Kaon Physics

What is this experiment all about?

This is experiment is designed to observe the rare, CP violating K-long to pi zero - nu - nu bar decay. We want to do this because the branching ratio directly measures the eta parameter of the Cabibbo - Kobayashi - Maskawa (CKM) Matrix. From the Wolfenstein parameterization of the CKM matrix we get the following result:

Wolfenstein Parameterization of the CKM Matrix

Currently, lambda and A are well determined. This leaves rho and eta left to be determined. What makes this experiment special is the fact that this branching ratio has a small theoretical uncertainty and it directly meausres eta squared.

Difficulties in Measuring the Branching Ratio

This experiment is very challenging. The biggest problem comes from the fact that you only get a signal from pi-zero decaying to 2 gamma rays, which occurs very rarely (approximately 3 in every 100 million events). The pi-zero decays into 2 photons but the two nu's have no charge and can't be detected by the calorimeter. The other problem is that this is a small branching ratio fraction and the majority of the time K-long will not decay into pi-zero - nu - nu bar. These other decay modes will often release photons that are detected. The challenge comes from differentiating between decay modes that we are not concerned with and correctly measuring the one decay mode that we are interested in.

Interesting Properties of the Kaon

The K-mason (kaon) is a particle with many interesting properties. The kaon is made up of a down quark and an anti-strange quark, whereas the anti-kaon is made up of a strange quark and an anti-down quark. While the kaon propegates, it will oscillate between the two possible states. K-short and K-long are superpositions of the kaon/anti-kaon states. K-short has a lifetime of ~0.8926*10^-10 seconds and K-long has a lifetime of ~5.17*10^-8 seconds.