Measurement of theKSbranching ratio into two pions

A spark-chamber experiment to measure the branching ratio of the short-lived $K_1$ meson into the modes ${\pi }^{+}{\pi }^{-}$ and ${\pi }^0{\pi }^0$ was performed at the Princeton-Pennsylvania Accelerator. A 1.0-GeV/c beam of incident ${\pi }^{-}$ mesons produced ${\Lambda }^0{K}^0$ from a polyethylene target. The decay of $\Lambda$ into ${\pi }^{-}p$ was used as a signal for the associated production of a $K^0$. A total of 160 000 pictures, taken in two views, were completely scanned for $V\text{'}\mathrm{s}$ (charged decays of neutral particles), and all tracks were measured. A final sample of 16 000 $K_1$ charged decays and 65 000 ${\Lambda }^0$ charged decays directly determined the branching fraction for $K_1$ to decay into ${\pi }^{+}{\pi }^{-}$ to be 0.684 ± 0.009. The branching ratio found by this experiment is then $\frac{\Gamma (K_1\to {\pi }^{+}{\pi }^{-})}{\Gamma (K_1\to {\pi }^0{\pi }^0)}=2.169\mathrm{}\pm 0.094$.