arXiv subjects

Cristina Manuel

Publications and source records attributed to Cristina Manuel.

89 records · Page 5Linked to original sources

arXiv2000

Lifetime Effects in Color Superconductivity at Weak Coupling

Present computations of the gap of color superconductivity in weak coupling assume that the quarks which participate in the condensation process are infinitely long-lived. However, the quasiparticles in a plasma are characterized by having a finite lifetime. In this article we take into account this fact to evaluate its effect in the computation of the color gap. By first considering the Schwinger-Dyson equations in weak coupling, when one-loop self-energy corrections are included, a general gap equation is written in terms of the spectral densities of the quasiparticles. To evaluate lifetime effects, we then model the spectral density by a Lorentzian function. We argue that the decay of the quasiparticles limits their efficiency to condense. The value of the gap at the Fermi surface is then reduced. To leading order, these lifetime effects can be taken into account by replacing the coupling constant of the gap equation by a reduced effective one.

hep-ph↗

arXiv2000

Dispersion Relations in Ultradegenerate Relativistic Plasmas

The propagation of excitation modes in a relativistic ultradegenerate plasma is modified by their interactions with the medium. These modifications can be computed by evaluating their on-shell self-energy, which gives (gauge-independent) dispersion relations. For modes with momentum close to the Fermi momentum, the one-loop fermion self-energy is dominated by a diagram with a soft photon in the loop. We find the one-loop dispersion relations for quasiparticles and antiquasiparticles, which behave differently as a consequence of their very different phase-space restrictions when they scatter with the electrons of the Fermi sea. In a relativistic system, the unscreened magnetic interactions spoil the normal Fermi liquid behavior of the plasma. For small values of the Fermi velocity, we recover the non-relativistic dispersion relations of condensed matter systems.

hep-ph↗

arXiv2000

Fluctuations from dissipation in a hot non-Abelian plasma

We consider a transport equation of the Boltzmann-Langevin type for non-Abelian plasmas close to equilibrium to derive the spectral functions of the underlying microscopic fluctuations from the entropy. The correlator of the stochastic source is obtained from the dissipative processes in the plasma. This approach, based on classical transport theory, exploits the well-known link between a linearized collision integral, the entropy and the spectral functions. Applied to the ultra-soft modes of a hot non-Abelian (classical or quantum) plasma, the resulting spectral functions agree with earlier findings obtained from the microscopic theory. As a by-product, it follows that Bödeker's effective theory is consistent with the fluctuation-dissipation theorem.

hep-ph↗

arXiv2000

Deriving effective transport equations for non-Abelian plasmas

Within classical transport theory, an approach is presented to derive systematically effective collision terms and noise sources for effective kinetic equations of non-Abelian plasmas. This procedure amounts to the `integrating-out' of fluctuations, and is applicable for in- and out-of-equilibrium situations. When applied to a hot non-Abelian plasma close to equilibrium we recover the collision integral and the noise source first obtained by Bödeker. It is also shown that our approach, and hence Bödeker's effective theory, is consistent with the fluctuation-dissipation theorem.

hep-ph↗

arXiv2000

Masses of the Goldstone modes in the CFL phase of QCD at finite density

We construct the U_L(3) x U_R(3) effective lagrangian which encodes the dynamics of the low energy pseudoscalar excitations in the Color-Flavor-Locking superconducting phase of QCD at finite quark density. We include the effects of instanton-induced interactions and study the mass pattern of the pseudoscalar mesons. A tentative comparison with the analytical estimate for the gap suggests that some of these low energy momentum modes are not stable for moderate values of the quark chemical potential.

hep-ph↗

arXiv1999

Effective Transport Equations for non-Abelian Plasmas

Starting from classical transport theory, we derive a set of covariant equations describing the dynamics of mean fields and their statistical fluctuations in a non-Abelian plasma in or out of equilibrium. A general procedure is detailed for integrating-out the fluctuations as to obtain the effective transport equations for the mean fields. In this manner, collision integrals for Boltzmann equations are obtained as correlators of fluctuations. The formalism is applied to a hot non-Abelian plasma close to equilibrium. We integrate-out explicitly the fluctuations with typical momenta of the Debye mass, and obtain the collision integral in a leading logarithmic approximation. We also identify a source for stochastic noise. The resulting dynamical equations are of the Boltzmann-Langevin type. While our approach is based on classical physics, we also give the necessary generalizations to study the quantum plasmas. Ultimately, the dynamical equations for soft and ultra-soft fields change only in the value for the Debye mass.

hep-ph↗

arXiv1999

Mean Field Dynamics in Non-Abelian Plasmas from Classical Transport Theory

Based on classical transport theory, we present a general set of covariant equations describing the dynamics of mean fields and their statistical fluctuations in a non-Abelian plasma in or out-of-equilibrium. A procedure to obtain the collision integrals for the Boltzmann equation from the microscopic theory is described. As an application, we study a hot non-Abelian plasma close to equilibrium, where the fluctuations are integrated out explicitly. For soft fields, and at logarithmic accuracy, we obtain Bödeker's effective theory.

hep-ph↗

arXiv1999

Hard Thermal Loops in the Electroweak Theory

We derive a thermal effective action for soft fields in the broken phase of the electroweak theory in the limit of a strongly interacting Higgs sector. This action is just the proper generalization of the hard thermal loop effective action of a Yang-Mills theory when there is a Higgs mechanism and for a heavy Higgs boson. One can obtain from this action the thermal corrections to the masses of the W, Z and the photon.

