T2K experiment shows strongest evidence yet of symmetry breaking in neutrinos

Inside Super Kamiokande detector

Inside the Super-Kamiokande detector, which receives the neutrino beam in the T2K experiment.

The T2K Collaboration, an international particle physics experiment that includes Colorado State University researchers, has revealed the strongest evidence yet of fundamental differences between matter and antimatter neutrino oscillations. The results were published April 15 in the journal Nature. T2K, based in Japan, includes over 500 scientists worldwide.

For over a decade, T2K scientists have used beams of muon neutrinos and muon antineutrinos to study how particles and antiparticles transition into electron neutrinos and electron antineutrinos, respectively.

The parameter governing the matter/antimatter symmetry breaking in neutrino oscillation, called charge parity violation, can take a value from -180º to 180º. For the first time, T2K has disfavored almost half of those possible values at a 99.7% confidence level and is starting to reveal a basic property of neutrinos that has not been measured until now.

This new evidence is an important step in knowing whether neutrinos and antineutrinos behave differently. Understanding how neutrinos and antineutrinos differ will help scientists explain why so much matter exists in the universe, and so little antimatter.

CSU contributions

Walter Toki, a professor in the Department of Physics, formed a group to contribute to the T2K experiment more than 10 years ago. The group, which included CSU professors Robert Wilson, Norm Buchanan and many former students and postdoctoral researchers, designed and built components of the U.S. detector, called the P0D, for which Toki served as operations manager from 2011-2017. CSU group members helped take the data for the recently published Nature paper.

It is generally believed that equal amounts of matter and antimatter were created at the beginning of the universe. However, the universe observed today is composed of matter with little antimatter.

For the universe to end up with more matter than antimatter, a violation of a law, the so-called charge parity symmetry, was necessary. Until now, CP symmetry violation has only been observed in the physics of subatomic particles called quarks, but the magnitude of the CP symmetry violation has not been large enough to explain the matter-antimatter imbalance.

T2K is now searching for a new source of CP symmetry violation in neutrino oscillations that would manifest as a difference in the measured oscillation probability for neutrinos and antineutrinos.

CP violation disfavoring balance illustration

The arrow indicates the value most compatible with the data. The gray region is disfavored at 99.7% (3s) confidence level. Nearly half of the possible values are excluded.

The T2K experiment uses a beam consisting primarily of muon neutrinos or muon antineutrinos created with the proton beam from the Japan Proton Accelerator Research Complex (J-PARC) located in Tokai village on the east coast of Japan. A small fraction of the neutrinos (or antineutrinos) are detected 295 kilometers away at the Super-Kamiokande detector, located under a mountain in Kamioka, near the west coast of Japan. As the muon neutrinos and muon antineutrinos traverse the distance from Tokai to Kamioka (hence the name T2K), a fraction will oscillate or change flavor into electron neutrinos or electron antineutrinos respectively.

The electron neutrinos and electron antineutrinos are identified in the Super-Kamiokande detector by the rings of Cherenkov light they produce. While Super-Kamiokande cannot identify each event as a neutrino or antineutrino interaction, T2K is able to study the neutrino and antineutrino oscillations separately by operating the beam in neutrino mode or antineutrino mode.

candidate neutrino and antineutrino events

Event displays of candidate electron neutrino (left) and electron antineutrino (right) events observed in Super-K from the T2K neutrino beam. When an electron neutrino or antineutrino interacts with water, an electron or positron is produced. They emit a faint ring pattern light, which is detected by about 11,000 photo-sensors. The color in the displays represents the photon detection timing.

Upgrades planned

Despite the Nature results showing a strong preference for enhancement of the neutrino rate in T2K, it is not yet clear if CP symmetry is violated or not. To further improve the experimental sensitivity, the T2K Collaboration will upgrade the near detector suite to reduce systematic uncertainties and accumulate more data, and J-PARC will increase the beam intensity by upgrading the accelerator and beamline.

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, which currently consists of nearly 500 scientists from 68 institutions in 12 countries – Canada, France, Germany, Italy, Japan, Poland, Russia, Spain, Switzerland, United Kingdom, the United States and Vietnam. This result is made possible by the efforts of J-PARC to deliver high-quality beam to T2K.

The U.S. T2K collaborating team is composed of 13 institutions – Boston University, University California, Irvine, University of Colorado, Colorado State University, Duke University, University of Houston, Louisiana State University, Michigan State University, University of Pennsylvania, University of Pittsburgh, University of Rochester, Stanford Linear Accelerator Center, and Stony Brook University – and is funded by the U.S. Department of Energy, Office of Science.

The U.S. 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. They are currently participating in an upgrade of the T2K near detector, SuperFGD. They are also part of the team that built, upgraded and operates the Super-Kamiokande detector.

More information: http://t2k-experiment.org