A study of the pion effective mass at finite temperature using the linear sigma model

We have used the Cornwall--Jackiw--Tomboulis method of composite operators and its formulation at finite temperature by Amelino-Camelia and Pi, in order to calculate the effective potential of the O(4) linear sigma model beyond the Hartree approximation. We have obtained a system of gap equations for the effective mass of sigma and the pions as well as for the condensate, the order parameter of the chiral phase transition. We find that the thermal effective mass of the pions at low temperatures remains lower than in the Hartree case, nevertheless deviates from the chiral limit. Our observation is consistent with other results which have been published previously.Comment: Typos in address and references corrected. Latex2e, 5 pages, 2 .eps figures. Contribution to the proceedings of the conference: X Hadron Physics and VII Relativistic Aspects of Nuclear Physics: A joint meeting on QCD and QGP, (HADRON-RANP 2004), 28 March--3 April 2004, Angra dos Reis, Rio de Janeiro, Brazi

Baryogenesis at the electroweak phase transition

A possible solution to the observed baryon asymmetry in the universe is described, based on the physics of the standard model of electroweak interactions. At temperatures high enough electroweak physics provides violation of baryon number, while C and CP symmetries are not exactly conserved, although in the context of the minimal electroweak model with one Higgs doublet the rate of CP violation is not sufficient enough to generate the observed asymmetry. The condition that the universe must be out of thermal equilibrium requires the electroweak phase transition (EWPT) to be first order. The dynamics of the phase transition in the minimal model is investigated through the effective potential, which is calculated at the one loop order. Finite temperature effects on the effective potential are treated numerically and within the high temperature approximation, which is found to be in good agreement with the exact calculation. At the one loop level the phase transition was found to be of the first order, while the strength of the transition depends on the unknown parameters of the theory which are the Higgs boson and top quark masses.Comment: 30 pages, 15 eps figures, revtex4, based on MSc thesis report submitted to Manchester Universit

Linear sigma model at finite temperature

The chiral phase transition is investigated within the framework of thermal field theory using the O(N) linear sigma model as an effective theory. We calculate the thermal effective potential by using the Cornwall-Jackiw-Tomboulis formalism of composite operators. The thermal effective potential is calculated for N=4 involving as usual the sigma and the three pions, and in the large-N approximation involving N-1 pion fields. In both cases, the system of the resulting gap equations for the thermal effective masses of the particles has been solved numerically, and we have investigated the evolution of the effective potential. In the N=4 case, there is indication of a first-order phase transition, while in the large N approximation the phase transition appears as second-order. In this analysis, we have ignored quantum fluctuations and have used the imaginary time formalism for calculations. We have extended our calculation in order to include the full effect of two loops in the calculation of the effective potential. In this case, the effective masses are momentum dependent. In order to perform the calculations, we found the real time formalism to be convenient. We have calculated the effective masses of pions at the low-temperature phase and we found a quadratic dependence on temperature, in contrast to the Hartree case, where the mass is proportional to temperature. The sigma mass was investigated in the presence of massive pions, and we found a small deviation compared to the Hartree case. In all cases, the system approaches the behaviour of the ideal gas at the high temperature limit.Comment: 33 pages, 40 eps figures, based on PhD thesis submitted to Manchester University in Oct 200

Thermal effective potential of the linear sigma model

We have attempted an approach to the chiral phase transition of QCD using the linear sigma model as an effective theory. In order to get some insight into how the phase transition could proceed, we have calculated the finite temperature effective potential of this model in the Hartree and large N approximations using the Cornwall-Jackiw-Tomboulis formalism of composite operators.Comment: 4 pages revtex, 5 eps figures, axodraw.sty, poster presented in the 5th International Workshop on Thermal Field Theories, Regensburg, German

One loop corrections to coupling constants in String Effective Field Theory

In the framework of a recently proposed method for computing exactly string amplitudes regularized in the infra-red, I determine the one-loop correlators for auxiliary fields in the symmetric $Z_2\times Z_2$ orbifold model. The $D$-field correlation function turns out to give the one-loop corrections for the gauge couplings, which amounts to a string-theory supersymmetry Ward identity. The two-point function for uncharged $F$ fields leads to the one-loop renormalization of the moduli K\"ahler metric, and eventually to the corrections for the Yukawa couplings.Comment: Plain TeX, 19p

Gravitational duality, topologically massive gravity and holographic fluids

Self-duality in Euclidean gravitational set ups is a tool for finding remarkable geometries in four dimensions. From a holographic perspective, self-duality sets an algebraic relationship between two a priori independent boundary data: the boundary energy-momentum tensor and the boundary Cotton tensor. This relationship, which can be viewed as resulting from a topological mass term for gravity boundary dynamics, survives under the Lorentzian signature and provides a tool for generating exact bulk Einstein spaces carrying, among others, nut charge. In turn, the holographic analysis exhibits perfect-fluid-like equilibrium states and the presence of non-trivial vorticity allows to show that infinite number of transport coefficients vanish.Comment: 37 page

String Theory on AdS3: Some Open Questions

String theory on curved backgrounds has received much attention on account of both its own interest, and of its relation with gauge theories. Despite the progress made in various directions, several quite elementary questions remain unanswered, in particular in the very simple case of three-dimensional anti-de Sitter space. I will very briefly review these problems, discuss in some detail the important issue of constructing a consistent spectrum for a string propagating on ADS3 plus torsion background, and comment on potential solutions.Comment: 18 pages, latex. To appear in the proceedings of the TMR European program meeting "Quantum aspects of gauge theories, supersymmetry and unification", Paris, France, 1--7 September, 1999; v2: comments and references adde

NS5-branes, holography and CFT deformations

A few supergravity solutions representing configurations of NS5-branes admit exact conformal field theory (CFT) description. Deformations of these solutions should be described by exactly marginal operators of the corresponding theories. We briefly review the essentials of these constructions and present, as a new case, the operators responsible for turning on angular momentum.Comment: 6 pages, 2 figures, Proceedings contribution to the 9th Hellenic School on Elementary Particle Physics and Gravity}, Corfu, Greece, September 200