T2K Near Detector Upgrade

T2K presents first CP violation search result

New data support growing hint of different oscillation probabilities for neutrinos and antineutrinos

The T2K Collaboration presented new results on neutrino and antineutrino oscillations at the 27^th International Conference on Neutrino Physics and Astrophysics (Neutrino 2016) at Imperial College London.  T2K’s new data continue to prefer maximal mixing in the atmospheric angle (θ₂₃), a value of the CP violating phase (δ_CP) near the maximally violating value -π/2, and the normal ordering of the neutrino mass hierarchy.

With nearly twice the antineutrino data in 2016 compared to their 2015 result, T2K has performed a new analysis of all data, as shown in Fig 1, fitting both neutrino and antineutrino modes simultaneously. If CP violation occurs in neutrinos, it will manifest itself as a difference in the oscillation probabilities of neutrinos and antineutrinos.  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.

When analyzed in a full framework of three neutrino and antineutrino flavors, and combined with measurements of electron antineutrino disappearance from reactor experiments, the size of the expected T2K 90% confidence interval for δ_CP with the current statistics ranges from approximately 2π (ie. the full range of δ_CP) to 1π depending on the true value of δ_CP  and the true mass ordering. The actual T2K data yield a 90% confidence interval for δ_CP of [–3.02; –0.49] ([–1.87 ; –0.98]) for  the normal (inverted) mass ordering, as shown in Fig 2. The CP conserving values (δ_CP=0 and δ_CP= π) lie outside of this interval.

This new result is based on a data set of 1.44×10²¹ protons on target (POT), which is 20% of the POT exposure that T2K is set to receive.  The full T2K exposure of 7.8×10²¹ POT is expected to come by ~2021, thanks to planned upgrades to the J-PARC Main Ring accelerator and the neutrino beamline. Moreover, T2K is proposing a run extension that will lead to a full exposure of 20×10²¹ POT, with 3σ sensitivity to CPV observation, by ~2025, when the next generation experiments are expected to begin operations.

Violation of CP symmetry could hold the key to one of the most profound questions in science, which is: why is the universe comprised of matter today even though the Big Bang produced equal parts matter and antimatter?  Although the new T2K result is not yet statistically significant, it is nevertheless an intriguing hint that the neutrino will continue to provide new breakthroughs in our understanding of the universe.

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 London Neutrino 2016 conference.

Figure 1. Neutrino(top) and antineutrino(bottom) event distributions at the T2K far detector (Super-K), for both muon (left) and electron (right) flavors. In each figure, the black points show T2K (anti)neutrino data, the black curves show the expectations for the case of no neutrino oscillation, and the blue curves show the expectation for the best fit oscillation parameter values.

Figure 2. Negative log likelihood values as a function of the CP violating phase parameter δCP; The black (red) curves show the case for the normal (inverted) mass ordering; the black (red) vertical lines with hatch marks show the 90% CL allowed regions for the normal (inverted) mass ordering. This figure shows the result for T2K neutrino and antineutrino data, combined with reactor antineutrino results. The CP conserving values (δCP =0 and δCP= π) lie outside the 90% region.

J-PARC Nu Beam Power Exceeds 400 kW!

The J-PARC neutrino beam, which provides neutrinos and antineutrinos for the T2K experiment, has exceeded 400 kW beam power.  A new Main Ring accelerator tune allowed the beam power to exceed the 400 kW threshold on May 20, and the T2K beam ran stably above 400 kW from May 23 until May 27, when the T2K beam shut down for the summer.

Snapshot of J-PARC accelerator status webpage, showing beam power above 400 kW.

T2K run extension being studied

The T2K collaboration is considering the improvements in physics sensitivity, especially for CP violation, from an extension of the running time with beam power of ~1.3 MW. An Expression of Interest for the T2K extension was presented to the J-PARC PAC held  January 13-15, 2016.

J-PARC Main Ring upgrade approved

A new facility in J-PARC to upgrade the Main Ring accelerator is approved. This facility will house a new power supply system with which the repetition rate of the Main Ring will be doubled―resulting in 750 kW beam power for T2K, with potential to exceed 1 MW.

Koichiro Nishikawa and members of T2K Collaboration awarded Breakthrough Prize

Dr Koichiro Nishikawa and the members of the T2K collaboration have been awarded the prestigious Breakthrough Prize for Fundamental Physics, for their role in the discovery and study of neutrino oscillation.

