27 research outputs found
New look at the Riemann-Cartan theory.
The geometry of torsion in the Riemann-Cartan (RC) theory can be described by an Abelian axial-vector field interacting with the axial-vector fermion current in a purely Riemannian background. On the basis of this observation we note that the Schwinger model formulated in curved spacetime can be interpreted as the two-dimensional version of the RC theory. In two dimensions as well as in four dimensions there is a one-parameter family of regulators that can be used to compute the axial anomaly. In four dimensions we set the value of the arbitrary parameter equal to zero and compute the axial anomaly, including counterterms, using Fujikawa's approach. The addition of the Wess-Zumino Lagrangian changes the original RC theory into a nonanomalous Abelian gauge theory of the torsion field. Guided by the analogy with the Schwinger model, we offer several forms of {ital L}{sub gravity} from which one can deduce the spin content of the quanta of torsion
Axionic Membranes
A metal ring removed from a soap-water solution encloses a film of soap which
can be mathematically described as a minimal surface having the ring as its
only boundary. This is known to everybody. In this letter we suggest a
relativistic extension of the above fluidodynamic system where the soap film is
replaced by a Kalb-Ramond gauge potential \b(x) and the ring by a closed
string. The interaction between the \b-field and the string current excites a
new configuration of the system consisting of a relativistic membrane bounded
by the string. We call such a classical solution of the equation of motion an
axionic membrane. As a dynamical system, the axionic membrane admits a
Hamilton-Jacobi formulation which is an extension of the H-J theory of
electromagnetic strings.Comment: 15 page
Classical and Quantum Shell Dynamics, and Vacuum Decay
Following a minisuperspace approach to the dynamics of a spherically
symmetric shell, a reduced Lagrangian for the radial degree of freedom is
derived directly from the Einstein-Hilbert action. The key feature of this new
Lagrangian is its invariance under time reparametrization. Indeed, all
classical and quantum dynamics is encoded in the Hamiltonian constraint that
follows from that invariance. Thus, at the classical level, we show that the
Hamiltonian constraint reproduces, in a simple gauge, Israel's matching
condition which governs the evolution of the shell. In the quantum case, the
vanishing of the Hamiltonian (in a weak sense), is interpreted as the
Wheeler-DeWitt equation for the physical states, in analogy to the
corresponding case in quantum cosmology. Using this equation, quantum tunneling
through the classical barrier is then investigated in the WKB approximation,
and the connection to vacuum decay is elucidated.Comment: 36 pages, ReVTeX, 10 Figs. in postscript format, in print on Class.&
Quant.Gra
Loop Quantum Mechanics and the Fractal Structure of Quantum Spacetime
We discuss the relation between string quantization based on the Schild path
integral and the Nambu-Goto path integral. The equivalence between the two
approaches at the classical level is extended to the quantum level by a
saddle--point evaluation of the corresponding path integrals. A possible
relationship between M-Theory and the quantum mechanics of string loops is
pointed out. Then, within the framework of ``loop quantum mechanics'', we
confront the difficult question as to what exactly gives rise to the structure
of spacetime. We argue that the large scale properties of the string condensate
are responsible for the effective Riemannian geometry of classical spacetime.
On the other hand, near the Planck scale the condensate ``evaporates'', and
what is left behind is a ``vacuum'' characterized by an effective fractal
geometry.Comment: 19pag. ReVTeX, 1fig. Invited paper to appear in the special issue of
{\it Chaos, Solitons and Fractals} on ``Super strings, M,F,S,...Theory''
(M.S. El Naschie and C.Castro, ed
Quantum effects of a massive 3-form coupled to a Dirac field
We consider the coupling of A_{\mu\nu\rho} to the generic current of matter
field, later identified with the spin density current of a Dirac field. In
fact, one of the objectives of this paper is to investigate the impact of the
quantum fluctuations of A_{\mu\nu\rho} on the effective dynamics of the spinor
field. The consistency of the field equations, even at the classical level,
requires the introduction of a mass term for A_{\mu\nu\rho}. In this case, the
Casimir vacuum pressure includes a contribution that is explicitly dependent on
the mass of A_{\mu\nu\rho} and leads us to conclude that the mass term plays
the same role as the infrared cutoff needed to regularize the finite volume
partition functional previously calculated in the massless case. Remarkably,
even in the presence of a mass term, A_{\mu\nu\rho} contains a mixture of
massless and massive spin-0 fields so that the resulting equation is still
gauge invariant. This is yet another peculiar, but physically relevant property
of A_{\mu\nu\rho} since it is reflected in the effective dynamics of the spinor
fields and confirms the confining property of A_{\mu\nu\rho} already expected
from the earlier calculation of the Wilson loop.Comment: 10 pages, Revtex, no figures; in print on Phys.Rev.D; added new
reference
The bosonized version of the Schwinger model in four dimensions: A blueprint for confinement?
