arXiv subjects

Ricardo A. Broglia

Publications and source records attributed to Ricardo A. Broglia.

7 recordsLinked to original sources

arXiv2016

40 years of the Nobel prize in physics: then and now

The findings for which Aage Bohr and Ben R. Mottelson became co-winners of the 1975 Nobel prize in physics provided the basis for a comprehensive and operative answer to the central problem in the study of the nuclear structure, namely the identification of the appropriate concepts and degrees of freedom that are suitable for describing the phenomena encountered. To do so they produced a breathtaking unification of a number of well established concepts, namely liquid drop and shell models, elementary modes of excitation, superconductivity and quantum electrodynamics, resulting eventually in the paradigm of broken symmetry restoration to determine the nuclear collective variables (CV, elementary modes of excitation): violation of translation invariance by the mean field and by scattering states (single-particle motion), of rotational invariance in the variety of spaces, in particular in 3D- and in gauge-space, leading to surface vibrations and to quadrupole rotations, as well as to pairing vibrations and rotations, with associated emergent properties of generalized rigidity in these spaces, resulting from the coupling to single-particle degrees of freedom.

physics.hist-ph↗

arXiv2015

Unified description of structure and reactions: implementing the Nuclear Field Theory program

The modern theory of the atomic nucleus results from the merging of the liquid drop (Niels Bohr and Fritz Kalckar) and of the shell model (Marie Goeppert Meyer and Axel Jensen), which contributed the concepts of collective excitations and of independent-particle motion respectively. The unification of these apparently contradictory views in terms of the particle-vibration (rotation) coupling (Aage Bohr and Ben Mottelson) has allowed for an ever increasingly complete, accurate and detailed description of the nuclear structure, Nuclear Field Theory (NFT, developed by the Copenhagen-Buenos Aires collaboration) providing a powerful quantal embodiment. In keeping with the fact that reactions are not only at the basis of quantum mechanics (statistical interpretation, Max Born) , but also the specific tools to probe the atomic nucleus, NFT is being extended to deal with processes which involve the continuum in an intrinsic fashion, so as to be able to treat them on an equal footing with those associated with discrete states (nuclear structure). As a result, spectroscopic studies of transfer to continuum states could eventually use at profit the NFT rules, extended to take care of recoil effects. In the present contribution we review the implementation of the NFT program of structure and reactions, setting special emphasis on open problems and outstanding predictions.

nucl-th↗

arXiv2014

Pairing interaction and two-nucleon transfer reactions

Making use of the fact that the collective modes associated with the spontaneous (static and dynamic) violation of gauge invariance in atomic nuclei (pairing rotations and pairing vibrations) are amenable to a simple, quite accurate nuclear structure description (BCS and QRPA respectively), it is possible to quantitatively test the reaction mechanism which is at the basis of two-nucleon transfer reactions, specific probe of pairing in nuclei. With the help of the static and dynamic mean field spectroscopic amplitudes, taking into account successive and simultaneous transfer channels properly corrected because of non-orthogonality effects, as well as describing the associated elastic channels in terms of experimentally determined optical potentials, one obtains absolute, two-particle transfer differential cross sections which provide an overall account of the data within experimental errors. One of the first results connected with such quantitative studies of pairing correlations in nuclei is the observation of phonon mediated pairing in the exotic halo nucleus 11Li, and the associated discovery of a new mechanism to break nuclear gauge symmetry: bootstrap, pigmy-resonance-mediated Cooper pair binding.

nucl-th↗

arXiv2012

Nuclear Field Theory predictions for 11Li and 12Be: shedding light on the origin of pairing in nuclei

Recent data resulting from studies of two-nucleon transfer reaction on 11Li, analyzed through a unified nuclear-structure-direct-reaction theory have provided strong direct as well as indirect confirmation, through the population of the first excited state of 9Li and of the observation of a strongly quenched ground state transition, of the prediction that phonon mediated pairing interaction is the main mechanism binding the neutron halo of the 8.5 ms-lived 11Li nucleus. In other words, the ground state of 11Li can be viewed as a neutron Cooper pair bound to the 9Li core, mainly through the exchange of collective vibration of the core and of the pigmy resonance arizing from the sloshing back and forth of the neutron halo against the protons of the core, the mean field leading to unbound two particle states, a situation essentially not altered by the bare nucleon-nucleon interaction acting between the halo neutrons. Two-neutron pick-up data, together with (t,p) data on 7Li, suggest the existence of a pairing vibrational band based on 9Li, whose members can be excited with the help of inverse kinematic experiments as was done in the case of 11Li(p,t)9Li reaction. The deviation from harmonicity can provide insight into the workings of medium polarization effects on Cooper pair nuclear pairing, let alone specific information concering the "rigidity" of the N=6 shell closure. Further information concerning these questions is provided by the predicted absolute differential cross sections associated with the reactions 12Be(p,t)10Be(gs) and 12Be(p,t)10Be(pv)(~10Be(p,t)8Be(gs)). In particular, concerning this last reaction, predictions of σ_{abs} can change by an order of magnitude depending on whether the halo properties associated with the d_{5/2} orbital are treated selfconsistently in calculating the ground state correlations of the (pair removal) mode, or not.

nucl-th↗

arXiv2012

Pairing correlations with single Cooper pair transfer to individual quantal states

With the help of the static and dynamic mean field spectroscopic amplitudes, taking into account successive and simultaneous transfer channels properly corrected because of non-orthogonality effects, as well as describing the associated elastic channels in terms of experimentally determined optical potentials, one obtains absolute, two-particle transfer differential cross sections which provide an overall account of the data within experimental errors. One of the first results connected with such quantitative studies of pairing correlations in nuclei is the observation of phonon mediated pairing in the exotic halo nucleus $^{11}$Li, and the associated discovery of a new mechanism to break nuclear gauge symmetry: bootstrap, pigmy-resonance-mediated Cooper pair binding.

nucl-th↗

arXiv2006

Hierarchy of events in protein folding: beyond the Go model

Simplified Go models, where only native contacts interact favorably, have proven useful to characterize some aspects of the folding of small proteins. The success of these models is limited by the fact that all residues interact in the same way, so that the folding features of a protein are determined only by the geometry of its native conformation. We present an extended version of a C-alpha based Go model where different residues interact with different energies. The model is used to calculate the thermodynamics of three small proteins (Protein G, SrcSH3 and CI2) and the effect of mutations on the wildtype sequence. The model allows to investigate some of the most controversial areas in protein folding such as its earliest stages, a subject which has lately received particular attention. The picture which emerges for the three proteins under study is that of a hierarchical process, where local elementary structures (LES) (not necessarily coincident with elements of secondary structure) are formed at the early stages of the folding and drive the protein, through the transition state and the postcritical folding nucleus (FN), resulting from the docking of the LES, to the native conformation.

q-bio.BM↗

arXiv2004

Quasi-particle properties of trapped Fermi gases

We develop a consistent formalism in order to explore the effects of density and spin fluctuations on the quasi-particle properties and on the pairing critical temperature of a trapped Fermi gas on the attractive side of a Feshbach resonance. We first analyze the quasi-particle properties of a gas due to interactions far from resonance (effective mass and lifetime, quasi-particle strength and effective interaction) for the two cases of a spherically symmetric harmonic trap and of a spherically symmetric infinite potential well. We then explore the effect of each of these quantities on $T_c$ and point out the important role played by the discrete level structure.

cond-mat.stat-mech↗