Search

Double parton scattering, occurring in high energy hadron-hadron collisions, e.g. at the LHC, is usually investigated through model dependent analyses of the so called effective cross section $\sigma_{eff}$. We present a dynamic approach to this fundamental quantity making use of a Light-Front model treatment. Within such a framework $\sigma_{eff}$ is initially evaluated at low energy scale using the model and then, through QCD evolution, at high energy scale to reach the experimental conditions. Our numerical outcomes are consistent with the present experimental analyses of data in kinematical region we investigate. An important result of the present work is the $x_i$ dependence of $\sigma_{eff}$, a feature directly connected to double parton correlations and which could unveil new information on the three dimensional structure of the proton., Comment: 5 pages, 2 figures, invited talk at the Conference "Light-Cone 2015", 21-25 September 2015, Frascati, Italy; Few-Body Systems, in press

Double parton distribution functions (dPDF), accessible in high energy proton-proton and proton nucleus collisions, encode information on how partons inside a proton are correlated among each other and could represent a tool to explore the 3D proton structure. In recent papers, double parton correlations have been studied in the valence quark region, by means of constituent quark models. This framework allows to understand clearly the dynamical origin of the correlations and to establish which, among the features of the results, are model independent. Recent relevant results, obtained in a relativistic light-front scheme, able to overcome some drawbacks of previous calculations, such as the poor support, will be presented. Peculiar transverse momentum correlations, generated by the correct treatment of the boosts, are obtained. The role of spin correlations will be also shown. In this covariant approach, the symmetries of the dPDFs are unambiguously reproduced. The study of the QCD evolution of the model results has been performed in the valence sector, showing that, in some cases, the effect of evolution does not cancel that of correlations., Comment: 6 pages, 3 figures, invited talk at the Conference "Light Cone 2014 (LC2014)", Raleigh, North Carolina, May 26-30, 2014. Submitted to Few-Body Systems

We study the response of the nucleon, as a system of three bound (constituent) non relativistic quarks, to external (quasi static) electric and magnetic fields. The approach, based on a sum rule technique, is applied to a large class of two and three-body interquark potentials. Lower and upper bounds to the electric polarizability and para-magnetic susceptibility are explicitly calculated within a large variety of constituent models and their values related to the features of the interquark interaction picture. The r\^ole of three-body forces is investigated in details as well as the effects of SU(6) breaking terms in the potential model. Our results can be used to extract the mesonic contributions to the static polarizabilty and susceptibility. The quark degrees of freedom give a quite sizeable contributions to both and the meson cloud accounts roughly for 30% and 60% of the electric proton and neutron polarizability respectively. The quark contribution to the paramagnetic susceptibility is even higher and the mesonic effects are rather uncertain., Comment: 21 pages, no figures, LaTEX

The inclusive quasi-elastic electron scattering off16O,40Ar,208Pb and238U is studied including the distortion of the incoming and outgoing electron wave due to Coulomb interaction with the nuclear charge. The total cross-section, containing a new term due to the interference between the longitudinal and transverse structure functions, is discussed within the impulse approximation where final-state interaction and binding effects are included by using sum rules techniques. The quasi-elastic scattering induced by positrons is also investigated.

A study of deep inelastic electron-nucleus scattering is presented. The approach assumes that the non-perturbative component of the parton distributions can be approximated by constituent quark model wave functions for both free and bound nucleons. The perturbative contribution is evaluated by using the renormalization group evolution equations. Results are obtained for a large class of nuclei and the effects of conventional short-range nuclear dynamics included in the discussion. The dynamically generated sea and gluon content of the nucleus is investigated and rescaling and change in confinement scale are discussed in connection with the evolution procedure. Results in the regionx>1 are also presented.

We study the link between the constituent quark masses and the parton distributions extracted from high-energy deep inelastic lepton-nucleon scattering. To this end the valence distributions at low-resolution scale are evaluated within a constituent quark model while the solution of the QCD evolution equations is discussed in terms of an effective-quark-mass approximation. The results are compared with the experimental data and the influence of the one-gluon-exchange mechanism discussed.

The momentum distributions of constituent quarks in nucleons are studied within the framework of non-relativistic quark models for baryons. The corrections due to one-gluon-exchange effects are discussed comparing different quark-quark potential approximations. Applications to deep inelastic lepton-nucleon scattering are discussed within a quark-parton model in the laboratory frame.

TheM1-transitions in nuclei are studied by means of sum rules. A useful sum rule for the total strength of theM1-excitations is given in terms of the number of the nucleons which are in the nonfilledl-shell. The well-known Kurath sum rule for the energy-weighted total strength is generalized to include the two-body potential contribution. The centroid of the excitation is studied; finally the isotopic spreading of theT upper andT lower component of the excitation is analysed and the «symmetry energy» for theM1-transitions is studied. Not only the centroid shift, but also the isotopic splitting is an effect due toSU4-breaking terms of the nuclear Hamiltonian, in particular the spinorbit, Bartlett, Heisenberg terms.

Dynamical-correlation effects are studied in relation to both elastic and inelastic electron scattering processes, using a sum rule approach. The role of tensor correlations turns out to be important especially in spin-isospin modes. Detailed results are presented for16O showing the different influence of tensor correlations in longitudinal and transverse spin-isospin excitations.

In the present paper we have evaluated the TRK sum rule in spherical nuclei using many variants of realistic potentials. The crucial role of correlations has been discussed, even for the most recent parametrizations of the nucleon-nucleon forces, within a phenomenological model for the correlated ground state. In addition, the RPA values of the integrated photonuclear cross-section are reviewed: both schematic zero-range effective forces (of Skyrme type) and finite-range density-dependent forces have been discussed. i) The theoretical evaluation of the TRK sum rule represents a good test for the nucleon-nucleon potential model and the ground-state description. In particular, the inclusion of dynamical medium-range correlations (of tensor type mainly) enhances the values of the total sum considerably. ii) The TRK sum can be studied even within the RPA framework of the nuclear excitations. The values obtained are strongly dependent on the effective potentials, while the self-consistent treatment of the nuclear ground state and effective forces is not crucial in the numerical results. iii) A clear comparison between theoretical calculations and experimental observations is limited by contradictory experimental results and by the difficulties of evaluating higher-multipole excitations and tail contribution to the integrated photonuclear cross-section.

The momentum distributions in nuclei like4He,16O and40Ca are explicitly calculated within a phenomenological model which includes dynamical short range and tensor correlation effects. The common behaviour of such distributions in the high momentum region, already established in light nuclei, is extended to the medium weight region. Comparison with existing calculations is discussed and, for completeness, also form factors are evaluated within the same framework.

The role of short-range and tensor correlations in spin isospin longitudinal (q·σ) and transverse (q×σ) responses of a finite nucleus is studied in detail by using a sum rule approach. The tensor effects concur in favouring a spin-isospin ordered phase in the longitudinal response which results enhanced. They give the opposite contribution in the transverse channel. The influence of the dynamical correlations on the mean energies of the two responses is also discussed.

The radiative pion capture in nuclei has been studied by sum rules techniques. Two simple sum rules for the radiative-capture rates from 1s and 2p orbits are given in terms of the mean photon momentum, the nuclear density and the wave function of the pion. Explicit calculations are performed for4He,6Li,10B,12C,14N,16O where experimental results are available.

The dipole polarizability sum rule has been evaluated by means of a restricted Hartree-Fock approach. The method leads to a simple and analytical expression for the dipole polarizability. Explicit calculations have been performed in16O and40Ca with different types of interaction.