The (magnetized) effective QCD phase diagram

I present the highlights of a recent study of the effective QCD phase diagram on the temperature T and quark chemical potential mu plane, where the strong interactions are modeled using the linear sigma model coupled to quarks. The phase transition line is found from the effective potential at finite T and mu taking into account the plasma screening effects. We find the location of the critical end point (CEP) to be (mu^CEP/T_c,T^CEP/T_c) \sim (1.2,0.8), where T_c is the (pseudo)critical temperature for the crossover phase transition at vanishing mu. This location lies within the region found by lattice inspired calculations. Since the linear sigma model does not exhibit confinement, I argue that the location is due to the proper treatment of the plasma screening effects and not to the size of the confining scale. I also comment on the extension of this study to determine the dependence of the CEP's location on the strength of an external magnetic field.Comment: 5 pages, 1 figure. Talk given at 18th International Conference in Quantum Chromodynamics (QCD 15, 30th anniversary), 29 june - 3 july 2015, Montpellier - F

In Medium modifications to rho from finite pion density effects and dilepton spectrum

The behavior of the pion dispersion relation in a pion medium is strongly modified by the introduction of a finite chemical potential associated to the finite pion number density. Such behavior is particularly important during the hadronic phase of a relativistic heavy-ion collision, between chemical and thermal freeze-out, where the pion number changing processes, driven by the strong interaction, can be considered to be frozen. We make use of an effective Lagrangian that explicitly respects chiral symmetry through the enforcement of the chiral Ward identities. The pion dispersion relation is computed through the computation of the pion self-energy in a non-perturbative fashion by giving an approximate solution to the Schwinger-Dyson equation for this self-energy. Given the strong coupling between rho vectors and pions, we argue that the modification of the pion mass due to finite pion density effects has to be taken into account self-consistently for the description of the in-medium modifications of rho's. We finally study some possible consequences of finite pion density effects for the low-mass dilepton spectrum produced in relativistic heavy-ion collisions.Comment: 4 pages 1 eps figure. Talk presented at the XXXVIIIth Rencontres de Moriond "QCD and high energy hadronic interactions", Les Arcs 1800, France, March 22-29, 200

Hadronic matter at the edge: A survey of some theoretical approaches to the physics of the QCD phase diagram

In the past few years a wealth of high quality data has made possible to test current theoretical ideas about the properties of hadrons subject to extreme conditions of density and temperature. The relativistic heavy-ion program carried out at the CERN-SPS and under development at the BNL-RHIC and CERN-LHC has provided results that probe the evolution of collisions of hadronic matter at high energies from the initially large density to the late dilute stages. In addition, QCD on the lattice has produced results complementing these findings with first principles calculations for observables in a regime where perturbative techniques cannot describe the nature of strongly coupled systems. This work aims to review some recent developments that make use of field theoretical methods to describe the physics of hadrons at finite temperature and density. I concentrate on two of the main topics that have been explored in the last few years: (1) The search for the structure of the phase diagram and (2) analytical signals linked to the chiral symmetry restoration/deconfinement.Comment: 14 pages, 1 figure. More references added. arXiv admin note: text overlap with arXiv:0908.2709 by other author

Chiral Ward identities and pion propagation at finite temperature in the linear sigma model

Working within the linear sigma model at finite temperature, we construct effective one-loop vertices and propagators satisfying the chiral Ward identities, for small pion momentum and mass compared to the sigma mass. We obtain an expansion for the pion dispersion relation up to second order in the parameter m_pi^2/4pi^2f_pi^2. At leading order, this expansion reproduces the dispersion curve obtained in chiral perturbation theory. We also study pion damping and compute the mean free path for pions traveling in the medium before forming a sigma resonance. These results are also in good agreement with chiral perturbation theory when using a value for the sigma mass of 600 MeV.Comment: 5 pages 2 eps figures, uses JHEP.cls (included). Proceedings of the Third Latin American Symposium on High Energy Physics, Cartagena de Indias, Colombia, April 2000, JHEP, to appea

Exact solution for charged-particle propagation during a first-order electroweak phase transition with hypermagnetic fields

