertainties in future oscillation experiments like DUNE.

The Gist

The Big Picture: Catching a Ghost in a Jar

Imagine you are trying to understand how a specific type of invisible ghost (a neutrino) interacts with a giant, solid block of ice (an argon atom). Neutrinos are notoriously difficult to catch; they usually pass right through matter without leaving a trace.

The scientists in this paper used a massive detector called MicroBooNE, which is essentially a giant, ultra-sensitive camera filled with liquid argon. They waited for a beam of neutrinos to shoot through it. Their goal was to catch a very specific, rare event: a neutrino hitting an argon atom and gently knocking out a "particle pair" (a muon and a pion) without breaking the argon atom apart.

The Special Event: The "Coherent" Dance

Usually, when a neutrino hits an atom, it's like a billiard ball hitting a rack of balls—it smashes them apart, sending pieces flying everywhere. This is messy and hard to study.

However, this paper focuses on Coherent Pion Production.

• The Analogy: Imagine the argon nucleus is a tightly packed group of dancers holding hands.
• The "Messy" Hit: If a neutrino hits just one dancer, the whole group might scatter, and the formation breaks.
• The "Coherent" Hit: In this rare event, the neutrino hits the entire group at once. The group doesn't break apart; they stay together (the nucleus remains intact). Instead, the whole group gently sways forward and releases two specific dancers (a muon and a pion) who fly off together in a straight line.

Because the nucleus stays intact, the two released particles fly in a very straight, predictable path. This makes them easy to spot, like seeing two skaters glide perfectly in sync while the crowd behind them stays still.

Why This Matters: The "Standard Candle"

The paper explains that scientists need to know exactly how many neutrinos are in their beam to measure other things accurately (like how neutrinos change "flavors" as they travel).

• The Problem: It's hard to count the neutrinos directly because they are invisible.
• The Solution: This specific "Coherent Dance" is so predictable that if you know the rules of the dance (the physics), you can count how many times it happens to figure out how many neutrinos were in the beam.
• The Paper's Claim: This is the first time anyone has measured this specific dance on an argon target at low energies (sub-GeV). Before this, scientists had to guess the rules based on models. Now, they have actual data.

How They Did It: Finding the Needle in the Haystack

The detector collected data from over a billion billion protons hitting a target.

1. The Filter: They looked for events where exactly two tracks (the muon and pion) came out of a single point, moving in almost the same direction, with no other debris.
2. The Background Noise: Most of the time, neutrinos cause messy collisions (like the billiard ball breaking the rack). These look similar but have particles flying off at weird angles.
3. The Trick: The scientists used a clever statistical method. They knew that the "Coherent Dance" particles fly very straight (forward), while the "Messy Collisions" scatter more widely. By looking at the angle of the particles, they could mathematically separate the clean signal from the noisy background, even without knowing the exact number of neutrinos beforehand.

The Results: Checking the Rulebook

After analyzing the data, they calculated the "cross-section" (a fancy word for the probability of this specific event happening).

• The Measurement: They found the probability to be 9.1 (in specific scientific units).
• The Comparison: They compared this real-world number against three different computer "rulebooks" (models) that scientists use to predict physics:
  • Rulebook A (NEUT) and Rulebook B (GENIE RS): These predicted a number very close to 9.1. The paper says, "Great, these models are correct!"
  • Rulebook C (GENIE BS) and Rulebook D (NuWro): These predicted numbers that were quite different (too low or too high). The paper says, "These models need to be updated."

The Bottom Line

This paper is a milestone because it provides the first real-world measurement of this specific neutrino interaction on argon at low energies. It proves that some of the computer models scientists use to design future experiments (like the DUNE experiment) are accurate, while others need fixing.

By understanding this "Coherent Dance" better, scientists can use it as a reliable tool to measure neutrino beams more precisely in the future, ensuring that their experiments on the nature of the universe are built on solid ground.

Technical Summary

Technical Summary: First Measurement of Sub-GeV $\nu_\mu$ Charged-Current Coherent Pion Production on Argon in MicroBooNE

Problem Statement
Precise knowledge of the neutrino flux is a prerequisite for the next generation of long-baseline oscillation experiments, such as the Deep Underground Neutrino Experiment (DUNE), which aim to measure oscillation parameters with unprecedented precision. Achieving this requires percent-level control over flux-related systematic uncertainties. Charged-current coherent pion production ($\nu_\mu$CC$\pi^+_c$) offers a potential handle for constraining the $\nu_\mu$ component of the neutrino flux in situ. This process is characterized by simple two-body final-state kinematics where the neutrino interacts coherently with the entire nucleus, leaving it intact while producing a forward-going muon and pion. However, the viability of this channel as a flux constraint depends on an accurate understanding of its cross section. While theoretical models (rooted in the Partially Conserved Axial Current hypothesis and the Berger-Sehgal formalism) exist, current event generator predictions exhibit significant differences, particularly when extended to lower neutrino energies where lepton-mass effects are non-negligible. Prior to this work, no measurements of $\nu_\mu$CC$\pi^+_c$ production on argon existed at sub-GeV energies, with previous measurements limited to multi-GeV regimes or other target materials.

