Dipion Production at Low Momentum Transfer inπ−−pCollisions at 1.5 BeV/c

Abstract

We have used precision spark-chamber spectrometers to measure the dipion mass spectrum from the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}+{\ensuremath{\pi}}^{+}+n$ at an incident momentum of 1.5 BeV/c. The spectrum was observed between 450 and 1000 MeV, with $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ scattering angles at 90\ifmmode\pm\else\textpm\fi{}30 deg, and with four-momentum transfer to the nucleon between the minimum ${\ensuremath{\Delta}}_{min}$ and ${\ensuremath{\Delta}}_{min}+2\ensuremath{\mu}$. The mass resolution of the apparatus was \ifmmode\pm\else\textpm\fi{}4 MeV. With more than 6000 events we observe a peak in the $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ mass spectrum at \ensuremath{\sim}750 MeV with a width $\ensuremath{\Gamma}\ensuremath{\cong}130$ MeV. We do not observe any other structure in the mass range explored. With these results we can set an upper limit for the branching ratio ($\ensuremath{\omega}\ensuremath{\rightarrow}\frac{{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}}{\ensuremath{\omega}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}$) of \ensuremath{\le}1.0%, based on the assumption that there is no interference between the $\ensuremath{\rho}$ and $\ensuremath{\omega}$ amplitudes. Interpreting the data with the one-pion exchange (OPE) model, we have computed the differential cross section $\frac{d{\ensuremath{\sigma}}_{\ensuremath{\pi}\ensuremath{\pi}}}{dcos{\ensuremath{\theta}}_{\ensuremath{\pi}\ensuremath{\pi}}}$ at 90 deg. This quantity shows a strong peak at the mass of the ${\ensuremath{\rho}}^{0}$ meson, but the peak is much larger than expected from a $J=1$ $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ resonant state produced by OPE. If this peak is really the ${\ensuremath{\rho}}^{0}$ meson, we must be observing it because of absorption phenomena in the initial $\ensuremath{\pi}\ensuremath{-}N$ and final $\ensuremath{\rho}\ensuremath{-}N$ states which tend to depolarize the $\ensuremath{\rho}$. Expressing the $\ensuremath{\rho}$ decay intensity as $a+b{cos}^{2}{\ensuremath{\theta}}_{\ensuremath{\pi}\ensuremath{\pi}}$, our data require $\frac{a}{b}\ensuremath{\approx}0.06$, while the simple OPE model predicts $\frac{a}{b}=0$. Other features of the data, the momentum-transfer distribution and the Treiman-Yang angular distribution, fit the OPE predictions fairly well. We discuss some evidence that the effect of a 1.4-BeV, $T=\frac{1}{2}$ isobar can be seen in the data.