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Register a new userStudy of $N(1440)$ structure via $gamma^*pto N(1440)$ transition
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We study the photoproduction of the $N(1440)$ resonance in $gamma^*pto N^*$ process in quark models, where the $N(1440)$ takes different wave functions: first radial excitation of the nucleon imported from low-lying baryon mass spectrum calculations, a general radial excitation of the nucleon, and a $q^3$ state with positive parity. The comparison between the theoretical results and experimental data on the helicity amplitudes $A_{1/2}$ and $S_{1/2}$ and the analysis of the spatial wave function of the $N(1440)$ resonance reveal that the $N(1440)$ resonance is mainly the $q^3$ first radial excitation.
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Using a covariant spectator quark model we estimate valence quark contributions to the F1*(Q2) and F2*(Q2) transition form factors for the gamma N -> P11(1440) reaction. The Roper resonance, P11(1440), is assumed to be the first radial excitation of the nucleon. The present model requires no extra parameters except for those already fixed by the previous studies for the nucleon. Our results are consistent with the experimental data in the high Q2 region, and those from lattice QCD. We also estimate the meson cloud contributions, focusing on the low Q2 region, where they are expected to be dominant.
The N*(1440) -> N pi pi decay is studied by making use of the chiral reduction formula. This formula suggests a scalar-isoscalar pion-baryon contact interaction which is absent in the recent study of Hern{a}ndez et al. The contact interaction is introduced into their model, and is found to be necessary for the simultaneous description of g_{RN pi pi} and the pi-pi and pi-N invariant mass distributions.
We calculate the axial $Nto Delta(1232)$ and $Nto N^{star}(1440)$ transition form factors in a chiral constituent quark model. As required by the partial conservation of axial current ($PCAC$) condition, we include one- and two-body axial exchange currents. For the axial $Nto Delta(1232)$ form factors we compare with previous quark model calculations that use only one-body axial currents, and with experimental analyses. The paper provides the first calculation of all weak axial $Nto N^{star}(1440)$ form factors. Our main result is that exchange currents are very important for certain axial transition form factors. In addition to improving our understanding of nucleon structure, the present results are relevant for neutrino-nucleus scattering cross section predictions needed in the analysis of neutrino mixing experiments.
The excitation of the Roper resonance $N^*(1440)$ in $NN$ collisions leading to single and double pion production exhibits a structure in the energy dependence of the total cross section, which does not correspond to the usual opening of the $N^*(1440)$ production channel, but rather to the formation of dibaryon states with $I(J^P) = 0(1^+)$ and $1(0^+)$ at the $N^*(1440)N$ threshold. This is very similar to the situation at the $Delta(1232)N$ threshold, where meanwhile a number of dibaryonic resonances have been found.
Motivated by a recent measurement of proton-proton elastic scattering observables up to 3.0 GeV, we investigate the description of those data within models of the nucleon-nucleon (NN) interaction valid above the pion production threshold. In addition to including the well known Delta resonance we incorporate two low-lying N* resonances, the N*(1440) and the N*(1535), and study their influence on pp and np observables for projectile laboratory kinetic energies up to 1.5 GeV.
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