Project title
Experiment ëåë-å246 "Search for T-violating transverse muon polarization in the decays of stopped positive kaons"

Members:institutes and universities from Japan, Russia, Canada,
USA, Korea and Taiwan

Scientific motivation of the project
The laws of nature are generally invariant under the symmetry operations

1) of parity (mirror reflection) P ;
2) charge conjugation (matter --> antimatter) C;
3) and time reversal T.
As found in the 1950s P is violated maximally by the weak force. Later, in 1964, a combined CP violation has been discovered. CP is violated at the level of 0.1% by the weak interaction, and has only been observed in the decays of neutral kaons. Recently, in 2001, the first reports on CP violation in B-meson systems appeared.
The origin of CP violation remains mysterious to this day. Within the framework of the Standard Model (which successfuly describes most of the phenomena of the high energy physics) CP violation can be introduced through quark mixing effects (Kobayashi-Maskawa mechanism). Such mixings allow us to give predictions that could be tested in K- and B-meson decays.

The Standard Model, however, does not really explain the underlying reason for CP violation. Furthermore, it appears that an additional source of CP violation from some unknown new physics may be necessary to explain the matter-antimatter asymmetry of our universe.

Testing the Standard Model's description of this phenomenon and searching for non-standard CP-violation are major goals of high-energy physics.
Following that path may lead us to an understanding of the origin of mass and our Universe.
Searches for an electric dipole moments and the transverse muon polarization in K+ => (pi^0)(mu^+)(nu) decay are the experiments that can be sensitive to CP violation beyond the Standard Model. E246 experiment is searching for the last process.

Gist of the project.
What we measure is a transverse polarization of the muon from Kmu3 decay of positive kaon (kaon decays into neutral pion, positive muon and muon neutrino). Muon polarization is determined by its spin which according to Standard Model should be parallel to the kaon decay plane: the other (transverse) direction is predicted to be very small - less than 10-6 (one millionth), which is 3-4 orders below the current experimental level of sensitivity. Some non-standard extensions, such as three Higgs doublet model (3HDM), Supersymmetrical models (SUSY) with/without R-parity violation and Leptoquark models predict muon transverse polarization in Kmu3 at the level 0.1-1%, which could be achieved in our experiment (its sensitivity is about 0.25%).
The experimental setup at K5 line of KEK, Japan, consists of the Cherenkov detector for kaon identification; active target, where the kaon stops and decays; plastic scintillation hodoscope; electromagnetic calorimeter (an array of CsI(Tl) modules ) for neutral pion detection; charged particle spectrometer based on the superconducting toroidal magnet with multi-wire proportional chambers (MWPC); polarimeter; a number of other detectors, electronics and DAQ.

Brief history.
1991 - a proposal has been approved
1992 - test of the prototype of the electromagnetic calorimeter
1992-1995 - R & D; creating the experimental setup
1996-2000 - data collecting; analysis.
2001-... - data analysis; systematics study;
data collecting for a new experiment KEK-E470 "Measurement of $K^+\to\pi^+\pi^0\gamma$ decay with direct emission photon" (using E246 experimental setup).