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L. Canton

Publications and source records attributed to L. Canton.

At least 55 records · Page 3Linked to original sources
arXiv2005

Spectator-model operators in point-form relativistic quantum mechanics

We address the construction of transition operators for electromagnetic, weak, and hadronic reactions of relativistic few-quark systems along the spectator model. While the problem is of relevance for all forms of relativistic quantum mechanics, we specifically adhere to the point form, since it preserves the spectator character of the corresponding transition operators in any reference frame. The conditions imposed on the construction of point-form spectator-model operators are discussed and their implications are exemplified for mesonic decays of baryon resonances within a relativistic constituent quark model.

hep-ph↗
arXiv2005

Role of the Pauli principle in collective-model coupled-channels calculations

A multi-channel algebraic scattering theory, to find solutions of coupled-channel scattering problems with interactions determined by collective models, has been structured to ensure that the Pauli principle is not violated. By tracking the results in the zero coupling limit, a correct interpretation of the sub-threshold and resonant spectra of the compound system can be made. As an example, the neutron-12C system is studied defining properties of 13C to 10 MeV excitation. Accounting for the Pauli principle in collective coupled-channels models is crucial to the outcome.

nucl-th↗
arXiv2005

Compound and quasi-compound states in low-energy scattering of nucleons from 12C

A multi-channel algebraic scattering theory has been used to study the properties of nucleon scattering from 12C and of the sub-threshold compound nuclear states, accounting for properties in the compound nuclei to ~10 MeV. All compound and quasi-compound resonances observed in total cross-section data are matched, and on seeking solutions of the method at negative energies, all sub-threshold states in 13C and 13N are predicted with the correct spin-parities and with reasonable values for their energies. A collective-model prescription has been used to define the initiating nucleon-12C interactions and via use of orthogonalizing pseudo-potentials, account is made of the Pauli principle. Information is extracted on the underlying structure of each state in the compound systems by investigating the zero-deformation limit of the results.

nucl-th↗
arXiv2004

The importance of few-nucleon physics at low energy

This manuscript originated from the discussion at the workshop on the "Future of Few-body Low Energy Experimental Physics" (FFLEEP), which was held at the University of Trento on December 4-7, 2002 and has been written in its present form on March 19, 2003. It illustrates a selection of theoretical advancements in the nuclear few-body problem, including two- and many-nucleon interactions, the three-nucleon bound and scattering system, the four-body problem, the A-body (A$>$4) problem, and fields of related interest, such as reactions of astrophysical interest and few-neutron systems. Particular attention is called to the contradictory situation one experiences in this field: while theory is currently advancing and has the potential to inspire new experiments, the experimental activity is nevertheless rapidly phasing out. If such a trend will continue, advancements in this area will become critically difficult.

nucl-th↗
arXiv2004

Hadronic Decays of Baryons in Point-Form Relativistic Quantum Mechanics

We discuss strong decays of baryon resonances within the concept of relativistic constituent quark models. In particular, we follow a Poincare-invariant approach along the point form of relativistic quantum mechanics. Here, we focus on pionic decay modes of N and Delta resonances. It is found that the covariant quark-model predictions calculated in the point-form spectator model in general underestimate the experimental data considerably. This points to a systematic defect in the used decay operator and/or the baryon wave functions. From a detailed investigation of the point-form decay operator it is seen that the requirement of translational invariance implies effective many-body contributions. Furthermore, one has to employ a normalization factor in the definition of the decay operator in the point-form spectator model. Our analysis suggests that this normalization factor is best chosen consistently with the one used for the electromagnetic and axial current operators for elastic nucleon form factors.

hep-ph↗
arXiv2004

Strong Decays of Baryons

A Poincare-invariant description of mesonic baryon resonance decays is presented following the point form of relativistic quantum mechanics. In this contribution we focus on pionic decay modes. It is found that the theoretical results in general underestimate the experimental ones considerably. Furthermore, the problem of a nontrivial normalization factor appearing in the definition of the decay operator is investigated. The present results for decay widths suggest a normalization factor that is consistent with the choice adopted for the current operator in the studies of electroweak nucleon form factors.

hep-ph↗
arXiv2004

On Raynal's posting in arXiv (nucl-th/0407060)

