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©2011 American Physical Society. We thank J. Nieves and E. Ruiz-Arriola for useful discussions, checks and suggestions and B. Mussallam for the results of his Roy-Steiner dispersive analysis of the K(800) channel., We show that the latest and very precise dispersive data analyses require a large and very unnatural finetuning of the 1/N_c expansion at N_c = 3if the f_o(600) and K(800) light scalar mesons are to be considered predominantly q̅q states, which is not needed for light vector mesons. For this, we use scattering observables whose 1/N_c corrections are suppressed further than one power of 1/N_c for q̅q or glueball states, thus enhancing contributions of other nature. This is achieved without using unitarized ChPT, but if it is used we can also show that it is not just that the coefficients of the 1/N_c expansion are unnatural, but that the expansion itself does not even follow the expected 1/N_c scaling of a glueball or a q̅q meson., Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

©2 008 The American Physical Society. Research was partially funded by Banco Santander/Complutense Contract No. PR27/05-13955- SCH and Spanish Contract No. FPA2007-29115-E, No. FPA2005-02327, No. FPA2004-02602, and No. UCM-CAM 910309. J. R. Peláez’s research is partly funded by Spanish Contract No. BFM2003-00856, and is part of the EU integrated infrastructure initiative HadronPhysics project, under Contract No. RII3-CT-2004-506078., 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., Banco Santander / Complutense, Spanish Contract, HadronPhysics project, UCM-CAM, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

©2002 The American Physical Society. Work supported by the Spanish CICYT projects, Grant Nos. FPA2000-0956, PB98-0782, and BFM2000-1326. J.R.P. acknowledges support from the CICYT-INFN collaboration, Grant No. 003P 640.15, and E. Oset for useful comments., We study the temperature evolution of the rho and sigma mass and width, using a unitary chiral approach. The one-loop pipi scattering amplitude in chiral perturbation theory at Tnot equal0 is unitarized via the inverse amplitude method. Our results predict a clear increase with T of both the rho and sigma widths. The masses decrease slightly for high T, while the rhopipi coupling increases. The rho behavior seems to be favored by experimental results. In the sigma case, it signals chiral symmetry restoration., Spanish CICYT, CICYT-INFN, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 2009 The American Physical Society. We acknowledge financial support from the Spanish research Projects No. FPA2007-29115-E, No. PR34- 1856-BSCH, No. CCG07-UCM/ESP-2628, No. FPA2008- 00592, No. FIS2008-01323, and from the FPI programme (No. BES-2005-6726)., We consider an interacting pion gas in a stage of the system evolution where thermal but not chemical equilibrium has been reached, i.e., for temperatures between thermal and chemical freeze-out T(ther) < T < T(chem) reached in relativistic heavy-ion collisions. Approximate particle number conservation is implemented by a nonvanishing pion number chemical potential mu(pi) within a diagrammatic thermal field-theory approach, valid in principle for any bosonic field theory in this regime. The resulting Feynman rules are derived here and applied within the context of chiral perturbation theory to discuss thermodynamical quantities of interest for the pion gas such as the free energy, the quark condensate, and thermal self-energy. In particular, we derive the mu(pi) not equal 0 generalization of Luscher and Gell-Mann-Oakes-Renner-type relations. We pay special attention to the comparison with the conventional kinetic theory approach in the dilute regime, which allows for a check of consistency of our approach. Several phenomenological applications are discussed, concerning chiral symmetry restoration, freeze-out conditions, and Bose-Einstein pion condensation., Spanish research Projects, FPI programme, BSCH, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 2006 The American Physical Society. We are grateful to R. F. lvarez-Estrada, A. Dobado, F. J. LLanes-Estrada, and J. M. Martínez Resco for their useful comments. We also acknowledge financial support from the Spanish research projects No. FPA2004-02602, No. BFM2002-01003, No. R27/05-13955-BSCH, No. FPA2005-02327, and from the Spanish F.P.I. programme (BES-2005-6726)., 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 sigma(T) is a decreasing function of T, behaving for very low T as sigma(T)similar to e(2)m(pi) root m pi/T, consistently with nonrelativistic kinetic theory. When unitarization effects are included, sigma(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., Spanish research projects, Spanish F.P.I. programme, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub