Two-photon production of pion pairs

Abstract

We present a new high-statistics measurement of the cross section for the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ at a center-of-mass energy of 29 GeV for invariant pion-pair masses $M({\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})$ between 350 MeV/${\mathit{c}}^{2}$ and 1.6 GeV/${\mathit{c}}^{2}$. We observe the ${f}_{2}(1270)$ and measure its radiative width to be 3.15\ifmmode\pm\else\textpm\fi{}0.04\ifmmode\pm\else\textpm\fi{}0.39 keV. We also observe an enhancement in the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ spectrum near 1 GeV. General agreement is found with unitarized models of the $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ reaction that include final-state interactions.