With nucl-th/0407060, Jacques Raynal uses the arXiv in a way which does not conform to standard professional practices. His posting contains many statements that are beyond the borders of acceptable scientific disputes, with the scope to defame colleagues by manifestly false or misleading statements. In this comment we reject the three ``critiques'' expressed by Raynal. 1. The fact that we possibly misquoted our references. 2. The role of the Pauli principle in these kind of calculations. 3. The nature and limits of our coupled-channel potential model. Raynal's postings unfairly detract from the importance of our work, which we published in Nuclear Physics A728, 65 (2003), on a new approach, Multi-Channel-Algebraic-Scattering (MCAS), for coupled-channel calculations. With the MCAS approach we were able to identify systematically all low-energy compound resonances, and to include effectively the Pauli principle in collective, geometrical-type, macroscopic models of multichannel interaction. This represents a clear advantage with respect to the current distribution of the ECIS formulation.

nucl-th↗
arXiv2004

Raynal's use of the word "aberrant" appears more appropriate for his ECIS formulation

Recently, we published a paper (Nucl. Phys. A 728 (2003) 65; see nucl-th/0306068) presenting a new calculational method for nucleon-nucleus elastic scattering at low energies. That method is particularly appropriate for analyses in the region of narrow resonances. The method is based upon the sturmian representation of the S-matrix, and allows inclusion of nonlocality effects due to Pauli principle. It also provides a systematic identification of narrow-resonance spectra and subthreshold bound states. A phenomenological test calculation for low-energy (below 4 MeV) neutrons on 12-C (including the first two excitations of the target) was presented to illustrate the validity of the approach. The model calculation received a violent criticism (see nucl-th/0312038) by the developer of a method (ECIS) which to date cannot handle nonlocality effects and cannot be used easily to identify all narrow resonances. We demonstrate that Raynal's opposition to our development is not well founded by the arguments he presents. Indeed the work we published shows, on rewording the title of nucl-th/0312038, that it is ``aberrant'' phenomenologically to analyze resonant low-energy nucleon-nucleus processes with coupled-channel methods without taking into account the nonlocalities due to the Pauli principle; problems typically encountered in the ECIS formulation.

nucl-th↗
arXiv2003

Nuclear effects in positive pion electroproduction on the deuteron near threshold

Positive pion electroproduction from the deuteron near threshold has been considered within an approach based on the unitary transformation method. The gauge independence of the treatment is provided by using an explicitly gauge independent expression for the reaction amplitude. The results of calculations for kinematics of the experiments on forward-angle $π^+$ meson electroproduction accomplished at Saclay and Jefferson Laboratory are discussed and compared with those given by the impulse approximation. It is shown that the observed behaviour of the cross sections is in accordance with the calculations based on the pion-nucleon dynamics. In particular, the pion production rate suppression in the $^2H(e,e'π^+)nn$ reaction compared to that for the $^1H(e,e'π^+)n$ one can be due to such ``nuclear medium'' effects as nucleon motion and binding along with Pauli blocking in the final $nn$ state.

nucl-th↗
arXiv2003

An algebraic solution of the multichannel problem applied to low energy nucleon-nucleus scattering

Compound resonances in nucleon-nucleus scattering are related to the discrete spectrum of the target. Such resonances can be studied in a unified and general framework by a scattering model that uses sturmian expansions of postulated multichannel interactions between the colliding nuclei. Associated with such expanded multichannel interactions are algebraic multichannel scattering matrices. The matrix structure of the inherent Green functions not only facilitates extraction of the sub-threshold (compound nucleus) bound state spin-parity values and energies but also readily gives the energies and widths of resonances in the scattering regime. We exploited also the ability of the sturmian-expansion method to deal with non-local interactions to take into account the strong non-local effects introduced by the Pauli principle. As an example, we have used the collective model (to second order) to define a multichannel potential matrix for low energy neutron-C12 scattering allowing coupling between the 0+ (ground), 2+ (4.4389 MeV), and 0+ (7.64 MeV) states. The algebraic S matrix for this system has been evaluated and the sub-threshold bound states as well as cross sections and polarizations as functions of energy are predicted. The results are reflected in the actual measured data, and are shown to be consistent with expectations as may be based upon a shell model description of the target and of the compound nucleus.

