Study of Chiral Invariance Using Pions

We review two manifestations of chiral symmetry breaking in the pion sector. First, we examine the $\pi \pi$ scattering amplitude at threshold, which is completely determined by the chiral symmetry breaking part of the strong interaction, and, thus, directly constrains the form of the low energy effective lagrangians. We review the current status of the $\pi \pi$ scattering lengths in view of the recent results on reactions $\pi N\to \pi \pi N$ near threshold, and discuss the needed improvements. Second, we focus on the rate of the pion beta decay, determined at the tree level by the CVC hypothesis, a remnant of chiral symmetry breaking. Precise knowledge of the $\pi \beta$ decay rate constrains CKM unitarity, and, thus, can be used to distinguish among certain extensions of the minimal Standard Model. Present status of CKM unitarity, and the PIBETA experiment at PSI are discussed.