Functional approach to the non-mesonic decay of Lambda-hypernuclei

We present an evaluation of the non-mesonic decay widths for Lambda-hypernuclei (Lambda N --> NN, Lambda NN --> NNN) within the framework of the polarization propagator method. The full Lambda self-energy is evaluated microscopically in nuclear matter by using the functional approach, which supplies a theoretically well grounded approximation scheme for the classification of the relevant diagrams, according to the prescriptions of the bosonic loop expansion. We employ average Fermi momenta, suitably adapted to different mass number regions (medium-light, medium and heavy hypernuclei). Moreover, we study the dependence of the decay rates on the NN and Lambda-N short range correlations. With a proper choice of the parameters which control these correlations in the new approximation scheme, it is possible to reproduce the experimental decay widths for A > 10 hypernuclei.Comment: 25 pages, 8 figure

A simple model for NN correlations in quasielastic lepton-nucleus scattering

We present a covariant extension of the relativistic Fermi gas model which incorporates correlation effects in nuclei. Within this model, inspired by the BCS descriptions of systems of fermions, we obtain the nuclear spectral function and from it the superscaling function for use in treating high-energy quasielastic electroweak processes. Interestingly, this model has the capability to yield the asymmetric tail seen in the experimental scaling function.Comment: 11 pages, 6 figures, Proceedings of the Twenty Seventh International Workshop on Nuclear Theory, June 23 - 28, 2008, Rila mountains, Bulgari

Functional approach to the electromagnetic response function: the Longitudinal Channel

In this paper we address the (charge) longitudinal electromagnetic response for a homogeneous system of nucleons interacting via meson exchanges in the functional framework. This approach warrants consistency if the calculation is carried on order-by-order in the mesonic loop expansion with RPA-dressed mesonic propagators. At the 1-loop order and considering pion, rho and omega exchanges we obtain a quenching of the response, in line with the experimental results.Comment: RevTeX, 18 figures available upon request - to be published in Physical Review

Connecting scaling with short-range correlations

We reexamine several issues related to the physics of scaling in electron scattering from nuclei. A basic model is presented in which an assumed form for the momentum distribution having both long- and short-range contributions is incorporated in the single-particle Green function. From this one can obtain saturation of nuclear matter for an NN interaction with medium-range attraction and short-range repulsion, and can obtain the density-density polarization propagator and hence the electromagnetic response and scaling function. For the latter, the shape of the scaling function and how it approaches scaling as a function of momentum transfer are both explored.Comment: 24 pages, 15 figures. A reference has been corrected and update

Fermion propagators in space-time

The one- and the two-particle propagators for an infinite non-interacting Fermi system are studied as functions of space-time coordinates. Their behaviour at the origin and in the asymptotic region is discussed, as is their scaling in the Fermi momentum. Both propagators are shown to have a divergence at equal times. The impact of the interaction among the fermions on their momentum distribution, on their pair correlation function and, hence, on the Coulomb sum rule is explored using a phenomenological model. Finally the problem of how the confinement is reflected in the momentum distribution of the system's constituents is briefly addressed.Comment: 26 pages, 9 figures, accepted for publication on Phys. Rev.

Photopion reactions on deltas preexisting in nuclei

Reactions A(\gamma, pi^+p) are considered to proceed through the formation of pion-proton pairs on delta constituents in nuclei. We develop the nonrelativistic operator for \Delta^{++}(\gamma, pi^+)p process in an arbitrary frame. The calculated cross section for C12(\gamma, \pi^+p) reaction is compared to the existing experimental data.Comment: 8 pages, 3 figure

