Neutrino-argon cross-section measurements from the MicroBooNE experiment

This proceeding presents an overview of recent MicroBooNE results on neutrino-argon cross-section measurements across inclusive, CC0$\pi$, and rare channels, utilizing advanced reconstruction tools to benchmark interaction models and constrain backgrounds for future liquid argon neutrino experiments like DUNE.

The Gist

Imagine the universe is filled with invisible, ghostly messengers called neutrinos. These particles zip through everything—stars, planets, and even you—without leaving a trace. They are so shy that they rarely bump into anything. But when they do crash into an atom, they leave behind a tiny, chaotic fingerprint.

The MicroBooNE experiment is like a giant, high-tech "crime scene" detector built deep underground at Fermilab in Illinois. Instead of a room full of cameras, it's a massive tank filled with 85 tons of liquid argon (which is like super-cold, frozen air turned into a liquid).

Here is what the MicroBooNE team did, explained simply:

1. The "Ghost" Hunt

The scientists fired beams of these neutrino ghosts at their tank of liquid argon. When a neutrino hit an argon atom, it caused a tiny explosion of energy and particles. Because the argon is liquid and electrically charged, this explosion creates a trail of electrons that the detector catches, turning the invisible crash into a 3D picture on a computer screen.

2. Why Do This? (The Puzzle)

Scientists want to understand why the universe is made of matter (us) instead of antimatter (the opposite). To do this, they need to measure how neutrinos change their "identity" (oscillate) as they travel.

However, there's a problem: We don't know exactly how fast the neutrinos are moving.
Think of it like trying to guess the speed of a car by only looking at the skid marks it leaves after crashing. If you don't know how the car's brakes work (the physics of the crash), you can't guess the speed accurately.

For decades, scientists had to guess how neutrinos crash into atoms (specifically argon atoms). The MicroBooNE team decided to stop guessing. They wanted to measure the crash itself with extreme precision.

3. The "Crash Report"

The paper presents a massive report card of these crashes. They didn't just look at the big, obvious crashes; they looked at everything:

• The Common Crashes (Inclusive & CC0π): They measured the most frequent types of collisions. It's like counting every car accident on a highway, not just the ones that total the car. They found that the "brakes" (theoretical models) scientists were using before were a bit off. MicroBooNE provided the real data to fix the math.
• The Rare "Alien" Crashes: Some crashes are incredibly rare. The team found evidence of neutrinos creating strange particles like Lambda (Λ) and K-plus (K+) particles.
  • Analogy: Imagine firing a ping-pong ball at a bowling ball and, instead of just bouncing off, the bowling ball suddenly sprouts a tiny, exotic flower. That's how rare and surprising these events are. The paper says they found these "flowers" with a precision never seen before.
• The "Eta" (η) Meson: They also spotted a particle called an eta meson. This is like finding a specific, rare type of spark in the crash. This helps scientists understand how heavy particles inside the atom behave.

4. The "Direction Finder"

One of the hardest things to figure out is: Where did the neutrino come from?
The team tested a new way to guess the direction. They looked at the "kick" given to a single proton and the muon (a heavy electron) after the crash.

• Analogy: If you throw a ball at a stationary object and it bounces off, you can guess where you threw it by looking at the angle of the bounce. MicroBooNE found that by looking at just the proton and the muon, they could guess the neutrino's direction with amazing accuracy (usually within 5 degrees). This is crucial for future experiments that need to know exactly where the neutrinos are coming from.

5. Why It Matters for the Future

The paper concludes that these measurements are the "instruction manual" for the next generation of giant neutrino experiments, like DUNE (Deep Underground Neutrino Experiment).

Before, scientists were driving a car with a blurry map. MicroBooNE has now provided a high-definition GPS. By understanding exactly how neutrinos crash into argon, future experiments can:

• Measure the speed of neutrinos more accurately.
• Solve the mystery of why the universe exists.
• Look for "sterile" neutrinos (ghosts that are even shyer than the ones we know).

In short: MicroBooNE took a giant tank of liquid argon, waited for invisible ghosts to crash into it, and took thousands of high-definition photos of the wreckage. These photos are teaching scientists exactly how the crash happens, which is the key to unlocking the biggest secrets of the universe.

Technical Summary

Technical Summary: Neutrino-argon cross-section measurements from the MicroBooNE experiment

Problem Statement
The primary scientific objectives of neutrino oscillation experiments include searching for charge–parity (CP) symmetry violation, determining the neutrino mass hierarchy, and exploring the existence of sterile neutrinos. A critical bottleneck in achieving the precision required for these goals is the accurate determination of the initial-state neutrino energy. Because neutrinos interact weakly, their energies must be inferred indirectly from detected final-state particles. Consequently, uncertainties in neutrino energy measurements stem not only from detector response but significantly from the modeling of neutrino–nucleus cross-sections. The complex quantum many-body nature of nuclear structure makes calculating these cross-sections from first principles extremely difficult. Current semi-classical theoretical models, implemented in event generators, often carry leading systematic uncertainties for experiments using nuclear targets at accelerator energies. Furthermore, understanding rare processes, such as neutrino-induced strange particle and $\eta$-meson production, is essential for Beyond the Standard Model (BSM) searches. There is a pressing need for high-sensitivity experimental data on neutrino-argon interactions to constrain these theoretical models and improve background estimation for future experiments like DUNE.

