⚛️ phenomenology

Probing T and CP Violation at DUNE and T2HK

This paper demonstrates that combining DUNE and T2HK data allows for a novel, neutrino-only search for time-reversal violation by analyzing the baseline dependence of oscillation probabilities, achieving up to 4σ significance and highlighting the complementary strengths of the two experiments in probing T and CP violation.

Original authors: Sabya Sachi Chatterjee, Sudhanwa Patra, Thomas Schwetz, Kiran Sharma

Published 2026-08-18

📖 4 min read🧠 Deep dive

CC0 1.0

Original authors: Sabya Sachi Chatterjee, Sudhanwa Patra, Thomas Schwetz, Kiran Sharma

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep within the subatomic world, particles known as neutrinos drift through the universe in vast numbers, rarely interacting with anything they pass. These ghostly travelers come in three distinct types, or "flavors," and as they move through space, they have a peculiar habit of changing from one flavor to another, a phenomenon called oscillation. For decades, physicists have known that this shapeshifting is governed by a complex mathematical rule that includes a hidden phase, a kind of internal clock that can run forward or backward. If this clock behaves differently depending on the direction of time, it would mean that the fundamental laws of nature are not perfectly symmetrical, a discovery that could help explain why our universe is made of matter rather than being an empty void of antimatter. The central question for the next generation of experiments is whether this time-reversal symmetry is broken, and if so, how strongly.

A team of researchers has proposed a fresh way to hunt for this symmetry breaking using two massive underground experiments currently being built or planned: DUNE in the United States and T2HK in Japan. Instead of the traditional method, which requires comparing how neutrinos behave against how their antimatter counterparts, antineutrinos, behave, the scientists suggest looking at how the probability of a neutrino changing flavor depends on the distance it has traveled. In a world where time symmetry holds true, the chance of a neutrino changing flavors over a certain distance should look the same whether you view the journey forward or backward. However, if time symmetry is broken, the pattern of these changes will contain a specific signature that looks different when the distance is reversed. By measuring this pattern using only neutrino data at two different distances, the researchers can isolate this unique signal without needing to generate difficult-to-produce antineutrino beams.

The study focuses on the specific energy range where these two experiments overlap, a window between 0.68 and 0.92 gigaelectronvolts. In this range, the DUNE experiment, which sends a beam of neutrinos 1,300 kilometers through the Earth, is uniquely positioned to observe the second peak of the oscillation pattern, while T2HK, with its shorter 295-kilometer baseline, captures the first peak. When the researchers combined the data from both facilities in their simulations, they found that the two experiments work together beautifully to reveal the time-reversal breaking signal. The long distance of DUNE allows it to see a specific component of the oscillation that is invisible to shorter experiments, and this component carries the most important information about the time-violating phase. The results indicate that if the universe is indeed violating time symmetry in the way the standard model predicts, the combination of DUNE and T2HK could detect it with a statistical certainty of about four standard deviations, a level of confidence that is very close to a definitive discovery.

The researchers also explored how much data is needed to reach this level of certainty and found that the sensitivity depends heavily on knowing the exact properties of the neutrinos, particularly the mixing angle that determines how the third flavor mixes with the others. If scientists can pin down this angle beforehand, the experiments become much more powerful. The simulations showed that with the planned exposure of the DUNE detector running in neutrino-only mode for a specific period, the team could reach the four-standard-deviation mark if the time-violating phase is at its maximum possible value. Interestingly, the study revealed a division of labor between the two experiments: DUNE is far more sensitive to the time-reversal signal when running with neutrinos alone, thanks to its long baseline, while T2HK is better suited for the traditional comparison between neutrinos and antineutrinos. This means the two projects are not competing but are instead complementary tools, each bringing a different strength to the search for the fundamental nature of time in the quantum world.

Ultimately, this work offers a new way to interpret the data that these massive detectors will soon collect. Rather than just fitting numbers into a complex model to guess at a hidden parameter, the researchers argue that scientists will be able to directly observe the specific signature of time violation in the oscillation pattern itself. This approach provides a clearer, more direct window into the fundamental laws governing the universe. The study emphasizes that optimizing the ability of the DUNE detector to measure low-energy neutrinos with high precision will be crucial for this success. If the detectors perform as expected, the combination of these two long-baseline experiments could soon provide the first direct evidence that the flow of time is not perfectly reversible for these elusive particles, marking a profound step forward in our understanding of the cosmos.

Technical Summary: Probing T and CP Violation at DUNE and T2HK

Problem Statement
The discovery of a non-trivial complex phase ( $\delta_{CP}$ ) in the PMNS neutrino mixing matrix is a primary objective of next-generation long-baseline experiments. Within the standard three-flavour framework, CP and Time Reversal (T) symmetry violations are equivalent, both governed by $\delta_{CP}$ . The conventional method for searching for T violation requires exchanging initial and final neutrino flavors (e.g., comparing $P_{\nu_\mu \to \nu_e}$ with $P_{\nu_e \to \nu_\mu}$ ), a process that necessitates a neutrino factory and is experimentally prohibitive. Furthermore, conventional CP violation searches, which compare neutrino and antineutrino probabilities, are contaminated by "environmental" asymmetries arising from the sign change of the matter potential. This paper addresses the challenge of establishing T violation using only neutrino data from existing long-baseline facilities, avoiding the need for flavor-exchange capabilities or antineutrino beams.

