Measurement of Neutral- and Charged-Pion Form-Factor Slopes

Abstract

In an experiment performed at Columbia University's Nevis cyclotron, we studied the reactions (1) ${\pi }^0\to \gamma +e^{+}+e^{-}$ and (2) ${\pi }^{-}+p\to n+e^{+}+e^{-}$ for ${\pi }^{-}$ at rest. Using a system of counters and spark chambers, we selected pairs with an opening angle near 120°. We analyzed the invariant-mass spectrum of the pairs (1) in 2200 ${\pi }^0$ decays and (2) in 400 radiative captures. For the ${\pi }^0$ form-factor slope, we find $a=\frac{(+0.01\mathrm{}\pm 0.11)}{m_{{\pi }^0}^{}{}_{}{}^2}$. We obtain the charged-pion form-factor slope for time like momentum transfers of the order of $m_{\pi }$. Our result for the pion charge radius is $r_{\pi }<1.9\mathrm{}$ F with a 90% confidence level.