4For a (3 + 1)-dimensional generalization of the Schwinger model, we compute the interaction energy between two test charges. The result shows that the static potential profile contains a linear term leading to the confinement of probe charges, exactly as in the original model in two dimensions. We further show that the same 4-dimensional model also appears as one version of the Bâ§F models in (3+1) dimensions under dualization of Stueckelberg-like massive gauge theories. Interestingly, this particular model is characterized by the mixing between a U (1) potential and an Abelian 3-form field of the type that appears in the topological sector of QCD.5 pages. arXiv admin note: text overlap with arXiv:1504.05810partially_openembargoed_20180512Aurilia, Antonio; Gaete, Patricio; HelayĂ«l-Neto, JosĂ© A.; Spallucci, EuroAurilia, Antonio; Gaete, Patricio; HelayĂ«l Neto, JosĂ© A.; Spallucci, Eur
Gauge Theory of the String Geodesic Field
A relativistic string is usually represented by the Nambu-Goto action in
terms of the extremal area of a 2-dimensional timelike submanifold of Minkowski
space. Alternatively, a family of classical solutions of the string equation of
motion can be globally described in terms of the associated geodesic field. In
this paper we propose a new gauge theory for the geodesic field of closed and
open strings. Our approach solves the technical and conceptual problems
affecting previous attempts to describe strings in terms of local field
variables. The connection between the geodesic field, the string current and
the Kalb-Ramond gauge potential is discussed and clarified. A non-abelian
generalization and the generally covariant form of the model are also
discussed.Comment: 38 pages, PHYZZX, UTS-DFT-92-2
Fuzzy dimensions and Planck's Uncertainty Principle for p-branes
The explicit form of the quantum propagator of a bosonic p-brane, previously
obtained by the authors in the quenched-minisuperspace approximation, suggests
the possibility of a novel, unified, description of p-branes with different
dimensionality. The background metric that emerges in this framework is a
quadratic form on a Clifford manifold. Substitution of the Lorentzian metric
with the Clifford line element has two far reaching consequences. On the one
hand, it changes the very structure of the spacetime fabric since the new
metric is built out of a Minimum Length below which it is impossible to resolve
the distance between two points; on the other hand, the introduction of the
Clifford line element extends the usual relativity of motion to the case of
Relative Dimensionalism of all p-branes that make up the spacetime manifold
near the Planck scale.Comment: 11 pages, LaTex, no figures; in print on Class.& Quantum Gra
Efficacy of a new technique - INtubate-RECruit-SURfactant-Extubate - "IN-REC-SUR-E" - in preterm neonates with respiratory distress syndrome: Study protocol for a randomized controlled trial
Background: Although beneficial in clinical practice, the INtubate-SURfactant-Extubate (IN-SUR-E) method is not successful in all preterm neonates with respiratory distress syndrome, with a reported failure rate ranging from 19 to 69 %. One of the possible mechanisms responsible for the unsuccessful IN-SUR-E method, requiring subsequent re-intubation and mechanical ventilation, is the inability of the preterm lung to achieve and maintain an "optimal" functional residual capacity. The importance of lung recruitment before surfactant administration has been demonstrated in animal studies showing that recruitment leads to a more homogeneous surfactant distribution within the lungs. Therefore, the aim of this study is to compare the application of a recruitment maneuver using the high-frequency oscillatory ventilation (HFOV) modality just before the surfactant administration followed by rapid extubation (INtubate-RECruit-SURfactant-Extubate: IN-REC-SUR-E) with IN-SUR-E alone in spontaneously breathing preterm infants requiring nasal continuous positive airway pressure (nCPAP) as initial respiratory support and reaching pre-defined CPAP failure criteria. Methods/design: In this study, 206 spontaneously breathing infants born at 24+0-27+6 weeks' gestation and failing nCPAP during the first 24 h of life, will be randomized to receive an HFOV recruitment maneuver (IN-REC-SUR-E) or no recruitment maneuver (IN-SUR-E) just prior to surfactant administration followed by prompt extubation. The primary outcome is the need for mechanical ventilation within the first 3 days of life. Infants in both groups will be considered to have reached the primary outcome when they are not extubated within 30 min after surfactant administration or when they meet the nCPAP failure criteria after extubation. Discussion: From all available data no definitive evidence exists about a positive effect of recruitment before surfactant instillation, but a rationale exists for testing the following hypothesis: a lung recruitment maneuver performed with a step-by-step Continuous Distending Pressure increase during High-Frequency Oscillatory Ventilation (and not with a sustained inflation) could have a positive effects in terms of improved surfactant distribution and consequent its major efficacy in preterm newborns with respiratory distress syndrome. This represents our challenge. Trial registration: ClinicalTrials.gov identifier: NCT02482766. Registered on 1 June 2015
Minisuperspace approach to quantum bubble dynamics
The minisuperspace approximation, widely used in quantum cosmology, is used to study the properties of the wave function of a spherically symmetric quantum bubble. The case of the free bubble illustrtes the method which is then applied to the case of a bubble minimally coupled to the totally anti-symmetric tensor gauge potential A\u3bcvp. The minisuperspace approximation fully preserves the relativistic properties of bubble dynamics but reduces the problem to that of the motion of a relativistic point-particle in an external electromagnetic potential in 1 + 1 dimensions. In this conncetion we discuss the analogue of the Klein effect for quantum bubbles: while a free bubble can exist in discrete levels of positive energy, with large bubbles being exponentially depressed, the interaction with the A\u3bcvp potential leads to the crossing of positive and negative energy levels; this phenomenon gives rise to a finite probability that a bubble might tunnel to a classical configuration of arbitrarily large radius