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 ${\pi }^{-}+p\to {\pi }^{-}+{\pi }^{+}+n$ at an incident momentum of 1.5 BeV/c. The spectrum was observed between 450 and 1000 MeV, with $\pi -\pi$ scattering angles at 90±30 deg, and with four-momentum transfer to the nucleon between the minimum ${\Delta }_{min}$ and ${\Delta }_{min}+2\mathrm{}\mu$. The mass resolution of the apparatus was ±4 MeV. With more than 6000 events we observe a peak in the $\pi -\pi$ mass spectrum at ∼750 MeV with a width $\Gamma \cong 130$ 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 ($\omega \to \frac{{\pi }^{+}{\pi }^{-}}{\omega }\to {\pi }^{+}{\pi }^{-}{\pi }^0$) of ≤1.0%, based on the assumption that there is no interference between the $\rho$ and $\omega$ amplitudes. Interpreting the data with the one-pion exchange (OPE) model, we have computed the differential cross section $\frac{d{\sigma }_{\pi \pi }}{dcos{\theta }_{\pi \pi }}$ at 90 deg. This quantity shows a strong peak at the mass of the ${\rho }^0$ meson, but the peak is much larger than expected from a $J=1\mathrm{}$ $\pi -\pi$ resonant state produced by OPE. If this peak is really the ${\rho }^0$ meson, we must be observing it because of absorption phenomena in the initial $\pi -N$ and final $\rho -N$ states which tend to depolarize the $\rho$. Expressing the $\rho$ decay intensity as $a+b{cos}^2{\theta }_{\pi \pi }$, our data require $\frac{a}{b}\approx 0.06$, while the simple OPE model predicts $\frac{a}{b}=0\mathrm{}$. 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.