Gaussian quantum fluctuations in the superfluid-Mott phase transition

Recent advances in cooling techniques make now possible the experimental study of quantum phase transitions, which are transitions near absolute zero temperature accessed by varying a control parameter. A paradigmatic example is the superfluid-Mott transition of interacting bosons on a periodic lattice. From the relativistic Ginzburg-Landau action of this superfluid-Mott transition we derive the elementary excitations of the bosonic system, which contain in the superfluid phase a gapped Higgs mode and a gappless Goldstone mode. We show that this energy spectrum is in good agreement with the available experimental data and we use it to extract, with the help of dimensional regularization, meaningful analytical formulas for the beyond-mean-field equation of state in two and three spatial dimensions. We find that, while the mean-field equation of state always gives a second-order quantum phase transition, the inclusion of Gaussian quantum fluctuations can induce a first-order quantum phase transition. This prediction is a strong benchmark for next future experiments on quantum phase transitions.Comment: 7 pages, 4 figures, to be published in Physical Review

Instantons, diquarks and non-leptonic weak decays of hyperons

This work is devoted to the study of the non-perturbative contributions in non-leptonic hyperon decays. We show that the instanton-induced 't Hooft interaction can naturally explain the Delta I=1/2 rule, by generating quark-diquark clustering inside octet baryons. We compute P-wave and S-wave amplitudes in the Instanton Liquid Model (ILM), and find good agreement with experiment. We propose a model-independent procedure to test on the lattice if the leading quark-quark attraction in the 0^+ anti-triplet channel responsible for diquark structures in hadrons is originated by the interaction generated by quasi-classical fields or it is predominantly due to other perturbative and/or confining forces.Comment: Final version to appear on PR

Instantons, Chiral Dynamics and Hadronic Resonances

We use the Interacting Instanton Liquid Model (IILM) as a tool to study the role played by the chiral interactions in the lowest-lying vector and axial vector meson resonances. We find that narrow a1 and rho meson resonances can be generated by instanton-induced chiral forces, even in the absence of confinement. In the IILM, these hadrons are found to have masses only about 30% larger than the experimental value and small width <10-50 MeV. This result suggests that chiral interactions are very important in these systems and provide most of their mass. We explore the decaying patterns of the rho meson, in the absence of confinement. We argue that, in our model where only chiral forces are switched on, this meson decays dissociating into its quark anti-quark constituents

Garigliano nuclear power plant: seismic evaluation of the turbine building

The Italian Garigliano Nuclear Power Plant (NPP) started its energy production in 1963. At present it is in the decommissioning stage. In order to get a proper management of the radioactive waste that will be produced during the dismantling operations it has been considered convenient to convert the turbine building of the plant into a temporary waste repository. This decision posed a remarkable seismic safety assessment issue. As a matter of fact, the challenge was to extend, in satisfactory safety conditions, the use of an important facility that has reached the end of its designed lifetime and to have this extended use approved by nuclear safety agencies. In this context many tasks have been accomplished, of which the most important are: (a) a new appraisal of site seismic hazard; (b) the execution of many investigations and testing on the construction materials; (c) the set up of a detailed 3D finite element model including the explicit representation of foundation piles and soil; (d) consideration of soil structure kinematic and dynamic nteraction effects. This paper describes the adopted seismic safety assessment criteria which are based on a performance objectives design approach. While performance based design is the approach currently recommended by European Regulations to manage seismic risk and it is fully incorporated in the Italian code for conventional buildings, bridges and plants, NPP are not explicitly considered. Therefore it was necessary to delineate a consistent interpretation of prescribed rules in order to properly select the maximum and operating design earthquakes on one side and corresponding acceptable limit states on the other side. The paper further provides an outline of the numerical analyses carried out, of the main results obtained and of the principal retrofitting actions that will be realized

Effective Field Theory for the Quantum Electrodynamics of a Graphene Wire

We study the low-energy quantum electrodynamics of electrons and holes, in a thin graphene wire. We develop an effective field theory (EFT) based on an expansion in p/p_T, where p_T is the typical momentum of electrons and holes in the transverse direction, while p are the momenta in the longitudinal direction. We show that, to the lowest-order in (p/p_T), our EFT theory is formally equivalent to the exactly solvable Schwinger model. By exploiting such an analogy, we find that the ground state of the quantum wire contains a condensate of electron-hole pairs. The excitation spectrum is saturated by electron-hole collective bound-states, and we calculate the dispersion law of such modes. We also compute the DC conductivity per unit length at zero chemical potential and find g_s =e^2/h, where g_s=4 is the degeneracy factor.Comment: 7 pages, 2 figures. Definitive version, accepted for publication on Phys. Rev.

Are There Diquarks in the Nucleon?

This work is devoted to the study of diquark correlations inside the nucleon. We analyze some matrix elements which encode information about the non-perturbative forces, in different color anti-triplet diquark channels. We suggest a lattice calculation to check the quark-diquark picture and clarify the role of instanton-mediated interactions. We study in detail the physical properties of the 0+ diquark, using the Random Instanton Liquid Model. We find that instanton forces are sufficiently strong to form a diquark bound-state, with a mass of ~500 MeV, which is compatible with earlier estimates. We also compute its electro-magnetic form factor and find that the diquark is a broad object, with a size comparable with that of the proton.Comment: Final version, accepted for publication on Phys. Rev.

The Effect of Interactions on the Conductance of Graphene Nanoribbons

We study the effects of the interaction between electrons and holes on the conductance G of quasi-one-dimensional graphene systems. We first consider as a benchmark the limit in which all interactions are negligible, recovering the predictions of the tight-binding approximation for the spectrum of the system, and the well-known result G=4 e^2/h for the lowest conductance quantum. Then we consider an exactly solvable field theoretical model in which the electro-magnetic interactions are effectively local. Finally, we use the effective field theory formalism to develop an exactly solvable model in which we also include the effect of non-local interactions. We find that such interactions turn the nominally metallic armchair graphene nanoribbon into a semi-conductor, while the short-range interactions lead to a correction to the G=4 e^2/h formula.Comment: 9 pages, 1 figur

Quantitative Protein Dynamics from Dominant Folding Pathways

We develop a theoretical approach to the protein folding problem based on out-of-equilibrium stochastic dynamics. Within this framework, the computational difficulties related to the existence of large time scale gaps in the protein folding problem are removed and simulating the entire reaction in atomistic details using existing computers becomes feasible. In addition, this formalism provides a natural framework to investigate the relationships between thermodynamical and kinetic aspects of the folding. For example, it is possible to show that, in order to have a large probability to remain unchanged under Langevin diffusion, the native state has to be characterized by a small conformational entropy. We discuss how to determine the most probable folding pathway, to identify configurations representative of the transition state and to compute the most probable transition time. We perform an illustrative application of these ideas, studying the conformational evolution of alanine di-peptide, within an all-atom model based on the empiric GROMOS96 force field.Comment: 4 pages, 1 figur

Dominant Reaction Pathways in High Dimensional Systems

This paper is devoted to the development of a theoretical and computational framework to efficiently sample the statistically significant thermally activated reaction pathways, in multi-dimensional systems obeying Langevin dynamics. We show how to obtain the set of most probable reaction pathways and compute the corrections due to quadratic thermal fluctuations around such trajectories. We discuss how to obtain predictions for the evolution of arbitrary observables and how to generate conformations which are representative of the transition state ensemble. We present an illustrative implementation of our method by studying the diffusion of a point particle in a 2-dimensional funneled external potential.Comment: 18 pages, 7 figures. Improvement in the text and in the figures. Version submitted for publicatio