T2K experiment enters a new phase with significantly improved sensitivity
for its world leading neutrino oscillation research- Started data taking with upgraded accelerator neutrino beam and new detectors The T2K Collaboration has started data taking using the enhanced neutrino beam and newneutrino near-detectors from December 2023. The KEK/J-PARC center has upgraded the main
ring accelerator and the neutrino beamline to increase the beam power. T2K has also upgraded its
neutrino production instruments. The stable operation of neutrino beam has been successfully
achieved at a record high beam intensity (about 710 kW), an increase of about 40% compared tobefore the upgrade. Furthermore, on December 25th, the continuous operation of neutrino beam
has been successfully achieved at 760kW, which is greater than the initial design beam power.
The pulsed electromagnet (electromagnetic horn) system, the heart of the neutrino generator, wasalso upgraded. The current applied to the electromagnetic horn has been increased from 250 kA
to 320 kA. This allowed us to increase the neutrino intensity by about 10%. In addition, T2Kinstalled new neutrino detectors that can measure neutrino interactions with even higher precision
than before. The newly installed detectors consist of SuperFGD, which detects tracks around aneutrino interaction point inside the detector, High-Angle TPC, which measures momentum ofparticles emitted over a wide range of angles, and Time-of-Flight, which can detect incoming oroutgoing particles and identify particles. Neutrino event candidates were successfully observed
during a technical run of the new detectors after the start of beam operation. In 2020, the T2K
gave the first hints that the symmetry between matter and antimatter could be violated in neutrino
oscillations. With these enhancements, T2K will continue to lead the world in advancing theunderstanding of neutrino properties and unraveling the mystery of the absence of antimatter inthe universe.

Fig. 1 : Illustration of the upgraded neutrino production devices (left) and the new neutrino
detectors (right).

Context
T2K is an experiment to study neutrino oscillations (*1) by sending neutrinos produced at the Japan
Proton Accelerator Research Complex (J-PARC) in Tokai, Ibaraki, to the neutrino near-detectors
and the Super-Kamiokande detector in Kamioka, Hida, Gifu, about 300 km away. The T2K
experiment started taking data in 2010 and directly observed electron neutrino appearance for thefirst time in the world in 2013. In 2014, we started measurements using an anti-neutrino beam toverify CP violation (*2), and in 2020 we strongly restricted possible values of the neutrino CP phase(*3)
for the first time. To obtain evidence of the CP violation, a more precise measurement isrequired to eliminate 0 and ±180 degrees from the possible range of CP phase with a high degreeof confidence. However, in order to realize this, it is necessary to produce more neutrinos and toobtain a deeper understanding of the interactions between neutrinos and nucleus.
(*1) Neutrino oscillation
It is a phenomenon in which a neutrino changes into another type of neutrino as it travels
through space. The discovery of this phenomenon showed that neutrinos have mass and earned
Prof. Takaaki Kajita the Nobel Prize in Physics in 2015 (shared with Prof. Arthur McDonald).
(*2) CP violation
The "C" in CP symmetry represents the "C transformation," which swaps the particle and theanti-particle (for example, swaps the electron and the positron), and the "P" represents the "Ptransformation," which swaps the directions of up, down, left, right, front and back with respect
to space, as if they were mirror images. When the same physical phenomenon occurs with thesame probability when the "C transformation" and the "P transformation" are performed, it iscalled "CP symmetry". When a phenomenon does not obey CP symmetry, it is called "CPviolation". The CP violation is one of the conditions that explain the fact that the current
universe is dominated by matter. However, the quark CP violation observed so far is so smallthat it cannot explain the amount of matter in the universe today. Therefore, the neutrino CP
violation is expected to provide a major hint for the mystery.
(*3) CP phase
The CP phase is a fundamental property of the "weak interaction" between elementary particles,
introduced by Prof. Makoto Kobayashi and Prof. Toshihide Maskawa to explain CP violationin quarks. The CP phase can take values between -180 and 180 degrees, but for leptons such as
electron and neutrino, the values have been completely unknown until recently. T2K
experiment excluded nearly half of the range of possible values of the CP phase in 2020 with aconfidence level of 99.7% (3 sigma).

