Publications: Oscillations

Updated T2K measurements of muon neutrino and antineutrino disappearance using 3.6E21 protons on target

Muon neutrino and antineutrino disappearance probabilities are identical in the standard three-flavor neutrino oscillation framework, but CPT violation and non-standard interactions can violate this symmetry. In this work we report the measurements of sin²θ₂₃ and Δm²₃₂ independently for neutrinos and antineutrinos. The aforementioned symmetry violation would manifest as an inconsistency in the neutrino and antineutrino oscillation parameters. The analysis discussed here uses a total of 1.97E21 and 1.63E21 protons on target taken with a neutrino and antineutrino beam respectively, and benefits from improved flux and cross-section models, new near detector samples and more than double the data reducing the overall uncertainty of the result. No significant deviation is observed, consistent with the standard neutrino oscillation picture.

• arXiv preprint: https://arxiv.org/abs/2305.09916, https://arxiv.org/abs/2305.09916
• Data release: https://zenodo.org/record/7929975

Measurements of neutrino oscillation parameters from the T2K experiment using 3.6E21 protons on target

The normal ordering and upper octant is favoured, with a nearly maximally CP violating phase, consistent with previous results. The analysis excludes delta CP=0 and pi at more than 90% confidence level. It also excludes a Jarlskog invariant of zero at more than 2 sigma credible level with a flat prior in delta CP, and just below 2 sigma using a flat prior in sin delta CP. Furthermore, the analysis finds a sin^2 theta_{13} consistent with the reactor experiments.

• Data release: https://doi.org/10.5281/zenodo.7741399, https://doi.org/10.5281/zenodo.7741399
• arXiv preprint: 2303.03222

Search for Electron Antineutrino Appearance in a Long-baseline Muon Antineutrino Beam

Electron antineutrino appearance is measured by the T2K experiment in an accelerator-produced antineutrino beam, using additional neutrino beam operation to constrain parameters of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix. T2K observes 15 candidate electron antineutrino events with a background expectation of 9.3 events. Including information from the kinematic distribution of observed events, the hypothesis of no electron antineutrino appearance is disfavored with a significance of 2.40σ and no discrepancy between data and PMNS predictions is found. A complementary analysis that introduces an additional free parameter which allows non-PMNS values of electron neutrino and antineutrino appearance also finds no discrepancy between data and PMNS predictions.

• arXiv preprint: https://arxiv.org/abs/1911.07283, https://arxiv.org/abs/1911.07283
• DOI: 10.1103/PhysRevLett.124.161802, 10.1103/PhysRevLett.124.161802
• Reference: Phys.Rev.Lett. 124 (2020) 16, 161802

Improved constraints on neutrino mixing from the T2K experiment with 3.13×10E21 protons on target

The T2K experiment reports updated measurements of neutrino and antineutrino oscillations using both appearance and disappearance channels. This result comes from an exposure of 14.9(16.4)×10E20 protons on target in neutrino (antineutrino) mode. Significant improvements have been made to the neutrino interaction model and far detector reconstruction. An extensive set of simulated data studies have also been performed to quantify the effect interaction model uncertainties have on the T2K oscillation parameter sensitivity. T2K performs multiple oscillation analyses that present both frequentist and Bayesian intervals for the Pontecorvo-Maki-Nakagawa-Sakata parameters. For fits including a constraint on sin2θ13 from reactor data and assuming normal mass ordering T2K measures sin2θ23=0.53-0.04+0.03 and Δm322=(2.45±0.07)×10-3  eV2 c-4. The Bayesian analyses show a weak preference for normal mass ordering (89% posterior probability) and the upper sin2θ23 octant (80% posterior probability), with a uniform prior probability assumed in both cases. The T2K data exclude CP conservation in neutrino oscillations at the 2σ level.

• arXiv preprint: https://arxiv.org/abs/2101.03779, https://arxiv.org/abs/2101.03779
• Reference: Phys.Rev.D 103 (2021) 11, 112008
• DOI: 10.1103/PhysRevD.103.112008

T2K measurements of muon neutrino and antineutrino disappearance using 3.13E21 protons on target

We report measurements by the T2K experiment of the parameters θ23 and Δm322, which govern the disappearance of muon neutrinos and antineutrinos in the three-flavor PMNS neutrino oscillation model at T2K’s neutrino energy and propagation distance. Utilizing the ability of the experiment to run with either a mainly neutrino or a mainly antineutrino beam, muon-like events from each beam mode are used to measure these parameters separately for neutrino and antineutrino oscillations. Data taken from 1.49×1021 protons on target (POT) in neutrino mode and 1.64×1021 POT in antineutrino mode are used. The best-fit values obtained by T2K were sin2(θ23)=0.51-0.07+0.06(0.43-0.05+0.21) and Δm322=2.47-0.09+0.08(2.50-0.13+0.18)×10-3 eV2/c4 for neutrinos (antineutrinos). No significant differences between the values of the parameters describing the disappearance of muon neutrinos and antineutrinos were observed. An analysis using an effective two-flavor neutrino oscillation model where the sine of the mixing angle is allowed to take nonphysical values larger than 1 is also performed to check the consistency of our data with the three-flavor model. Our data were found to be consistent with a physical value for the mixing angle.

