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

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Abstract

In an experiment carried out at the CERN Proton Synchrotron and using the CERN polarized deuteron target, the reaction ${\ensuremath{\pi}}^{+}$${n}_{\ensuremath{\uparrow}}$\ensuremath{\rightarrow}${\ensuremath{\pi}}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$p has been measured in the region -t=0.1--1.0 (GeV/c${)}^{2}$ and m(${\ensuremath{\pi}}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$)=0.36--1.04 GeV at incident momenta of 5.98 and 11.85 GeV/c. We present the m and t dependence of the measured 14 linearly independent spin-density-matrix elements and of the bounds on the moduli squared of the S- and P-wave recoil transversity amplitudes. The results show the presence of ``${A}_{1}$'' exchange in the unnatural nucleon-helicity-nonflip amplitudes. The natural ``${A}_{2}$''-exchange amplitudes dominate at large t. In the range 0.2\ensuremath{\le}-t\ensuremath{\le}0.4 (GeV/c${)}^{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 ${\ensuremath{\rho}}^{0}$ 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 ${\ensuremath{\rho}}^{0}$ mass range and variations of the width and the position of the ${\ensuremath{\rho}}^{0}$ peak in spin-averaged partial-wave cross sections. These structures have not been seen in previous polarization experiments and reveal spin dependence of ${\ensuremath{\rho}}^{0}$ 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.

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