4,572 research outputs found
Barrier Paradox in the Klein Zone
We study the solutions for a one-dimensional electrostatic potential in the
Dirac equation when the incoming wave packet exhibits the Klein paradox (pair
production). With a barrier potential we demonstrate the existence of multiple
reflections (and transmissions). The antiparticle solutions which are
necessarily localized within the barrier region create new pairs with each
reflection at the potential walls. Consequently we encounter a new paradox for
the barrier because successive outgoing wave amplitudes grow geometrically.Comment: 10 page
Investigating Biological Matter with Theoretical Nuclear Physics Methods
The internal dynamics of strongly interacting systems and that of
biomolecules such as proteins display several important analogies, despite the
huge difference in their characteristic energy and length scales. For example,
in all such systems, collective excitations, cooperative transitions and phase
transitions emerge as the result of the interplay of strong correlations with
quantum or thermal fluctuations. In view of such an observation, some
theoretical methods initially developed in the context of theoretical nuclear
physics have been adapted to investigate the dynamics of biomolecules. In this
talk, we review some of our recent studies performed along this direction. In
particular, we discuss how the path integral formulation of the molecular
dynamics allows to overcome some of the long-standing problems and limitations
which emerge when simulating the protein folding dynamics at the atomistic
level of detail.Comment: Prepared for the proceedings of the "XII Meeting on the Problems of
Theoretical Nuclear Physics" (Cortona11
On radio-bright Active Galactic Nuclei in a complete Spectroscopic Redshift Survey
Analysis of the frequency and physical properties of galaxies with
star-formation and AGN activity in different environments in the local universe
is a cornerstone for understanding structure formation and galaxy evolution. We
have built a new multiwavelength catalog for galaxies in a complete redshift
survey (the 15R Survey), gathering information on their H-alpha, R-band, radio,
far-infrared, and X-ray emission, as well as their radio and optical
morphologies, and have developed a classification scheme to compare different
selection methods and to select accurately samples of radio emitting galaxies
with AGN and star-forming activity. While alternative classification schemes do
not lead to major differences for star-forming galaxies, we show that
spectroscopic and photometric classifications of AGN lead to incomplete
samples. In particular, a large population of AGN-containing galaxies with
absorption-line spectra, and in many cases extended radio structures (jets,
lobes), is missed in the standard Baldwin-Phillips-Terlevich emission-line
classification of active galaxies. This missed class of objects accounts for
roughly half of the radio AGN population. Similarly, for X-ray selected AGN in
our sample, we find that absorption-line AGN account for half of the sample.
Spectroscopically unremarkable, passive galaxies with AGN activity are not an
exception, but the norm, and we show that although they exist in all
environments, these systems preferentially reside in higher density regions.
Because of the existence of this population, the fractional abundance of AGN
increases with increasing density, in contrast to some published results based
on emission-line AGN extracted from the 15R, Sloan and 2DF redshift surveys.Comment: 26 pages with 8 figures. Accepted for publication on the
Astrophysical Journa
Fermi-Walker gauge in 2+1 dimensional gravity.
It is shown that the Fermi-Walker gauge allows the general solution of
determining the metric given the sources, in terms of simple quadratures. We
treat the general stationary problem providing explicit solving formulas for
the metric and explicit support conditions for the energy momentum tensor. The
same type of solution is obtained for the time dependent problem with circular
symmetry. In both cases the solutions are classified in terms of the invariants
of the Wilson loops outside the sources. The Fermi-Walker gauge, due to its
physical nature, allows to exploit the weak energy condition and in this
connection it is proved that, both for open and closed universes with
rotational invariance, the energy condition imply the total absence of closed
time like curves. The extension of this theorem to the general stationary
problem, in absence of rotational symmetry is considered. At present such
extension is subject to some assumptions on the behavior of the determinant of
the dreibein in this gauge. PACS number: 0420Comment: 28 pages, RevTex, no figure
Pharmacology and clinical drug candidates in redox medicine
SIGNIFICANCE
Oxidative stress is suggested to be a disease mechanism common to a wide range of disorders affecting human health. However, so far, the pharmacotherapeutic exploitation of this, for example, based on chemical scavenging of pro-oxidant molecules, has been unsuccessful. Recent Advances: An alternative emerging approach is to target the enzymatic sources of disease-relevant oxidative stress. Several such enzymes and isoforms have been identified and linked to different pathologies. For some targets, the respective pharmacology is quite advanced, that is, up to late-stage clinical development or even on the market; for others, drugs are already in clinical use, although not for indications based on oxidative stress, and repurposing seems to be a viable option.