Methodology
The authors propose a novel approach to detect T violation by analyzing the baseline ( $L$ ) dependence of the $\nu_\mu \to \nu_e$ transition probability at a fixed neutrino energy ( $E_\nu$ ), utilizing neutrino data exclusively.

Formalism: Under T conjugation, the oscillation probability transforms as $P_{\alpha \to \beta}(L) \to P_{\alpha \to \beta}(-L)$ . Consequently, T violation manifests as an $L$ -odd component in the appearance probability. The probability is decomposed into $P_{even}$ (T-even) and $P_{odd}$ (T-odd) parts. T violation is established if experimental data cannot be described by $P_{even}$ alone.
Experimental Setup: The study utilizes the DUNE (Deep Underground Neutrino Experiment) and T2HK (Tokai to Hyper-Kamiokande) configurations.
- DUNE: 1300 km baseline, 40 kton Liquid Argon TPC, 1.2 MW beam power. Default exposure: 336 kt MW yr (neutrino mode only, 7 years).
- T2HK: 295 km baseline, 187 kton Water Cherenkov detector, 30 GeV J-PARC beam. Default exposure: 608 kt MW yr (neutrino mode only, ~2.5 years).
Analysis Strategy: The analysis assumes Normal Ordering and specific benchmark oscillation parameters. The sensitivity is evaluated by comparing the $\Delta\chi^2$ between the T-violating hypothesis (allowing an $L$ -odd component) and the T-conserving hypothesis (fitting only with $P_{even}$ ). The study focuses on the energy range $E_\nu \in [0.68, 0.92]$ GeV, splitting this into two bins to optimize sensitivity. The authors also investigate the impact of prior knowledge on mixing angles, specifically the $\theta_{23}$ octant degeneracy, and the effect of DUNE's energy resolution and exposure.

Key Contributions and Results

Establishment of T Violation: The combined analysis of DUNE and T2HK data can establish the presence of an $L$ -odd component in the oscillation probability with a significance of up to $\sim 4\sigma$ .
Optimal Energy Window: The highest sensitivity is found in the energy range $E_\nu \in [0.68, 0.92]$ $E_{ν} \in [0.68, 0.92]$ GeV.
- The lower bin ($0.68 - 0.80$ GeV) benefits from the synergy between T2HK (spectral peak) and DUNE (second oscillation maximum).
- The higher bin ($0.80 - 0.92$ GeV) is dominated by DUNE's sensitivity.
- The second oscillation maximum of DUNE plays a crucial role in this analysis.
Role of Prior Knowledge: The sensitivity to T violation is highly dependent on the prior knowledge of mixing angles. Specifically, resolving the $\theta_{23}$ octant degeneracy (knowing whether $\theta_{23}$ is in the lower or higher octant) is essential to achieving $>3\sigma$ sensitivity. Assuming a prior on $\sin^2 \theta_{23}$ yields significantly better results than a prior on $\sin^2 2\theta_{23}$ , which leaves the degeneracy unresolved.
Exposure and Resolution: For a true $\delta_{CP} = 270^\circ$ , a sensitivity of $\sim 4\sigma$ is reached at a DUNE exposure of approximately 400 kt MW yr in neutrino-only mode, assuming default energy resolution. Better energy resolution allows this significance to be reached at smaller exposures.
Complementarity of Experiments:
- DUNE: Demonstrates superior sensitivity to T violation when running in neutrino-only mode. This is attributed to its longer baseline and broad spectrum covering both the first and second oscillation maxima, where the $L$ -odd component carries dominant information on $\delta_{CP}$ .
- T2HK: Demonstrates superior sensitivity to CP violation when comparing neutrino and antineutrino spectra. Due to its shorter baseline, the first oscillation maximum dominates, making the CP-asymmetric observable more powerful in this configuration.
- The two experiments are thus complementary: DUNE is optimized for T violation via baseline dependence, while T2HK is optimized for CP violation via beam polarity comparison.

Significance
The paper claims that this analysis offers a "new, physically transparent interpretation of future long-baseline data." Rather than merely fitting for the parameter $\delta_{CP}$ within a model, this method allows for the direct observation of the $L$ -odd component of the oscillation probability, which is the specific hallmark of fundamental T violation in neutrino mixing. The results highlight the importance of optimizing the low-energy performance of the DUNE detector for T-violation searches and demonstrate that the combination of DUNE and T2HK can robustly probe T violation using only neutrino beam data, circumventing the experimental challenges associated with flavor-exchange or antineutrino beam requirements.

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