[en] We report on a recent investigation on the dynamics of light quarks, at intermediate and low-energy. By measuring some appropriate correlation functions on the lattice, it is possible to probe the Dirac and flavor structure of the non-perturbative quark-quark interaction and look directly for signatures of the 't Hooft Lagrangian. Results obtained with chiral fermions strongly support the instanton picture and the phenomenological parameters of the Instanton Liquid Model

[en] We study the instanton contribution to the proton and neutron electric form factors. Using the single instanton approximation, we perform the calculations in a mixed time-momentum representation in order to obtain the form factors directly in momentum space. We find good agreement with the experimentally measured electric form factor of the proton. For the neutron, our result falls short of the experimental data. We argue that this discrepancy is due to the fact that we neglect the contribution of the sea quarks. We compare to lattice calculations and a relativistic version of the quark-diquark model

[en] We show that a light-front covariant treatment of the spin in the nucleon gives rise to significant differences between transverse and longitudinal polarized parton distributions at low energies. These differences survive evolution to higher Q² scales and the importance of these relativistic spin effects can be assessed by measuring double-spin asymmetries in lepton pair production in polarized hadron-hadron collisions

[en] Lattice QCD predicts that, above a certain critical energy density or temperature, strongly interacting matter undergoes a phase transition from the hadronic world to a quark-gluon plasma state, where the coloured quarks and gluons are no longer bound to colourless hadrons. The suppression of quarkonium production in high-energy nuclear collisions is one of the most interesting signatures of QGP formation, for two reasons: due to their large masses, charm and beauty quarks are created only in the initial hard scattering processes, before the QGP is formed; and the Q(bar Q) binding potential should be screened in the deconfined colour medium. Until the LHC starts colliding Pb nuclei, charm is the heaviest quark that can check the validity of the finite temperature QCD predictions, given the much smaller beauty production cross sections. However, the interpretation of the presently available results on charmonium suppression in heavy-ion collisions, obtained at the SPS and RHIC, is hampered by a multitude of other 'nuclear effects', which exist even in the absence of QGP formation, such as the badly understood nuclear modifications of the gluon distribution functions, the level of energy lost by the partons traversing the nuclei before producing the Q(bar Q) pair, the rate at which the nascent quarkonium state is broken up by the surrounding nuclear matter, etc. Fortunately, most of these 'cold nuclear matter' effects can be studied on the basis of proton-nucleus measurements. However, care must be taken when converting the p-A observations into a reference baseline that can be used in the analysis of the heavy-ion data. In particular, it has recently been shown (1) that it is wrong to assume that the rate of final-state Glauber-like J/ψ absorption, usually called the 'J/ψ absorption cross section', σ_abs^J/ψ, is independent of the collision energy and of the charmonium kinematics, as was previously assumed in the analysis of the SPS heavy-ion data.

[en] We report the first nonperturbative calculation of proton electromagnetic form factors in the random instanton liquid model and in the interacting instanton liquid model. By calculating the ratio of appropriate three-point to two-point functions, we divide out the coupling constants and compare our results directly to some integral of the form factors. Using various parametrizations of the electric form factor G_E(Q²) at large Q²>3.5 GeV², where it is not yet measured, we compare those with expected theoretical dependence. We find from this comparison that some distributions of charge are clearly excluded (e.g., the same as of the magnetic moment, μG_E/G_M=1, as well as the opposite scenario in which μG_E/G_M rapidly approach zero), restricting possible behavior of the form factor to a rather narrow band. Furthermore, we found that our calculation of the nucleon form factors is dominated by a configuration in which two out of three quarks interact with a single instanton, in spite of the fact that the evaluated three-point function has rather large distances (∼1.2 fm) between points. We also estimate the size of the scalar diquark and found it to be very small, comparable to the typical instanton size

[en] 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(e²/h), where g_s=4 is the degeneracy factor.

[en] CP violation in the loop-mediated photon-photon interactions would modify the birefringence of the quantum vacuum. We discuss the implications of this effect on an experiment in which light propagates in a Fabry-Perot cavity permeated by a time-dependent electric and magnetic field: for some suitable orientation of the electric field, the effect does not cancel out in a round-trip. We show that the violation of CP would imply the existence of characteristic Fourier components, in the intensity spectrum of the outgoing wave. In order to estimate the magnitude of this effect within the standard model, we use chiral perturbation theory to compute the θ-term contribution to the coefficient of the leading CP-odd vertex, in the low-energy effective field theory for photon dynamics.

[en] We study the effects of instantons on the charged pion electromagnetic form factor at intermediate momenta. In the single instanton approximation (SIA), we predict the pion form factor in the kinematic region Q²=2-10 GeV². By developing the calculation in a mixed time-momentum representation, it is possible to maximally reduce the model dependence and to calculate the form factor directly. We find the intriguing result that the SIA calculation coincides with the vector dominance monopole form, up to a surprisingly high momentum transfer Q²∼10 GeV². This suggests that vector dominance for the pion holds beyond low energy nuclear physics

[en] A standard approach to investigate the nonperturbative QCD dynamics is through vacuum models which emphasize the role played by specific gauge field fluctuations, such as instantons, monopoles, or vortexes. The effective Hamiltonian describing the dynamics of the low-energy degrees of freedom in such approaches is usually postulated phenomenologically, or obtained through uncontrolled approximations. In a recent paper, we have shown how lattice field theory simulations can be used to rigorously compute the effective Hamiltonian of arbitrary vacuum models by stochastically performing the path integral over all the vacuum field fluctuations which are not explicitly taken into account. In this work, we present the first illustrative application of such an approach to a gauge theory and we use it to compute the instanton size distribution in SU(2) gluon dynamics in a fully model independent and parameter-free way.

[en] We study the electromagnetic form factors of the nucleon, from small to large momentum transfer, in the context of the instanton liquid model (ILM). As a first step, we analyze the role of single-instanton effects, and show that they dominate the form factors at large momentum transfer. Then, we go beyond the single-instanton approximation and perform a calculation to all orders in the 't Hooft interaction. We find that the ILM is in good agreement with the available experimental data. Based on these results, we argue that instantons provide a microscopic mechanism that explains the delay of the onset of the asymptotic perturbative regime in the electromagnetic form factors