CRITICAL ISSUES
For all other targets, reliable preclinical validation and drug ability are key factors for any translation into the clinic. In this study, specific pharmacological agents with optimal pharmacokinetic profiles are still lacking. Moreover, these enzymes also serve largely unknown physiological functions and their inhibition may lead to unwanted side effects.
FUTURE DIRECTIONS
The current promising data based on new targets, drugs, and drug repurposing are mainly a result of academic efforts. With the availability of optimized compounds and coordinated efforts from academia and industry scientists, unambiguous validation and translation into proof-of-principle studies seem achievable in the very near future, possibly leading towards a new era of redox medicine
A spatial model of autocatalytic reactions
Biological cells with all of their surface structure and complex interior
stripped away are essentially vesicles - membranes composed of lipid bilayers
which form closed sacs. Vesicles are thought to be relevant as models of
primitive protocells, and they could have provided the ideal environment for
pre-biotic reactions to occur. In this paper, we investigate the stochastic
dynamics of a set of autocatalytic reactions, within a spatially bounded
domain, so as to mimic a primordial cell. The discreteness of the constituents
of the autocatalytic reactions gives rise to large sustained oscillations, even
when the number of constituents is quite large. These oscillations are
spatio-temporal in nature, unlike those found in previous studies, which
consisted only of temporal oscillations. We speculate that these oscillations
may have a role in seeding membrane instabilities which lead to vesicle
division. In this way synchronization could be achieved between protocell
growth and the reproduction rate of the constituents (the protogenetic
material) in simple protocells.Comment: Submitted to Phys. Rev.
Qualitative Properties of the Dirac Equation in a Central Potential
The Dirac equation for a massive spin-1/2 field in a central potential V in
three dimensions is studied without fixing a priori the functional form of V.
The second-order equations for the radial parts of the spinor wave function are
shown to involve a squared Dirac operator for the free case, whose essential
self-adjointness is proved by using the Weyl limit point-limit circle
criterion, and a `perturbation' resulting from the potential. One then finds
that a potential of Coulomb type in the Dirac equation leads to a potential
term in the above second-order equations which is not even infinitesimally
form-bounded with respect to the free operator. Moreover, the conditions
ensuring essential self-adjointness of the second-order operators in the
interacting case are changed with respect to the free case, i.e. they are
expressed by a majorization involving the parameter in the Coulomb potential
and the angular momentum quantum number. The same methods are applied to the
analysis of coupled eigenvalue equations when the anomalous magnetic moment of
the electron is not neglected.Comment: 22 pages, plain Tex. In the final version, a section has been added,
and the presentation has been improve
N=2 supergravity models with stable de Sitter vacua
In the present talk I shall review the construction of N=2 supergravity
models exhibiting stable de Sitter vacua. These solutions represent the first
instance of stable backgrounds with positive cosmological constant in the
framework of extended supergravities (N >=2). After briefly reviewing the role
of de Sitter space--times in inflationary cosmology, I shall describe the main
ingredients which were necessary for the construction of gauged N=2
supergravity models admitting stable solutions of this kind.Comment: Prepared for Workshop on the Quantum Structure of Spacetime and the
Geometric Nature of Fundamental Interactions, Leuven, Belgium, September
13-19 200
Constructive algebraic renormalization of the abelian Higgs-Kibble model
We propose an algorithm, based on Algebraic Renormalization, that allows the
restoration of Slavnov-Taylor invariance at every order of perturbation
expansion for an anomaly-free BRS invariant gauge theory. The counterterms are
explicitly constructed in terms of a set of one-particle-irreducible Feynman
amplitudes evaluated at zero momentum (and derivatives of them). The approach
is here discussed in the case of the abelian Higgs-Kibble model, where the zero
momentum limit can be safely performed. The normalization conditions are
imposed by means of the Slavnov-Taylor invariants and are chosen in order to
simplify the calculation of the counterterms. In particular within this model
all counterterms involving BRS external sources (anti-fields) can be put to
zero with the exception of the fermion sector.Comment: Jul, 1998, 31 page
Remarks upon the mass oscillation formulas
The standard formula for mass oscillations is often based upon the
approximation and the hypotheses that neutrinos have been
produced with a definite momentum or, alternatively, with definite energy
. This represents an inconsistent scenario and gives an unjustified
reduction by a factor of two in the mass oscillation formulas. Such an
ambiguity has been a matter of speculations and mistakes in discussing flavour
oscillations. We present a series of results and show how the problem of the
factor two in the oscillation length is not a consequence of gedanken
experiments, i.e. oscillations in time. The common velocity scenario yields the
maximum simplicity.Comment: 9 pages, AMS-Te
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