Overview of the experiments
based on the Toroidal spectrometer

The experiment uses a stopped K+ beam in conjunction with a 12-sector iron core superconducting toroidal spectrometer. The kaons are slowed down by a degrader and stopped in the active target. Charged particles from the target are tracked and momentum-analyzed using position trackers such as MWPC and GEM. The particles are identified by TOF and Cherenkov counters. The photon detector, an assembly of 768 CsI(Tl) crystals, covers 75% of the total system.
Because of the rotational symmetry of the 12 identical gaps in the spectrometer and the large directional acceptance of the calorimeter, spectral distortions due to detector acceptance are cancelled out by integrating over all directions of charged and neutral particles and the systematic errors are greatly suppressed. Thanks to these features, this spectrometer is suited to symmetry violation test experiments.

General features of a stopped K+ experiment

The kaon beam history such as beam momentum, emittance, etc., does not contribute to the systematic error at all, and secondary particles from kaon decay are independent of the initial kaon momentum. The momentum range of decay particles is confined within 250 MeV/c enabling rather easy spectroscopy with a mid-sized detector system. Generally speaking, the kinematical resolution of stopped K+ experiments is much better than in-flight K+ experiments due to the above feature. On the other hand, in the case of the in-flight method, events are boosted and concentrated in the forward direction, and thus the detector system covers the overall phase space and the acceptance is higher compared to stopped K+ experiments, while the phase space coverage is usually limited by the detector acceptance in the case of stopped K+ experiments.

History of stopped K+ experiments

Several stopped K+ experiments such as E10 (K+ → π+ ν ν(bar)), E89(Heavy neutrino search) and E99 / E195 (Right-handed current search) have been performed at the KEK-PS using low momentum K+ beams. In the USA, the experiment to search for the rare K+ → π+ ν ν(bar) decay was also carried out using the stopped K+ method at the BNL-AGS (E787 and E949).

E246 and E470 at KEK-PS K5

We have been searching for a new CP phase by measuring the T-violating transverse muon polarization in the K+→ π0 μ+ ν decay (PT) using a stopped K+ beam at the K5 area in KEK(E246). The detector was based on the Toroidal spectrometer and the CsI(Tl) calorimeter. We measured the PT polarization as a positron asymmetry from muon decay using a passive muon polarimeter. This experiment took advantage of a stopped K+ beam by comparing π0-going forward and backward events (double ratio measurement) to drastically reduce the systematic error. The E246 final result was reported as PT=−0.0017±0.0023(stat)±0.0011(syst).

The chiral anomaly, which is a basic feature of quantum field theory, has been the subject of extensive theoretical investigation. The K+→ π+ π0 decay is hindered because it violates Δ I=1/2 rule, hence the internal bremsstrahlung (IB) to the radiative K+→ π+ π0 γ (Kπ2γ) decay is also suppressed. This feature, in turn, enhances the direct emission (DE) which is sensitive to the chiral anomaly. Using the same detector system as E246, the E470 experiment was performed. The branching ratio was derived as Br(DE, partial) = [3.2 ± 1.3(stat.) ± 1.0(syst.)] ×10-6, which is consistent with the previous stopped experiment.

E06 and E36 at J-PARC

The planned E06 experiment at J-PARC is an advanced measurement of the previous E246 experiment with reasonable upgrading of the E246 detector system. Keeping the experimental scheme to compare forward/backward π0-going events using a stopped kaon method, E06 is aiming at measuring PT with a sensitivity of 10-4. A new active polarimeter to measure the incoming muon and outgoing positron tracks will be installed. The E36 experiment is a precise measurement ratio of the decay width RK=( K+ → e+ ν)/( K+ → μ+ ν) to test lepton universality using a sub-system of the E06 experiment. A possible mechanism to produce a Lepton Flavor Violating effect through a minimal SUSY extension to the Standard Model has been intensively discussed. We aim at achieving an uncertainty for RK of better than Δ RK/RK=2×10-3.