Success of the Belle experiment at KEK was highlighted by an overwhelming confirmation of Kobayashi-Maskawa mechanism for CP violation in 2001, for which the two were awarded the 2008 Nobel Prize. It established that the CP violation arises from the quark mixing in the Standard Model. However, somewhat under-stressed but equally important accomplishments by this experiment was to over-constrain the Cabibbo-Kobayashi-Maskawa unitary matrix through high precision measurements of a variety of B-meson decays, providing a detailed phenomenological description of the flavor sector in the Standard Model.
Although the statistics is limited, some of these measurements seem to be deviated from the Standard Model prediction, which indicate an existence of a hidden mechanism at a high energy scale, which is also pointed out by several other experimental and theoretical studies.
Flavor physics has provided several critical breakthroughs in the history of establishing the Standard Model. In particular, the Flavor- Changing Neutral Current processes, which appear only in quantum loop corrections, proved powerful for observing effects of heavier particles. Study of B meson decays is a natural place to investigate a wide range of the FCNC processes because the b quark belongs to the third generation and hence is involved in all existing generations of quarks. Powerfulness of flavor physics to uncover physics beyond the Standard Model should be fully exploited, and this situation remains the same even after energy frontier machines discover new particles. In addition to the B-meson decay studies, we search for a lepton-flavor violating decay in τ-lepton decays. The lepton-flavor violating decay is highly suppressed within the Standard Model, but a new physics may enhance the process to a detectable level. Specifically one needs to address:
- Is there any new CP-violating phase?
- Is there any new right-handed current?
- Is there any effect from new Higgs fields?
- Is there any new flavor violation such as lepton-flavor violation?
- Is there any new flavor symmetry that explains the CKM hierarchy?
The Belle II experiment at the SuperKEKB accelerator offers an excellent opportunity for the further exploration into flavor physics. The SuperKEKB is an asymmetric-energy e^+ e^- collider, expected to be commissioned in 2014. It operates at the Upsilon (4S)-resonance energy with the luminosity 40 times higher than the KEKB accelerator and produces 50 times more B and anti-B meson pairs of the Belle experiment in a boosted center-of-mass frame. The boost allows to measure the decay time of each B meson. The original Belle detector is being upgraded into Belle II so that it can maximally exploit the high luminosity operation of SuperKEKB.
At the moment, the Kavli IPMU team plays a central role for assembling the silicon vertex detector that is used to measure the decay vertices of B mesons. Once the construction period is over, the group will study a wide range of subjects in flavor physics and search for a hint of new physics beyond the Standard Model.