Measurement of the reactionπ+n↑→π+π−p at 5.98 and 11.85 GeV/cusing a transversely polarized deuteron target

In an experiment carried out at the CERN Proton Synchrotron and using the CERN polarized deuteron target, the reaction ${\pi }^{+}$ $n_{\uparrow }$→${\pi }^{+}$ ${\pi }^{\mathrm{-}}$p has been measured in the region -t=0.1–1.0 (GeV/c${)}^2$ and m(${\pi }^{+}$ ${\pi }^{\mathrm{-}}$)=0.36–1.04 GeV at incident momenta of 5.98 and 11.85 GeV/c. We present the m and t dependence of the measured 14 linearly independent spin-density-matrix elements and of the bounds on the moduli squared of the S- and P-wave recoil transversity amplitudes. The results show the presence of ‘‘$A_1$’’ exchange in the unnatural nucleon-helicity-nonflip amplitudes. The natural ‘‘$A_2$’’-exchange amplitudes dominate at large t. In the range 0.2≤-t≤0.4 (GeV/c${)}^2$ the mass dependence shows that the unnatural exchange amplitudes with transversity ‘‘down’’ are generally larger than those with transversity ‘‘up.’’ The opposite is true for the natural exchange. In this range of t and at the ${\rho }^0$ mass, the P-wave unnatural amplitudes with both transversities contribute in equal amounts while the production by natural exchange proceeds entirely with transversity up. We observe rapid changes of the moduli within the ${\rho }^0$ mass range and variations of the width and the position of the ${\rho }^0$ peak in spin-averaged partial-wave cross sections. These structures have not been seen in previous polarization experiments and reveal spin dependence of ${\rho }^0$ production. Our bounds cannot exclude an S-wave resonance in the range 700–800 MeV. The results emphasize the need for a better experimental and theoretical understanding of the mass dependence of the production mechanism.