#
Latest Results from the T2K Neutrino Experiment^{ †}

^{*}

^{†}

## Abstract

**:**

## 1. Introduction to Lepton Mixing

## 2. Experiments with Artificial Neutrinos

## 3. The T2K Experiment

## 4. T2K Results

#### 4.1. Neutrino Oscillation Analyses

#### 4.2. Neutrino Oscillation Results

## 5. Conclusions and Outlook

## Conflicts of Interest

## References

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**2011**, 659, 106. [Google Scholar] [CrossRef] - Abe, K.; et al. [T2K Collaboration] Indication of Electron Neutrino Appearance from an Accelerator-Produced Off-Axis Muon Neutrino Beam. Phys. Rev. Lett.
**2011**, 107, 041801. [Google Scholar] [CrossRef] [PubMed][Green Version] - Sanchez, M. Nova results and prospects. In Proceedings of the XXVIII International Conference on Neutrino Physics and Astrophysics, Heidelberg, Germany, 4–9 June 2018. [Google Scholar]

**Figure 1.**The spectrum of neutrinos passing through the T2K far detector, peaking at approximately 0.6 GeV due to the 2.5${}^{\circ}$ off axis angle, is compared with the energy dependence of the muon–neutrino oscillation probabilities. (

**a**) The muon–neutrino survival probability is near zero at the peak of the neutrino spectrum. (

**b**) The electron–neutrino appearance probability is near maximum at the peak. The different curves correspond to different CP violation parameters for normal mass ordering for neutrinos and anti-neutrinos. The difference between neutrino and anti-neutrino oscillation probability for ${\delta}_{CP}={0}^{\circ}$ gives an indication of T2K sensitivity to mass ordering, while the difference for ${\delta}_{CP}={270}^{\circ}$ shows the much larger effect that would arise with maximum CP violation.

**Figure 2.**Two event displays for neutrino interactions observed by the (

**a**) near and (

**b**) far detectors of T2K. (

**a**) A charged current muon–neutrino interaction in the first Fine Grained Detector produces tracks in the two downstream Time Projection Chambers; (

**b**) An energetic electron from a neutrino interaction inside the Super-Kamiokande detector generates Cerenkov light which is detected by photomultipliers in a cylindrical structure. The display shows the pattern of light on the wall (unwrapped) and the base.

**Figure 3.**${\pi}^{0}$ production in muon–neutrino charged current interactions is measured by the T2K near detector, ND280. (

**a**) An illustration of the criteria applied to select a sample enhanced with CC${\pi}^{0}$ events and a typical topolgy of these events in ND280; (

**b**) Observed ${\mu}^{-}$ spectrum (points) compared to the NEUT model predictions broken down by interaction category.

**Figure 4.**The muon spectrum in ND280 event samples enhanced in quasi-elastic charged current interactions is compared to the model expectations for the (

**a**) nominal systematic parameters and (

**b**) optimized systematic parameters. The optimization takes into account constraints from external data.

**Figure 5.**The rate of muon–neutrinos observed in the far detector is significantly suppressed due to neutrino oscillation. (

**a**) The expected rate without neutrino oscillation (unoscillated prediction) is compared to the observed rate. Also shown are expected rates for the best fit oscillation parameters. There is no observable difference if reactor data is included in the fit. (

**b**) Confidence intervals for the atmospheric oscillation parameters are shown separately for the normal and inverted mass ordering hypotheses.

**Figure 6.**The three pairs of narrow ellipses show the expected numbers of anti electron–neutrino events and electron–neutrino events for optimized systematic parameter values. The solid (dashed) ellipses are for normal (inverted) mass ordering. The value of ${\delta}_{CP}$ locates the prediction on any of the ellipses. The observed number of events is shown by the inverted triangle. The region with the blue jagged border centred around the expectation for (${sin}^{2}{\theta}_{23}=0.5$, ${\delta}_{CP}=-\pi /2$, normal ordering) shows the region containing 68% of the predictions (for that choice of physics parameters) when systematic parameters are treated as random variables constrained by T2K and external data. The larger region with the red jagged border represents 68% of experimental outcomes distributed according to Poisson random variables whose expectation values are distributed due to the systematic uncertainties.

**Figure 7.**The posterior probability density for $sin{\delta}_{CP}$ marginalized over all other parameters including mass ordering (

**a**–

**c**). The shaded regions show 68%, 90%, and 95.5% credible intervals. (

**a**) The outcome for an experiment observing the expected number of events for (${sin}^{2}{\theta}_{23}=0.45$, ${\delta}_{CP}=0$, normal ordering). The posterior odds is 1:1 (not favoring either mass ordering); (

**b**) The outcome for an experiment observing the expected number of events for (${sin}^{2}{\theta}_{23}=0.53$, ${\delta}_{CP}=-\pi /2$, normal ordering). The posterior odds are 2.7 in favor of normal ordering; (

**c**) The outcome for the T2K observation. The posterior odds are 7.9 in favor of normal ordering and $sin{\delta}_{CP}=0$ is outside of the 95.5% credible interval; (

**d**) The frequentist $2\sigma $ confidence intervals on ${\delta}_{CP}$ (selected using the likelihood ratio) shown separately for the two mass orderings.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Karlen, D.; on behalf of the T2K Collaboration. Latest Results from the T2K Neutrino Experiment. *Universe* **2019**, *5*, 21.
https://doi.org/10.3390/universe5010021

**AMA Style**

Karlen D, on behalf of the T2K Collaboration. Latest Results from the T2K Neutrino Experiment. *Universe*. 2019; 5(1):21.
https://doi.org/10.3390/universe5010021

**Chicago/Turabian Style**

Karlen, Dean, and on behalf of the T2K Collaboration. 2019. "Latest Results from the T2K Neutrino Experiment" *Universe* 5, no. 1: 21.
https://doi.org/10.3390/universe5010021