Indication of νμ → νe Oscillation by Electron Neutrino Appearance
 Figure 1 : Overview of the T2K experiment. |
 Figure 2 : Flavor mixing of neutrinos. |
 Figure 3 : Candidate event of electron neutrino reaction observed at Super-Kamiokande originating from J-PARC. A Cherenkov ring from an electron produced by an electron neutrino reaction with a proton is seen. |
Previously, in the High Energy Physics Laboratory News (Starting Up J-PARC Neutrino Beam), we discussed the two major objectives of the T2K experiment. One of these objectives was “to search for the unconfirmed neutrino appearance events using νμ→νe“. We are pleased to announce that we have successfully captured events that can be considered as signs of this phenomenon.
The T2K experiment, as shown in Figure 1, measures flavor oscillation phenomena occurring between a neutrino beam produced at the J-PARC high-intensity proton accelerator facility in Tokai Village, Ibaraki Prefecture, and observed at the Super-Kamiokande in Kamioka Town, Gifu Prefecture. Neutrino oscillation occurs because the mass eigenstates (ν1, ν2, ν3) and the flavor eigenstates (νe, νμ, ντ) of neutrinos are different (Figure 2). Each flavor eigenstate is a linear combination of the three mass eigenstates, and the degree of mixing is represented by mixing angles (θ12, θ23, θ13). From previous experiments, θ12 and θ23 had been measured with some precision and were known to be non-zero, but θ13 had only an upper limit known. The T2K experiment aimed to discover the νμ→νe oscillation due to the as-yet-undetected electron neutrino appearance phenomenon and measure θ13 from its occurrence frequency. This was achieved thanks to the extremely high-purity νμ beam produced at J-PARC and the high sensitivity of Super-Kamiokande in distinguishing between νμ and νe reactions. Additionally, to ensure the events were caused by neutrinos produced at J-PARC, the beam timing of the accelerator and the observation time at Super-Kamiokande were verified using GPS. Figure 3 shows an example of a νe reaction event candidate observed at Super-Kamiokande, where a Cherenkov ring caused by an electron from a reaction between an electron neutrino and a proton is observed.
The T2K experiment began in earnest in January 2010 and collected data representing a statistical amount of 1.43×1020 POT until the accelerator was halted by the Great East Japan Earthquake on March 11, 2011. “POT” stands for “Protons on Target,” representing the amount of proton beam initially irradiated on a graphite target to produce neutrinos. This amount already exceeds what the pioneering K2K experiment collected over five years, demonstrating the excellent performance of the J-PARC accelerator.
Figure 4 : Comparison between the data (points with error bars) and the Monte Carlo simulation (histogram) in terms of the electron neutrino appearance phenomenon. The red hatched area shows the predicted amount of νe appearance phenomenon assuming sin22θ13 = 0.1.
Analyzing all the data collected so far, the T2K experiment detected a total of 88 neutrino events believed to originate from the J-PARC beam at Super-Kamiokande. After applying several criteria to exclude background, six events were identified as electron production by electron neutrinos. On the other hand, if we simulate assuming no νμ→νe neutrino oscillation, the number of electron production events detected at Super-Kamiokande is estimated to be 1.5±0.3 events. Statistically, the probability that the six detected events are entirely due to statistical error is 0.7%. In other words, the probability that these six events indicate electron neutrino appearance is 99.3%. This result can be considered the first in the world to show signs of electron neutrino appearance. Figure 4 shows the reconstructed neutrino energy distribution, with data points shown with error bars and histograms representing the predicted distribution from Monte Carlo simulations. The red hatched area represents the amount of electron neutrino appearance expected assuming sin22θ13 = 0.1. The absence of electron neutrino oscillation, represented by the absence of the red hatched area, does not fit well with the experimental results, as shown in the plot.
Figure 5 : The allowed region of sin2θ13 and δCP obtained from the current result. The light and deep red regions represent the regions with significance levels of 68% and 90% respectively.
Measuring the neutrino mixing angle θ13 is significant as it is a crucial step in probing the CP asymmetry, which is fundamental to understanding why the current universe is mostly made of matter. While the CP asymmetry in the quark sector has been measured, it is known to be insufficient to explain the matter-antimatter asymmetry of the universe. Therefore, it is believed that there are other sources of matter-antimatter asymmetry, one of which is the CP asymmetry in the lepton sector. Theoretical studies have shown that the CP asymmetry in the lepton sector cannot be observed if θ13 is zero. Hence, the measurement of θ13 from this result is highly significant as it opens the way for measuring the CP asymmetry in the lepton sector. Figure 5 shows the possible regions for θ13 and the CP phase angle δCP based on this result.
Currently, the amount of data collected is about 2% of the initial goal. Moving forward, we plan to increase the data volume to make this result more certain.