Measurement of neutrino CP violating angle
The international research collaboration of the T2K experiment, where the High Energy Physics Laboratory at Osaka City University is joining, has demonstrated with 95% confidence level that there is a violation of CP symmetry in the neutrino sector. This result provides a key to solving one of the most important issues in modern physics: why only matter exists in the universe today when there should have been equal amounts of matter and antimatter at the beginning of the universe with the Big Bang.
Figure 1 : The energy spectrum of electron neutrino appearance phenomenon. The negative area of vertical region represents the spectrum of electron anti-neutrinos.
The CP symmetry violation in neutrinos manifests as a difference between the oscillation probabilities of neutrinos and anti-neutrinos. In the T2K experiment, data obtained from generating neutrino beams from 2010 to 2013 were compared with data obtained in anti-neutrino mode from May 2014 to May 2016. It was announced in August 2016 with 90% confidence level that the probability of muon neutrinos oscillating into electron neutrinos differs from the probability of muon anti-neutrinos oscillating into electron anti-neutrinos. Subsequently, the T2K experiment conducted further experiments in neutrino mode from October 2016 to April 2017. During this period, the beam intensity of the J-PARC accelerator increased, and the stability of beam transport improved, allowing the generation of about two years’ worth of neutrino beams in about one year. Consequently, data collection was carried out very efficiently. As a result, from the start of the T2K experiment in 2010, 1.4×10²¹ POT of neutrino beam data and 7×10²⁰ POT of anti-neutrino beam data were obtained. By incorporating newly developed analysis methods, a reanalysis of all this data was performed. It was found that, while the predicted number of electron neutrino events in Super-Kamiokande was 67, the actual observed number was higher at 89. On the other hand, for electron anti-neutrinos, the predicted number was 9, but the actual observed number was 7. Figure 1 illustrates this situation.
Figure 2 : The allowed region of sin2(θ13) and δCP obtained from the T2K experiment. The black lines show the region for the normal mass hierarchy and the red lines show the region for the inverted mass hierarchy. The yellow band represents the precise value of sin2(θ13) obtained from the reactor neutrino experiments.
Figure 3 : The allowed regions of sin2(θ13) and δCP for the combined analysis between the T2K experiment and reactor experiments.
図４：The value of -2Δln(L) as a function of δCP. The hatched region is an allowed range of δCP.
In addition to these observations, a comprehensive analysis was conducted by simultaneously fitting the observations of muon neutrino oscillations and muon anti-neutrino oscillations, their energy spectra, and considering the results of reactor neutrino experiments in China, South Korea, and France, which showed electron anti-neutrino deficit events. Figure 2 shows the plot of sin2(θ13) and δCP obtained from the T2K experiment alone. The black lines correspond to the normal mass hierarchy of neutrinos, while the red lines correspond to the inverted mass hierarchy. The areas enclosed by solid lines indicate the 90% confidence level regions, and those enclosed by dashed lines indicate the 68% confidence level regions. The yellow band represents the precise value of sin2(θ13) obtained from reactor neutrino experiments. Figure 3 shows the results when combining the T2K experiment results with those of reactor neutrino experiments. The meanings of the solid and dotted lines are the same as before. Furthermore, based on these results, a statistical analysis using the Feldman-Cousins method was performed, successfully rejecting the hypothesis that CP symmetry is conserved (i.e., δCP is 0 or π) at the 95% confidence level. As shown in Figure 4, the allowable CP phase angle δCP at 95% confidence level is –171° < δCP < –34° (–88° < δCP < –68°) for the normal (inverted) mass hierarchy. This result only accounts for about 30% of the data that the T2K experiment is approved to collect. The remaining 70% of the data is expected to be obtained over the next few years, promising even more precise results.