Measurement of muon-antineutrino disappearance
The T2K experiment, conducted using the J-PARC High Intensity Proton Accelerator Facility and the Super-Kamiokande detector, has been continuing the operation of generating anti-neutrino beams since May 2014.
Figure 1 : The number of protons exposed to the pion production target of the T2K experiment. This corresponds to statistics of the produced neutrinos. The red dots and the purple dots represent the beam power of neutrino mode and anti-neutrino mode respectively.
As previously introduced in the High Energy Physics Laboratory News “Production of Anti-Neutrino Beam“, a muon anti-neutrino (νμ) is an antiparticle of the muon neutrino, generated through the decay of negatively charged pions produced when accelerated high-energy protons collide with a target. Figure 1 represents the amout of statistics obtained by the T2K experiment, expressed in terms of the number of protons collided with the target (Protons on Target, POT). The red dots indicate muon neutrino beam operation, while the purple dots indicate muon anti-neutrino beam operation. Currently, the muon anti-neutrino beam is at 3.12×1020 POT, with approximately half of the statistics of the accumulated muon neutrino beam obtained so far. Recently, the T2K experiment successfully observed the phenomenon of muon anti-neutrino disappearance due to muon anti-neutrino oscillation through the analysis of these data.
 Figure 2 : The distribution of neutrino interaction events observed at Super-Kamiokande during the anti-neutrino operation. While the red histogram shows the expectation without muon anti-neutrino oscillation, the black dots show data. The blue histograms represent the distribution obtained by fitting the anti-neutrino oscillation parameters assuming existence of muon anti-neutrino oscillation based on the actual data. |
 Figure 3 : The blue contours show the allowed regions of the measured anti-neutrino oscillation parameters, sin2θ23 and Δm232. The black contours show the measured results of neutrino oscillation parameters, sin2θ23 and Δm232. |
 Figure 4 : Comparison between muon anti-neutrino oscillation parameters, sin2θ23, Δm232 and the same parameters obtained from the MINOS experiment. |
Based on measurements of the muon anti-neutrino beam generated at J-PARC using the near detector on the J-PARC site, and estimating the expected number of reaction events observed by Super-Kamiokande, it was anticipated that 59.8 events would be observed assuming no muon anti-neutrino oscillation, whereas with the assumption of muon anti-neutrino oscillation, 19.9 events were expected. However, it is assumed that the oscillation behavior is the same as that of muon neutrino oscillation. On the other hand, 17 events were observed from the actual measurement.
Figure 2 illustrates the number of events obtained by Super-Kamiokande as a function of neutrino energy. In Figure 2(a), the red histogram represents the case without muon anti-neutrino oscillation, while the black points with error bars represent the actual data obtained. It is evident that muon anti-neutrinos are disappearing around 0.6 GeV. The blue histogram is the distribution obtained by fitting the anti-neutrino oscillation parameters based on the actual data. Figure 2(b) compares the black points and the blue fit result from Figure 2(a) with the red histogram assuming no muon anti-neutrino oscillation, showing a clear oscillation pattern. Figure 3 represents the measured oscillation parameters sin2θ23 and Δm232 obtained from fitting the data. The blue solid and dashed lines indicate the 90% and 68% confidence level allowed regions. For reference, the sin2θ23 and Δm232 obtained from muon neutrino oscillation measurements are shown with black solid and dashed lines. Although the measurement of muon anti-neutrino oscillation is not as precise as that of muon neutrino oscillation, both are consistent within the error range, suggesting no significant difference in oscillation behavior between muon anti-neutrinos and muon neutrinos. Additionally, Figure 4 compares the results of the MINOS experiment in the United States that also measured muon anti-neutrino oscillation (red line), showing a good agreement within the error range between the two results.
In the future, efforts will be made to accumulate more statistics and improve measurement accuracy, while also exploring the search for the appearance of electron anti-neutrinos sensitive to the lepton CP asymmetry.