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Register a new userOn the microscopic structure of $pi NN$, $pi NDelta$ and $piDeltaDelta$ vertices
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We use a hybrid constituent-quark model for the microscopic description of $pi N N$, $pi N Delta$ and $pi Delta Delta$ vertices. In this model quarks are confined by an instantaneous potential and are allowed to emit and absorb a pion, which is also treated as dynamical degree of freedom. The point form of relativistic quantum mechanics is employed to achieve a relativistically invariant description of this system. Starting with an $SU(6)$ spin-flavor symmetric wave function for $N_0$ and $Delta_0$, i.e. the eigenstates of the pure confinement problem, we calculate the strength of the $pi N_0 N_0$, $pi N_0 Delta_0$ and $pi Delta_0 Delta_0$ couplings and the corresponding vertex form factors. Interestingly the ratios of the resulting couplings resemble strongly those needed in purely hadronic coupled-channel models, but deviate significantly from the ratios following from SU(6) spin-flavor symmetry in the non-relativistic constituent-quark model.
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We present a microscopic description of the strong $pi NN$, $pi NDelta$ and $piDeltaDelta$ vertices. Our starting point is a constituent-quark model supplemented by an additional $3qpi$ non-valence component. In the spirit of chiral constituent-quark models, quarks are allowed to emit and reabsorb a pion. This multichannel system is treated in a relativistically invariant way within the framework of point-form quantum mechanics. Starting with a common $SU(6)$ spin-flavor-symmetric wave function for $N$ and $Delta$, we calculate the strength of the $pi NN$, $pi NDelta$ and $piDeltaDelta$ couplings and the corresponding vertex form factors. Our results are in accordance with phenomenological fits of these quantities that have been obtained within purely hadronic multichannel models for baryon resonances.
We report on shell-model calculations employing effective interactions derived from a new realistic nucleon-nucleon (NN) potential based on chiral effective field theory. We present results for 18O, 134Te, and 210Po. Our results are in excellent agreement with experiment indicating a remarkable predictive power of the chiral NN potential for low-energy microscopic nuclear structure.
As a step toward performing a complete coupled-channels analysis of the world data of pi N, gamma^* N --> pi N, eta N, pi pi N reactions, the pi N --> pi pi N reactions are investigated starting with the dynamical coupled-channels model developed in Phys. Rev. C76, 065201 (2007). The channels included are pi N, eta N, and pi pi N which has pi Delta, rho N, and sigma N resonant components. The non-resonant amplitudes are generated from solving a set of coupled-channels equations with the meson-baryon potentials defined by effective Lagrangians. The resonant amplitudes are generated from 16 bare excited nucleon (N^*) states which are dressed by the non-resonant interactions as constrained by the unitarity condition. The data of total cross sections and pi N and pi pi invariant mass distributions of pi^+ p --> pi^+ pi^+ n, pi^+ pi^0p and pi^- p --> pi^+ pi^- n, pi^- pi^0 n, pi^0 pi^0 n reactions from threshold to the invariant mass W = 2 GeV can be described to a very large extent. We show the importance of the coupled-channels effects and the strong interference between the contributions from the pi Delta, sigma N, and rho N channels. The large interference between the resonant and non-resonant amplitudes is also demonstrated. Possible future developements are discussed.
Fully constrained bubble chamber data on the pp -> pi+ pn and pp -> pi+ d reactions are used to investigate the ratio of the counting rates for the two processes at low pn excitation energies. Whereas the ratio is in tolerable agreement with that found in a high resolution spectrometer experiment, the angular distribution in the final pn rest frame shows that the deviation from the predictions of final state interaction theory must originate primarily from higher partial waves in the pn system. These considerations might also be significant for the determination of the S-wave Lambda:p scattering length from data on the pp -> K+ Lambda p reaction.
Recent data from the PIONS$@$MAX-lab Collaboration, measuring the total cross section of the pion incoherent photoproduction $gamma dtopi^-pp$ near threshold, have been used to extract the E$_{0+}$ multipole and total cross section of the reaction $gamma n!to!pi^-p$, also near threshold. These are the first measurements of the reaction $gamma dtopi^-pp$ in the threshold region. The value of E$_{0+}$ is extracted through a fit to the deuteron data in a photoproduction model accounting for final-state interactions. The model takes an $S$-wave approximation for the elementary reaction $gamma n!to!pi^-p$ with E$_{0+}! = $ const in the threshold region. The obtained value E$_{0+} = -31.86pm 0.8$ (in $10^{-3}/m_{pi^+}$ units) is in agreement with other existing results. Model predictions for the total cross section $sigma(gamma n!to!pi^-p)$ are also given.
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