Remarks on the U(1) axial symmetry in QCD at zero and non-zero temperature

This paper is organized in two parts. The first part (Sections 2-5) is dedicated to the theory at T=0 and contains a pedagogical review of some fundamental aspects related with the chiral symmetries of QCD, the U(1) problem and its solution proposed by 'tHooft, Witten and Veneziano. In the second part (Sections 6-14) we discuss the role of the U(1) axial symmetry for the phase structure of QCD at finite temperature. One expects that, above a certain critical temperature, also the U(1) axial symmetry will be 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, we analyse a scenario in which the U(1) axial symmetry is still broken above the chiral transition. We will show that this scenario can be consistently reproduced in the full respect of the relevant QCD Ward Identities and also using an effective Lagrangian model. A new order parameter is introduced for the U(1) axial symmetry.Comment: Expanded version of a talk given at the ``Workshop on Quark-Gluon Plasma and Relativistic Heavy Ions'', Frascati (Italy), January 14th-18th, 2002 (QGP2002); 61 pages, LaTeX file, + 6 PS figure

A remark on the high--energy quark--quark scattering and the eikonal approximation

In this paper we calculate the high--energy quark--quark scattering amplitude, first in the case of scalar QCD, using Fradkin's approach to derive the scalar quark propagator in an external gluon field and computing it in the eikonal approximation. (This approach was also recently used by Fabbrichesi, Pettorino, Veneziano and Vilkovisky to study the four--dimensional Planckian--energy scattering in gravity.) We then extend the results to the case of ``real'' ({\it i.e.} fermion) QCD, thus deriving again, in a rather direct way, the results previously found by Nachtmann. The abelian case (QED) is also discussed in the Appendix.Comment: 24 pages (no figures), LaTeX--file. New references have been added and commented and some other minor corrections have been performe

Theta dependence of the vacuum energy density in chiral effective Lagrangian models at finite temperature, above TcT_c

In this work, extending a previous study at zero temperature ($T=0$), we perform a systematic study of the modifications to the QCD vacuum energy density $\epsilon_{vac}$ in the finite-temperature case, above the chiral transition at $T_c$, caused by a nonzero value of the parameter $\theta$, using two different effective Lagrangian models which implement the $U(1)$ axial anomaly of the fundamental theory and which are both well defined also above $T_c$. In particular, we derive (and critically compare) the expressions for the topological susceptibility $\chi$ and for the second cumulant $c_4$ starting from the $\theta$ dependence of $\epsilon_{vac}(\theta)$ in the two models.Comment: 23 pages; revised versio

Eikonal propagators for the high-energy parton-parton scattering in gauge theories

By a direct resummation of perturbation theory in the limit of very high energy and small transferred momentum (the so-called ``eikonal'' limit), we derive expressions for the truncated-connected quark, antiquark and gluon propagators in an external gluon field, both for scalar and fermion gauge theories. These are the basic ingredients to derive ``soft'' high-energy parton-parton scattering amplitudes, using the LSZ reduction formulae and a functional integral approach.Comment: Talk given at the ``Sixth Workshop on Non-Perturbative Quantum Chromodynamics'', Paris (France), June 5th-9th 2001 (NPQCD 01); 8 pages, LaTeX file, + 1 PS figur

High-energy scattering amplitudes in QCD: from Minkowskian to Euclidean space

We shall discuss about some analytic properties of the high-energy parton-parton (and hadron-hadron) scattering amplitudes in gauge theories, when going from Minkowskian to Euclidean theory, and we shall see how they can be related to the still unsolved problem of the s-dependence of the total cross-section.Comment: Talk given at the ``26th Johns Hopkins Workshop on current problems in particle theory: high energy reactions'', Heidelberg (Germany), 1-3 August 2002 (JHW2002); 7 pages, LaTeX file. Revised version with two errors in Eqs. (26) and (27) correcte

High-energy quark-quark scattering and the eikonal approximation

The high-energy quark-quark scattering amplitude is calculated first in the case of scalar QCD, using Fradkin's approach to derive the scalar quark propagator in an external gluon field and computing it in the eikonal approximation. The results are then extended to the case of ``real'' (i.e., fermion) QCD. The high-energy quark-quark scattering amplitude turns out to be described by the expectation value of two lightlike Wilson lines, running along the classical trajectories of the two colliding particles. Interesting analytic properties of the high-energy quark-quark scattering amplitude can be derived, going from Minkowskian to Euclidean theory: they could open the possibility of evaluating the high-energy scattering amplitude directly on the lattice.Comment: Talk given at the ``High Energy Conference on Quantum Chromodynamics'', Montpellier (France), July 3rd-9th 1997 (QCD 97); 6 pages, LaTeX file, uses ``espcrc2.sty'

The topological susceptibility of QCD: from Minkowskian to Euclidean theory

We show how the topological susceptibility in the Minkowskian theory of QCD is related to the corresponding quantity in the Euclidean theory, which is measured on the lattice. We discuss both the zero-temperature case (T = 0) and the finite-temperature case (T > 0). It is shown that the two quantities are equal when T = 0, while the relation between them is much less trivial when T > 0. The possible existence of ``Kogut-Susskind poles'' in the matrix elements of the topological charge density between states with equal four-momenta turns out to invalidate the equality of these two quantities in a strict sense. However, an equality relation is recovered after one re-defines the Minkowskian topological susceptibility by using a proper infrared regularization.Comment: 21 pages, LaTeX file, + 1 PS figur

Comments on high-energy total cross sections in QCD

We discuss how hadronic total cross sections at high energy depend on the details of QCD, namely on the number of colours $N_c$ and the quark masses. We find that while a "Froissart"-type behaviour $\sigma_{\rm tot}\sim B\log^2s$ is rather general, relying only on the presence of higher-spin stable particles in the spectrum, the value of $B$ depends quite strongly on the quark masses. Moreover, we argue that $B$ is of order ${\cal O}(N_c^0)$ at large $N_c$, and we discuss a bound for $B$ which does not become singular in the $N_f=2$ chiral limit, unlike the Froissart-\L ukaszuk-Martin bound.Comment: Revised version; matches published versio

Hadronic total cross sections at high energy and the QCD spectrum

We show how to obtain the leading energy dependence of hadronic total cross sections, in the framework of the nonperturbative approach to soft high-energy scattering based on Wilson-loop correlation functions, if certain nontrivial analyticity assumptions are satisfied. The total cross sections turn out to be of "Froissart" type, $\sigma_{\rm tot}^{(hh)}(s) \mathop\sim B\log^2 s$ for ${s \to \infty}$. We also discuss under which conditions the coefficient $B$ is universal, i.e., independent of the hadrons involved in the scattering process. In the most natural scenarios for universality, $B$ can be related to the stable spectrum of QCD, and is predicted to be $B_{\rm th}\simeq 0.22~{\rm mb}$, in fair agreement with experimental results. If we consider, instead, the stable spectrum of the quenched (i.e., pure-gauge) theory, we obtain a quite larger value $B^{(Q)}_{\rm th} \ge 0.42~{\rm mb}$, suggesting (quite surprisingly) large unquenching effects due to the sea quarks.Comment: Revised version; 43 pages, 3 figure