Measurement of the reaction K+n

Abstract

In an experiment carried out at the CERN Proton Synchrotron and using the CERN polarized deuteron target, the reaction ${K}^{+}$${n}_{\ensuremath{\uparrow}}$\ensuremath{\rightarrow}${K}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$p was measured in the region -t=0.1--1.0 (GeV/c${)}^{2}$ and m(${K}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$)=0.812--0.972 GeV at incident momenta of 5.98 and 11.8 GeV/c. The experiment yields the m and t dependence of 14 linearly independent spin-density-matrix elements describing the coherent production of ${K}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ states with dimeson spins J=0 and J=1. This first measurement of the KN\ensuremath{\rightarrow}K\ensuremath{\pi}N reaction on a polarized target enables us to study experimentally pion production on the level of production amplitudes. Although the mass dependence of partial cross sections averaged over nucleon spins is smooth, we observe large and systematic structures in the moduli squared of individual nucleon transversity amplitudes which reveal an unexpected but essential role of nucleon spin in the pion production process. Our polarization data suggest the possibility of a new state I=(1/2 ${0}^{++}$(860) with a width of 20--40 MeV. At present we consider this conclusion as only tentative. The predictions of the additive quark model relating ${K}^{+}$${n}_{\ensuremath{\uparrow}}$\ensuremath{\rightarrow}${K}^{\mathrm{*}0}$p and ${p}_{\ensuremath{\uparrow}}$p\ensuremath{\rightarrow}${\ensuremath{\Delta}}^{++}$n are well satisfied at 6 GeV/c. Our results emphasize the need for a systematic study of single-pion production in a new generation of dedicated experiments with spin at the recently proposed high-intensity hadron facilities.