arXiv subjects

M. Radici

Publications and source records attributed to M. Radici.

At least 37 records · Page 2Linked to original sources

arXiv2002

Nucleon electromagnetic and axial form factors in point-form relativistic quantum mechanics

Results for the proton and neutron electric and magnetic form factors as well as the nucleon axial form factor are presented for constituent quark models, based on either one-gluon-exchange and Goldstone-boson-exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The only input to the calculations is the nucleon wave function of the corresponding constituent quark model. A comparison is given to results of the instanton-induced constituent quark model treated with the Bethe-Salpeter equation.

hep-ph↗

arXiv2002

Polynomiality of unpolarized off-forward distribution functions and the D-term in the chiral quark-soliton model

Mellin moments of off-forward distribution functions are even polynomials of the skewedness parameter. This constraint, called polynomiality property, follows from Lorentz- and time-reversal invariance. We prove that the unpolarized off-forward distribution functions in the chiral quark-soliton model satisfy the polynomiality property. The proof is an important contribution to the demonstration that the description of off-forward distribution functions in the model is consistent. As a byproduct of the proof we derive explicit model expressions for moments of the D-term and compute the first coefficient in the Gegenbauer expansion for this term.

hep-ph↗

arXiv2002

Addressing transversity with interference fragmentation functions

The class of interference fragmentation functions, arising from interference among different hadron production channels, is reconsidered. Their symmetry properties with respect to chiral transformations allow building spin asymmetries where the quark transversity distribution can be factorized out at leading twist. For the case of two leading spinless hadrons inside the same current jet, the pair system is expanded in relative partial waves. The cross section is represented on the helicity basis of the target and the fragmenting quark, as well as on the relative orbital angular momentum of the pair. From the decay matrix being positive semi-definite, new bounds on the interference fragmentation functions can be derived. The expansion in partial waves allows to naturally incorporate in a unified formalism specific cases already studied in the literature, such as the fragmentation functions arising from the interference of two mesons in relative s and p waves, as well as the fragmentation of a spin-1 hadron.

hep-ph↗

arXiv2002

Polynomiality of off-forward distribution functions in the chiral quark soliton model

Mellin moments of off-forward distribution functions are, at t = 0, even polynomials of the skewedness parameter xi. It is proven that the unpolarized off-forward distribution functions in the chiral quark soliton model satisfy this so called polynomiality property. The proof is an important contribution to the demonstration that the description of off-forward distribution functions in the model is consistent.

hep-ph↗

arXiv2002

Spectral function and relativistic mean-field description of (un)polarized (e,e'p) reactions: a consistent picture

We analyze the unpolarized and polarized electron-induced proton-knockout reactions on $O^{16}$ in different kinematical settings using two theoretical approaches. The first one is based on a relativistic mean-field distorted-wave description of the bound and scattering states of the proton, including a fully relativistic electromagnetic current operator. The second approach adopts the same current operator, but describes the proton properties consistently on the basis of microscopic calculations of the self-energy in $O^{16}$ below the Fermi energy and final-state damping in nuclear matter above the Fermi energy, using the same realistic short-range and tensor correlations. Good agreement with all unpolarized and polarized data is obtained at low and high $Q^2$ by using the same spectroscopic factors fixed by the low-$Q^2$ analysis, indicating that a high degree of internal consistency has been reached.

nucl-th↗

arXiv2002

Electroweak properties of the nucleon in a chiral constituent quark model

Results for all elastic electroweak nucleon form factors are presented for the chiral constituent quark model based on Goldstone-boson-exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The direct predictions of the model yield a remarkably consistent picture of the electroweak nucleon structure.

hep-ph↗

arXiv2001

Accessing transversity with interference fragmentation functions

We discuss in detail the option to access the transversity distribution function $h_1(x)$ by utilizing the analyzing power of interference fragmentation functions in two-pion production inside the same current jet. The transverse polarization of the fragmenting quark is related to the transverse component of the relative momentum of the hadron pair via a new azimuthal angle. As a specific example, we spell out thoroughly the way to extract $h_1(x)$ from a measured single spin asymmetry in two-pion inclusive lepton-nucleon scattering. To estimate the sizes of observable effects we employ a spectator model for the fragmentation functions. The resulting asymmetry of our example is discussed as arising in different scenarios for the transversity.

hep-ph↗

arXiv2001

Covariant electroweak nucleon form factors in a chiral constituent quark model

Results for the proton and neutron electric and magnetic form factors as well asthe nucleon axial and induced pseudoscalar form factors are presented for the chiral constituent quark model based on Goldstone-boson-exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The direct predictions of the model yield a remarkably consistent picture of the electroweak nucleon structure.

hep-ph↗

arXiv2001

Relevance of high-momentum nucleons for nuclear phenomena

A brief review is given concerning the status of the theoretical work on nucleon spectral functions. A recent concern about the validity of the concept of spectroscopic factors as deduced from (e,e'p) reactions at higher Q^2, is discussed in some detail. The consequences of the observed spectral strength are then considered in the context of nuclear saturation. It is argued that short-range correlations are mainly responsible for the actual value of the observed charge density in 208Pb and by extension for the empirical value of the saturation density of nuclear matter. This observation combined with the general understanding of the spectroscopic strength suggests that a renewed study of nuclear matter, emphasizing the self-consistent determination of the spectral strength due to short-range and tensor correlations, may shed light on the perennial nuclear saturation problem. First results using such a scheme are presented.

