π−p→π−π0pReaction at 2.77 GeV/cfor Low Momentum Transfers to the Proton

The reaction ${\pi }^{-}p\to {\pi }^{-}{\pi }^{0}p$ produced by 2.77-GeV/c pions has been studied for low squared four-momentum transfers to the proton (${\Delta }^2<~13{\mu }^2$). A selection based on a fast premeasurement method has allowed the accumulation of high statistics (7666 events) and a detailed study of the ${\rho }^{-}$ properties. In particular, the ${\rho }^{-}$ mass and width are determined as a function of ${\Delta }^2$, showing essentially no variation of these parameters in the whole ${\Delta }^2$ range. On the other hand, the mass and width values deduced from the mass spectra (${\omega }_r=773\mathrm{}\pm 2$ MeV, ${\Gamma }_r=150\mathrm{}\pm 5$ MeV) differ from the corresponding values for the resonant $P$ wave (${\omega }_r=755\mathrm{}\pm 5$ MeV, ${\Gamma }_r=110\mathrm{}\pm 9$ MeV) obtained by extrapolation to the pion pole, following the Chew-Low method. Essential results on the ${\pi }^{-}{\pi }^0$ elastic cross section are summarized. The sensitivity of the determination of the ${\rho }^{-}$ production and decay parameters to various mass and ${\Delta }^2$ cuts is shown.