SOLUZIONE DINAMICA DELLA RETE ASSOGEO E INQUADRAMENTO EUREF-RDN

L’importante sviluppo avuto recentemente dai servizi GPS di posizionamento differenziale in tempo reale (RTK e VRS), ha permesso di diminuire in modo considerevole i tempi e quindi i costi per un rilievo topografico, questo senza incidere in modo significativo sulla precisione delle misure eseguite [Pesci et al., 2008]. L’infrastruttura che permette la realizzazione di questi servizi è costituita da una rete GPS di stazioni permanenti (SP) distribuite più o meno regolarmente su di una griglia la cui maglia, cioè la distanza tra le stazioni, è di qualche decina di chilometri. Assogeo S.p.a. a partire dal 2006 ha sviluppato nell’Italia centro-settentrionale una rete di stazioni permanenti (Fig. 1a) in grado di supportare i diversi servizi di posizionamento in tempo reale.Attualmente questa rete è costituita da 32 stazioni equipaggiate con ricevitore ed antenne geodetiche a doppia frequenza (Tab. 1) in continua espansione. Un singolo operatore, anche se equipaggiato di un solo ricevitore GPS, ha la possibilità di connettersi al centro operativo Assogeo ed eseguire il proprio rilievo topografico, in tempo reale. Il sistema di riferimento in cui vengono forniti i risultati del rilievo è quello su cui vengono calcolate le posizioni delle diverse SP, per questo motivo è importante che tale sistema di riferimento sia compatibile con quello utilizzato in ambito cartografico e topografico definito dall’Istituto Geografico Militare Italiano (IGMI). Inoltre, è necessario che la stima della posizione delle diverse SP sia la più precisa possibile e che quindi venga calcolata utilizzando tutte le informazioni acquisite da tali stazioni. A questo proposito IGMI ha istituito la Rete Dinamica Nazionale [RDN, Baroni et al. 2009] formata da 99 SP già operanti sul territorio e omogeneamente distribuite, offrendo un riferimento per allineare le diverse reti GPS per il servizio di posizionamento in tempo reale sorte sul territorio italiano. La rete RDN è a sua volta allineata ad un sistema di riferimento convenzionale ufficializzato in Europa, cioè il sistema ETRF2000 [Bruyninx et al., 2009], e riferito all’epoca 2005.0, secondo le più recenti direttive EUREF [Bruyninx, 2004, Kenyeres and Bruyninx, 2004]. Per uniformare le soluzioni allo stesso sistema di riferimento utilizzato dall’IGMI, è necessario quindi che anche le posizioni delle SP della rete Assogeo siano calcolate in ETRF2000 all’epoca 2005.0 e poiriportate al 2008.0. In questo lavoro vengono proposte e confrontate due strategie di calcolo per ottenere, in un modo semplice e intuitivo, il sistema di riferimento desiderato. Si tratta di procedure differenti che analizzano il medesimo insieme di osservazioni utilizzando lo stesso software, ma utilizzando strategie diverse, questo per verificare la reale precisione con cui può essere stimata la posizione di una stazione GPS permanente

GPS-based monitoring of land subsidence in the Po Plain (Northern Italy)

We use regional and local networks of continuously-operating GPS stations (CGPS) distributed in the northern–central part of the Italian peninsula to investigate the subsidence phenomenon of the Po plain sedimentary basin and vertical movements of the surrounding areas. The observations of 146 scientific and commercial stations are analyzed and compared, adopting analytical techniques widely used to study GPS coordinate time series. The use of simple antenna supports in commercial installations, instead of a more rigorous geodetic monument, does not seem to induce significant differences in the noise characteristics and in the amplitudes of annual and semi-annual periodic signals. The vertical velocity field deduced from 129 sites with observation time spans greater than one year, located in the Central–Northern Italian Peninsula, indicates the presence of two mainly subsidence areas: the Po Plain and the Arno Plain, while the sites located in the Alps and Apennine domains show relatively low uplift. The areas of the Po Plain monitored by GPS seem to indicate that the subsidence rate is constant or, in some cases, decreasing with respect to the values obtained from the last measurements, performed up to 2006 by means of both SAR and levelling techniques. Only the central part of the eastern Po Plain close to the Apennine border (Modena city area) is characterized by a peak in subsidence consisting in a velocity of about 15 mm/yr

Singular Structure and Enhanced Friedel Oscillations in the Two-Dimensional Electron Gas

We calculate the leading order corrections (in $r_s$) to the static polarization $\Pi^{*}(q,0,)$, with dynamically screened interactions, for the two-dimensional electron gas. The corresponding diagrams all exhibit singular logarithmic behavior in their derivatives at $q=2 k_F$ and provide significant enhancement to the proper polarization particularly at low densities. At a density of $r_s=3$, the contribution from the leading order {\em fluctuational} diagrams exceeds both the zeroth order (Lindhard) response and the self-energy and exchange contributions. We comment on the importance of these diagrams in two-dimensions and make comparisons to an equivalent three-dimensional electron gas; we also consider the impact these finding have on $\Pi^{*}(q,0)$ computed to all orders in perturbation theory