• arXiv preprint: https://arxiv.org/abs/2008.07921, https://arxiv.org/abs/2008.07921
• DOI: https://doi.org/10.1103/PhysRevD.103.L011101

Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations

Nature Nature 580, 339–344 (2020)

The charge-conjugation and parity-reversal (CP) symmetry of fundamental particles is a symmetry between matter and antimatter. Violation of this CP symmetry was first observed in 1964¹, and CP violation in the weak interactions of quarks was soon established². Sakharov proposed³ that CP violation is necessary to explain the observed imbalance of matter and antimatter abundance in the Universe. However, CP violation in quarks is too small to support this explanation. So far, CP violation has not been observed in non-quark elementary particle systems. It has been shown that CP violation in leptons could generate the matter–antimatter disparity through a process called leptogenesis⁴. Leptonic mixing, which appears in the standard model’s charged current interactions^5,6, provides a potential source of CP violation through a complex phase δ_CP, which is required by some theoretical models of leptogenesis^7,8,9. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible using accelerator-produced beams as established by the Tokai-to-Kamioka (T2K) and NOvA experiments^10,11. Until now, the value of δ_CPhas not been substantially constrained by neutrino oscillation experiments. Here we report a measurement using long-baseline neutrino and antineutrino oscillations observed by the T2K experiment that shows a large increase in the neutrino oscillation probability, excluding values of δ_CP that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δ_CP, which is cyclic and repeats every 2π, is [−3.41, −0.03] for the so-called normal mass ordering and [−2.54, −0.32] for the inverted mass ordering. Our results indicate CP violation in leptons and our method enables sensitive searches for matter–antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger datasets will test whether leptonic CP violation is larger than the CP violation in quarks.

• Data release: https://t2k-experiment.org/results/2020-constraint-cp-phase/, https://t2k-experiment.org/results/2020-constraint-cp-phase/
• arXiv preprint: https://arxiv.org/abs/1910.03887
• DOI: https://doi.org/10.1038/s41586-020-2177-0
• Reference: Nature 580 (2020) 7803, 339-344

Search for CP violation in Neutrino and Antineutrino Oscillations by the T2K experiment with 2.2E21 protons on target

Updated measurements of muon neutrino and antineutrino disappearance

We report measurements by the T2K experiment of the parameters θ₂₃ and ∆m²₃₂ governing the disappearance of muon neutrinos and antineutrinos in the three flavor neutrino oscillation model. Utilizing the ability of the experiment to run with either a mainly neutrino or a mainly antineutrino beam, the parameters are measured separately for neutrinos and antineutrinos. Using 7.482 × 10²⁰ POT in neutrino running mode and 7.471 × 10²⁰ POT in antineutrino mode, T2K obtains sin²(θ₂₃) = 0.51^+0.08_−0.07 and ∆m²₃₂ = 2.53^+0.15_−0.13 × 10⁻³ eV²/c⁴ for neutrinos, and sin²(θ̅₂₃) = 0.47^+0.25_−0.07  and ∆m̅²₃₂ = 2.55^+0.33_−0.27 × 10⁻³ eV²/c⁴ for antineutrinos (assuming normal mass ordering). No significant differences between the values of the parameters describing the disappearance of muon neutrinos and antineutrinos were observed.

Top panels show the reconstructed energy distribution of the 135 far detector ν_μ-CCQE candidate events (left) and 66 ν̅_μ-CCQE candidate events (right), with predicted spectra for best fit and no oscillation cases. Bottom panels show the ratio to unoscillated predictions.

First combined analysis of neutrino and antineutrino oscillations at T2K

T2K reports its first results in the search for CP violation in neutrino oscillations using appearance and disappearance channels for neutrino- and antineutrino-mode beam. The data include all runs from Jan 2010 to May 2016 and comprise 7.482×10207.482×1020,protons on target in neutrino mode, which yielded in the far detector 32 e-like and 135 μμ-like events, and 7.471×10207.471×1020,protons on target in antineutrino mode which yielded 4 e-like and 66 μμ-like events. Reactor measurements of sin22θ13sin2⁡2θ13 have been used as an additional constraint. The one-dimensional confidence interval at 90% for δCPδCP spans the range (−3.13−3.13, −0.39−0.39) for normal mass ordering. The CP conservation hypothesis (δCP=0,πδCP=0,π) is excluded at 90% C.L.

• arXiv preprint: arXiv:1701.00432, arXiv:1701.00432