The inverse amplitude method is a powerful unitarization technique to enlarge the energy applicability region of effective Lagrangians. It has been widely used to describe resonances in hadronic physics, combined with chiral perturbation theory, as well as in the strongly interacting symmetry breaking sector. In this work we show how it can be slightly modified to also account for the subthreshold region, incorporating correctly the Adler zeros required by chiral symmetry and eliminating spurious poles. These improvements produce negligible effects on the physical region.

Abstract. We present recent results on a systematic method to calculate transport coefficients for a meson gas (in particular, we analyze a pion gas) at low temperatures in the context of Chiral Perturbation Theory (ChPT). Our method is based on the study of Feynman diagrams taking into account collisions in the plasma by means of the non-zero particle width. This implies a modification of the standard ChPT power counting scheme. We discuss the importance of unitarity, which allows for an accurate high energy description of scattering amplitudes, generating dynamically the ρ(770) and f0(600) mesons. Our results are compatible with analyses of kinetic theory, both in the non-relativistic very low-T regime and near the transition. We show the behavior with temperature of the electrical and thermal conductivities as well as of the shear and bulk viscosities. We obtain that bulk viscosity is negligible against shear viscosity, except near the chiral phase transition where the conformal anomaly might induce larger bulk effects. Different asymptotic limits for transport coefficients, large-Nc scaling and some applications to heavy-ion collisions are studied. PACS

We present a complete one-loop calculation of all the two-meson scattering amplitudes within the framework of SU͑3͒ chiral perturbation theory, which includes pions, kaons, and the eta. In addition, we have unitarized these amplitudes with the coupled channel inverse amplitude method, which simultaneously ensures the good low energy properties of chiral perturbation theory and unitarity. We show how this method provides a remarkable description of meson-meson scattering data up to 1.2 GeV including the scattering lengths and the generation of seven light resonances, which is consistent with previous determinations of the chiral parameters. Particular attention is paid to discussing the differences and similarities of this work with previous analyses in the literature.

The electrical conductivity of a pion gas at low temperatures is studied in the framework of linear response and chiral perturbation theory. The standard ChPT power counting has to be modified to include pion propagator lines with a nonzero thermal width in order to properly account for collision effects typical of kinetic theory. With this modification, we discuss the relevant chiral power counting to be used in the calculation of transport coefficients. The leading order contribution is found and we show that the dominant higher order ladder diagrams can be treated as perturbative corrections at low temperatures. We find that the DC conductivity T is a decreasing function of T, behaving for very low T as T e 2 m m =T p , consistently with nonrelativistic kinetic theory. When unitarization effects are included, T increases slowly as T approaches the chiral phase transition. We compare with related works and discuss some physical consequences, especially in the context of the low-energy hadronic photon spectrum in relativistic heavy ion collisions.

We study the temperature evolution of the ρ and σ mass and width, using a unitary chiral approach. The one-loop ππ scattering amplitude in Chiral Perturbation Theory at T = 0 is unitarized via the Inverse Amplitude Method. Our results predict a clear increase with T of both the ρ and σ widths. The masses decrease slightly for high T , while the ρππ coupling increases. The ρ behavior seems to be favored by experimental results. In the σ case, it signals chiral symmetry restoration.PACS numbers: 11.10. Wx, 12.39.Fe, 11.30.Rd, One of the outstanding phenomena related to heavy ion collisions is the flatness of the dilepton spectrum near the mass of the ρ meson, which is so clearly visible in many processes involving hadrons and electromagnetic probes. This flatness has been observed by the HELIOS and CERES collaborations [1,2] and has been the subject of widespread discussion. Dileptons and photons provide neat signals of the early stages of the quark-gluon plasma and its subsequent evolution into a hadron gas [3]. In fact, the most credible explanation of the absence of a prominent hill in the dilepton spectrum is a change in the mass and width of the ρ due to its interactions with the hot hadron gas [4,5,6,7]. Since the baryons, with a large forward momentum, have almost escaped the central collision region, this gas is composed mainly of pions. Our aim is to study the thermal evolution of the ρ mass M ρ and width Γ ρ , from the first principles of chiral symmetry and unitarity in ππ scattering.What happens to the ρ in extreme conditions is a hadronic physics problem, involving non perturbative physics and hence difficult to be treated. Prior to this work, a copious number of models and estimations have appeared. In most of them Γ ρ increases with temperature, simply as a consequence of stimulated emission in the pion thermal bath or, equivalently, because the effective phase space increases [8,9]. This behavior is often interpreted as a deconfining effect, or hadron "melting". As for the mass, Vector Meson Dominance (VMD) implies that M ρ changes very little at low temperatures [8,10,11]. As T approaches the critical temperature, earlier works claimed that M ρ increases [8,11,12] but the analysis of experimental dilepton data seems to favor a decreasing behavior [5,7]. Let us remark that in all these works, the ρ is introduced as an explicit degree of freedom and often a dilute pion gas is assumed, so that the thermal effects appear, to leading order, only through the pion distribution function and not through the interaction details. Other approaches include the NJL model [13], where M ρ and Γ ρ slightly decrease (but there is an spurious quark threshold near M ρ ) as well as qq wavefunction analysis in the π channel yielding a decreasing width [14].In this work we will use a thermal treatment of the effective degrees of freedom, the pions in the aftermath of the collision at moderate temperatures. The guiding fundamental principles will be just chiral symmetry and unitarity. We will build on a previous work ...