T2K Experiment Enters a New phase with Significantly Improved Sensitivity for Its World-leading Neutrino Oscillation Research

Data Taken with Upgraded Accelerator Neutrino Beam and New Detectors

STONY BROOK, NY - (Press Embargoed, Wednesday, January 17, 2024, Midnight, EST) - The T2K Collaboration has started data taking using the enhanced neutrino beam and new neutrino near-detectors from December 2023. Stony Brook University SUNY Distinguished Professor, Chair of the Department of Physics and Astronomy and United States lead Principal Investigator of the T2K Collaboration Chang Kee Jung is available to discuss this recent advancement regarding the T2K experiment.

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 to before the upgrade.

Furthermore, on December 25^th, the continuous operation of neutrino beam was 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, was also 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, T2K installed 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 a neutrino interaction point inside the detector, High-Angle TPC, which measures momentum of particles emitted over a wide range of angles, and Time-of-Flight, which can detect incoming or outgoing 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, 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 the understanding of neutrino properties and unraveling the mystery of the absence of antimatter in the universe.

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

Context

T2K is an experiment developed 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 the first time in the world in 2013.

In 2014, researchers started measurements using an anti-neutrino beam to verify CP violation ^(*2), and in 2020 they 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 is required to eliminate 0 and ±180 degrees from the possible range of CP phase with a high degree of confidence. However, in order to realize this, it is necessary to produce more neutrinos and to obtain 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 Professor Takaaki Kajita the Nobel Prize in Physics in 2015 (shared with Professor Arthur McDonald).

(*2) CP violation

The "C" in CP symmetry represents the "C transformation," which swaps the particle and the anti-particle (for example, swaps the electron and the positron), and the "P" represents the "P transformation," 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 the same probability when the "C transformation" and the "P transformation" are performed, it is called "CP symmetry". When a phenomenon does not obey CP symmetry, it is called "CP violation". 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 small that 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 Professor Makoto Kobayashi and Professor Toshihide Maskawa to explain CP violation in 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. The T2K experiment excluded nearly half of the range of possible values of the CP phase in 2020 with a confidence 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 the experiment 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 a graphite 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 beam facility such as targets, electromagnetic horns, and beam monitors. The beam commissioning was started 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 the upgrade. Furthermore, on December 25^th, the continuous operation of neutrino beam has been successfully 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 three electromagnetic horns was increased from 250kA to 320 kA by upgrading the power supply and other 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 Super-Kamiokande detector while increasing the number of neutrinos observed by about another 10%.

Fig. 2: Upgraded 2^nd 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 near-detectors(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 new detector, SuperFGD with a mass of approximately 2 tons of sensitive volume, is located at the center of the upgraded detectors. It has an innovative structure consisting of approximately 2 million 1cm³ cubes each with 3 holes, made of plastic scintillator. Approximately 56,000 optical fibers 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 be reconstructed. Below the SuperFGD, the first High Angle Time Projection Chambers (HATPC) has been installed. The HATPC is a gaseous detector 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 and hence 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 to determine the direction of particles and particle identification. T2K started measurements with the 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.

Outlook

With 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 be possible to observe about three times as many neutrino interactions (per unit time) as before, and to reduce the error originating from statistical variations (statistical error) in the observed data. In addition, 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 neutrino-nucleus interactions and therefore reduces the systematic errors. Furthermore, the Super-Kamiokande 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.

The T2K experiment is supported by the Japanese Ministry for Culture, Sports, Science, and Technology (MEXT), and is jointly hosted by the High Energy Accelerator Research Organization (KEK) and the University of Tokyo's Institute for Cosmic Ray Research (ICRR). The T2K experiment was constructed and is operated by an international collaboration including US institutions.

The current US T2K collaborating team is composed of 14 institutions [Boston University, University California, Irvine, University of Colorado, Duke University, University of Houston, Lawrence Berkeley National Lab, Louisiana State University, Michigan State University, University of Pennsylvania, University of Pittsburgh, University of Rochester, South Dakota School of Mines and Technology, Stanford Linear Accelerator Center and Stony Brook University] and is funded by the US Department of Energy, Office of Science. The US groups have built super-conducting corrector magnets, proton beam monitor electronics, the second neutrino horn and a GPS time synchronization system for the T2K neutrino beamline; and a pi-zero detector and a side muon range detector (partial detector) in the T2K near detector complex for the original T2K experiment. They were also part of the team that built, upgraded and operated the Super-Kamiokande detector.