The prize, presented by the Breakthrough Prize Foundation, was awarded “for the fundamental discovery of neutrino oscillations, revealing a new frontier beyond, and possibly far beyond, the standard model of particle physics”. The prize is valued at 3 million USD, and is shared with four other international experimental collaborations studying neutrino oscillation: The Daya Bay, KamLAND, SNO, and Super-Kamiokande scientific collaborations. The T2K collaboration is named together with the K2K collaboration for its share of the prize. Dr. Nishikawa is the founding spokesperson of the T2K and K2K collaborations.

The award was presented at a ceremony at the NASA Ames Research Centre in Moffett Field, California. The ceremony was broadcast live in the U.S. on the National Geographic Channel, and was hosted by comedian Seth Macfarlane. A one-hour version of the broadcast is scheduled for Fox on Nov. 29, at 7 p.m. ET.

T2K is an accelerator-based long-baseline neutrino experiment in Japan. It uses the J-PARC Main Ring proton accelerator to create an intense beam of muon neutrinos. The neutrinos are directed to the Super-Kamiokande detector in the Kamioka mine deep inside Mt Ikeno, 295 km away from J-PARC. T2K’s citation for the prize was given for the observation of electron neutrino appearance in the muon neutrino beam, which is the first observation of the appearance of a neutrino flavour. This discovery sets the stage for the study of differences in the neutrino oscillation process relative to their antiparticles (antineutrinos), called CP violation, that may elucidate how the universe came to be matter dominated. T2K has recently started data-taking with an antineutrino beam to study antineutrino oscillations.

The T2K Collaboration has included over 500 members from 64 institutions in 12 countries:
University of Alberta; University of British Columbia; University of Regina; University of Toronto; TRIUMF; University of Victoria; University of Winnipeg; York University (Canada);
IPN Lyon (IN2P3); IRFU, CEA Saclay; LLR Ecole polytechnique (IN2P3); LPNHE, UPMC, Paris (France);
RWTH Aachen University (Germany);
INFN Sezione di Bari; INFN Sezione di Roma; Napoli University and INFN; Padova University and INFN (Italy);
ICRR, Kamioka Observatory; ICRR, RCCN; ICRR, University of Tokyo; Kavli IPMU (WPI); University of Tokyo; KEK; Kobe University; Kyoto University; Miyagi University of Education; Okayama University; Osaka City University; Tokyo Metropolitan University; University of Tokyo (Japan);
Chonnam National University, Donhshin University, Seoul National University (Republic of Korea);
IFJ PAN, Krakow; NCBJ, Warsaw; University of Silesia, Katowice; Warsaw University of Technology; Wroclaw University; University of Warsaw (Poland);
INR (Russia);
IFAE, Barcelona; IFIC, Valencia (Spain);
ETH Zurich; University of Bern; University of Geneva (Switzerland);
Imperial College London; Oxford University; Queen Mary, University of London; STFC Daresbury Laboratory; STFC Rutherford Appleton Laboratory; University of Lancaster; University of Liverpool; University of Sheffield; University of Warwick (UK);
Boston University; Brookhaven National Laboratory, Colorado State University; Duke University; Louisiana State University; Michigan State University; Stony Brook University; University of California, Irvine; University of Colorado; University of Pittsburgh; University of Rochester; University of Washington (USA).

T2K is hosted jointly by the High Energy Accelerator Research Organization (KEK) and the Institute for Cosmic Ray Research, University of Tokyo (ICRR).

For more information, please visit https://breakthroughprize.org

Takaaki Kajita wins 2015 Nobel prize in physics

The  Royal Swedish Academy of Sciences has announced that T2K collaboration member Takaaki Kajita will be awarded the 2015 Nobel prize in physics.  Prof Kajita, Director of  The Institute for Cosmic Ray Research (ICRR), University of Tokyo, shares the award with  Prof  Arthur McDonald  (Queen’s University, CA) “for the discovery of neutrino oscillations, which shows that neutrinos have mass.”

Prof Kajita’s Nobel-prize winning work was on the Super-Kamiokande (Super-K) experiment.  Super-K, which serves as the far detector for T2K, is a gigantic underground water Cherenkov detector that discovered atmospheric neutrino oscillation.  When cosmic rays interact in the earth’s atmosphere, they create both electron and muon neutrinos, which Super-K can distinguish with high accuracy. Although the rate of electron neutrinos coming from above is consistent with rate of electron neutrinos from below, Prof Kajita and his Super-K colleagues found that the rate of muon neutrinos coming from below, which is to say those that travel a long distance through the earth, is much smaller than the rate of muon neutrinos from above.  We now know that the muon neutrinos are turning into tau neutrinos, and because the neutrino energy is too low to create the heavy tau particles, they do not interact.  This phenomenon is called neutrino disappearance and its discovery by Prof Kajita and Super-K is considered the dawn of a new field of particle physics, which T2K is now continuing.

Congratulations to Kajita-san!