2023 Nishina Memorial Prize
Dr. Atsuko Ichikawa
Professor, Department of Physics, Graduate School of Science, Tohoku University“Constraining CP violating phase δ in neutrino oscillations”

The T2K experiment searches for a violation of the symmetry between particles andantiparticles, called CP violation, in the neutrino oscillation phenomena. In the experiment, theJ-PARC 30 GeV, high-power proton accelerator located at Tokai, Ibaraki Prefecture launches
neutrinos and antineutrinos separately toward the Super-Kamiokande at Kamioka, Gifu
Prefecture, located 295 km away. The difference between neutrino oscillations and antineutrinooscillations, if observed, is the signature of CP violation.
Matter in the universe was created through the Big Bang. According to particle theory,
particles and antiparticles, whose signs of electric charges (or particle numbers in general) arereversed to each other, are always created in pairs and annihilate in pairs. Matter is composedof particles, while antimatter is composed of antiparticles. Therefore, the fact that only matterand no antimatter remains in the current universe is known as the "mystery of thepredominance of matter in the universe," and is considered as a big problem in the cosmic andelementary particle theory. Sakharov's three conditions are known to be necessary for matterto be predominant in the universe; they are baryon number violation, C and CP violation, andnon-equilibrium. CP symmetry holds if the laws of physics are invariant under simultaneoustransformations of Charge (particle to antiparticle) and Parity (spatial inversion). Although CP
violation has been discovered in quarks, it is considered insufficient to explain thepredominance of matter in the universe, and accordingly, CP violation in leptons, including
neutrinos, has attracted attention. CP violation in neutrinos is represented by the complexphase  of the neutrino mixing matrix, and if  has a value other than 0 or ± (with a period of2), CP symmetry is violated.

A strong neutrino beam is indispensable to the search for CP violation in neutrinos. Dr.
Ichikawa has been involved in the conceptualization, design, and fabrication of neutrino beam
generators since the beginning of the T2K experiment and played a central role in therealization of a device called an electromagnetic horn to focus the neutrino beam and increase
the beam intensity. The electromagnetic horn is a highly technical device that operates in theharshest radiation environment of any neutrino beam facility. In addition to serving as a physics
analysis leader, she has been the spokesperson of the T2K experiment from 2019 to 2023 leading
an international experimental group consisting of about 500 collaborators. She has led theresearch in all aspects of the experiment.
Results published in 2020 [1] rejected nearly half of the possible values  with a confidence
level of 3 or higher. This is the first experimental result in the world that provided a constrainton . The results also prefer =−/2 corresponding to CP violation, and the range of possible
values  at a confidence level of 3 has been provided to be [−3.41,−0.03] for the case of thenormal ordering of three neutrino masses (𝑚1 < 𝑚2 ≪ 𝑚3 ), and [−2.54, −0.32] for the case of theinverted ordering (𝑚3 ≪ 𝑚1 < 𝑚2 ), respectively. The symmetry-preserving values, 0 or ± arerejected at 95% confidence level, which means that the results indicate CP violation in neutrinos
with 95% confidence.
In the world, the U.S. NOA experiment of competing performance is following up the T2K
experiment, and furthermore, next-generation projects are being prepared in the world. In
Japan, Hyper-Kamiokande is under construction and the measurement of  value is one of its
main objectives. In addition to the enlargement of the neutrino detector at Hyper-Kamiokande,
the further enhancement of neutrino beam intensity is planned to discover CP violation and tomeasure CP phase with high precision. Another large-scale project, DUNE, is also prepared in
the United States for the measurement of . The results of T2K will have a significant impact
on these large-scale projects as well as on future research in particle physics and astrophysics.
References[1] The T2K Collaboration, “Constraint on the matter-antimatter symmetry-violating phase inneutrino oscillations”, Nature 580, 339 (2020).