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Register a new userNeutron-Proton Scattering in the Context of the $d^*$(2380) Resonance
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New data on quasifree polarized neutron-proton scattering, in the region of the recently observed $d^*$ resonance structure, have been obtained by exclusive and kinematically complete high-statistics measurements with WASA at COSY. This paper details the determination of the beam polarization, checks of the quasifree character of the scattering process, on all obtained $A_y$ angular distributions and on the new partial-wave analysis, which includes the new data producing a resonance pole in the $^3D_3$-$^3G_3$ coupled partial waves at ($2380pm10 - i40pm5$) MeV -- in accordance with the $d^*$ dibaryon resonance hypothesis. The effect of the new partial-wave solution on the description of total and differential cross section data as well as specific combinations of spin-correlation and spin-transfer observables available from COSY-ANKE measurements at $T_d$ = 2.27 GeV is discussed.
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Differential cross sections have been extracted from exclusive and kinematically complete high-statistics measurements of quasifree polarized $vec{n}p$ scattering performed in the energy region of the $d^*(2380)$ dibaryon resonance covering the the range of beam energies $T_n$ = 0.98 - 1.29 GeV ($sqrt s$ = 2.32 - 2.44 GeV). The experiment was carried out with the WASA-at-COSY setup having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process $dp to np + p_{spectator}$. That way the $np$ differential cross section $sigma(Theta)$ was measured over a large angular range. The obtained angular distributions complement the corresponding analyzing power $A_y(Theta)$ measurements published previously. A SAID partial-wave analysis incorporating the new data strengthens the finding of a resonance pole in the coupled $^3D_3 - ^3G_3$ waves.
Exclusive measurements of the quasi-free $pn to pppi^-$ and $pp to pppi^0$ reactions have been performed by means of $pd$ collisions at $T_p$ = 1.2 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region $T_p = 0.95 - 1.3$ GeV ($sqrt s$ = 2.3 - 2.46 GeV), which includes the regions of $Delta(1232)$, $N^*(1440)$ and $d^*(2380)$ resonance excitations. From these measurements the isoscalar single-pion production has been extracted, for which data existed so far only below $T_p$ = 1 GeV. We observe a substantial increase of this cross section above 1 GeV, which can be related to the Roper resonance $N^*(1440)$, the strength of which shows up isolated from the $Delta$ resonance in the isoscalar $(Npi)_{I=0}$ invariant-mass spectrum. No evidence for a decay of the dibaryon resonance $d^*(2380)$ into the isoscalar $(NNpi)_{I=0}$ channel is found. An upper limit of 90 $mu$b (90 $%$ C.L.) corresponding to a branching ratio of 5 $%$ has been deduced.
Based on measurements the branching ratios for the decay of the recently discovered dibaryon resonance $d^*(2380)$ into two-pion production channels and into the $np$ channel are evaluated. Possibilities for a decay into the isoscalar single-pion channel are discussed. Finally also the electromagnetic decay of $d^*(2380)$ is considered.
We report a measurement of the spin polarisation of the recoiling neutron in deuterium photodisintegration, utilising a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300~--~700~MeV provides the first measurement of recoil neutron polarisation at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarisation in a narrow structure centred around $E_{gamma}sim$~570~MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behaviour could be related to the excitation of the $d^*(2380)$ hexaquark.
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 $d^*$ with $I(J^P) = 0(3^+)$, $M approx$ 2380 MeV/$c^2$ and $Gamma approx$ 70 MeV observed recently in the double-pionic fusion channels $pn to dpi^0pi^0$ and $pn to dpi^+pi^-$. 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 $^3D_3 - ^3G_3$ waves as expected from the $d^*$ resonance hypothesis.
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