Most recently a group of US institutions participated in an upgrade of the T2K near detector as described above, and made significant contributions to the construction, installation and commissioning of the SuperFGD detector. The specific US contributions to SuperFGD are: (1) Scintillator Cubes (Stony Brook U.); (2) Wave Length Shifting Fibers (U. of Rochester); (3) Photon Sensors (Louisiana State U.); (4) Mechanical Structure (U. of Colorado); (5) Electronics (Louisiana State U, U. of Pennsylvania and U. of Pittsburgh); and (6) Project Management (Stony Brook U.).

"The neutrino is our most fascinating and mysterious fundamental constituent of matter," said Nobel Prize in Physics Laureate and Stony Brook University President's Distinguished Endowed Chair in Physics Professor Barry Barish. "It was proposed almost 100 years ago to explain the apparent mysterious disappearance of energy in nuclear beta decay. Since its experimental discovery, the mysterious neutrino has become both our most fascinating particle and a powerful probe of the fundamental laws of nature. The newly upgraded T2K experiment, a marvel of modern technology and large collaborative science, is poised to resolve puzzles and make new discoveries that result from the neutrinos having a very tiny unexplained mass."

"Absence of a antimatter or matter dominated universe is one of the most profound mysteries in science that is directly related to our own existence," said Jung. "Discovery of CP violation in neutrinos is currently our best path to obtain a critical clue to this mystery. T2K has taken the first step in quest of CP violation in neutrinos in 2020, and now with the upgraded J-PARC accelerator, neutrino beamline and the near detector, it is poised to take significant steps toward the discovery. I eagerly look forward to seeing what new data reveals in the next few years with a more powerful neutrino beam and new state-of-art detectors."

More information about the T2K experiment can be found on the T2K public website (http://t2k- experiment.org).

Media Contacts for Further Inquiries in U.S.:

(For contacts in Japan and elsewhere in the world, see global media contact information below.)

• Prof. Chang Kee Jung, US Representative, Stony Brook University (Stony Brook, NY), [email protected], Phone: +1 631-707-2018.
• Prof. Daniel Cherdack, University of Houston (Houston, TX), [email protected], Phone: 201-693-6117.
• Prof. Edward Kearns, Boston University (Boston, MA), [email protected], Phone: 617-353-3425.
• Prof. Thomas Kutter, Louisiana State University (Baton Rouge, LA), [email protected], Phone: 225-578-8310.
• Prof. Kendall Mahn, Michigan State University (East Lansing, MI), [email protected], Phone: 920-915-4417.
• Prof. David Martinez, South Dakota School of Mines and Technology (Rapid City, SD), [email protected], Phone: 219-316-9749.
• Prof. Christopher Mauger, University of Pennsylvania (Philadelphia, PA), [email protected],Phone: 505-500-7541.
• Prof. Kevin McFarland, University of Rochester (Rochester, NY), [email protected], Phone: 775-538-4668, 585-275-7076.
• Prof. Vittorio Paolone, University of Pittsburgh (Pittsburgh, PA), [email protected], Phone: 412-877-4079.
• Prof. Kate Scholberg, Duke University (Durham, NC), [email protected], Phone: 919-660-2962.
• Prof. Hank Sobel, University of California, Irvine (Irvine, CA), [email protected], Phone: 949- 824-6431.
• Prof. Hiro Tanaka, Stanford Linear Accelerator Center, Stanford University (Menlo Park, CA), [email protected], Phone: 650-885-1565
• Dr. Callum Wilkinson, Lawrence Berkeley National Lab (Berkeley, CA), [email protected]
• Prof. Eric Zimmerman, University of Colorado (Boulder, CO), [email protected], Phone: 303-735-5338.

Media Contacts for Further Inquiries (Global):

Globally:

• Dr. Ken Sakashita, Spokesperson, KEK/J-PARC (Tokai, Japan), Email: [email protected], Phone: +81-29-284-4656
• Dr. Kendall Mahn, International Co-Spokesperson, Michigan State University (Michigan, USA), Email: [email protected], Phone: +1-920-915-4417

About KEK:

PR office,High Energy Accelerator Research Organization (KEK, Japan), Email: [email protected], Phone: +81-29-879-6047

About ICRR:

Makio Nakamura, PR Office, Institute for Cosmic Ray Research, The University of Tokyo (ICRR, Japan), Email: [email protected], Phone: +81-80-4869-4539

About J-PARC:

PR section, J-PARC, Email: [email protected], Phone : +81-29-284-4578

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