Methodology
The MicroBooNE experiment utilizes a Liquid Argon Time Projection Chamber (LArTPC) located at Fermi National Accelerator Laboratory. The detector features an active volume of 85 tonnes of liquid argon with a uniform 273 V/cm drift field and a three-plane anode readout assembly. It is exposed to neutrino fluxes from two Fermilab beamlines: the Booster Neutrino Beam (BNB) and the Neutrinos at the Main Injector (NuMI).

MicroBooNE has developed pioneering reconstruction tools to analyze a large dataset containing hundreds of thousands of events. The analysis strategy involves:

• Flux-Integrated Measurements: Utilizing the full exposure (e.g., $1.3 \times 10^{21}$ POT for BNB) to measure total and differential cross-sections.
• Event Topology Selection: Categorizing events based on final-state particles, such as inclusive charged-current (CC) interactions, zero-pion (CC0$\pi$) channels, and specific proton multiplicities (e.g., "0p" and "Np" with kinetic energy $>35$ MeV).
• Unfolding Techniques: Deriving unfolded differential cross-sections to correct for detector effects and resolution, presenting results in regularized truth space.
• Model Comparison: Comparing unfolded data against various theoretical models and event generators (e.g., GiBUU, NEUT, GENIE) using $\chi^2$ and $p$-values to assess agreement.
• Rare Channel Identification: Employing high-sensitivity reconstruction to identify ultra-rare channels, including $K^+$, $\Lambda$, and $\eta$ production, often at cross-section levels of $\mathcal{O}(10^{-41})$ cm$^2$/nucleon.

Key Contributions and Results
The paper presents a broad program of neutrino-argon cross-section measurements spanning inclusive, semi-inclusive, and rare channels:

1. Inclusive $\nu_\mu$ Charged-Current (CC): MicroBooNE reports the first double-differential cross-section measurements for inclusive $\nu_\mu$ CC interactions with respect to hadronic final states containing zero or multiple reconstructed protons.

   • Results show that while most models struggle to describe the data simultaneously across all kinematic regions, the GiBUU model provides the best description of the "0p" bin data (events with no reconstructed protons above 35 MeV), despite underestimating high-energy contributions.
   • Measurements include the kinetic energy of the leading proton and proton multiplicity.

2. Charged-Current Zero-Pion (CC0$\pi$): A high-statistics measurement of the flux-integrated double-differential cross-section with respect to $\cos \theta_\mu$ and $p_\mu$ was performed.

   • The GiBUU and NEUT models show agreement with the correlated joint distribution ($p$-value $> 5\%$), while other models yield poorer descriptions.

3. Neutrino Angle Reconstruction: Using the CC1$p$0$\pi$ channel (single muon-proton pair), the experiment investigated the precision of reconstructing the incoming neutrino direction.

   • The vector sum of the muon and proton momenta ($\vec{b} = \vec{p}_\mu + \vec{p}_p$) serves as an estimator for the neutrino direction.
   • For the majority of events, the resolution of this estimator is better than $5^\circ$, with no major biases observed. Current simulations are found to model the bias in the inferred direction well for energies similar to atmospheric neutrinos.

4. Pion Production:

   • $\nu_e$ CC1$\pi^\pm$: The first measurement of electron neutrino charged pion production on an argon target yielded a total cross-section of $(0.93 \pm 0.13(\text{stat}) \pm 0.27(\text{syst})) \times 10^{-39}$ cm$^2$/nucleon. The differential cross-sections are consistent with generator predictions, though sensitivity is currently limited by flux modeling systematics.
   • NC$\pi^0$: The first double-differential cross-section measurement for neutral-current neutral pion production was reported. Data indicates that the 0.2–0.5 GeV/$c$ momentum range is strongly impacted by final-state interactions (FSIs), suggesting a need for refined FSI modeling.

5. Rare Processes:

   • Strange Particle Production: Cross-sections for $\bar{\nu}_\mu$-induced $\Lambda$ production and $\nu_\mu$-induced $K^+$ production were measured. These are poorly constrained by existing data and are sensitive to nuclear effects and nucleon form factors.
   • $\eta$ Meson Production: A first measurement of $\eta$ meson production on argon yielded a cross-section of $3.22 \pm 0.84(\text{stat}) \pm 0.86(\text{syst}) \times 10^{-41}$ cm$^2$/nucleon. This process probes higher-order resonances ($N(1535)$, $N(1650)$, $N(1710)$) and offers a novel tool for electromagnetic energy scale calibration in LArTPCs.

Significance
The paper asserts that these measurements provide invaluable datasets for constraining backgrounds and improving the modeling of neutrino scattering. By benchmarking models at very high sensitivity across the interaction phase space—including ultra-rare channels—MicroBooNE addresses the leading source of systematic errors in neutrino oscillation experiments. These results are claimed to be crucial inputs for the physics research of the future Short-Baseline Neutrino (SBN) program and the Deep Underground Neutrino Experiment (DUNE), enabling the precision necessary to achieve their ambitious goals regarding CP violation, mass hierarchy, and sterile neutrino searches. Additionally, the ability to observe rare decays like $\eta$ and strange particles in a LArTPC provides new tools for calibration and background rejection in proton decay searches.