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The aim of this work is to shed light to some less known aspects of Polyakov-loop extended chiral models, especially on the correlation of the quark sector and the Polyakov loop. We show that the order of chiral and Polyakov-loop transition and their difference in temperature as seen in LQCD calculations could be realised with a critical scale of the Polyakov-loop potential larger than the one in pure gauge theory. Comparing results for the Polyakov-loop susceptibility with the self-consistent medium-dependent quark mass and keeping the later at a fixed value allows to disentangle chiral-symmetry restoration and center-symmetry breaking effects. This indicates a confined chirally restored phase which is also seen as a plateau in the quark contribution to thermodynamics and by sigma and pion spectral functions that coincide but have a small width. Furthermore, we discuss that from some large chemical potential on the explicit center symmetry breaking is so strong that statistical deconfinement is realised at infinitely small temperatures. Both, the missing sensitivity of the Polyakov-loop to the quark mass except close to the chiral transition and the Polyakov loop being zero at zero temperature at all chemical potentials it is interpreted as an indication of a missing mechanism to take into account the quark back-reaction on the Polyakov loop.

The aim of this work is to shed light on some lesser known aspects of Polyakov-loop--extended chiral models (namely the Polyakov-loop extended Nambu--Jona-Lasinio and Quark-Meson models), especially on the correlation of the quark sector with the Polyakov loop. We show that the ordering of chiral and Polyakov-loop transitions and their difference in temperature as seen in lattice QCD calculations could be realized with a critical scale of the Polyakov-loop potential that is larger than the one in pure gauge theory. The comparison of the results for the Polyakov-loop susceptibility obtained using the self-consistent medium-dependent quark mass with those obtained while keeping these masses at a fixed value allows to disentangle chiral-symmetry restoration and center-symmetry breaking effects. Furthermore, a confined chirally restored phase is identified by a plateau in the quark contribution to thermodynamics and by sigma and pion spectral functions that coincide but have a small width. We also discuss that, for some large chemical potential values, the explicit center-symmetry breaking is so strong that statistical deconfinement is realized at infinitely small temperatures. Both the missing sensitivity of the Polyakov loop to the quark mass, except at close to the chiral transition, and the Polyakov loop being zero at zero temperature at all chemical potentials, can be interpreted as indications of a missing mechanism which accounts for the quark back-reaction on the Polyakov loop., Comment: 18 pages, 13 figures, version published in Phys. Rev. D

Fluctuations play an important role in diffractive production of vector mesons. It was in particular recently suggested, based on the Impact-Parameter dependent Saturation model (IPSat), that geometrical fluctuations triggered by the motion of the constituent quarks within the protons could explain incoherent diffractive processes observed at HERA. We propose a variant of the IPSat model which includes spatial and symmetry correlations between constituent quarks, thereby reducing the number of parameters needed to describe diffractive vector meson production to a single one, the size of the gluon cloud around each valence quark. The application to $J/\Psi$, $\rho$ and $\phi$ diffractive electron and photon production cross sections reveal the important role of geometrical fluctuations in incoherent channels, while other sources of fluctuations are needed to fully account for electroproduction of light mesons, as well as photo production of $J/\Psi$ mesons at small momentum transfer., Comment: 17 pages, 15 figures, major revision, title changed, new sections included , figures added to discuss vector meson electroproduction, new references, version accepted for publication in EPJ C

International audience; We discuss the leading-twist quark Wigner distributions in the nucleon, both in the T-even and T-odd sectors, considering all the possible configurations of the quark and nucleon polarizations. We identify the basic multipole structures associated with each distribution in the transverse phase space, providing a transparent interpretation of the spin-spin and spin-orbit correlations of quarks and nucleons encoded in these functions.

Fluctuations play an important role in diffractive production of vector mesons. It was in particular recently suggested, based on the Impact-Parameter dependent Saturation model (IPSat), that geometrical fluctuations triggered by the motion of the constituent quarks within the protons could explain incoherent diffractive processes observed at HERA. We propose a variant of the IPSat model which includes spatial and symmetry correlations between constituent quarks, thereby reducing significantly the number of parameters needed to describe diffractive vector meson production. The application to $J/\Psi$ diffractive photoproduction cross sections in coherent and incoherent channels reveals the r\^ole of correlations and their interplay with perturbative DGLAP evolution implemented up to NNLO.

International audience; We discuss different kinds of parton distributions, which allow one to obtain a multidimensional picture of the internal structure of the nucleon. We use the concept of generalized transverse momentum dependent parton distributions and Wigner distributions, which combine the features of transverse-momentum dependent parton distributions and generalized parton distributions. We show examples of these functions within a phenomenological quark model, with focus on the role of the spin-spin and spin-orbit correlations of quarks.

In two-photon collisions at LEP2 and a future e + e − linear collider heavy quarks (mainly charm) will be pair-produced rather copiously. The production via direct and resolved photons can be distinguished experimentally via a remnant-jet tag. We study correlations of the heavy quarks at next-to-leading order in QCD in the direct channel, which is free from phenomenological parton densities in the photon. These correlations are therefore directly calculable in perturbative QCD and provide a stringent test of the production mechanism.