Search for an eV-scale sterile neutrino with the first six detection units of KM3NeT/ORCA

Using data from the first six detection units of the KM3NeT/ORCA telescope, this study searched for eV-scale sterile neutrinos through atmospheric neutrino oscillations and established new constraints on the mixing elements $|U_{\mu4}|^2$ and $|U_{\tau4}|^2$ that are consistent with the absence of such neutrinos.

The Gist

The Ghost in the Machine: Searching for the "Invisible" Neutrino

Imagine you are watching a massive, high-stakes game of billiards. You see the white cue ball strike a cluster of colored balls, and they scatter across the table exactly how the laws of physics say they should. Everything makes sense.

But suddenly, you notice something strange: occasionally, a ball seems to vanish mid-roll, or a ball that should have bounced left instead zips right for no apparent reason. You check the table—no holes, no magnets, no trickery. You are left with a haunting possibility: there is an "invisible" player on the table, a ghost ball that is bumping into the others, changing the game in ways you can’t see directly.

In the world of particle physics, scientists are currently hunting for exactly that kind of "ghost." This paper describes the first attempt by the KM3NeT/ORCA team to find a specific type of ghost called an eV-scale sterile neutrino.

---

1. The Cast of Characters

To understand the search, we need to meet the players:

• The Neutrinos (The Standard Players): These are tiny, ghostly particles that fly through everything—including you—by the trillions every second. We know there are three "flavors" of them (let’s call them Electron, Muon, and Tau). They are famous for "oscillating," which is a fancy way of saying they shape-shift from one flavor to another as they travel.
• The Sterile Neutrino (The Ghost Player): For years, some experiments have seen "glitches" in the shape-shifting process. To explain these glitches, scientists proposed a fourth neutrino. But this one is different: it’s "sterile," meaning it doesn't interact with the forces of nature the way the others do. It’s even more invisible than a regular neutrino. It only reveals itself by how its presence messes up the dance of the other three.
• KM3NeT/ORCA (The Giant Underwater Eye): How do you catch a ghost? You build a massive trap. ORCA is a giant telescope sitting on the bottom of the Mediterranean Sea. It uses the deep, dark water to detect the tiny flashes of light produced when neutrinos crash into water molecules.

2. The Experiment: Watching the Dance

The researchers used a small portion of this giant underwater detector (about 5% of it, called "ORCA6") to watch atmospheric neutrinos—particles created when cosmic rays hit Earth's atmosphere.

They weren't looking for the sterile neutrino itself (which is nearly impossible to see). Instead, they were looking for disturbances in the rhythm.

If the "Standard Model" of physics is correct, the neutrinos should shape-shift in a very predictable pattern as they travel through the Earth. But if a sterile neutrino exists, it would act like a "thief" in the night, stealing some of the flavor from the regular neutrinos. This would cause the "dance" to look slightly off—some neutrinos would disappear or reappear at the wrong times or energies.

3. The Results: No Ghost... Yet

After analyzing a massive amount of data (equivalent to 433,000 tons of water being watched for a year), the team looked for these rhythmic glitches.

What did they find?
They found that the neutrinos were dancing almost exactly according to the old rules. While they did find a tiny "best-fit" point that could suggest a ghost, it wasn't strong enough to claim a discovery. In scientific terms, the results are "compatible with the absence of mixing."

In plain English: The ghost didn't show up. The "billiards game" looked normal, and the standard three-flavor dance held steady.

4. Why This Matters

You might ask, "If they didn't find it, was it a waste of time?"

Quite the opposite! This paper is like a scout reporting back from the frontier. Even though they didn't find the sterile neutrino with this small piece of the detector, they proved that the KM3NeT/ORCA telescope is incredibly sensitive and works exactly as intended.

They have set "speed limits" (constraints) on how much these ghosts can interact with regular matter. They’ve essentially told the scientific community: "If the ghost is hiding, it’s much smaller and more subtle than we previously thought."

As the full, massive detector is completed, the "eye" under the sea will get much larger and sharper. The hunt for the invisible player continues, and the next time the rhythm breaks, we might finally see the ghost.

Technical Summary

This technical summary details the research paper titled "Search for an eV-scale sterile neutrino with the first six detection units of KM3NeT/ORCA," prepared for submission to the Journal of High Energy Physics (JHEP).

