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) ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}+{e}^{+}+{e}^{\ensuremath{-}}$ and (2) ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}n+{e}^{+}+{e}^{\ensuremath{-}}$ for ${\ensuremath{\pi}}^{\ensuremath{-}}$ at rest. Using a system of counters and spark chambers, we selected pairs with an opening angle near 120\ifmmode^\circ\else\textdegree\fi{}. We analyzed the invariant-mass spectrum of the pairs (1) in 2200 ${\ensuremath{\pi}}^{0}$ decays and (2) in 400 radiative captures. For the ${\ensuremath{\pi}}^{0}$ form-factor slope, we find $a=\frac{(+0.01\ifmmode\pm\else\textpm\fi{}0.11)}{m_{{\ensuremath{\pi}}^{0}}^{2}}$. We obtain the charged-pion form-factor slope for time like momentum transfers of the order of ${m}_{\ensuremath{\pi}}$. Our result for the pion charge radius is ${r}_{\ensuremath{\pi}}<1.9$ F with a 90% confidence level.