arXiv subjects

G. Pisent

Publications and source records attributed to G. Pisent.

23 records · Page 2Linked to original sources

arXiv2002

Production mechanisms and polarization observables for p + d -> He(3) + pi(o) near threshold

Pion production at threshold from nucleon-deuteron collisions is considered, with reference to the outgoing channel where the three-nucleon system is bound. The available experimental data are compared with calculations using accurate nuclear wavefunctions coming from rigorous solutions of the three-nucleon quantum mechanical equations. The dominant contributions for pion production are obtained through matrix elements involving pion-nucleon rescattering mechanisms in S- and P-waves. S-wave rescattering includes also an isoscalar contribution which is enhanced because of the off-shell effects. P-wave rescattering includes also diagrams involving explicitly the Delta degrees of freedom. It is found that the pion-nucleon S-wave off-shell effects in the isospin-even channel are of considerable importance. Initial-state interactions between the proton and the deuteron have in general minor effects on the spin-averaged and spin-dependent observables, except when the polarization observable involves interference terms amongst the various helicity amplitudes. Keywords: Polarization phenomena. Pion production. Few-nucleon systems.

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arXiv2001

Three-nucleon portrait with pion

We report on recent results obtained by the above collaboration on the collision processes involving three nucleons, where we pay particular attention on the dynamical role of the pion. After discussing the case at intermediate energies, where real pions can be produced and detected, we have considered the case at lower energies, where the pions being exchanged are virtual. The study has revealed the presence of some new pion-exchange mechanisms, which leads to a new three-nucleon force of tensor structure. Recently, the effect of this tensor three-nucleon force to the spin observables for neutron-deuteron scattering at low energy has been analyzed, and will be briefly reviewed.

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arXiv2000

Spin observables for pion production from pd collisions

We have calculated the proton analyzing power A_{y0} of the pion-production reaction from pd collisions for one energy close to threshold and for another in the region of the $Δ$-resonance. A fair reproduction of the experimental data could be obtained in both cases with a model which includes isoscalar and isovector $πN$ rescatterings in s waves, as well as the p-wave rescattering mechanisms mediated by the $πNN$ and $πN Δ$ vertices. For the analyzing power at threshold we found that the initial-state interaction (ISI) is also quite important.

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arXiv1998

Spin observables for the pd <-> pi+ t process around the Delta resonance

The proton analyzing power Ay0 and the deuteron tensor analyzing power T20 are evaluated for the pd <-> pi+ t process, in the energy region around and above the Delta resonance. These calculations extend a previous analysis of the excitation function and differential cross-section, based on a model embodying one-- and two-body p-wave absorption mechanisms and isobar excitation. The three-nucleon bound state and the pd scattering state are evaluated through Faddeev techniques for both the Bonn and Paris potentials. The spin variables exhibit a greater sensitivity to the number of included three-nucleon partial waves than the cross-sections, while the role played by the initial-- or final-state interactions appears to be small. The results for the tensor analyzing power at backward angles show a non-negligible dependence on the potentials employed, consistently with what has been previously found for the cross-sections. The calculation of spin observables gives a clear indication that other reaction mechanisms (presumably s-wave two-body absorption) have to be included in the model, in order to reproduce the experimental data below the Delta-resonance, in analogy with the simpler pp <-> pi+ d process.

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arXiv1993

Complex Conjugate Pairs in Stationary Sturmians

Sturmian eigenstates specified by stationary scattering boundary conditions are particularly useful in contexts such as forming simple separable two nucleon t matrices, and are determined via solution of generalised eigenvalue equation using real and symmetric matrices. In general, the spectrum of such an equation may contain complex eigenvalues. But to each complex eigenvalue there is a corresponding conjugate partner. In studies using realistic nucleon--nucleon potentials, and in certain positive energy intervals, these complex conjugated pairs indeed appear in the Sturmian spectrum. However, as we demonstrate herein, it is possible to recombine the complex conjugate pairs and corresponding states into a new, sign--definite pair of real quantities with which to effect separable expansions of the (real) nucleon--nucleon reactance matrices.

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