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E. Meggiolaro

Publications and source records attributed to E. Meggiolaro.

29 records · Page 2Linked to original sources
arXiv2004

On the loop-loop scattering amplitudes in Abelian and non-Abelian gauge theories

The high-energy scattering amplitude of two colour-singlet quark-antiquark pairs is governed by the correlation function of two Wilson loops, which follow the classical straight lines for quark (antiquark) trajectories. This quantity is expected to be free of IR divergences, differently from what happens for the parton-parton scattering amplitude, described, in the high-energy limit, by the expectation value of two Wilson lines. We shall explicitly test this IR finitness by a direct non-perturbative computation of the loop-loop scattering amplitudes in the (pedagogic, but surely physically interesting) case of quenched QED. The results obtained for the Abelian case will be generalized to the case of a non-Abelian gauge theory with N_c colours, but stopping to the fourth order in perturbation theory. In connection with the above-mentioned IR finitness, we shall also discuss some analytic properties of the loop-loop scattering amplitudes in both Abelian and non-Abelian gauge theories, when going from Minkowskian to Euclidean theory, which can be relevant to the still unsolved problem of the s-dependence of hadron-hadron total cross-sections.

hep-ph↗
arXiv2003

The effects of an extra U(1) axial condensate on the radiative decay eta' --> gamma gamma at finite temperature

Supported by recent lattice results, we consider a scenario in which a U(1)-breaking condensate survives across the chiral transition in QCD. This scenario has important consequences on the pseudoscalar-meson sector, which can be studied using an effective Lagrangian model. In particular, generalizing the results obtained in a previous paper (where the zero-temperature case was considered), we study the effects of this U(1) chiral condensate on the radiative decay eta' --> gamma gamma at finite temperature.

hep-ph↗
arXiv2003

Reviewing the problem of the U(1) axial symmetry and the chiral transition in QCD

We discuss the role of the U(1) axial symmetry for the phase structure of QCD at finite temperature. We expect that, above a certain critical temperature, also the U(1) axial symmetry will be (effectively) restored. We will try to see if this transition has (or has not) anything to do with the usual chiral transition: various possible scenarios are discussed. In particular, supported by recent lattice results, we analyse a scenario in which a U(1)-breaking condensate survives across the chiral transition. This scenario can be consistently reproduced using an effective Lagrangian model. The effects of the U(1) chiral condensate on the slope of the topological susceptibility in the full theory with quarks are studied: we find that this quantity (in the chiral limit of zero quark masses) acts as an order parameter for the U(1) axial symmetry above the chiral transition. Further information on the new U(1) chiral order parameter is derived from the study (at zero temperature) of the radiative decays of the pseudoscalar mesons in two photons: a comparison of our results with the experimental data is performed.

hep-ph↗
arXiv2003

Gauge-invariant field-strength correlators in pure Yang-Mills and full 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, both for the pure-gauge SU(3) theory and for full QCD with two flavours. Quenched data agree within errors with previous determinations, but have much higher statistics. A best-fit analysis of the data has been performed, both for the quenched and the full-QCD case, showing that the electric gluon condensate drops to zero at the deconfining phase transition.

hep-lat↗
arXiv2000

The gauge invariant quark correlator in QCD sum rules and lattice QCD

Taking the gauge invariant quark correlator as an example, in this work we perform a direct comparison of lattice QCD simulations with the QCD sum rule approach. The quark correlator is first investigated in the framework of QCD sum rules and the correlation length of the quark field is calculated. Comparing the phenomenological part of the sum rule with previous measurements of the quark correlator on the lattice, we are able to obtain an independent result for the correlation length. From a fit of the lattice data to the operator product expansion, the quark condensate and the mixed quark-gluon condensate can be extracted.

hep-ph↗
arXiv1999

Structure functions at small x_Bj in a Euclidean field theory approach

The small-x_Bj limit of deep inelastic scattering is related to the high-energy limit of the forward Compton amplitude in a familiar way. We show that the analytic continuation of this amplitude in the energy variable is calculable from a matrix element in Euclidean field theory. This matrix element can be written as a Euclidean functional integral in an effective field theory. Its effective Lagrangian has a simple expression in terms of the original Lagrangian. The functional integral expression obtained can, at least in principle, be evaluated using genuinely non-perturbative methods, e.g., on the lattice. Thus, a fundamentally new approach to the long-standing problem of structure functions at very small x_Bj seems possible. We give arguments that the limit x_Bj -> 0 corresponds to a critical point of the effective field theory where the correlation length becomes infinite in one direction.

hep-ph↗
arXiv1998

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.

hep-lat↗
arXiv1998

Gauge-invariant quark-antiquark nonlocal condensates in lattice QCD

We study, by numerical simulations on a lattice, the behaviour of the gauge-invariant quark-antiquark nonlocal condensates in the QCD vacuum with dynamical fermions. A determination is also done in the quenched approximation and the results are compared with the full-QCD case. The fermionic correlation length is extracted and compared with the analogous gluonic quantity.

hep-lat↗
arXiv1998

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.

hep-lat↗
arXiv1994

Exact solution (by algebraic methods) of the lattice Schwinger model in the strong-coupling regime

Using the monomer--dimer representation of the lattice Schwinger model, with $N_f =1$ Wilson fermions in the strong--coupling regime ($β=0$), we evaluate its partition function, $Z$, exactly on finite lattices. By studying the zeroes of $Z(k)$ in the complex plane $(Re(k),Im(k))$ for a large number of small lattices, we find the zeroes closest to the real axis for infinite stripes in temporal direction and spatial extent $S=2$ and 3. We find evidence for the existence of a critical value for the hopping parameter in the thermodynamic limit $S\rightarrow \infty$ on the real axis at about $k_c \simeq 0.39$. By looking at the behaviour of quantities, such as the chiral condensate, the chiral susceptibility and the third derivative of $Z$ with respect to $1/2k$, close to the critical point $k_c$, we find some indications for a continuous phase transition.

hep-lat↗