arXiv subjects

S. Simula

Publications and source records attributed to S. Simula.

At least 199 records · Page 11Linked to original sources

arXiv1997

Rare exclusive semileptonic b -> s transitions in the Standard Model

We study long-distance effects in rare exclusive semileptonic decays B -> (K, K*) (l+ l-, nu bar{nu}) and analyze dilepton spectra and asymmetries within the framework of the Standard Model. The form factors, describing the meson transition amplitudes of the effective Hamiltonian are calculated within the lattice-constrained dispersion quark model: the form factors are given by dispersion representations through the wave functions of the initial and final mesons, and these wave functions are chosen such that the B -> K* transition form factors agree with the lattice results at large q**2. We calculate branching ratios of semileptonic B -> K, K* transition modes and study the sensitivity of observables to the long-distance contributions. The shape of the forward-backward asymmetry and the longitudinal lepton polarization asymmetry are found to be independent of the long-distance effects and mainly determined by the values of the Wilson coefficients in the Standard Model.

hep-ph↗

arXiv1997

The nucleon Drell-Hearn-Gerasimov sum rule within a relativistic constituent quark model

The Drell-Hearn-Gerasimov sum rule for the nucleon is investigated within a relativistic constituent quark model formulated on the light-front. The contribution of the N - Delta(1232) transition is explicitly evaluated using different forms for the baryon wave functions and adopting a one-body relativistic current for the constituent quarks. It is shown that the N - Delta(1232) contribution to the sum rule is sharply sensitive to the introduction of anomalous magnetic moments for the constituent quarks, at variance with the findings of non-relativistic and relativized quark models. The experimental value of the isovector-isovector part of the sum rule is almost totally reproduced by the N - Delta(1232) contribution, when the values of the quark anomalous magnetic moments are fixed by fitting the experimental nucleon magnetic moments. Our results are almost independent of the adopted form of the baryon wave functions and only slightly sensitive to the violation of the angular condition caused by the use of a one-body current. The calculated average slope of the generalized sum rule around the photon point results to be only slightly negative at variance with recent predictions of relativized quark models.

nucl-th↗

arXiv1997

Isgur-Wise form factors of heavy baryons within a light-front constituent quark model

The space-like elastic form factors of baryons containing a heavy quark are investigated within a light-front constituent quark model in the limit of infinite heavy-quark mass, adopting a gaussian-like ansatz for the three-quark wave function. The results obtained for the Isgur-Wise form factors corresponding both to a spin-0 and a spin-1 light spectator pair are presented. It is found that the Isgur-Wise functions depend strongly on the baryon structure, being sharply different in case of diquark-like or collinear-type configurations in the three-quark system. It is also shown that the relativistic effects lead to a saturation property of the form factors as a function of the baryon size. Our results are compared with those of different models as well as with recent predictions from QCD sum rules and lattice QCD simulations; the latter ones seem to suggest the dominance of collinear-type configurations, in which the heavy-quark is sitting close to the center-of-mass of the light quark pair.

hep-ph↗

arXiv1997

Bloom-Gilman duality of inelastic structure functions in nucleon and nuclei

The Bloom-Gilman local duality of the inelastic structure function of the proton, the deuteron and light complex nuclei is investigated using available experimental data in the squared four-momentum transfer range from 0.3 to 5 (GeV/c)**2. The results of our analysis suggest that the onset of the Bloom-Gilman local duality is anticipated in complex nuclei with respect to the case of the protonand the deuteron. A possible interpretation of this result in terms of a rescaling effect is discussed with particular emphasis to the possibility of reproducing the damping of the nucleon-resonance transitions observed in recent electroproduction data off nuclei.

nucl-th↗

arXiv1997

Exclusive and inclusive weak decays of the B-meson

A relativistic quark model is applied to the description of semileptonic and non-leptonic charmed decays of the B-meson. The exclusive semileptonic modes B -> D l νand B -> D* l νare described through the universal Isgur-Wise form factor, which is calculated in terms of a constituent quark model wave function for the B-meson. Different approximations for the latter, either based on a phenomenological ansatz or derived from analyses of the meson spectra, are adopted. In particular, two wave functions, constructed via the Hamiltonian light-front formalism using a relativized and a non-relativistic constituent quark model, are considered, obtaining a link between standard spectroscopic quark models and the B-meson decay physics. The inclusive semileptonic and non-leptonic branching ratios are calculated within a convolution approach, inspired by the partonic model and involving the same B-meson wave function used for the evaluation of the exclusive semileptonic modes. Our results for the major branching ratios are consistent with available experimental data and the sum of all the calculated branching ratios turns out to be close to unity. In particular, we found that: i) a remarkable fraction (~ 35%) of semileptonic decay modes occur in non-D, non-D* final states; ii) non-perturbative effects enhance the inclusive b -> c \bar{u} d decay channels, with a sizable contribution provided by internal decays into heavy mesons and baryon-antibaryon pairs; the resulting reduction of the semileptonic branching ratio brings the theoretical prediction in agreement with the experimental value without increasing at the same time the charm counting.

