Study of rare K-meson decays

We search for and measure the transitions of K mesons (kaons) into lighter particles via specific patterns, on very rare occasions, due to broken symmetries in nature.

Purpose and Vision

Due to interactions among quarks that constitute a heavy particle, the particle makes a quantum transition into lighter particles. The transition is called "decay" in particle physics. The decay of a particle proceeds via several patterns of "decay modes";by studying the decay modes precisely,we are able to understand the interactions among quarks.

Our research is to search for the decay of the particles named "K mesons (kaons)" into lighter particles via specific patterns, on very rare occasions as once every forty billion decays, and to discover new laws of physics that are observable only through the rare processes.


We perform new Kaon Decay experiments at Japan Proton Accelerator Research Complex (J-PARC), Tokai village, Ibaraki, with newly-constructed beam lines in the Hadron Experimental Facility.

The E14 KOTO experiment measures rare neutral-kaon decays to reveal new sources of symmetry breaking that can explain the matter-antimatter asymmetry in the universe.

In particular, the decay of a long-lived neutral kaon into a neutral pi meson (pion) and pair of neutrons will be observed for the first time. "KOTO" is the abbreviated name for "K0 at TOkai".

At present,37 experimental physicists from the institutes in Japan (KEK, Osaka University, Kyoto University, Saga University, Yamagata University, and NDA) and 28 from abroad (USA, Taiwan, Korea, Russia) participate in the KOTO experiment.They completed the detector in JFY 2012 and started the experiment in JFY 2013.

Moreover, the E06 TREK experiment measures, with a high statistical precision, the polarization of the muon spin in the decay of a charged kaon into a neutral pion, a muon, and a neutrino to reveal the breaking of Time reversal. "TREK" is the abbreviated name for "Time Reversal Experiment with Kaons".

At present, 15 experimental physicists from the institutes in Japan(KEK, Osaka University, Kyoto University, Tokyo Institute of Technology, Tohoku University, NDA) and 31 from abroad (Canada, USA, Russia, Vietnam, Thailand) participate in the TREK experiment.

They performed the beam survey in JFY 2010 and 2011, and continue the preparation of the detector.

Related WEB sites

J-PARC E14 KOTO Experiment
J-PARC TREK Experiment

KEK-PS E391a Experiment
KEK-PS E246 T-violation Experiment
The J-PARC KOTO experiment

Related facilities

KL beam line