T2K presents first CP violation search result

August 30, 2016

The international T2K Collaboration announced today their findings on the symmetry between neutrino and antineutrino oscillation. With nearly twice the antineutrino data in 2016 compared to their initial results using antineutrino data in 2015, T2K has performed a new analysis of all data, fitting both neutrino and antineutrino modes simultaneously. Today’s announcement was made by Konosuke Iwamoto, of the University of Rochester, who presented the results at the 38th International Conference on High Energy Physics in Chicago. T2K’s new data continue the trends observed in 2015, which is a preference for maximal disappearance of muon neutrinos, as well as a discrepancy between the electron neutrino and electron antineutrino appearance rates.

Why the universe is dominated by matter today, instead of being comprised of equal parts matter and antimatter, is one of the most intriguing questions in all of science. One of the conditions required for the observed dominance of matter over antimatter to develop is the violation of Charge-Parity (CP) symmetry, which is the principle that the laws of physics should be the same if viewed upside-down in a mirror, with all matter exchanged with antimatter. If CP violation occurs in neutrinos, it will manifest itself as a difference in the oscillation probabilities of neutrinos and antineutrinos.

In the T2K experiment in Japan, a muon neutrino beam is produced at the Japan Proton Accelerator Research Complex (J-PARC) located in Tokai village, Ibaraki prefecture, on the east coast of Japan. The neutrino beam is created by directing 30 GeV protons from the J-PARC Main Ring (MR) accelerator onto a cylindrical target to produce an intense secondary particle beam that is focused and filtered by strong magnetic fields called neutrino horns. The focused particle beam decays into a beam of muon neutrinos or antineutrinos, depending on the sign of the filtering done by the neutrino horns. The neutrino/antineutrino beam is monitored by a detector complex in Tokai, 280 m away from the neutrino target and aimed at the gigantic Super-Kamiokande underground detector in Kamioka, near the west coast of Japan, 295 kilometers (185 miles) away from Tokai.

T2K’s observed electron antineutrino appearance event rate is lower than would be expected based on the electron neutrino appearance event rate, assuming that CP symmetry is conserved. T2K observes 32 electron neutrinos and 4 electron antineutrinos, when they expect around 23 neutrinos and 7 antineutrinos with no CP violation. When analyzed in a full framework of three neutrino and antineutrino flavors, and combined with measurements of electron antineutrino disappearance from reactor experiments, the T2K data favor maximal CP violation.

More specifically, the size of the expected T2K 90% confidence interval for the CP violation parameter δCP with the current data set ranges from approximately 2π (ie. the full range of δCP) to 1π, depending on the true value of δCP and the true mass ordering. The actual T2K data yield a 90% confidence interval for δCP of [–0.99π ; –0.12π] ([–0.67π ; –0.24π]) for the normal (inverted) mass ordering. The best fit points lie near the maximally CP violating value δCP=–0.5π , with the CP conserving values (δCP=0 and δCP= π) lying outside of the T2K 90% confidence level interval.

The T2K experiment is primarily supported by the Japanese Ministry for Culture, Sports, Science, and Technology, 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. The current T2K collaboration consists of over 450 scientists from 61 institutions in 11 countries [Canada, France, Germany, Italy, Japan, Poland, Russia, Spain, Switzerland, UK, and USA].

This observation is made possible by the tireless efforts of J-PARC to deliver high quality beam to T2K despite many difficulties, including the devastating March 2011 earthquake in eastern Japan, which caused severe damage to the accelerator complex at J- PARC and abruptly discontinued the data-taking run of the T2K experiment.

The search for CP symmetry violation with neutrinos leverages T2K’s 2013 discovery of electron neutrino appearance in a muon neutrino beam, which was the first observation of the appearance of a different neutrino flavour from the original. This appearance is an example of neutrino oscillation, a purely quantum mechanical phenomena that comes about because of the interference of neutrino flavor and mass; neutrino oscillation can only occur if neutrinos have non-zero mass. T2K’s 2013 electron neutrino appearance discovery resulted in a share of the 2016 Breakthrough Prize for Fundamental Physics being awarded to Koichiro Nishikawa and the entire T2K collaboration.

This 2016 result is based on a total data set of 1.51x1021 protons on target (POT), which is 19% of the POT exposure that T2K is set to receive. The probability that this observation is a result of random statistical fluctuations that would mimic a neutrino-antineutrino asymmetry when none exists is about 1 in 20, motivating the need for more neutrino and antineutrino data to explore and solidify these intriguing results. The full T2K exposure of 7.8x1021 POT is expected to come by ~2021, thanks to planned upgrades to the J-PARC MR accelerator and the neutrino beamline . Moreover, T2K is proposing a run extension that will lead to a full exposure of 20x1021 POT, with 3σ sensitivity to CP violation observation (for certain values of oscillation parameters) by ~2025, when the next generation experiments are expected to begin operations.

More details on the new T2K result, as well as prospects for future running of the experiment, can be found in the presentation file from the 38th International Conference on High Energy Physics in Chicago, and more information about the T2K experiment can be found on the T2K public website .