NOvA's Current and Future Sterile Neutrino Searches

This paper outlines NOvA's current search strategy for eV-scale sterile neutrinos using NuMI beam data and details future plans to incorporate Booster Neutrino Beam data to enhance sensitivity in the systematics-limited high $\Delta m^2_{41}$ parameter space favored by experimental anomalies.

The Gist

Imagine the universe is filled with a ghostly swarm of particles called neutrinos. These particles are like invisible ninjas; they zip through planets, stars, and even your body without ever bumping into anything. Scientists have known for a while that these ninjas can change their "costumes" (flavors) as they travel, a phenomenon called oscillation.

However, there's a nagging mystery in the physics world. Some experiments have seen weird glitches that suggest there might be a fourth type of neutrino hiding in the shadows. This hypothetical particle is called a sterile neutrino. It's called "sterile" because, unlike the other three types, it doesn't interact with anything at all—not even the weak force that usually governs these particles. It's the ultimate ghost, detectable only by the fact that it makes the other neutrinos disappear.

This paper is a report from the NOvA experiment, a massive scientific detective agency in the United States, on how they are hunting for this invisible fourth ninja.

The Detective Agency: NOvA

NOvA has two main "cameras" (detectors):

1. The Near Camera (ND): Located right next to the particle factory at Fermilab, Illinois. It sees the neutrinos fresh out of the oven.
2. The Far Camera (FD): Located 500 miles away in Minnesota. It catches the neutrinos after they've had a long journey.

By comparing what the Near Camera sees with what the Far Camera sees, scientists can spot if any neutrinos have vanished or changed into something else during the trip.

The Problem: The "Blind Spot"

For a long time, NOvA has been very good at finding these ghosts, but they hit a wall.

• The Wall: When the "ghost" neutrinos are very heavy (specifically, when their mass difference is high), they oscillate (change) so incredibly fast that by the time they reach the Far Camera, the signal is all jumbled up and invisible.
• The Analogy: Imagine trying to watch a hummingbird's wings. If you stand far away, the wings move so fast they just look like a blur. You can't tell if the wings are actually flapping or if the bird is just hovering. NOvA's current setup is like standing too far away to see the rapid flapping of these heavy sterile neutrinos.

The New Strategy: A Second Beamline

To solve this, NOvA is getting creative. They are turning on a second particle beam called the Booster Neutrino Beam (BNB).

Think of the main beam (NuMI) as a highway with fast cars (high-energy neutrinos). The new beam (BNB) is like a local street with slower, smaller cars (lower-energy neutrinos).

Here is why this helps:

1. Different Speeds, Same Distance: The new beam shoots neutrinos at a different angle and energy. Because the neutrinos are moving slower, the "blur" of their rapid oscillations happens closer to the source.
2. The "Off-Axis" Trick: The NOvA Near Detector is positioned slightly to the side of this new beam. This is like looking at a spotlight from an angle; it filters out the bright glare and lets you see the specific colors you are interested in.
3. Cross-Checking: By comparing the "highway" data with the "local street" data, scientists can tell the difference between a real ghost (sterile neutrino) and a glitch in their equipment (systematic errors). It's like checking a suspect's alibi with two different witnesses who saw them at different times and places.

The Results So Far

The paper presents a "test run" of this new idea. They simulated what would happen if they used this new beam data.

• The Good News: Adding this new data source is like giving the detectives a 30% better magnifying glass. It allows them to see clearly into the "heavy mass" region where they were previously blind.
• The Challenge: The new beam produces fewer neutrinos than the main highway, so the data is a bit "noisy" (statistically limited). Also, the neutrinos are lower energy, which makes them harder to catch with their current software.
• The Fix: They are currently training their computer algorithms (AI) to get better at spotting these low-energy, fast-moving neutrinos, much like teaching a dog to hunt a specific type of small rabbit instead of a big deer.

The Big Picture

The global picture of neutrinos is currently a bit messy. Some experiments say "Yes, there's a fourth ghost!" while others say "No, we don't see it." This tension is frustrating but exciting.

NOvA's goal is to be the tie-breaker. By combining their massive existing data with this new, cleverly angled beam from the Booster, they hope to finally confirm if these sterile neutrinos exist or if the "ghosts" are just illusions. If they find them, it would be a massive discovery, proving that our understanding of the universe is incomplete and that there are invisible particles shaping the cosmos in ways we never imagined.

In short: NOvA is upgrading its search party. They are adding a new, specialized flashlight to look for a ghost that moves too fast to be seen with their old gear. If they find it, it changes everything we know about the universe.

Technical Summary

1. Problem Statement

The existence of eV-scale sterile neutrinos remains one of the most significant open questions in neutrino physics. While several experiments (e.g., MiniBooNE, LSND) have reported anomalies consistent with a "3+1" oscillation model (three active neutrinos + one sterile neutrino), other experiments (e.g., IceCube, MINOS) have found no evidence, creating significant tension in global data.

The NOvA experiment has historically searched for sterile neutrinos by looking for the disappearance of Neutral Current (NC) interactions at its Far Detector (FD). However, this approach has limitations:

• Systematic Limitations: At high mass-squared splittings ($\Delta m^2_{41} \gtrsim 1 \text{ eV}^2$), oscillations occur rapidly between the beam target and the Near Detector (ND). In this regime, the FD sensitivity is lost because oscillations are averaged out, leaving the analysis dominated by systematic uncertainties in the ND.
• Energy Reconstruction Bias: NC events involve outgoing neutrinos that do not deposit energy, making energy reconstruction difficult and smeared, which reduces sensitivity.
• Parameter Space Gaps: Current anomalies suggest $\Delta m^2_{41}$ values in the range of $1 \text{ eV}^2$ to $100 \text{ eV}^2$, a region where NOvA's previous single-detector analyses were less sensitive.