hep-ph↗

arXiv1997

Rare decays B -> (K, K*) (l+ l-, nu bar{nu}) in the quark model

Long-distance effects in exclusive rare semileptonic transitions B -> (K, K*) are analysed within a relativistic quark model. The meson transition form factors, describing the meson amplitudes of the effective weak Hamiltonian, are calculated within the dispersion formulation of the quark model as relativistic double spectral representations through the wave functions of the initial and final mesons. The dilepton spectra and lepton asymmetries are considered within the framework provided by the Standard Model. It is found that, while the non-resonant decay rates are very sensitive to quark model parameters, the model dependence of the predicted dilepton forward-backward and lepton polarization asymmetries is remarkably small, providing only an overall ~10% uncertainty.

hep-ph↗

arXiv1997

Electroproduction of the Roper resonance and the constituent quark model

A parameter-free evaluation of the $N - P_{11}(1440)$ electromagnetic transition form factors is performed within a light-front constituent quark model, using for the first time the eigenfunctions of a mass operator which generates a large amount of configuration mixing in baryon wave functions. A one-body electromagnetic current, which includes the phenomenological constituent quark form factors already determined from an analysis of pion and nucleon experimental data, is adopted. For $Q^2$ up to few $(GeV/c)^2$, at variance with the enhancement found in the elastic channel, the effect of configuration mixing results in a significant suppression of the calculated helicity amplitudes with respect to both relativistic and non-relativistic calculations, based on a simple gaussian-like ansatz for the baryon wave functions.

nucl-th↗

arXiv1996

Light-front CQM calculations of baryon electromagnetic form factors

The parameter-free predictions for the $N - P_{11}(1440)$ and $N - P_{33}(1232)$ electromagnetic transition form factors, obtained within our light-front constituent quark model using eigenfunctions of a baryon mass operator which includes a large amount of configuration mixing, are reported. The effects due to small components in the baryon wave functions, such as S'- and D-wave, are also investigated.

nucl-th↗

arXiv1996

Charmed decays of the B-meson in the quark model

Exclusive and inclusive, semileptonic and non-leptonic, charmed decays of the B-meson are investigated in the context of a phenomenological quark model. Bound-state effects are taken care of by adopting a single (model-dependent) non-perturbative wave function, describing the motion of the light spectator quark in the B-meson. A nice reproduction of both exclusive and inclusive semileptonic data is obtained. Our predictions for the electron spectrum are presented and compared with those of the Isgur-Scora-Grinstein-Wise quark model. Finally, our approach is applied to the calculation of inclusive non-leptonic widths, obtaining a remarkable agreement with experimental findings.

hep-ph↗

arXiv1996

Weak Decay Form Factors of Heavy Pseudoscalar Mesons within a Light-Front Constituent Quark Model

The transition form factors describing the semileptonic decays of heavy pseudoscalar mesons are investigated within a relativistic constituent quark model formulated on the light-front. For the first time, the form factors are calculated in the whole accessible kinematical region, adopting meson wave functions derived from various effective $q \bar{q}$ interactions able to describe the meson mass spectra. It is shown that the decay rates of the $B \to D \ell ν_{\ell}$, $D \to K \ell ν_{\ell}$ and $D \to π\ell ν_{\ell}$ weak decays are mainly governed by the effects of the confinement scale, whereas the $B \to π\ell ν_{\ell}$ decay rate is sensitive to the high-momentum components generated in the meson wave functions by the effective one-gluon-exchange interaction. Our results are consistent with available experimental data and predictions of lattice $QCD$ calculations and $QCD$ sum rules.

hep-ph↗

arXiv1996

Heavy-to-heavy and heavy-to-light weak decay form factors in the light-front approach: the exclusive $ 0^- $ to $ 0^- $ case

Weak transition form factors among heavy pseudoscalar mesons are investigated within a relativistic quark model formulated on the light-front. It is shown that the light-front result derived in the time-like region for the matrix elements of the plus component of the weak vector current coincides with the spectator pole term of the quark triangle diagram. For the first time, the dependence of the form factors on the squared four-momentum transfer $q^2$ is calculated in the whole accessible kinematical region $0 \leq q^2 \leq q^2_{max}$. For the numerical investigations of the semileptonic $B \to D \ell ν_{\ell}$, $B \to π\ell ν_{\ell}$, $D \to K \ell ν_{\ell}$ and $D \to π\ell ν_{\ell}$, the equal-time wave functions corresponding to the updated version of the ISGW model are adopted. Our results for the form factors and the decay rates are presented and compared with available experimental data and predictions of different approaches. Moreover, the $K^0_L \to π^{\pm} ~ e^{\mp} ~ ν_e$ decay is briefly discussed. Our approach is consistent with experimental data.

