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Observation of ν_μ→ν_e Oscillation by Electron Neutrino Appearance

Figure 1：Overview of the T2K experiment.

Figure 2：Flavor mixing of neutrinos.

In the previous article from the High Energy Physics Laboratory News (Indication of ν_μ → ν_e Oscillation by Electron Neutrino Appearance)​, it was mentioned that “Signs of the yet unconfirmed neutrino appearance event ν_μ→ν_e have been captured”. We are now pleased to announce that we have successfully discovered this phenomenon.

The T2K experiment measures the phenomenon of neutrino flavor oscillation by observing a neutrino beam created at the J-PARC and the Super-Kamiokande.  J-PARC is a high-intensity proton accelerator facility in Tokai Village, Ibaraki Prefecture, and Super-Kamiokande is a neutrino observatory located in Kamioka Town, Gifu Prefecture, as illustrated in Figure 1.

Neutrino oscillation is a phenomenon that occurs due to the difference between neutrino mass eigenstates (ν₁, ν₂, ν₃) and flavor eigenstates (ν_e, ν_μ, ν_τ)​ (Figure 2). Each flavor eigenstate is a linear combination of the three mass eigenstates, and the degree of mixing is expressed by mixing angles (θ₁₂, θ₂₃, θ₁₃). From previous experiments, it was known with some precision that θ_12​ and θ₂₃​ are non-zero, but only an upper limit was known for θ₁₃​ for a long time. This θ₁₃​ has become measurable recently through experiments detecting neutrinos from reactors around last year.  The T2K experiment discovered the ν_μ→ν_e oscillation phenomenon associated with electron neutrino appearance that had not yet been confirmed and measured the angle from its frequency of appearance. This achievement was made possible by the extremely high-purity ν_μ​ beam generated at J-PARC and the high sensitivity of Super-Kamiokande in distinguishing ν_μ​ and ν_e​ reactions. Furthermore, to ensure that the events are due to neutrinos generated at J-PARC, the timing of the accelerator beam and the observation time at Super-Kamiokande are confirmed using GPS. Figure 3 shows an example of an event candidate for ν_e​ reaction by neutrinos from J-PARC observed at Super-Kamiokande, where an electron neutrino reacts with a neutron and creates a Cherenkov ring from the resulting electron.

Since starting its full-fledged experiments in January 2010 until the accelerator shutdown due to the March 11, 2011, Great East Japan Earthquake, the T2K experiment has obtained data with a statistical quantity represented by 1.43×10²⁰ POT. “POT” stands for “Protons on Target” and represents the amount of proton beam irradiated on a graphite target initially to create neutrinos. This amount already exceeds the data accumulated by the K2K experiment over five years, which is a pioneering experiment of T2K. This indicates the excellent performance of the J-PARC accelerator.

Analyzing all the data obtained so far by the T2K experiment, a total of 532 neutrino events presumed to be originated from the beam at J-PARC were detected within Super-Kamiokande. Among these, after applying several criteria to exclude background events, 28 events of electron neutrino appearance by electron generation were detected. On the other hand, assuming that ν_μ→ν_e neutrino oscillation did not occur and conducting simulations, it was estimated that there would be 4.6 events of electron generation detected at Super-Kamiokande. Statistically considering this result, the probability that these 28 detected events are solely due to statistical errors is less than one in a trillion. In other words, the probability that these 28 events are indeed due to electron neutrino appearance is more than 99.9999999999%. This result conclusively confirms the existence of the electron neutrino appearance phenomenon, marking a world-first achievement. Figure 3 shows the reconstructed energy distribution of neutrinos, where the points with error bars represent the data, and the histogram represents the distribution expected from Monte Carlo simulations. The green area represents the part due to background events unrelated to ν_μ→ν_e​ oscillation, while the red area represents the expected amount of electron neutrino appearance assuming ν_μ→ν_e oscillation. The absence of the red area, i.e., oscillation to electron neutrinos, is evident from this plot, indicating a complete mismatch with the experimental results.

The significance of measuring the neutrino mixing angle θ₁₃ lies in its potential to probe CP asymmetry, which is considered one of the fundamental origins of the vast predominance of matter over antimatter in the current universe. While CP asymmetry in the quark sector has been measured, it is not large enough to explain the entire matter-antimatter asymmetry in the universe. Therefore, it is believed that there are other sources contributing to this asymmetry, with one candidate being CP asymmetry in the lepton sector. Theoretical understanding suggests that the CP asymmetry in the lepton sector cannot be observed if θ₁₃​ were zero. Hence, the measurement of θ₁₃ in this study opens the door to measuring CP asymmetry in the lepton sector, which is of significant importance.

The current amount of data acquired is approximately 2% of the initial goal. In the future, we plan to increase the data volume to further confirm the results, explore the oscillation phenomenon of antineutrinos in detail, and approach CP asymmetry in the lepton sector.

• T2K public web page : “New results from T2K conclusively show muon neutrinos transform to electron neutrinos”