nucl-th↗
arXiv2002

Three-Body Dynamics in One Dimension

We discuss the general three-particle quantum scattering problem, for motion restricted to the full line. Specifically, we formulate the three-body problem in one dimension in terms of the (Faddeev-type) integral equation approach. As a first application, we develop a spinless, one-dimensional (1-D) model that mimics three-nucleon dynamics in one dimension. In addition, we investigate the effects of pionic 3NF diagrams.

nucl-th↗
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.

nucl-th↗
arXiv2002

Three-body dynamics in one dimension: A test model for the 3-Nucleon system with irreducible pionic diagrams

We formulate the three-body problem in one dimension in terms of the (Faddeev-type) integral equation approach. As an application, we develop a spinless, one-dimensional (1-D) model that mimics three-nucleon dynamics in one dimension. Using simple two-body potentials that reproduce the deuteron binding, we obtain that the three-body system binds at about 7.5 MeV. We then consider two types of residual pionic corrections in the dynamical equation; one related to the 2PI-exchange three-body diagram, the other to the 1PI-exchange three-body diagram. We find that the first contribution can produce an additional binding effect of about 0.9 MeV. The second term produces smaller binding effects, which are, however, dependent on the uncertainty in the off-shell extrapolation of the two-body t-matrix. This presents interesting analogies with what occurs in three dimensions. The paper also discusses the general three-particle quantum scattering problem, for motion restricted to the full line.

quant-ph↗
arXiv2001

Irreducible pionic effects in nucleon-deuteron scattering below 20 MeV

The consequences of a recently introduced irreducible pionic effect in low energy nucleon-deuteron scattering are analyzed. Differential cross-sections, nucleon (vector) and deuteron (vector and tensor) analyzing powers, and four different polarization transfer coefficients have been considered. This 3NF-like effect is generated by the pion-exchange diagram in presence of a two-nucleon correlation and is partially cancelled by meson-retardation contributions. Indications are provided that such type of effects are capable to selectively increase the vector (nucleon and deuteron) analyzing powers, while in the considered energy range they are almost negligible on the differential cross sections. These indications, observed with different realistic nucleon-nucleon interactions, provide additional evidences that such 3NF-like effects have indeed the potential to solve the puzzle of the vector analyzing powers. Smaller but non negligible effects are observed for the other spin observables. In some cases, we find that the modifications introduced by such pionic effects on these spin observables (other than the vector analyzing powers) are significant and interesting and could be observed by experiments.

nucl-th↗
arXiv2001

Meson Dynamics and the resulting "3-Nucleon-Force" diagrams: Results from a simplified test case

A simplified 1D (one-dimensional) model for a generalized 3N system is considered, as a testing ground for the explicit treatment of the meson dynamics in this system. We focus attention on the irreducible diagrams generated by the pion dynamics in the 3N system, and in particular to a new type of three-nucleon force discussed recently in the literature, and generated by the one-pion-exchange mechanism in presence of a nucleon-nucleon correlation. It is found that these new terms in the simplified model have an approximately 30% effect compared to the standard three-nucleon force terms in a `Triton' binding energy calculation. It is suggested that this effect should also not be ignored in realistic calculations.

nucl-th↗
arXiv2001

The one-pion-exchange three-nucleon force and the $A_y$ puzzle

We consider a new three-nucleon force generated by the exchange of one pion in the presence of a 2N correlation. The underlying irreducible diagram has been recently suggested by the authors as a possible candidate to explain the puzzle of the vector analyzing powers $A_y$ and $iT_{11}$ for nucleon-deuteron scattering. Herein, we have calculated the elastic neutron-deuteron differential cross section, $A_y$, $iT_{11}$, $T_{20}$, $T_{21}$, and $T_{22}$ below break-up threshold by accurately solving the Alt-Grassberger-Sandhas equations with realistic interactions. We have also studied how $A_y$ evolves below 30 MeV. The results indicate that this new 3NF diagram provides one possible additional contribution, with the correct spin-isospin structure, for the explanation of the origin of this puzzle.

nucl-th↗
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.

nucl-th↗