Properties of nucleon resonances by means of a genetic algorithm

Fernández-Ramírez, C.; Guerra, E. Moya de; Údias, Ángel; Udı́as, J. M.

doi:10.1103/physrevc.77.065212

Cited by 30 publications

(26 citation statements)

References 43 publications

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“…On the other hand, our results are hardly comparable with the low momentum Effective Field Theory and Chiral Perturbation Theory [55,56,57,58,59]. Those models include pion degrees of freedom consistently at least at one pion loop level.…”

Section: Discussioncontrasting

confidence: 48%

A covariant model for the nucleon and the Δ

2008

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The covariant spectator formalism is used to model the nucleon and the ∆(1232) as a system of three constituent quarks with their own electromagnetic structure. The definition of the "fixed-axis" polarization states for the diquark emitted from the initial state vertex and absorbed into the final state vertex is discussed. The helicity sum over those states is evaluated and seen to be covariant. Using this approach, all four electromagnetic form factors of the nucleon, together with the magnetic form factor, G * M , for the γN → ∆ transition, can be described using manifestly covariant nucleon and ∆ wave functions with zero orbital angular momentum L, but a successful description of G * M near Q 2 = 0 requires the addition of a pion cloud term not included in the class of valence quark models considered here. We also show that the pure S-wave model gives electric, G * E , and coulomb, G * C , transition form factors that are identically zero, showing that these form factors are sensitive to wave function components with L > 0.

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Section: Discussioncontrasting

confidence: 48%

A covariant model for the nucleon and the Δ

2008

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“…As a fitting procedure we have used a hybrid optimization code based on a genetic algorithm (GA) combined with the E04FCF routine from the NAG library [21]. In recent years, increased credit is being given in nuclear and particle physics to modern optimization procedures [22,23] and the error analysis for the parameters resulting from the fits. Modern and sophisticated optimization techniques such as GAs [24] have been developed over the past 20 years and have been applied to problems that were impossible to face with conventional tools.…”

Section: General Structure Of the Observables In Terms Of The Elementioning

confidence: 99%

Unexpected impact ofDwaves in low-energy neutral pion photoproduction from the proton and the extraction of multipoles

2009

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Contributions of D waves to physical observables for neutral pion photoproduction from the proton in the near-threshold region are studied and means to isolate them are proposed. Various approaches to describe the multipoles are employed-a phenomenological one, a unitary one, and heavy baryon chiral perturbation theory. The results of these approaches are compared and found to yield essentially the same answers. D waves are seen to enter together with S waves in a way that any means which attempt to obtain the E 0+ multipole accurately must rely on knowledge of D waves and that consequently the latter cannot be dismissed in analyses of low-energy pion photoproduction. It is shown that D waves have a significant impact on double-polarization observables that can be measured. This importance of D waves is due to the soft nature of the S wave and is a direct consequence of chiral symmetry and the Nambu-Goldstone nature of the pion. F -wave contributions are shown to be negligible in the near-threshold region.

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“…First of all, the relations between the different distributions are resumed. We chose to compare in all cases g3dr), determined from data of any other positron distribution using a genetic algorithm [10].…”

Section: Previous Work and Monv A Tionmentioning

confidence: 99%

A general framework to study positron range distributions

Cal-González¹,

Herraiz

España

et al. 2011

2011 IEEE Nuclear Science Symposium Conference Record

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The positron range, that is, the distance from the positron emission point to the annihilation one in p+ decay, limits the spatial resolution of PET images. Thus, any realistic simulation of PET acquisitions must include this effect as accurately as possible. Positron range distributions have been computed and compared in the literature. Not all authors present it in the same way. For instance, accumulated range distributions, ID projection of the positrons annihilation coordinates, or angular integrated 3D radial distributions, have been used. Any of these presentations may emphasize a different physical property. Thus it is difficult to perform a meaningful comparison of the results obtained by different authors. The aim of this work is to present a general framework to compare positron range distributions. We use a genetic algorithm to fit the radial, angle integrated, g3D(r) distribution of the annihilation points to any positron annihilation distribution chosen as reference. Therefore, given a positron range distribution presented in a certain way, we can obtain the distributions, first fitting g3D and then using the relations between distributions. Using this procedure, positron range distributions available in the literature are compared with the ones computed with the simulation package PeneioPET I. PREVIOUS WORK AND MOnV A TIONAs positron range limits the spatial resolution of PET images [1], good quantitative estimates of it should be included in any realistic simulation of PET acquisitions and reconstructions. Different positron range distributions were studied in the literature, each one with different physical significance. For a given radioactive point source emitting positrons in random directions, the 3D Cartesian coordinates (x, y, z) can be registered for each annihilation event and then provide 3D annihilation Point Spread Function (aPSF3D). A theoretical model for this 3D distribution was proposed by Palmer and Brownell. They supposed that the aPSF Manuscript

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Properties of nucleon resonances by means of a genetic algorithm

Cited by 30 publications

References 43 publications

A covariant model for the nucleon and the Δ

A covariant model for the nucleon and the Δ

Unexpected impact ofDwaves in low-energy neutral pion photoproduction from the proton and the extraction of multipoles

A general framework to study positron range distributions

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