The Experiment
The goal of the J-Parc KL experiment is to discover and measure the rate of the rare decay 
. This flavor changing neutral current decay proceeds through second-order weak interactions. Other, as yet undiscovered particles, which can mediate the decay could provide an enhancement to the branching ratio, which in the Standard Model predicted to be about 
. The experiment is expected to observe 100 events at the Standard Model branching ratio for a 10% measurement. The experiment is a follow-up to E391a at KEK and has been approved as experiment E14 at J-PARC.
Physics Motivation
CP violation is currently recognized as one of the most forefront issues in elementary particle physics. Together with obtaining a better understanding of quark mixing as well as neutrino mixing phenomena, it is one of the central goals of particle physics, and has been pursued vigorously in many experiments. The large number of experiments and theory papers, particularly for B mesons, illustrates the intense interest in CP violation. Out of myriad of ways to study CP violation, there are four “golden” processes: asymmetries in 
decays, the ratio of Bs to Bd mixing, and the decays 
and 
The very rare decay 
provides one of the best probes for understanding the original of CP violation in the quark sector. It is an FCNC process that is induced through a 
transition as expressed by the electroweak penguin and box diagrams as show in Figure 1. Long-distance contributions have been shown to be negligible. Theoretical uncertainties are extremely small in the Standard Model calculations.
Figure 1 . SM diagrams for the 
decay
In the Standard Model, the decay amplitude is proportional to the imaginary part of a product of CKM matrix elements and is proportional to the height of the unitarity triangle as shown in Figure 2. The unitarity of the CKM matrix has been considered as one of the most critical checks for new physics beyond the Standard Model. By using the current estimates for SM parameters, the branching ratio is expected to lie in the range 
. The error is dominated by the uncertainties in other CKM matrix parameters A and l. A stringent indirect constraint can be derived by using the information on the charged mode and isospin symmetry, the so called “Grossman-Nir (GN) bound” :
which gives 
.
As discussed in reference 6, this bound is valid in virtually any extension of the Standard Model. By comparing this model-independent bound and SM prediction, it is clear that there is still considerable room for new physics.
Figure 2. Kaon unitarity triangle
Various models beyond the SM predict sizable effects on the 
decay. For example, in the Minimal Supersymmetric extensions (MSSM) with new sources of flavor mixing, there are possible enhancements in the branching ratio even after taking into account all the available constraints from other CP-violating observables and rare decay. The ranges of branching ratios for both the 
and the 
are shown for various models in Figure 3.
Figure 3.Branching ratios for 
and 
for various models
The first experimental measurement 
was from Fermilab E799-I in 1992 with an upper limit of 
. This was the situation until 1999 when Fermilab E799-II improved the limit with basically the same technique to 
. The KTeV detector further pushed the limit to 
in 2000. With 50% of collected data, KEK E391a (the pilot and predecessor of E14) published the best limit so far with a result of 
. It should be pointed out that unlike all previous experiments, E391a and E14 are specifically designed to measure
.
