Gauge-invariant nonlocal quark condensates in QCD

We study, by numerical simulations on a lattice, the behaviour of the gauge-invariant nonlocal quark condensates in the QCD vacuum both in the quenched approximation and with four flavours of dynamical staggered fermions. The correlation length of the condensate is determined to be roughly twice as big as in the case of the gluon field strength correlators.Comment: LATTICE98(confine

Field strength correlators in QCD at zero and non-zero temperature

We study, by numerical simulations on a lattice, the behaviour of the gauge--invariant field strength correlators in QCD both at zero temperature, down to a distance of 0.1 fm, and at finite temperature, across the deconfinement phase transition.Comment: Talk given at the ``High Energy Conference on Quantum Chromodynamics'', Montpellier (France), 4-12 July 1996 (QCD 96); 5 pages, LaTeX file, uses ``espcrc2.sty''+ 5 PS figure

Field strength correlators in QCD with dynamical fermions

We determine, by numerical simulations on a lattice, the gauge-invariant two-point correlation functions of the gauge field strengths in the QCD vacuum with four flavours of dynamical staggered fermions.Comment: Talk given at the ``XVth International Symposium on Lattice Field Theory'', Edinburgh (UK), July 22nd-26th 1997 (LATTICE 97); 3 pages, LaTeX file, uses ``espcrc2.sty''+ 2 PS figure

On the dependence of the gauge-invariant field-strength correlators in QCD on the shape of the Schwinger string

We study, by numerical simulations on a lattice, the dependence of the gauge-invariant two-point field-strength correlators in QCD on the path used to perform the color parallel transport between the two points.Comment: 9 pages, LaTeX file, + 5 PS figures. Revised version (accepted for publication in Physics Letters B) with new comments on the result

Improving the topological charge density operator on the lattice

We analyze the properties of a class of improved lattice topological charge density operators, constructed by a smearing-like procedure. By optimizing the choice of the parameters introduced in their definition, we find operators having (i) a much better statistical behavior as estimators of the topological charge density on the lattice, i.e. much less noisy; (ii) a multiplicative renormalization much closer to one; (iii) a large suppression of the perturbative tail in the corresponding lattice topological susceptibility.Comment: 4 pages, to be published in the Proceedings of Lattice 95, uuencoded postscript fil

Gauge--invariant field correlators in QCD at finite temperature

We study by numerical simulations on a lattice the behaviour of the gauge--invariant two--point correlation functions of the gauge field strengths across the deconfinement phase transition.Comment: 12 pages, LaTeX file, + 5 PS figures, uuencoded-tar-compresse

Gauge invariant Field Strength Correlators in QCD

Gauge invariant correlators in QCD are studied on the lattice.A Systematic determination of the correlation lengths for gluon field strength correlators and quark correlators is made.The measurement of the gluon and quark condensates is discussed.Comment: 5 pages,4 figures.Presented by A.Di Giacomo to ICHEP98. To appear in the proceedings Misprints correcte

Field strength correlators in full QCD

We study, by numerical simulations on a lattice, the behaviour of the gauge-invariant two-point correlation functions of the gauge field strengths in the QCD vacuum with dynamical fermions.Comment: 11 pages, LaTeX file, + 2 PS figure

Topological susceptibility in Yang-Mills theory in the vacuum correlator method

We calculate the topological susceptibility of the Yang-Mills vacuum using the field correlator method. Our estimate for the SU(3) gauge group, \chi^{1/4} = 196(7) MeV, is in a very good agreement with the results of recent numerical simulations of the Yang-Mills theory on the lattice.Comment: 5 pages (JETP Letters style

Two flavor QCD and Confinement

We argue that the order of the chiral transition for N_f=2 is a sensitive probe of the QCD vacuum, in particular of the mechanism of color confinement. A strategy is developed to investigate the order of the transition by use of finite size scaling analysis. An in-depth numerical investigation is performed with staggered fermions on lattices with N_t=4 and N_s=12,16,20,24,32 and quark masses am_q ranging from 0.01335 to 0.307036. The specific heat and a number of susceptibilities are measured and compared with the expectations of an O(4) second order and of a first order phase transition. A second order transition in the O(4) and O(2) universality classes are excluded. Substantial evidence emerges for a first order transition. A detailed comparison with previous works is performed.Comment: 46 pages, 20 eps figures, 9 tables, REVTeX