1. Problem Statement

The standard three-flavor neutrino oscillation paradigm (the PMNS model) has been highly successful but fails to explain several "short-baseline (SBL) anomalies" observed over the last 25 years. These anomalies suggest the existence of a fourth, "sterile" neutrino state with a mass squared difference ($\Delta m^2_{41}$) of approximately $1 \text{ eV}^2$.

Because sterile neutrinos do not participate in weak interactions, they can only be detected through their effect on the oscillation probabilities of active neutrinos ($\nu_e, \nu_\mu, \nu_\tau$). This paper seeks to perform the first search for such a state using the KM3NeT/ORCA detector, specifically looking for deviations in GeV-scale atmospheric neutrino oscillations caused by the presence of an eV-scale sterile neutrino.

2. Methodology

The study utilizes data from the ORCA6 configuration—an early, partial deployment of the KM3NeT/ORCA detector consisting of only 6 detection units (5% of the final detector size).

• Data Sample: The analysis is based on 5828 neutrino candidates collected between January 2020 and November 2021, representing an exposure of $433 \text{ kton-years}$.
• Event Classification: Using Boosted Decision Trees (BDTs) trained on Monte Carlo simulations, the researchers classified events into three categories:
  1. High-purity tracks: Primarily $\nu_\mu$-CC interactions (high angular resolution).
  2. Low-purity tracks: $\nu_\mu$-CC interactions with higher muon contamination.
  3. Showers: A mixture of $\nu_e$-CC, $\nu_\tau$-CC, and Neutral Current (NC) interactions.
• Oscillation Modeling: The researchers employed a 3+1 model, extending the PMNS matrix to a $4 \times 4$ unitary matrix. They focused on the impact of the active-sterile mixing elements $|U_{\mu4}|^2$ and $|U_{\tau4}|^2$, assuming $\Delta m^2_{41} \geq 1 \text{ eV}^2$ and $\theta_{14} = 0$ (as the detector has limited sensitivity to $\nu_e$ disappearance).
• Statistical Framework: A frequentist approach was used, minimizing a negative log-likelihood function that accounts for 16 nuisance parameters (including uncertainties in atmospheric flux, cross-sections, detector energy scale, and standard oscillation parameters like $\Delta m^2_{31}$ and $\theta_{23}$).

3. Key Contributions

• First ORCA Search: This represents the inaugural sterile neutrino search using the KM3NeT/ORCA infrastructure.
• Sensitivity to $\nu_\tau$ Mixing: Unlike many other experiments, this analysis provides specific constraints on the mixing between the $\nu_\tau$ flavor and the sterile state ($|U_{\tau4}|^2$), a parameter that is notoriously difficult to constrain.
• Methodological Validation: The paper demonstrates that even with a highly limited detector configuration (6 units), the detector can effectively reconstruct the first $\nu_\mu \to \nu_\tau$ oscillation maximum, which is the primary driver for these constraints.

4. Results

• Compatibility with Standard Model: The observed data is fully compatible with the standard three-flavor oscillation model. The best-fit point for the sterile parameters was found at $|U_{\mu4}|^2 = 6.89 \times 10^{-2}$ and $|U_{\tau4}|^2 = 2.35 \times 10^{-4}$, but the $(0,0)$ point (no sterile neutrino) remains within the allowed region.
• Upper Limits (90% CL): The study established the following constraints on the mixing elements:
  • $|U_{\mu4}|^2 < 0.138$
  • $|U_{\tau4}|^2 < 0.076$
• Comparative Performance: The limits on $|U_{\tau4}|^2$ are highly competitive, being surpassed only by the DeepCore experiment. While other experiments (IceCube, Super-Kamiokande) provide tighter limits on $|U_{\mu4}|^2$, they utilized much larger datasets and complete detector configurations. The ORCA6 results are significant given they were achieved with only 5% of the final detector mass.

5. Significance

This paper serves as a "proof of concept" for the full KM3NeT/ORCA observatory. It demonstrates that the detector's energy and angular resolution are sufficient to probe Beyond the Standard Model (BSM) physics.

As the detector grows to its full 115-unit configuration, the increased statistics and volume will allow ORCA to:

1. Provide much more stringent constraints on sterile neutrino mixing.
2. Search for the TeV-scale $\bar{\nu}_\mu$ disappearance resonance, a distinctive signature of sterile neutrinos induced by matter effects in the Earth's mantle.
3. Deepen the understanding of the neutrino mass ordering and CP violation.