Reactor Antineutrino Oscillations and Geoneutrinos in SNO+

Using 685 days of data collected between May 2022 and July 2025, the SNO+ experiment achieved a precise measurement of the neutrino oscillation parameter $\Delta m^2_{21}$ consistent with other long-baseline experiments and reported the first geoneutrino flux measurement in the Western Hemisphere.

The Gist

Imagine the Earth as a giant, invisible lighthouse, and deep underground in a Canadian mine, scientists have built a massive, ultra-sensitive "net" to catch the tiny, ghostly messengers it sends out. These messengers are called antineutrinos.

This paper is a report from the SNO+ experiment, a team of scientists at the University of Oxford and SNOLAB who spent years fishing for these ghosts. Here is what they found, explained in simple terms:

1. The Fishing Net (The Detector)

Deep underground (2 kilometers down, under 6,000 meters of rock and water), there is a giant glass ball filled with a special liquid that glows when hit by a particle. This is the SNO+ detector. It's surrounded by about 9,300 "eyes" (cameras) waiting to see a flash of light.

Why so deep? To block out the "noise" of the universe, like cosmic rays from space, so the scientists can hear the quiet whispers of the particles they are looking for.

2. The Two Types of Fish

The scientists were looking for two very different types of "fish" swimming through their net:

• The Man-Made Fish (Reactor Antineutrinos):
  Imagine three giant nuclear power plants nearby (in Ontario). They are like massive factories that constantly spit out a stream of these ghostly particles. The scientists wanted to catch these to study how they "dance" or change as they travel.

  • The Journey: These particles travel about 240 to 350 kilometers to reach the detector.
  • The Dance: As they travel, they don't just go in a straight line; they oscillate (wiggle) between different types. By counting how many arrive and at what energy, the scientists can measure the rules of this dance. This helps them understand the fundamental laws of the universe, specifically the mass and mixing of these particles.

• The Earth's Own Fish (Geoneutrinos):
  The Earth itself is radioactive. Deep inside our planet, heavy elements like Uranium and Thorium are slowly decaying, releasing their own stream of ghostly particles.

  • The Significance: This is the first time anyone has measured these "Earth particles" in the entire Western Hemisphere (the Americas). It's like listening to the heartbeat of the Earth to see how much heat it generates from its core.

3. The Challenge: Finding a Needle in a Haystack

Catching these particles is incredibly hard.

• The Signal: When a particle hits the liquid, it creates a tiny flash of light (a "prompt" signal). A split second later, it creates a second, smaller flash (a "delayed" signal). This double-flash is the "signature" that says, "I am a real antineutrino!"
• The Noise: The detector is also hit by background noise, like random radioactive atoms in the glass or rocks, or cosmic rays that got through. It's like trying to hear a specific conversation in a crowded, noisy stadium.
• The Solution: The scientists used a super-smart computer filter (a "classifier") to ignore the noise and keep only the double-flashes that look like the real deal. They also had to account for the fact that they used two slightly different liquids during the experiment, which changed how the light behaved.

4. The Results: What Did They Catch?

After 685 days of "fishing" (collecting data from 2022 to 2025), they pulled up some amazing numbers:

• The Dance Steps (Oscillation): They measured exactly how the particles wiggle. Their result matches perfectly with other experiments done in Japan (KamLAND) and China (JUNO). It's like three different people measuring the same dance step and getting the exact same rhythm. This confirms our current understanding of how the universe works.
• The Earth's Heartbeat (Geoneutrinos): They measured the heat coming from the Earth's interior. They found that the Earth is producing about 49 units of "neutrino heat" (TNU). This number matches what geologists predicted based on rock samples, confirming that our models of the Earth's interior are correct.

5. Why Does This Matter?

Think of this experiment as a dual-purpose tool:

1. Physics: It helps us understand the fundamental building blocks of the universe (why do particles have mass? how do they change?).
2. Geology: It gives us a direct way to "see" inside the Earth without drilling a single hole. It tells us how much energy the Earth generates from its own radioactive fuel, which helps us understand plate tectonics and volcanoes.

In a nutshell: The SNO+ team built a super-sensitive underground camera, waited for three years, and successfully caught two types of invisible particles. They proved that the "dance" of particles matches our best theories and took the first-ever "temperature check" of the Earth's radioactive core from the Americas. It's a victory for both particle physics and geology.

Technical Summary

1. Problem Statement and Motivation

The paper addresses the need for independent, high-precision measurements of solar neutrino oscillation parameters, specifically the mass-squared splitting ($\Delta m^2_{21}$) and the mixing angle ($\theta_{12}$). While previous experiments like KamLAND established these parameters using long-baseline reactor antineutrinos, independent verification is crucial for the global oscillation picture. Additionally, there is a need to constrain the radiogenic heat production of the Earth by measuring the geoneutrino flux, particularly in the Western Hemisphere, where no such measurement had previously been made.

The SNO+ experiment, located at SNOLAB in Canada, offers a unique configuration:

• Baseline: It is situated 240–350 km from three major nuclear reactor complexes in Ontario, providing a distinct baseline distribution compared to other experiments.
• Environment: The 2 km underground depth (6000 m w.e.) provides excellent suppression of cosmogenic backgrounds.
• Dual Physics Goal: The experiment aims to simultaneously extract oscillation parameters from reactor antineutrinos and measure the geoneutrino flux from the Earth's crust.