We obtain the exact solution of the Klein-Gordon equation describing the propagation of a particle in two regions of different constant magnetic field, separated by an infinite plane wall. The continuity of the wave function and of its derivative at the interface is satisfied when including evanescent-wave terms. We analyze solutions on truncated spaces and compare them with previously obtained approximate solutions. The findings of this work have applications in the problem of the propagation of particles in the presence of a bubble wall in the midsts of an electroweak phase transition, where the two regions separated by the wall are influenced by different (hyper)magnetic field strengths.Comment: 15 pages, 5 figure

Dynamical mass generation for fermions in quenched Quantum Electrodynamics at finite temperature

We study the dynamical generation of masses for fundamental fermions in quenched quantum electrodynamics (qQED) at finite temperature in the bare vertex approximation, using Schwinger-Dyson equations (SDE). Motivated by perturbation theory, a further simplification is introduced by taking the wave function renormalization to be unity. In the zeroth mode approximation, the SDE for the fermion propagator resembles QED in 2+1 dimensions (QED3) at zero temperature with an effective dimensionful coupling alpha'=alpha T. For a fixed temperature, mass is dynamically generated above a certain critical value of this coupling. As expected, raising the temperature restores chiral symmetry and fermions become massless again. We also argue that by summing over the frequency modes and under suitable simplifications, qualitative aspects of the result do not undergo significant changes.Comment: 9 pages 6 Postscript figures, uses RevTeX 4. Expanded discussion, published versio

Electroweak baryogenesis in the standard model with strong hypermagnetic fields

We show that in the presence of large scale primordial hypermagnetic fields, it is possible to generate a large amount of CP violation to explain the baryon to entropy ratio during the electroweak phase transition within the standard model. The mechanism responsible for the existence of a CP violating asymmetry is the chiral nature of the fermion coupling to the background field in the symmetric phase which can be used to construct two-fermion interference processes in analogy to the Bohm-Aharanov effect. We estimate that for strong hypermagnetic fields B_Y=(0.3-0.5)T^2 the baryon to entropy ratio can be rho_B/s=(3 - 6)X10^-11 for slowly expanding bubble walls.Comment: 4 pages, uses RevTe

Strange Baryon to Meson Ratio

We present a model to compute baryon and meson transverse momentum distributions, and their ratios, in relativistic heavy-ion collisions. The model allows to compute the probability to form colorless bound states of either two or three quarks as functions of the evolving density during the collision. The qualitative differences of the baryon to meson ratio for different collision energies and for different particle species can be associated to the different density dependent probabilities and to the combinatorial factors which in turn depend on whether the quarks forming the bound states are heavy or light. We compare to experimental data and show that we obtain a good description up to intermediate values of $pt$.Comment: Four pages, four figures. It should appear as proceedings of the SQM 2013, in Journal of Physics: Conference Serie

Inverse magnetic catalysis in the linear sigma model with quarks

We compute the critical temperature for the chiral transition in the background of a magnetic field in the linear sigma model, including the quark contribution and the thermo-magnetic effects induced on the coupling constants at one loop level. We show that the critical temperature decreases as a function of the field strength. The effect of fermions on the critical temperature is small and the main effect on this observable comes from the charged pions. The findings support the idea that the anticatalysis phenomenon receives a contribution due only to quiral symmetry effects independent of the deconfinement transition.Comment: 8 pages, 8 figures. arXiv admin note: substantial text overlap with arXiv:1406.3885. Accepted for publication in Phys. Rev.

Damping and reaction rates and wave function renormalization of fermions in hot gauge theories

We examine the relation between the damping rate of a chiral fermion mode propagating in a hot plasma and the rate at which the mode approaches equilibrium. We show how these two quantities, obtained from the imaginary part of the fermion self-energy, are equal when the reaction rate is defined using the appropriate wave function of the mode in the medium. As an application, we compute the production rate of hard axions by Compton-like scattering processes in a hot QED plasma starting from both, the axion self-energy and the electron self-energy. We show that the latter rate coincides with the former only when this is computed using the corresponding medium spinor modes.Comment: Modified version, includes new section on the hard axion photoproduction rate in a hot QED plasma. Version to appear in Int. Jour. Mod. Phys.