Person: Gómez Nicola, Ángel
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 10 of 69
PublicationErratum: Effective chiral lagrangian from QCD at nonzero chemical potential (vol 355, pg 288, 1995)(Elsevier Science, 1996-07-11) Fernández Álvarez-Estrada, Ramón; Gómez Nicola, Ángel PublicationLight quarks at finite temperature: chiral restoration and the fate of the U(1)(A) symmetry(Springer Heidelberg, 2021-05-28) Gómez Nicola, ÁngelWe review recent results on the role of light quark states within the QCD phase diagram. In particular, we will discuss how the combined use of theoretical techniques such as Effective Theories, Unitarization and Ward Identities helps to shed light on several important issues regarding chiral symmetry restoration, building bridges with recent lattice analyses. Special attention will be paid to the role of chiral and U(1)(A) partners in the interplay between those symmetries, which is crucial to properly understand the transition pattern. Light scalar mesons at finite temperature will be shown to be responsible for the description of susceptibilities encoding chiral and U(1)(A) restoration properties. PublicationBreaking the configurational anisotropy in Fe single crystal nanomagnets(American Institute of Physics, 2014-03-10) Gómez Nicola, Ángel; Cebollada, F.; Palomares, F. J.; Sánchez, N.; González Herrera, Elvira María; Gonzalez, J. M.; Vicent López, José LuisIn this work, we improve the ability to tailor the switching mechanism in nanomagnets by introducing an additional, highly controlled source of anisotropy: magnetocrystalline anisotropy. We analyze the vortex dynamics in single crystal Fe nanotriangles with different orientations of the crystalline axes. By experimental studies and simulation, we show that the angular dependence of the vortex annihilation field springs from the convolution of the crystalline and configurational anisotropies. In contrast, the remanence and the nucleation field present a much simpler behavior controlled by the existence of a single symmetry axis when shape and crystalline orientation are taken into account. PublicationTransport coefficients and resonances for a meson gas in chiral perturbation theory(Springer, 2009-06) Fernández Fraile, Daniel; Gómez Nicola, ÁngelWe 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 rho(770) and f(0)(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 the result that the bulk viscosity is negligible against the shear viscosity, except near the chiral phase transition where the conformal anomaly might induce larger bulk effects. Various asymptotic limits for transport coefficients, large-N(c) scaling and some applications to heavy-ion collisions are studied. PublicationImproved unitarized Heavy Baryon Chiral Perturbation Theory for π N scattering(Elsevier science, 2000-07-27) Gómez Nicola, Ángel; Nieves, J.; Peláez Sagredo, José Ramón; Ruiz Arriola, E.We show how the unitarized description of pion nucleon scattering within Heavy Baryon Chiral Perturbation Theory can be considerably improved, by a suitable reordering of the expansion over the nucleon mass. Within this framework, the ∆ resonance and its associated pole can be recovered from the chiral parameters obtained from low-energy determinations. In addition, we can obtain a good description of the six S and P wave phase shifts in terms of chiral parameters with a natural size and compatible with the Resonance Saturation Hypothesis. PublicationTransport properties of a meson gas(World Scientific Publ Co Pte Ltd, 2007-10) Fernández Fraile, Daniel; Gómez Nicola, ÁngelWe 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. Our method is based on the study of Feymnan diagrams with a power counting which takes into account collisions in the plasma by means of a non-zero particle width. In this way, we obtain results compatible with analysis of Kinetic Theory with just the leading order diagram. We show the behavior with temperature of electrical and thermal conductivities and shear and bulk viscosities, and we discuss the fundamental role played by unitarity. We obtain that bulk viscosity is negligible against shear viscosity near the chiral phase transition. Relations between the different transport coefficients and bounds on them based on different theoretical approximations are also discussed. We also comment on some applications to heavy-ion collisions. PublicationThe role of strangeness in chiral and U(1)A restoration(Springer, 2021-07) Gómez Nicola, Ángel; Ruiz de Elvira Carrascal, Jacobo; Vioque Rodríguez, Andrea; Álvarez Herrero, D.© The Author(s) 2021 We use recently derived Ward identities and lattice data for the light- and strange-quark condensates to reconstruct the scalar and pseudoscalar susceptibilities (chi S kappa, chi PK) in the isospin 1/2 channel. We show that chi S kappa develops a maximum above the QCD chiral transition, after which it degenerates with chi PK. We also obtain chi S kappa within Unitarized Chiral Perturbation Theory (UChPT) at finite temperature, when it is saturated with the K0(700) (or kappa) meson, the dominant lowest-energy state in the isospin 1/2 scalar channel of pi K scattering. Such UChPT result reproduces the expected peak structure, revealing the importance of thermal interactions, and makes it possible to examine the chi S kappa dependence on the light- and strange-quark masses. A consistent picture emerges controlled by the ml/ms ratio that allows one studying K-kappa degeneration in the chiral, two-flavor and SU(3) limits. These results provide an alternative sign for O(4)xU(1)A restoration that can be explored in lattice simulations and highlight the role of strangeness, which regulated by the strange-quark condensate helps to reconcile the current tension among lattice results regarding U(1)A restoration. PublicationComplete meson-meson scattering within one loop in chiral perturbation theory: unitarization and resonances(Amer Inst Physics, 2002) Peláez Sagredo, José Ramón; Gómez Nicola, ÁngelWe review our recent one-loop calculation of all the two meson scattering amplitudes within SU(3) Chiral Perturbation Theory, i.e. with pions, kaons and etas. By unitarizing these amplitudes we are able to generate dynamically the lightest resonances in meson-meson scattering. We thus obtain a remarkable description of the meson-meson scattering data right from threshold up to 1.2 GeV, in terms of chiral parameters in good agreement with previous determinations. PublicationAula Multimedia para el apoyo de la docencia a distancia en la Facultad de Ciencias Físicas(2021-06) Gómez Nicola, Ángel; Gil de Paz, Armando; García Payo, María del Carmen; Serna Galán, Julio; Echevarría Flores, María Teresa; Fernández Sanz, David PublicationNonequilibrium chiral perturbation theory and pion decay functions(Elsevier science, 1999-03-11) Gómez Nicola, Ángel; Galán González, V.We extend chiral perturbation theory to study a meson gas out of thermal equilibrium. Assuming that the system is initially in equilibrium at T-i < T-c and working within the Schwinger-Keldysh contour technique, we define consistently the time-dependent temporal and spatial pion decay functions, the counterparts of the pion decay constants, and calculate them to next to leading order. The link with curved space-time QFT allows to establish nonequilibrium renormalisation. The short-time behaviour and the applicability of our model to a heavy-ion collision plasma are also discussed in this work.