Reactor Neutrino Oscillations

• Reactor neutrino oscillations are the quantum process by which electron antineutrinos change flavor as they propagate, driven by nontrivial mixing between mass and flavor states.
• Detection via inverse beta decay in experiments like Daya Bay, RENO, and Double Chooz enables precise measurements of parameters such as θ13 and Δm²ee.
• Next-generation experiments like JUNO target sub-percent spectroscopy and neutrino mass hierarchy determination through enhanced detector calibration and large-scale liquid scintillator setups.

Reactor neutrino oscillations refer to the phenomenon of quantum-mechanical flavor conversion exhibited by electron antineutrinos ($\bar\nu_e$) emitted from nuclear reactors as they propagate over macroscopic distances. This process is a direct consequence of the nontrivial mixing between neutrino flavor and mass eigenstates and has profound implications for the Standard Model and its extensions.

1. Theoretical Framework and Vacuum Oscillation Probability

In the Standard Model three-flavor paradigm, the weak-interaction (flavor) eigenstates ($\nu_e$, $\nu_\mu$, $\nu_\tau$) are related to the mass eigenstates ($\nu_1$, $\nu_2$, $\nu_3$) by the unitary PMNS matrix $U$:

$$U_{\text{PMNS}} = \begin{pmatrix} c_{12} c_{13} & s_{12} c_{13} & s_{13} e^{-i\delta} \ -s_{12} c_{23} - c_{12} s_{23} s_{13} e^{i\delta} & c_{12} c_{23} - s_{12} s_{23} s_{13} e^{i\delta} & s_{23} c_{13} \ s_{12} s_{23} - c_{12} c_{23} s_{13} e^{i\delta} & -c_{12} s_{23} - s_{12} c_{23} s_{13} e^{i\delta} & c_{23} c_{13} \end{pmatrix}$$

where $s_{ij} \equiv \sin \theta_{ij}$, $c_{ij} \equiv \cos \theta_{ij}$, and $\delta$ is the Dirac CP-violating phase (Lu, 2014).

The survival probability for a reactor $\bar\nu_e$ in vacuum is given by

$$P_{ee}(L,E) = 1 - 4|U_{e1}|^2|U_{e2}|^2 \sin^2 \Delta_{21} -4|U_{e1}|^2|U_{e3}|^2 \sin^2 \Delta_{31} -4|U_{e2}|^2|U_{e3}|^2 \sin^2 \Delta_{32}$$

with $$\Delta_{ji} = 1.267\, \Delta m_{ji}^2\;[\mathrm{eV}^2]\,L\;[\mathrm{m}]/E\;[\mathrm{MeV}]$$, $\Delta m_{ji}^2 = m_j^2-m_i^2$ (Roskovec, 2018, Lu, 2014, Bezerra, 2022).

For most practical purposes, this can be explicitly expanded in terms of the PMNS parameters: $$P_{ee}(L,E) = 1 - \sin^2 2\theta_{13} \bigl[ \cos^2\theta_{12} \sin^2 \Delta_{31} + \sin^2\theta_{12} \sin^2 \Delta_{32} \bigr] - \cos^4\theta_{13} \sin^2 2\theta_{12} \sin^2 \Delta_{21}$$ (Lu, 2014, Balantekin et al., 2013, Bezerra, 2022).

At short to intermediate baselines (L ≲ 2 km), the term driven by $\Delta m_{31}^2$ dominates; for long baselines (L ≈ 50–200 km), the solar splitting $\Delta m_{21}^2$ becomes dominant (Balantekin et al., 2013, Lu, 2014).

2. Experimental Realizations: Detection, Baselines, and Systematics

Nuclear reactors provide a high-intensity $\bar\nu_e$ source via β-decay of neutron-rich fission fragments. The principal detection channel is inverse beta decay (IBD): $$\bar\nu_e + p \rightarrow e^+ + n, \quad E_\nu > 1.8\,\text{MeV}$$ The positron carries $E_e \approx E_\nu - 1.8\,\text{MeV}$ of kinetic energy. Detection involves time-correlated prompt (e$^+$) and delayed (n-capture, usually on Gd or H) signals, suppressing backgrounds (Lu, 2014, Bezerra, 2022, Qian et al., 2018).

State-of-the-art experiments deploy multiple, functionally identical detectors at different baselines to cancel out reactor and detection systematics. Notable examples include:

Experiment	Near Baseline	Far Baseline	Target Mass	Notable Features
Daya Bay	360, 500 m	1.6 km	20 t ×8	Gd-LS, 192 PMTs/module
RENO	290 m	1.4 km	16 t ×2	Gd-LS, double veto
Double Chooz	400 m	1.05 km	8.6 t (far)	Gd-LS, multi-layer buffer

Each experiment achieves <1–2% relative uncertainties via identical designs, redundant calibration (LEDs, radioactive sources), and systematic control on energy nonlinearity, efficiency, and background rates (Lu, 2014, Bezerra, 2022).

3. Precision Results and Core Phenomenology

These setups have directly measured both the “reactor angle” $\theta_{13}$ and the effective mass-squared splitting $$\Delta m^2_{ee} \equiv \cos^2\theta_{12}\Delta m_{31}^2 + \sin^2\theta_{12}\Delta m_{32}^2$$:

• Daya Bay: $\sin^2 2\theta_{13} = 0.090^{+0.008}_{-0.009}$, $$|\Delta m^2_{ee}| = (2.59^{+0.19}_{-0.20}) \times 10^{-3}$$ eV$^2$ (Lu, 2014)
• RENO: $$\sin^2 2\theta_{13} = 0.113\pm0.013(\rm stat)\pm0.019(\rm syst)$$ (Kim, 2012)
• Double Chooz: $\sin^2 2\theta_{13} = 0.105\pm0.014$ (Bezerra, 2022)

These findings established that all three mixing angles are sizable, enabling comprehensive global fits and opening a path to CP violation studies in the lepton sector. The direct measurement of $|\Delta m^2_{ee}|$ complements accelerator muon neutrino disappearance measurements (Lu, 2014, Roskovec, 2018).

Global fits incorporating reactor and accelerator neutrino data have now pinned down $\theta_{13}$ and $\Delta m_{31}^2$ to a few percent, while KamLAND and solar neutrino experiments provide $\Delta m_{21}^2$ and $\theta_{12}$ to similar precision (Roskovec, 2018, Balantekin et al., 2013).

4. Reactor Antineutrino Anomaly and Searches for Sterile Neutrinos

Absolute flux measurements by multiple short-baseline (L < 100 m) experiments have shown a $\sim$6% deficit relative to the state-of-the-art Huber+Mueller model predictions, significant at 2.5σ. This “reactor antineutrino anomaly” (RAA) motivates searches for eV-scale sterile neutrinos (Buck, 2017, Roskovec, 2018).

For baselines below 100 m, the three-flavor oscillation probability predicts no disappearance ($P_{ee}^{3\nu}\approx1$), so an oscillatory signal would necessitate at least one sterile state. Precision very-short-baseline experiments (NEOS, DANSS, STEREO, PROSPECT, SoLid, NEUTRINO-4) use segmented or movable Gd/6Li-doped detectors, and have collectively excluded much of the parameter space favored by the RAA for $\Delta m^2_{41} \sim 0.5$–2 eV$^2$, $\sin^2 2\theta_{14} \sim 0.05$–0.2 (Buck, 2017, Serebrov et al., 2013, Derbin et al., 2012, collaboration et al., 2020).

The survival probability in the 3+1 model is: $$P_{ee}^{3+1}(L,E) \simeq 1 - \sin^2 2\theta_{14} \sin^2 \left(1.27 \Delta m_{41}^2 \frac{L}{E}\right)$$ (Buck, 2017, Roskovec, 2018).

No definitive evidence for light sterile neutrinos has been found to date, with current and future VSB reactor experiments expected to resolve or close the remaining allowed parameter space at high significance (Buck, 2017).

5. Probing the Neutrino Mass Hierarchy and Sub-percent Spectroscopy

The moderately large value of $\theta_{13}$ enables next-generation medium-baseline reactor experiments (L ≈ 52–53 km, e.g., JUNO, RENO-50) to resolve the interference between the $\Delta m_{31}^2$ and $\Delta m_{32}^2$ oscillations. The mass ordering (hierarchy) changes the interference pattern by $O(1\%)$; resolving this requires:

• Target mass: $\sim$20 kton ultra-pure liquid scintillator
• Energy resolution: $\sigma_E/E\lesssim3\%/\sqrt{E\,(\mathrm{MeV})}$
• Baseline uniformity: $<500$ m (to avoid washing out spectral structure)
• Statistics: $O(10^5)$ IBD events in $\sim$6 years

JUNO, with an energy resolution goal of 3%/$\sqrt{E(\mathrm{MeV})}$ and $\sim$20 kton LS at 52.5 km from $\sim$36 GWth of reactors, reported first oscillation results, achieving $\sin^2\theta_{12} = 0.3092 \pm 0.0087$ and $\Delta m^2_{21}= (7.50 \pm 0.12)\times10^{-5}$ eV$^2$, a factor 1.6 improvement over the global average (Abusleme et al., 18 Nov 2025). JUNO aims for $>3\sigma$ sensitivity to the mass ordering and sub-percent accuracy on $\Delta m^2_{21}, \Delta m^2_{31}, \theta_{12}$ (Lu, 2014, Abusleme et al., 18 Nov 2025, Capozzi et al., 2013).

The fine oscillatory structure and the relative phases encode the hierarchy. Systematics (energy-scale nonlinearity, absolute calibration) and reactor core geometry become the dominant challenges at this precision (Abusleme et al., 18 Nov 2025, Capozzi et al., 2013).

6. Reactor Flux Anomalies, Detector R&D, and Systematic Challenges

Uncertainties and anomalies in the predicted reactor $\bar\nu_e$ spectrum remain. The “5 MeV bump” is a persistent $2-3\%$ excess in the window $5-7$ MeV, seen by Daya Bay, Double Chooz, RENO, and NEOS, and is likely due to deficiencies in fission-product $\beta$-spectrum modeling (Roskovec, 2018, Bezerra, 2022).

Systematic uncertainties affecting reactor oscillation measurements include:

• Reactor flux modeling (β-conversion, ab initio summation): $2-6\%$
• Energy-scale: non-linearity, absolute calibration, spatial/temporal uniformity
• Backgrounds: cosmogenic isotopes, accidental coincidences, fast neutrons
• Detector efficiency and event selection

Modern strategies for systematic control employ redundant calibration, precision modeling, and relative measurements (near/far), suppressing normalization errors to the sub-percent level (Lu, 2014, Roskovec, 2018, Bezerra, 2022).

Advanced detection concepts, such as the “LiquidO” opaque scintillator, aim for background-free event identification and topological positron tagging, enabling $\mathcal O(0.1\%)$ precision on $\sin^2 2\theta_{13}$ and new applications in nonproliferation monitoring and sterile-neutrino searches (Bezerra, 2022).

7. Beyond the Standard Paradigm: Decoherence, Nonstandard Interactions, and Prospects

Reactor oscillation data provide leading experimental bounds on quantum-coherence loss (wave-packet decoherence): Daya Bay + RENO + KamLAND set $\sigma_x > 2.1 \times 10^{-4}$ nm (90% CL) on the neutrino wave-packet width. The extraction of oscillation parameters is robust against such hypothetical decoherence, but future data (e.g., JUNO) will probe orders of magnitude closer to the nuclear-localization scale (Gouvêa et al., 2021, Gouvêa et al., 2020).

Reactor oscillations have also been used to constrain nonstandard interactions. Linear-order sensitivity to certain off-diagonal flavor tensor and scalar SMEFT operators is unique to oscillation experiments and is robust against cancellations affecting the absolute rate or $\beta$-decay data (Falkowski et al., 2019).

Prospects for the field include:

• Precision tests of PMNS unitarity via $\nu_e$ disappearance
• Improvement of theoretical flux models and reactor modeling
• Comprehensive searches for sterile neutrinos and exotic effects
• Ultra-high-statistics oscillation spectroscopy for mass-orbit determination

The combination of experimental advances, theoretical refinement, and new detector capabilities positions reactor neutrino oscillation physics as a continuing probe of the Standard Model and its possible extensions (Roskovec, 2018, Bezerra, 2022, Abusleme et al., 18 Nov 2025).