hep-ph↗

arXiv1998

Hard Thermal Loops in Chiral Perturbation Theory

It is shown how the hard thermal loop approximation can be used in chiral perturbation theory to study some thermal properties of Goldstone bosons. Hard thermal effects are first studied in the non-linear sigma model. Then those results are used to obtain the thermal corrections to the transverse and longitudinal gauge field masses in the electroweak theory in the limit of a strongly interacting Higgs boson.

hep-ph↗

arXiv1998

On the Thermal Gauge Boson Masses of the Electroweak Theory in the Broken Phase

Thermal effects in the broken phase of the electroweak theory are studied in the strongly interacting Higgs boson limit. In that limit and at tree level the bosonic sector of the theory is a gauged non-linear sigma model. The associated one-loop thermal effective action for soft fields is then computed by using the background field method together with the Stueckelberg formalism. This effective action describes thermal corrections to the masses of the gauge bosons W, Z and the photon. It is the proper generalization of the hard thermal effective action of a Yang-Mills theory when there is a Higgs mechanism for a heavy Higgs particle.

hep-ph↗

arXiv1997

Hard Thermal Loops and Chiral Lagrangians

Chiral symmetry is used as the guiding principle to derive hard thermal loop effects in chiral perturbation theory. This is done by using a chiral invariant background field method for the non-linear sigma model and the Wess-Zumino-Witten lagrangian, with and without external vector and axial vector sources. It is then shown that the n-point hard thermal loop is the leading thermal correction for the Green function of n point vector soft quark currents.

hep-ph↗

arXiv1997

Magnetic Screening at Finite Temperature

It is shown that at finite temperature and in the presence of magnetic sources magnetic fields are screened. This is proven within the framework of classical transport theory both for the Abelian and non-Abelian plasmas. Magnetic screening arises in this formalism as a consequence of polarization effects occurring in the plasmas, and it is proportional to the inverse of the gauge coupling constant. It is then discussed whether this mechanism could be relevant in realistic quantum gauge field theories, such as QCD.

hep-ph↗

arXiv1996

Hard Dense Loops in a Cold Non-Abelian Plasma

Classical transport theory is used to study the response of a non-Abelian plasma at zero temperature and high chemical potential to weak color electromagnetic fields. In this article the parallelism between the transport phenomena occurring in a non-Abelian plasma at high temperature and high density is stressed. Particularly, it is shown that at high densities it is also possible to relate the transport equations to the zero-curvature condition of a Chern-Simons theory in three dimensions, even when quarks are not considered ultrarelativistic. The induced color current in the cold plasma can be expressed as an average over angles, which represent the directions of the velocity vectors of quarks having Fermi energy. From this color current it is possible to compute $n$-point gluonic amplitudes, with arbitrary $n$. It is argued that these amplitudes are the same as the ones computed in the high chemical potential limit of QCD, that are then called hard dense loops. The agreement between the two different formalisms is checked by computing the polarization tensor of QED due to finite density effects in the high density limit.

hep-ph↗

arXiv1995

Classical Kinetics of Hard Thermal Phenomena in High Temperature QCD

Classical transport theory for colored particles is reviewed and used to derive the hard thermal loops of QCD. A perturbative study of the non-Abelian transport equations that preserves their gauge symmetry is used to compute the induced color current in a hot quark-gluon plasma. From this approach the effective action of hard thermal loops can be derived. This derivation is more direct than alternative ones based on perturbative quantum field theory, and shows that hard thermal effects in hot QCD are essentially classical.

hep-ph↗

arXiv1995

Consequences of Supergravity with Gauged $\rm U(1)_R$ Symmetry

The structure of gauged R supergravity Lagrangians is reviewed, and we consider models with a hidden sector plus light fields of the MSSM. A simple potential for the hidden sector is presented which has a global minimum with zero cosmological constant and spontaneously broken SUSY and R-symmetry. The $\rm U(1)_R$ vector multiplet acquires a Planck scale mass through the Higgs mechanism, and it decouples at low energy. Due to very interesting cancellations, the $\rm U(1)_R$ D-terms also drop out at low energy. Thus no direct effects of the gauging of R-symmetry remain in the low energy effective Lagrangian, and this result is model independent, requiring only that R-symmetry be broken at the Planck scale and $ = 0$, where $D$ is the auxiliary field of the $\rm U(1)_R$ vector multiplet. The low energy theory is fairly conventional with soft SUSY breaking terms for the MSSM fields. As a remnant of the gauging of R-symmetry, it also contains light fields, some required to cancel R-anomalies and others from the hidden sector.

hep-ph↗

arXiv1993

Perturbative Renormalization in Quantum Mechanics

Some quantum mechanical potentials, singular at short distances, lead to ultraviolet divergences when used in perturbation theory. Exactly as in quantum field theories, but much simpler, regularization and renormalization lead to finite physical results, which compare correctly to the exact ones. The Dirac delta potential, because of its relevance to triviality, and the Aharonov-Bohm potential, because ot its relevance to anyons, are used as examples here.

hep-th↗

arXiv1992

Differential Renormalization of a Yukawa Model with $γ_5$

We present a two-loop computation of the beta functions and the anomalous dimensions of a $γ_5$-Yukawa model using differential renormalization. The calculation is carried out in coordinate space without modifying the space-time dimension and no ad-hoc prescription for $γ_5$ is needed. It is shown that this procedure is specially suited for theories involving $γ_5$, and it should be considered in analyzing chiral gauge theories.

hep-th↗