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

Abstract

To understand the various results on the $N^{*}\left(1440\mathrm{}\mathrm{M}\mathrm{e}\mathrm{V}\right)$ resonance in a consistent way, the data on $\alpha -p$ scattering at $E_{\alpha }=4.2\mathrm{}\mathrm{GeV}$ 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 $M=1390\mathrm{}\pm 20\mathrm{MeV}$ and $\Gamma =190\mathrm{}\pm 30\mathrm{MeV}.$ This, however, is not consistent with the data on π-N scattering which, in general, require a higher resonance mass and a larger width. Both systems, α-p and π-N, can be described consistently in a T-matrix formalism, assuming two structures in the $P_{11}\left(1440\mathrm{}\mathrm{MeV}\right)$ resonance, from which only the first one is observed in α-p. For this structure the elastic π-N width is small and the decay into the $2\pi \mathrm{}\left(s\right)-N$ channel is large. This strongly supports the conclusions drawn from α-p scattering. The second structure at higher mass has a strong decay into the πΔ channel and can be well understood as a second-order excitation of the $\Delta \left(1230\mathrm{}\mathrm{MeV}\right).\mathrm{}$ The two resonance picture of the $P_{11}\left(1440\mathrm{}\mathrm{MeV}\right)$ resonance is supported by γ-induced reactions; no evidence is found for the first $N^{*},$ however, the second resonance is observed (although more or less obscured by nonresonant π-Δ production). A further crucial test of the existence of two structures in the Roper resonance is provided by exclusive α-p experiments; a $N^{*}$ decay pattern should be found quite different from π-N with a very strong 2π decay.