Evidence for a New Resonance from Polarized Neutron-Proton Scattering

Abstract

Exclusive and kinematically complete high-statistics measurements of quasifree polarized $\vec{n}p$ scattering have been performed in the energy region of the narrow resonance structure $d^{\ast }$ with $I(J^P)=0(3^{+})$, $M\approx$ 2380 MeV/$c^2$ and $\hat{I}“\approx$ 70 MeV observed recently in the double-pionic fusion channels $pn\to d\ddot{I}{€}^0\ddot{I}{€}^0$ and $pn\to d\ddot{I}{€}^{+}\ddot{I}{€}^{-}$. 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 $\vec{d}p\to np+p_{spectator}$. That way the $np$ analyzing power $A_y$ was measured over a large 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 ${}^3{D}_3{-}^3{G}_3$ waves as expected from the $d^{\ast }$ resonance hypothesis.