Evidence for a New Resonance from Polarized Neutron-Proton Scattering

Exclusive and kinematically complete high-statistics measurements of quasifree polarized $\stackrel{\ensuremath{\rightarrow}}{n}p$ scattering have been performed in the energy region of the narrow resonancelike structure ${d}^{*}$ with $I({J}^{P})=0({3}^{+})$, $M\ensuremath{\approx}2380\text{ }\text{ }\mathrm{MeV}$, and $\mathrm{\ensuremath{\Gamma}}\ensuremath{\approx}70\text{ }\text{ }\mathrm{MeV}$ observed recently in the double-pionic fusion channels $pn\ensuremath{\rightarrow}d{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ and $pn\ensuremath{\rightarrow}d{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$. The experiment was carried out with the WASA detector setup at COSY having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process $\stackrel{\ensuremath{\rightarrow}}{d}p\ensuremath{\rightarrow}np+{p}_{\text{spectator}}$. This allowed the $np$ analyzing power, ${A}_{y}$, to be measured over a broad angular range. The obtained ${A}_{y}$ angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new ${A}_{y}$ data into the SAID analysis produces a pole in the $^{3}{D}_{3}\ensuremath{-}^{3}{G}_{3}$ waves in support of the ${d}^{*}$ resonance hypothesis.