2. Methodology

The paper outlines a two-pronged strategy to address these limitations:

A. Current Analysis Strategy (3+1 Model)

• Simultaneous Fit: The analysis simultaneously fits four datasets: Near Detector (ND) $\nu_\mu$ Charged Current (CC), ND NC, Far Detector (FD) $\nu_\mu$ CC, and FD NC.
• Oscillation Channels:
  • NC Disappearance: Searches for a deficit in total NC events (sterile neutrinos do not interact weakly).
  • $\nu_\mu$ CC Disappearance: Uses the survival probability of muon neutrinos, which includes terms for standard three-flavor oscillations and additional sterile-induced disappearance.
• Parameter Space: The analysis covers a broad range of $10^{-3} \text{ eV}^2 < \Delta m^2_{41} < 100 \text{ eV}^2$.

B. Future Strategy: Booster Neutrino Beam (BNB) Integration

To overcome systematic limitations in the high-$\Delta m^2_{41}$ region, NOvA is incorporating data from the Booster Neutrino Beam (BNB):

• Geometry: The NOvA ND is located $\sim770$ m from the BNB target at an off-axis angle of $9.2^\circ$ (160 mrad).
• Flux Characteristics: The BNB flux is dual-peaked:
  • Low energy ($\sim200$ MeV) from $\pi^\pm$ decay.
  • High energy ($\sim1.4$ GeV) from $K^+$ decay.
• Systematics Disentanglement: The BNB provides a different $L/E_\nu$ (baseline/energy) ratio compared to the NuMI beam at different absolute energies. Since systematic uncertainties generally scale with energy ($E_\nu$) while oscillations scale with $L/E_\nu$, comparing NuMI and BNB data allows the experiment to distinguish between true oscillation signals and systematic flux/model errors.
• Event Selection: A dedicated trigger has been used since 2015. The analysis uses a Convolutional Neural Network (CNN) to select $\nu_\mu$ CC interactions, achieving $>95\%$ purity with over 5,000 events in $2.5 \times 10^{21}$ POT.

3. Key Contributions

• Unified Analysis Framework: The presentation of a simultaneous fit of ND and FD data across both NC and $\nu_\mu$ CC channels, extending sensitivity to higher $\Delta m^2_{41}$ values than previous FD-only NC searches.
• BNB Data Utilization: The first technical demonstration of using the BNB off-axis flux at the NOvA ND for sterile neutrino searches. This includes:
  • Validation of BNB event timing (peaking at 319.4 $\mu$s).
  • Flux simulation generation using MiniBooNE's EventWeight package.
  • Reconstruction of $\nu_\mu$ CC events with high purity.
• Sensitivity Projections: Quantitative assessment of how adding BNB data improves exclusion limits, specifically in the systematics-dominated region ($\Delta m^2_{41} > 1 \text{ eV}^2$).

4. Results

• Current Status: The latest analysis (presented in Ref [10]) is consistent with the standard three-flavor oscillation model at the 90% confidence level. It sets world-leading exclusion limits on mixing angles ($\sin^2 \theta_{24}$, $\sin^2 \theta_{34}$, and effective $\sin^2 \theta_{\mu\tau}$) across the $10^{-3}$ to $100 \text{ eV}^2$ range.
• BNB Impact:
  • Sensitivity Gain: Asimov sensitivity studies indicate that adding the BNB sample improves NOvA's sensitivity by approximately 30% in specific regions of $\Delta m^2_{41}$, even without further optimization.
  • Complementarity: The BNB sample provides significant power for $\log_{10}(\Delta m^2_{41}) \lesssim 0.5$. While it is less sensitive at very high energies compared to NuMI, its distinct $L/E$ profile helps break degeneracies between oscillation parameters and systematic uncertainties.
  • Data Quality: The selected BNB sample contains $\sim5,000$ events with $>95\%$ purity, sufficient to begin constraining sterile neutrino parameters.

5. Significance and Outlook

• Resolving Anomalies: By improving sensitivity in the $\Delta m^2_{41} \gtrsim 1 \text{ eV}^2$ region (where current anomalies are most prominent), this work is critical for confirming or ruling out the 3+1 sterile neutrino hypothesis.
• Systematics Control: The unique capability to probe the same $L/E$ at different energies (NuMI vs. BNB) offers a robust method to reduce systematic uncertainties, which is the primary bottleneck for current sterile neutrino searches.
• Future Plans:
  • Immediate: Incorporate twice the neutrino-mode data, antineutrino-mode data, and a $\nu$-on-$e$ sample (for in-situ flux constraints) into the next analysis.
  • Long-term: Optimize the BNB selection and retrain neural networks for low-energy events ($\sim200$ MeV) to capture more $\pi$-decay neutrinos.
  • Broader Impact: The BNB $\nu_\mu$ sample can also constrain the $K^+$ component of the on-axis BNB flux, benefiting other Short-Baseline Neutrino (SBN) experiments (SBND, MicroBooNE, ICARUS) by reducing uncertainties in intrinsic electron neutrino backgrounds.

In summary, this paper demonstrates NOvA's evolution from a single-detector, NC-focused search to a multi-detector, multi-beam analysis strategy, significantly enhancing its ability to probe the sterile neutrino parameter space and resolve long-standing experimental tensions.