Measurement of CP Asymmetries at BELLE

Abstract

The Belle experiment at the KEK $B$ factory has collected 93 fb$^{-1}$ of electron positron collisions at $\sqrt s = 10.6$ GeV. This has produced a sample of 85 million $B {\bar B}$ meson pairs that can be used to study $CP$ violation in rare (and not so rare) B decay modes. Here I report on a measurement of indirect CP violation in the decay $B^0 \to J/\psi K^0_S$, as well as time dependent CP asymmetries in rarer modes such as $B^0 \to \pi^+ \pi^-, \eta' K_S^0$ and $\phi K^0_S$. I summarise the prospects for improving the precision on these and related measurements.