Points of improvement
The T2K Collaboration, an international collaborative experiment involving approximately 570
researchers from 78 research institutions in 14 countries, has launched a new phase of theexperiment using an enhanced neutrino beam and new near-detectors. Neutrinos are produced
from decayed pions or other particles produced in interactions between proton beams and agraphite target. The KEK/J-PARC Center upgraded the J-PARC main ring accelerator, including
the power supply for the main magnet, to increase the repetition rate of the proton beam from
2.48 seconds to 1.36 seconds, which supplies more protons to the neutrino production target. The
T2K experimental group upgraded, modified, and exchanged instruments in the neutrino beamfacility such as targets, electromagnetic horns, and beam monitors. The beam commissioning wasstarted in November 2023. The stable production of neutrino beam has been successfully achieved
at a record high beam power (about 710 kW), an increase of about 40% compared to before theupgrade. Furthermore, on December 25th, the continuous operation of neutrino beam has beensuccessfully achieved at 760kW, which is greater than the initial design beam power. The heart of
the neutrino generator is the electromagnetic horns (Fig.2). The current applied to the threeelectromagnetic horns was increased from 250kA to 320 kA by upgrading the power supply andother components, thereby improving the focusing efficiency of parent particles such as pions
produced at the target. This improves the quality of the neutrino beam delivered to the SuperKamiokande detector while increasing the number of neutrinos observed by about another 10%.

Fig. 2: Upgraded 2nd electromagnetic horn with improved cooling capacity to enable neutrino
production by high-intensity proton beams.
In addition, the T2K Collaboration has started observation using new neutrino neardetectors(Fig.3) at the Neutrino Monitor Building located 280 m downstream of the neutrino
production target. By October 2023, three new types of detectors have been installed. A newdetector, SuperFGD with a mass of approximately 2 tons of sensitive volume, is located at thecenter of the upgraded detectors. It has an innovative structure consisting of approximately 2millions 1cm3 cubes each with 3 holes, made of plastic scintillator. Approximately 56,000 opticalfibers penetrate the cubes from three directions and photodetectors are at the ends of the fibers.
Charged particles can be observed in high resolution from three projections and 3D tracks can bereconstructed. Below the SuperFGD, the first High Angle Time Projection Chambers (HATPC)
has been installed. The HATPC is a gaseous detectors composed by two field cages that produce
a uniform electric field and that uses, as readout system, resistive Micromegas modules. HATPC
is an innovative detector that allow for an excellent reconstruction of the track trajectory andhence of the momentum of particles emitted by neutrino interactions in the SuperFGD. Finally,
the detectors surrounding the SuperFGD and HATPC are Time-of-Flight detectors. It is used todetermine the direction of particles and particle identification. T2K started measurements withthe upgraded neutrino beam in December 2023 and succeeded in observing neutrino event
candidates from the newly acquired data (Fig. 4, 5).

Fig. 3: A photo of the new detectors.

Fig. 4: An event display of a neutrino interaction candidate in the SuperFGD with a track
entering the bottom HATPC while another track is entering the original detectors.

Fig. 5: The beam timing structure observed with one of the new detectors, Time-of-Flight.

OutlookWith these improvements, the T2K experiment enters a new phase with an enhanced neutrino
beam and novel new detectors. The J-PARC accelerator and neutrino experimental facility are
undergoing an upgrade plan to further increase the output power to 1.3 MW (= 1300 kW) while
supplying beams for the T2K experiment. Together with the upgraded neutrino production
instruments such as the electromagnetic horns with improved focusing efficiency, it will bepossible to observe about three times as many neutrino interactions (per unit time) as before, andto reduce the error originating from statistical variations (statistical error) in the observed data. Inaddition, the new detector can detect large angle scattering in the neutrino interactions, which was
not possible with our original detector. This will enable a better understanding of the neutrinonucleus interactions and therefore reduces the systematic errors. Furthermore, the SuperKamiokande detector has also improved its detector performance with a much higher neutron
detection efficiency by loading gadolinium in the water. The T2K experiment will significantly
improve the sensitivity of the measurements through these improvements and proceed to verify

the differences in neutrino and antineutrino behavior. The J-PARC high-intensity proton
accelerator and neutrino experimental facility are expected to play a key role in the next
generation of neutrino research. The new phase of the T2K experiment is an important step toward
the next generation of experiments and T2K is expected to continue to lead the world in neutrino
research unraveling the mystery of the missing antimatter from our universe.

Media contacts for further inquires
Globally:
Dr. Ken Sakashita, Spokesperson, KEK/J-PARC (Tokai, Japan)
Dr. Kendall Mahn, International Co-Spokesperson, Michigan State University (Michigan, USA)
About KEK:
PR office, High Energy Accelerator Research Organization (KEK, Japan)
Email: press@kek.jp
Phone: +81-29-879-6047
About ICRR:
Makio Nakamura, PR Office, Institute for Cosmic Ray Research, The University of Tokyo (ICRR,Japan)
Email: icrr-pr@icrr.u-tokyo.ac.jp
Phone: +81-80-4869-4539
About J-PARC:
PR section, J-PARC
Email: pr-section@j-parc.jp
Phone : +81-29-284-4578