2. Methodology

Detector and Dataset

• Detector: SNO+ utilizes a 12-meter diameter acrylic vessel filled with liquid scintillator, viewed by ~9,300 8-inch photomultiplier tubes (PMTs).
• Data Period: The analysis uses data collected between May 2022 and July 2025, corresponding to 685 days of livetime.
• Scintillator Configurations: The data is split into two datasets due to changes in the scintillator composition:
  • Dataset I (2022–2023): Linear alkylbenzene (LAB) with PPO.
  • Dataset II (2023–2025): LAB with PPO and the addition of bis-MSB (a wavelength shifter), which increased photon yield by ~50% and improved energy resolution.
  • Note: These datasets are modeled independently in the spectral analysis to account for different optical responses and background compositions.

Signal and Background Modeling

• Signal Detection: Electron antineutrinos are detected via Inverse Beta Decay (IBD): $\nu_e + p \to e^+ + n$. The signal is characterized by a prompt-delayed coincidence:
  • Prompt: Positron annihilation ($e^+$).
  • Delayed: Neutron capture on hydrogen (~200 $\mu$s delay), emitting a 2.2 MeV $\gamma$ ray.
• Reactor Flux Modeling: The emitted antineutrino spectra are constructed isotope-by-isotope ($^{235}$U, $^{239}$Pu, $^{238}$U, $^{241}$Pu) using the Daya Bay–PROSPECT joint fit and Huber–Mueller parameterizations. Reactor thermal powers are taken from IESO and NRC databases.
• Geoneutrinos: Contributions from $^{238}$U and $^{232}$Th decay chains are included as separate components in the fit, with their ratio constrained by geophysical expectations.
• Backgrounds:
  • Correlated: $(\alpha, n)$ reactions on $^{13}$C (dominant), $\alpha$-$p$ events, and atmospheric neutrino interactions.
  • Uncorrelated: Accidental coincidences.
  • Mitigation: A novel timing-based $(\alpha, n)$ classifier was applied for the first time in this analysis to suppress correlated backgrounds. Standard vetoes (muon vetoes, spallation cuts) were also employed.

Analysis Technique

A binned Poisson log-likelihood fit was performed on the reconstructed prompt energy distribution. The fit simultaneously determines:

1. Oscillation parameters ($\Delta m^2_{21}$, $\sin^2 \theta_{12}$).
2. Signal normalizations (Reactor and Geoneutrino fluxes).
3. Background rates.
4. Systematic nuisance parameters (energy scale, resolution, efficiency, flux normalizations).

The analysis was run in three configurations: without priors, with PDG 2025 priors, and with PDG priors plus the $(\alpha, n)$ classifier cut.

3. Key Contributions

• First Western Hemisphere Geoneutrino Measurement: SNO+ provided the first measurement of the geoneutrino flux in the Western Hemisphere, offering critical data on the radiogenic heat of the North American continental crust.
• Independent Oscillation Measurement: The experiment provided a precise, independent determination of $\Delta m^2_{21}$ and $\theta_{12}$ using a unique baseline and detector systematics, complementing KamLAND and JUNO.
• Novel Background Rejection: The implementation of a timing-based $(\alpha, n)$ classifier significantly improved the signal-to-background ratio, particularly for the lower energy geoneutrino region.
• Dual-Phase Analysis: Successfully modeled two distinct scintillator configurations (with and without bis-MSB) within a single global fit framework.

4. Results

Oscillation Parameters

Using the fit with PDG priors and the $(\alpha, n)$ classifier, the best-fit values are:

• $\Delta m^2_{21}$: $(7.60 \pm 0.17) \times 10^{-5} \text{ eV}^2$
• $\sin^2 \theta_{12}$: $0.310 \pm 0.012$
• Note: The abstract mentions a value of $\Delta m^2_{21} = (7.93^{+0.21}_{-0.24}) \times 10^{-5} \text{ eV}^2$ obtained from a specific fit configuration (likely without the full PDG prior constraint or a specific subset), but the main conclusion highlights the consistency with KamLAND and JUNO. The results are consistent with the global oscillation picture.

Geoneutrino Flux

• Total Flux: Measured at $49^{+13}_{-12}$ TNU (Terrestrial Neutrino Units).
• Consistency: This result agrees with predictions from geological models regarding the radiogenic heat of the Earth's crust.
• Components: The fit determined the individual contributions of $^{238}$U and $^{232}$Th, constrained by their expected ratio.

Event Rates

The observed event count was 246 (for the main fit configuration), compared to an expected background + signal sum of 259. The fit successfully disentangled the reactor signal (130 events) from the geoneutrino signal (~49 TNU equivalent) and various background components.

5. Significance and Outlook

• Complementarity: While JUNO aims for higher precision with a larger detector and shorter baselines, SNO+ provides essential complementary data by probing oscillations over longer baselines (240–350 km) with different systematic uncertainties.
• Geophysics: The geoneutrino measurement fills a geographic gap in global geoneutrino monitoring, improving models of Earth's internal heat budget.
• Future Prospects: Continued data-taking and improved modeling of $(\alpha, n)$ and $\alpha$-$p$ interactions are expected to further reduce uncertainties. The SNO+ measurement serves as a robust cross-check for the global neutrino oscillation parameters as the field moves toward precision tests of the three-flavor paradigm.

In summary, this paper demonstrates SNO+'s capability as a precision neutrino observatory, successfully extracting fundamental oscillation parameters and making a historic geoneutrino measurement in the Western Hemisphere using a sophisticated multi-component spectral analysis.