hep-ph↗

arXiv1996

Quasi-elastic and inelastic inclusive electron scattering from an oxygen jet target

The results of an experiment on inclusive electron scattering from an oxygen jet target, performed in a wide range of energy and momentum transfer covering both quasi-elastic and $Δ$(1232) resonance regions, are reported. In the former region the theoretical predictions, obtained including effects of nucleon-nucleon correlations in both initial and final states, give a good description of the experimental data. In the inelastic region a broadening as well as a damping of the resonant part of the cross section with respect to the free nucleon case is observed. The need of more detailed calculations including nuclear structure effects on the electroproduction cross section of nucleon resonances is highlighted.

nucl-th↗

arXiv1996

Calculation of the Isgur-Wise function from a light-front constituent quark model

The space-like elastic form factor of heavy-light pseudoscalar mesons is investigated within a light-front constituent quark model in order to evaluate the Isgur-Wise form factor. The relativistic composition of the constituent quark spins is properly taken into account using the Melosh rotations, and various heavy-meson wave function are considered, including the eigenfunctions of an effective light-front mass operator reproducing meson mass spectra. It is shown that in a wide range of values of the recoil the Isgur-Wise form factor exhibits a moderate dependence upon the choice of the heavy-meson wave function and is mainly governed by the effects of the confinement scale.

hep-ph↗

arXiv1995

Electromagnetic $N - Δ$ transition form factors in a light-front constituent quark model

A parameter-free evaluation of the $N - Δ$ electromagnetic transition form factors is carried out in a light-front constituent quark model, based on $N$ and $Δ$ eigenstates of a mass operator which reproduces a large set of hadron energy levels. A one-body electromagnetic current, including the constituent quark form factors previously determined within the same approach from nucleon and pion experimental data, is used. Our prediction for the magnetic transition form factor is compared with the results both of phenomenological analyses of experimental data for the $Δ$ electroproduction, and of an analysis which includes the effects of off-energy-shell $π- N$ final state interaction.

nucl-th↗

arXiv1995

Electromagnetic Structure of Mesons in a Light-Front Constituent Quark Model

The elastic and transition electromagnetic form factors of mesons are investigated within a light-front constituent quark model. The general formulae for the space-like matrix elements of the one-body component of the electromagnetic current, including both Dirac and Pauli form factors of the constituent quarks, are presented. Our results for the pion charge form factor and the $πρ$ transition form factor are reported and compared with those obtained within various relativistic approaches in a range of values of the four-momentum transfer accessible to $CEBAF$.

nucl-th↗

arXiv1995

Nucleon-Nucleon Correlations and Multiquark Cluster Effects in Deep Inelastic Electron Scattering off Few-Nucleon Systems at $x>1$

Inclusive $A(e,e')X$ and semi-inclusive $A(e,e'N)X$ deep inelastic electron scattering processes off few-nucleon systems are investigated at $x > 1$, showing some of the relevant features of the cross section which are sensitive to the effects arising from nucleon-nucleon correlations and possible exotic multiquark cluster configurations at short internucleon separations.

nucl-th↗

arXiv1995

Nucleon and pion electromagnetic structure and constituent quark form factors

We have evaluated nucleon and pion electromagnetic form factors using for the first time eigenfunctions of a light-front mass operator, reproducing a large set of energy levels, and constituent quark form factors in the one-body current. The comparison with the experimental data yields valuable information on the electromagnetic structure of light constituent quarks and on the different role played by their Dirac and Pauli form factors.

nucl-th↗

arXiv1995

Nucleon-Nucleon Correlations and Inclusive Electron Scattering off Few-Nucleon Systems at $x>1$

Inclusive electron scattering off few-nucleon systems is investigated at $x>1$ and high momentum transfer, including the contributions from quasi-elastic and deep inelastic scattering processes. It is shown that at $x>1$ the inclusive cross section is dominated by the process of virtual photon absorption on a pair of correlated nucleons both in case of quasi-elastic and deep inelastic scattering events. The sensitivity of the nuclear response to the effects arising from the possible presence of multiquark cluster configurations at short internucleon separations as well as from possible medium-dependent modifications of the nucleon structure function, is illustrated.

nucl-th↗