nucl-th↗

arXiv2001

Covariant axial form factor of the nucleon in a chiral constituent quark model

The axial form factor G_A of the nucleon is investigated for the Goldstone-boson-exchange constituent quark model using the point-form approach to relativistic quantum mechanics. The results, being covariant, show large contributions from relativistic boost effects. The predictions are obtained directly from the quark-model wave functions, without any further input such as vertex or constituent-quark form factors, and fall remarkably close to the available experimental data.

nucl-th↗

arXiv2001

T-odd fragmentation functions

We discuss the properties of fragmentation functions arising from the interference of two leading hadrons produced inside the same jet in the current fragmentation region of a hard process. For the case of semi-inclusive Deep Inelastic Scattering (DIS), a proper folding of the cross section, integrated over the azimuthal position of the detected hadrons, produces a factorized form that allows for the extraction of the quark transversity distribution at leading twist. Using an extended spectator model, explicit calculations are shown for the hadron pair being two pions with invariant mass inside the $ρ$ resonance width.

hep-ph↗

arXiv2001

Interference fragmentation functions and spin asymmetries

A new class of fragmentation functions, arising from the interference of different hadron production channels, is analyzed. Their symmetry properties with respect to naive time-reversal transformations allow for the exploration of final-state interactions occurring during and after the hadronization. Their symmetry properties with respect to chiral transformations allow building spin asymmetries where the quark transversity distribution can be factorized out at leading twist. Explicit calculations will be shown for the interference fragmentation functions arising from final-state interactions of two pions detected in the same current jet for the case of semi-inclusive Deep-Inelastic Scattering (DIS).

hep-ph↗

arXiv2001

Electroweak properties of baryons in a covariant chiral quark model

The proton and neutron electromagnetic form factors and the nucleon axial form factor have been calculated in the Goldstone-boson exchange constituent-quark model within the point-form approach to relativistic quantum mechanics. The results, obtained without any adjustable parameter nor quark form factors, are, due to the dramatic effects of the boost required by the covariant treatment, in striking agreement with the data.

hep-ph↗

arXiv2000

Elastic Form Factors in Point-form Approach

Using the point-form approach to relativistic quantum mechanics, a covariant framework is presented for the calculation of proton and neutron electromagnetic form factors. Results for charge radii, magnetic moments, and electric as well as magnetic form factors are produced using the wave functions obtained in the so-called Goldstone-boson-exchange constituent quark model. All the predictions are found in remarkable agreement with existing experimental data.

nucl-th↗

arXiv2000

Covariant nucleon electromagnetic form factors from the Goldstone-boson-exchange quark model

We present a study of proton and neutron electromagnetic form factors for the recently proposed Goldstone-boson-exchange constituent quark model. Results for charge radii, magnetic moments, and electric as well as magnetic form factors are reported. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. All the predictions by the Goldstone-boson-exchange constituent quark model are found in remarkably good agreement with existing experimental data.

nucl-th↗

arXiv2000

Calculation of fragmentation functions in two-hadron semi-inclusive processes

We investigate the properties of interference fragmentation functions arising from the emission of two leading hadrons inside the same jet for inclusive lepton-nucleon deep-inelastic scattering. Using an extended spectator model for the mechanism of the hadronization, we give a complete calculation and numerical estimates for the examples of a proton-pion pair produced with invariant mass on the Roper resonance, and of two pions produced with invariant mass close to the $ρ$ mass. We discuss azimuthal angular dependence of the leading order cross section to point up favourable conditions for extracting transversity from experimental data.

hep-ph↗

arXiv2000

Nucleon magnetic moments in an extended chiral constituent quark model

We present results for the nucleon magnetic moments in the context of an extended chiral constituent quark model based on the mechanism of the Goldstone boson exchange, as suggested by the spontaneous breaking of chiral symmetry in QCD. The electromagnetic charge-current operator is consistently deduced from the model Hamiltonian, which includes all force components for the pseudoscalar, vector and scalar meson exchanges. Thus, the continuity equation is satisfied for each piece of the interaction, avoiding the introduction of any further parameter. A good agreement with experimental values is found. The role of isoscalar two-body operators, not constrained by the continuity equation, is also investigated.

hep-ph↗

arXiv1999

Nucleon Electromagnetic Form Factors in a chiral constituent-quark model

The electromagnetic form factors of the nucleon are calculated in an extended chiral constituent-quark model where the effective interaction is described by the exchange of pseudoscalar, vector, and scalar mesons. Two-body current-density operators, constructed consistently with the extended model Hamiltonian in order to preserve gauge invariance and current conservation, are found to give a significant contribution to the nucleon magnetic form factors and improve the estimates of the nucleon magnetic moments.

hep-ph↗