Properties ofη-meson production in nucleon-nucleon collisions near threshold

Abstract

Polarization observables in $\ensuremath{\eta}$ production induced by $\mathrm{NN}$ collisions $p+p\ensuremath{\rightarrow}p+p+\ensuremath{\eta}$, $n+p\ensuremath{\rightarrow}n+p+\ensuremath{\eta}$, and $n+p\ensuremath{\rightarrow}d+\ensuremath{\eta}$, are derived. The unique $S$ wave spin structure of the threshold amplitude for $p+p\ensuremath{\rightarrow}p+p+\ensuremath{\eta}$ allows us to calculate all polarization observables in an universal form. Asymmetries in $n\ensuremath{\rightarrow}p\ensuremath{\rightarrow}$ collisions, characterized by noninterfering singlet and triplet amplitudes, are calculated. From the large experimental ratio of the differential cross sections of the processes $n+p\ensuremath{\rightarrow}n+p+\ensuremath{\eta}$ and $p+p\ensuremath{\rightarrow}p+p+\ensuremath{\eta}$, near threshold, one predicts large spin correlation coefficients ${C}_{\mathrm{xx}}$ ${(C}_{\mathrm{yy}})$. The spin structure of $S$- and $P$- wave contributions is derived.