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تسجيل مستخدم جديدEvidence for a Narrow Roper Resonance - the Breathing Mode of the Nucleon
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All the time since its discovery the N$^*$(1440) baryon state, commonly known as Roper resonance, has been a state with many question marks - despite of its 4-star ranking in the particle data book. One reason is that it does not produce any explicit resonance-like structures in the observables of $pi$N or $gamma$N reactions. Only in partial wave analyses of $pi$N scattering data a clear resonance strcuture gets obvious in the $P_{11}$ partial wave. Very recent measurements of the J/$Psi$ decay by the BES collaboration and of the $pp to nppi^+$ reaction at 1.3 GeV by the CELSIUS-WASA collaboration show for the first time a clear resonance structure in the invariant $npi^+$ mass spectrum for the Roper resonance at M $approx$ 1360 MeV with a width of about 150 MeV. These values agree very favorably with the pole position results of recent $pi$N phase shift analyses. In consequence of this very low-lying pole postion, which is roughly 100 MeV below the nominal value, the decay properties have to be reinvestigated. From our two-pion production data we see that the decay mainly proceeds via N$^* to $N$sigma$, i.e. a monopole transition as expected for the breathing mode of the nucleon.
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Spectra of p-p and pi-p scattering at beam momenta between 6 and 30 GeV/c have been reanalysed. These show strong excitation of N* resonances, the strongest one corresponding to the scalar P11 excitation (breathing mode) at m_o= 1400+-10 MeV with Gam
ma= 200+-25 MeV. The result of a strong scalar excitation is supported by a large longitudinal amplitude S_1/2 extracted from e-p scattering. From exclusive data on p+p=>p p pi+ pi- a large 2pi-N decay branch for the P_11 resonance of B_2pi= 75+-20 % has been extracted. The differential cross sections were described in a double folding approach, assuming multi-gluon exchange as the dominant part of the effective interaction between the constituents of projectile and target. First, the parameters of the interaction were fitted to elastic scattering; then with this interaction the inelastic cross sections were described in the distorted wave Born approximation. A good description of the data requires a surface peaked transition density, quite different from that of a pure radial mode. In contrast, the electron scattering amplitude S_1/2 is quite well described by a breathing mode transition density with radial node. This large difference between charge and matter transition density suggests, that in p-p scattering the coupling to the multi-gluon field is much more important than the coupling to the valence quarks. A multi-gluon (or sea-quark) transition density is derived, which shows also breathing, indicating a rather complex multi-quark structure of N and N* including multi-glue (or q^2n qbar ^-2n) creation out of the g.s. vacuum.
We investigate the structure of the nucleon resonance N^*(1440) (Roper) within a coupled-channel meson exchange model for pion-nucleon scattering. The coupling to pipiN states is realized effectively by the coupling to the sigmaN, piDelta and rhoN ch
annels. The interaction within and between these channels is derived from an effective Lagrangian based on a chirally symmetric Lagrangian, which is supplemented by well known terms for the coupling of the Delta isobar, the omega meson and the sigma, which is the name given here to the strong correlation of two pions in the scalar-isoscalar channel. In this model the Roper resonance can be described by meson-baryon dynamics alone; no genuine N^*(1440) (3 quark) resonance is needed in order to fit piN phase shifts and inelasticities.
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Exclusive measurements of the p p -> p p pi+ pi- reaction have been carried out at Tp = 775 MeV at CELSIUS using the PROMICE/WASA setup. Together with data obtained at lower energy they point to a dominance of the Roper excitation in this process. Fr
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