Real null coframes in general relativity and GPS type coordinates

Based on work of Derrick, Coll, and Morales, we define a `symmetric' null coframe with {\it four real null covectors}. We show that this coframe is closely related to the GPS type coordinates recently introduced by Rovelli.Comment: Latex script, 9 pages, 4 figures; references added to work of Derrick, Coll, and Morales, 1 new figur

Inhomogeneous chiral symmetry breaking in noncommutative four fermion interactions

The generalization of the Gross-Neveu model for noncommutative 3+1 space-time has been analyzed. We find indications that the chiral symmetry breaking occurs for an inhomogeneous background as in the LOFF phase in condensed matter.Comment: 17 pages, 2 figures, published version, minor correction

Out-of-equilibrium electromagnetic radiation

We derive general formulas for photon and dilepton production rates from an arbitrary non-equilibrated medium from first principles in quantum field theory. At lowest order in the electromagnetic coupling constant, these relate the rates to the unequal-time in-medium photon polarization tensor and generalize the corresponding expressions for a system in thermodynamic equilibrium. We formulate the question of electromagnetic radiation in real time as an initial value problem and consistently describe the virtual electromagnetic dressing of the initial state. In the limit of slowly evolving systems, we recover known expressions for the emission rates and work out the first correction to the static formulas in a systematic gradient expansion. Finally, we discuss the possible application of recently developed techniques in non-equilibrium quantum field theory to the problem of electromagnetic radiation. We argue, in particular, that the two-particle-irreducible (2PI) effective action formalism provides a powerful resummation scheme for the description of multiple scattering effects, such as the Landau-Pomeranchuk-Migdal suppression recently discussed in the context of equilibrium QCD.Comment: 34 pages, 9 figures, uses JHEP3.cl

The self-consistent bounce: an improved nucleation rate

We generalize the standard computation of homogeneous nucleation theory at zero temperature to a scenario in which the bubble shape is determined self-consistently with its quantum fluctuations. Studying two scalar models in 1+1 dimensions, we find the self-consistent bounce by employing a two-particle irreducible (2PI) effective action in imaginary time at the level of the Hartree approximation. We thus obtain an effective single bounce action which determines the rate exponent. We use collective coordinates to account for the translational invariance and the growth instability of the bubble and finally present a new nucleation rate prefactor. We compare the results with those obtained using the standard 1-loop approximation and show that the self-consistent rate can differ by several orders of magnitude.Comment: 28 pages, revtex, 7 eps figure

Brane-skyrmions and wrapped states

In the context of a brane world and including an induced curvature term in the brane action, we obtain the effective lagrangian for the Goldstone bosons (branons) associated with the spontaneous breaking of the translational invariance in the bulk. In addition to the branons, this effective action has Skyrmion-like solitonic states which can be understood as holes in the brane. We study their main properties such as mass and size, the Skyrmion-branon interaction and their possible fermionic quantization. We also consider states where the brane is wrapped around the extra dimensions and their relation with the brane-skyrmions. Finally, we extend our results to higher-dimensional branes, such as those appearing in M-theory, where brane-skyrmions could also be present.Comment: 35 pages, 7 figures. Revised version. Appendix and new references included. To appear in Phys. Rev.

The Non-Trivial Effective Potential of the `Trivial' lambda Phi^4 Theory: A Lattice Test

The strong evidence for the `triviality' of (lambda Phi^4)_4 theory is not incompatible with spontaneous symmetry breaking. Indeed, for a `trivial' theory the effective potential should be given exactly by the classical potential plus the free-field zero-point energy of the shifted field; i.e., by the one-loop effective potential. When this is renormalized in a simple, but nonperturbative way, one finds, self-consistently, that the shifted field does become non-interacting in the continuum limit. For a classically scale-invariant (CSI) lambda Phi^4 theory one finds m_h^2 = 8 pi^2 v^2, predicting a 2.2 TeV Higgs boson. Here we extend our earlier work in three ways: (i) we discuss the analogy with the hard-sphere Bose gas; (ii) we extend the analysis from the CSI case to the general case; and (iii) we propose a test of the predicted shape of the effective potential that could be tested in a lattice simulation.Comment: 22 pages, LaTeX, DE-FG05-92ER40717-

Partial Deconfinement in Color Superconductivity

We analyze the fate of the unbroken SU(2) color gauge interactions for 2 light flavors color superconductivity at non zero temperature. Using a simple model we compute the deconfining/confining critical temperature and show that is smaller than the critical temperature for the onset of the superconductive state itself. The breaking of Lorentz invariance, induced already at zero temperature by the quark chemical potential, is shown to heavily affect the value of the critical temperature and all of the relevant features related to the deconfining transition. Modifying the Polyakov loop model to describe the SU(2) immersed in the diquark medium we argue that the deconfinement transition is second order. Having constructed part of the equation of state for the 2 color superconducting phase at low temperatures our results are relevant for the physics of compact objects featuring a two flavor color superconductive state.Comment: 9 pp, 4 eps-figs, version to appear in PR

Universality, the QCD critical/tricritical point and the quark number susceptibility

The quark number susceptibility near the QCD critical end-point (CEP), the tricritical point (TCP) and the O(4) critical line at finite temperature and quark chemical potential is investigated. Based on the universality argument and numerical model calculations we propose a possibility that the hidden tricritical point strongly affects the critical phenomena around the critical end-point. We made a semi-quantitative study of the quark number susceptibility near CEP/TCP for several quark masses on the basis of the Cornwall-Jackiw-Tomboulis (CJT) potential for QCD in the improved-ladder approximation. The results show that the susceptibility is enhanced in a wide region around CEP inside which the critical exponent gradually changes from that of CEP to that of TCP, indicating a crossover of different universality classes.Comment: 18 pages, 10 figure

A Critical Analysis of the Proton Form Factor with Sudakov Suppression and Intrinsic Transverse Momentum

The behavior of the proton magnetic form factor is studied within the modified hard scattering picture, which takes into account gluonic radiative corrections in terms of transverse separations. We parallel the analysis given previously by Li and make apparent a number of serious objections. The appropriate cut-off needed to render the form-factor calculation finite is both detailed and analyzed by considering different cut-off prescriptions. The use of the maximum interquark separation as a common infrared cut-off in the Sudakov suppression factor is proposed, since it avoids difficulties with the $\alpha _{s}$-singularities and yields a proton form factor insensitive to the inclusion of the soft region which therefore can be confidently attributed to perturbative QCD. Results are presented for a variety of proton wave functions including also their intrinsic transverse momentum. It turns out that the perturbative contribution, although theoretically self-consistent for $Q^{2}$ larger than about $6$~GeV${}^{2}$ to $10$~GeV${}^{2}$, is too small compared to the data.Comment: 31 pages (RevTex) + 6 figures in PS-file; preprint BUGH Wuppertal WU-B-94-06, preprint Ruhr-Universit\"at Bochum RUB-TPII-01/9