Novel Approach to Measuring $CP$ Violation in $B$ Meson Decays: A Case Study of Weak Measurement in High Energy Physics

The weak measurement, proposed by Aharonov {\it et al.}\ in 1988, has been applied for various fields of physics for the purpose of precision measurement, which is made possible with the help of the 'postselection' specifying actively the final state in the physical process. Here we report for the first time the feasibility of applying the weak measurement in high energy particle physics, especially in measuring the $CP$-violating parameters in $B$ meson decays, where the effective lifetime of the decay mode is expected to be prolonged statistically due to the postselection. Our analysis shows that, when adopted in the Belle II experiment at the SuperKEKB collider, the effective lifetime may be prolonged up to $2.6$ times, and that the measurement precision of the $CP$-violating parameters will be improved by about 20\%.