Manifestation of the P-wave diproton resonance in single-pion production in pp collisions

It is demonstrated that many important features of single-pion production in $pp$ collisions at intermediate energies (${T}_{p}\ensuremath{\simeq}400--800\text{ }\text{ }\mathrm{MeV}$) can naturally be explained by supposing excitation of intermediate diproton resonances in $pp$ channels ${^{1}D}_{2}$, ${^{3}F}_{3}$ and ${^{3}P}_{2}$, in addition to conventional mechanisms involving an intermediate $\mathrm{\ensuremath{\Delta}}$-isobar. We predict for the first time the crucial role of the ${^{3}P}_{2}$ diproton resonance, found in recent experiments on the single-pion production reaction $pp\ensuremath{\rightarrow}pp({^{1}S}_{0}){\ensuremath{\pi}}^{0}$, in reproducing the proper behavior of spin-correlation parameters in the reaction $pp\ensuremath{\rightarrow}d{\ensuremath{\pi}}^{+}$ which were poorly described by conventional meson-exchange models to date. The possible quark structure of the $P$-wave diproton resonances is also discussed.