arXiv subjects

S. Boffi

Publications and source records attributed to S. Boffi.

At least 37 records · Page 2Linked to original sources

arXiv2005

On the treatment of the $Δ$-contribution in electromagnetic $pp$-knockout reactions

The treatment of the $Δ$-current and its contribution in the exclusive $^{16}$O$(e,e'pp)^{14}$C and $^{16}$O$(γ,pp)^{14}$C knockout reactions are investigated in combination with the effects of correlations. Different parametrizations of the effective $Δ$-current and different treatments of correlations in the two-nucleon overlap function are considered. The results are presented and discussed for a suitable choice of kinematics. It is found that the investigation of different mutually supplementing kinematics is necessary to resolve the uncertainties in the theoretical ingredients and extract clear and unambiguous information on correlations.

nucl-th↗

arXiv2005

Generalized parton distributions of the nucleon in constituent quark models

Generalized parton distributions (GPDs) are studied at the hadronic (nonperturbative) scale within different assumptions based on a relativistic constituent quark model. In particular, by means of a meson-cloud model we investigate the role of nonperturbative antiquark degrees of freedom and the valence quark contribution. A QCD evolution of the obtained GPDs is used to add perturbative effects and to investigate the GPDs' sensitivity to the nonperturbative ingredients of the calculation at larger (experimental) scale.

hep-ph↗

arXiv2005

Nonperturbative versus perturbative effects in generalized parton distributions

hep-ph↗

arXiv2003

The NN-final-state-interaction in two-nucleon-knockout reactions

nucl-th↗

arXiv2003

Helicity-dependent generalized parton distributions in constituent quark models

Helicity-dependent generalized parton distributions of the nucleon are derived from the overlap representation of generalized parton distributions using light-cone wave functions obtained in constituent quark models. Results from two different quark models are used also to study the angular momentum sum rule and the spin asymmetry in polarized electron scattering.

hep-ph↗

arXiv2003

NN correlations and final-state interactions in (e,e'NN) reactions

After a brief overview of relevant studies on one-nucleon knockout showing the importance of quantitatively understanding the origin of the quenched spectroscopic factors extracted from data, attention is focussed on two-nucleon emission as a suitable tool to investigate nucleon-nucleon correlations inside complex nuclei. In particular, direct (e,e$'$pp) and (e,e$'$pn) reactions are discussed, and the role of final-state interactions is studied. The influence of the mutual interaction between the two outgoing nucleons is shown to depend on the kinematics and on the type of the considered reaction.

nucl-th↗

arXiv2003

NN final-state interaction in two-nucleon knockout from $^{16}O$

The influence of the mutual interaction between the two outgoing nucleons (NN-FSI) in electro- and photoinduced two-nucleon knockout from $^{16}O$ has been investigated perturbatively. It turns out that the effect of NN-FSI depends on the kinematics and on the type of reaction considered. The effect is generally larger in pp- than in pn-knockout and in electron induced than in photoinduced reactions. In superparallel kinematics NN-FSI leads in the $(e,e'pp)$ channel to a strong increase of the cross section, that is mainly due to a strong enhancement of the $Δ$-current contribution. In pn-emission, however, this effect is partially cancelled by a destructive interference with the seagull current. For photoreactions NN-FSI is considerably reduced in superparallel kinematics and can be practically negligible in specific kinematics.

nucl-th↗

arXiv2003

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

The polynomiality condition -- i.e. the property that Mellin moments of off-forward distribution functions are even polynomials in the skewedness parameter -- is a demanding check of consistency for model approaches. We demonstrate that the helicity off-forward distribution functions in the chiral quark-soliton model satisfy the polynomiality property. The proof contributes to the demonstration that the description of off-forward distribution functions in the model is consistent.

hep-ph↗

arXiv2002

On the NN-final-state-interaction in the $^{16}{O}(e,e' pp)$ reaction

The influence of the mutual interaction between the two outgoing nucleons (NN-FSI) in the $^{16}{O}(e,e' pp)$ reaction has been investigated. Results for various kinematics are discussed. In general, the effect of NN-FSI depends on kinematics and the chosen final state in the excitation spectrum of $^{14}{C}$.

nucl-th↗

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

Linking generalized parton distributions to constituent quark models

The link between the nucleon generalized parton distributions and the non-diagonal one-body density matrix in momentum space is studied. Attention is focussed on the region where quark generalized parton distributions (GPD's) describe emission and reabsorption of a single active quark by the target nucleon. The correct covariant connection with wave functions used in any constituent quark model is established. Results obtained with different constituent quark models are presented for the unpolarized quark GPD's.

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

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

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↗

arXiv2002

Spin force dependence of the parton distributions: the ratio $F_2^n(x,Q^2)/F_2^p(x,Q^2)$

Light-front Hamiltonian dynamics is used to relate low-energy constituent quark models to deep inelastic unpolarized structure functions of the nucleon. The approach incorporates the correct Pauli principle prescription consistently and it allows a transparent investigation of the effects due to the spin-dependent SU(6)-breaking terms in the quark model Hamiltonian. Both Goldstone-boson-exchange interaction and hyperfine-potential models are discussed in a unified scheme and a detailed comparison, between the two(apparently) different potential prescriptions, is presented.

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

(e,e'N) and (e,e'NN) experiments at NIKHEF, Mainz and JLab and open problems in one- and two-nucleon emission

A critical review is presented of recent data on direct one- and two-nucleon emission obtained in electron scattering at NIKHEF, Mainz and JLab. In the case of (e,e$'$p) reactions attention is focussed on extracting spectroscopic factors, looking at medium effects on the bound nucleon form factors, and investigating nuclear transparency. The possibility of obtaining information on nucleon-nucleon correlations in (e,e$'$pp) and (e,e$'$pn) reactions is also discussed.

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↗