Methodology
The measurement utilizes data from the MicroBooNE liquid argon time projection chamber (LArTPC) exposed to the Fermilab Booster Neutrino Beam (BNB). The dataset corresponds to an exposure of $1.26 \times 10^{21}$ protons on target (POT) collected between 2015 and 2020, with a mean neutrino energy of 0.8 GeV.

• Signal Definition: The signal is defined as interactions on argon producing exactly one muon and one charged pion, with the nucleus remaining in its ground state and no additional mesons or nucleons produced.
• Event Selection: To isolate the rare coherent signal (predicted to be $\sim$0.15% of interactions), the analysis applies strict kinematic and topological cuts:
  • Reconstructed vertex within the fiducial volume.
  • Two well-reconstructed tracks (muon and pion) originating within 10 cm of the vertex.
  • Tracks must be contained in the transverse ($x, y$) directions but are not required to be contained in the downstream ($z$) direction to maximize efficiency.
  • Particle identification (PID) via calorimetric $dE/dx$ and log-likelihood ratios to reject protons.
  • Track lengths $>20$ cm to ensure reliable momentum reconstruction via Multiple Coulomb Scattering (MCS).
  • Angular cuts: An opening angle $\theta_{\mu\pi} < 55^\circ$ and a cone angle $\theta_{cone}$ (angle between the beam and the $\mu+\pi$ system) $< 16^\circ$ to select the forward-peaked coherent topology.
• Signal Extraction: A data-driven fitting strategy is employed using the distribution of $1 - \cos\theta_{cone}$. The signal and background templates are modeled as exponential functions with shape parameters fixed from simulation (GENIE v3.0.6). The normalization of the signal ($S$) and background ($B$) components are floated in a simultaneous fit to the data. This approach minimizes reliance on simulated cross-section rates, leveraging the distinct angular shapes of coherent (sharp peak) versus non-coherent (smoothly falling) processes.
• Systematics: Uncertainties are evaluated by propagating variations in detector response (light yield, recombination, space-charge), neutrino flux, hadronic reinteraction modeling, and cross-section modeling through the fitting procedure.

Key Contributions

• First Measurement: This work presents the first measurement of the flux-averaged $\nu_\mu$CC$\pi^+_c$ production cross section on an argon target at sub-GeV neutrino energies.
• Data-Driven Extraction: The analysis successfully isolates the coherent signal from dominant resonant backgrounds using the unique kinematic properties of the LArTPC, achieving a signal purity of 32% with 14.5% efficiency in the signal-enhanced region.
• Model Validation: The extracted cross section is compared against predictions from GENIE (using both Berger-Sehgal and Rein-Sehgal formalisms), NEUT, and NuWro event generators.

Results
The measured flux-averaged cross section is:
$$\sigma_{CC\,Coh} = (9.1 \pm 1.2_{stat} \pm 1.2_{syst}) \times 10^{-40} \, \text{cm}^2/\text{Ar}$$

The total systematic uncertainty is 13.1%, dominated by neutrino flux prediction (8.1%), cross-section modeling (7.5%), and detector effects (6.6%).

Comparison with model predictions reveals:

• The measurement is in good agreement with the NEUT 6.1.4 and GENIE 3.6.2 (Rein-Sehgal) predictions.
• The measurement shows tension with the GENIE 3.0.6 (Berger-Sehgal) and NuWro 25.11 predictions, which underestimate the cross section.
• The spread in predictions across generators highlights the need for precise experimental constraints to resolve differences in the implementation of coherent pion production models.

Significance
This result establishes a benchmark for neutrino-nucleus interaction models at sub-GeV energies. By providing the first precise constraint on $\nu_\mu$CC$\pi^+_c$ production on argon, the measurement supports the use of this channel as a "standard candle" for constraining neutrino flux uncertainties in current and future oscillation experiments, including DUNE. The paper notes that future measurements from SBND and ICARUS, utilizing the same beam at different baselines, will enable further multi-detector constraints on coherent pion modeling.