Current Status of the direct CP Violation Experiment at Fermilab

Abstract

Twenty four years have passed since the the violation of CP symmetry was first established by the observation of the decay K L → π + π - in 1964[1]; yet, all the CP violating effects that have been confirmed so far can be described by a single parameter e which characterizes the CP contamination in the K L state. If this contamination is the only source of the CP violation (superweak hypothesis[2]), then the size of the CP violation is expected to be the same for the π + π - and π 0 π 0 final states. On the other hand, if the decay amplitude K L →2π itself violates CP (“direct” CP violation), then in general the size of CP violation is expected to be different for the neutral and charged final states. The goal of this experiment is to measure the double ratio $$ {{\Gamma ({K_L} \to {\pi ^ + }{\pi ^ - })/\Gamma ({K_S} \to {\pi ^ + }{\pi ^ - })} \over {\Gamma ({K_L} \to {\pi ^0}{\pi ^0})/\Gamma ({K_S} \to {\pi ^0}{\pi ^0})}} \simeq 1 + 6| \in '/ \in | $$ with an error of 0.005. The Standard model prediction for |∊′/∊| ranges from 0.001 to 0.007[3], while the superweak hypothesis leads to |∊′/∊| = 0.