Structure of theP11(1440MeV) resonance from α-pand π-Nscattering

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To understand the various results on the ${N}^{*}(1440\mathrm{M}\mathrm{e}\mathrm{V})$ resonance in a consistent way, the data on $\ensuremath{\alpha}\ensuremath{-}p$ scattering at ${E}_{\ensuremath{\alpha}}=4.2\mathrm{GeV}$ were reanalyzed assuming projectile and target excitation, and their interference. A quantitative fit of the spectrum is obtained, assuming for the ${N}^{*}$ mass distribution a threshold modified Breit-Wigner shape with momentum-dependent width and resonance parameters $M=1390\ifmmode\pm\else\textpm\fi{}20\mathrm{MeV}$ and $\ensuremath{\Gamma}=190\ifmmode\pm\else\textpm\fi{}30\mathrm{MeV}.$ This, however, is not consistent with the data on \ensuremath{\pi}-N scattering which, in general, require a higher resonance mass and a larger width. Both systems, \ensuremath{\alpha}-p and \ensuremath{\pi}-N, can be described consistently in a T-matrix formalism, assuming two structures in the ${P}_{11}(1440\mathrm{MeV})$ resonance, from which only the first one is observed in \ensuremath{\alpha}-p. For this structure the elastic \ensuremath{\pi}-N width is small and the decay into the $2\ensuremath{\pi}(s)\ensuremath{-}N$ channel is large. This strongly supports the conclusions drawn from \ensuremath{\alpha}-p scattering. The second structure at higher mass has a strong decay into the \ensuremath{\pi}\ensuremath{\Delta} channel and can be well understood as a second-order excitation of the $\ensuremath{\Delta}(1230\mathrm{MeV}).$ The two resonance picture of the ${P}_{11}(1440\mathrm{MeV})$ resonance is supported by \ensuremath{\gamma}-induced reactions; no evidence is found for the first ${N}^{*},$ however, the second resonance is observed (although more or less obscured by nonresonant \ensuremath{\pi}-\ensuremath{\Delta} production). A further crucial test of the existence of two structures in the Roper resonance is provided by exclusive \ensuremath{\alpha}-p experiments; a ${N}^{*}$ decay pattern should be found quite different from \ensuremath{\pi}-N with a very strong 2\ensuremath{\pi} decay.