Photoproduction of ${\pi }^{+}{\pi }^{-}$ meson pairs on the proton

Abstract

The exclusive reaction $\gamma p\to p{\pi }^{+}{\pi }^{-}$ was studied in the photon energy range 3.0 - 3.8 GeV and momentum transfer range $0.4<-t<1.0$ GeV${}^2$. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. In this kinematic range the integrated luminosity was about 20 pb${}^{-1}$. The reaction was isolated by detecting the ${\pi }^{+}$ and proton in CLAS, and reconstructing the ${\pi }^{-}$ via the missing-mass technique. Moments of the di-pion decay angular distributions were derived from the experimental data. Differential cross sections for the $S$, $P$, and $D$-waves in the $M_{{\pi }^{+}{\pi }^{-}}$ mass range $0.4-1.4$ GeV were derived performing a partial wave expansion of the extracted moments. Besides the dominant contribution of the $\rho (770)$ meson in the $P$-wave, evidence for the $f_0(980)$ and the $f_2(1270)$ mesons was found in the $S$ and $D$-waves, respectively. The differential production cross sections $d\sigma /dt$ for individual waves in the mass range of the above-mentioned mesons were extracted. This is the first time the $f_0(980)$ has been measured in a photoproduction experiment.