482 research outputs found
Proposal to improve the behaviour of self-energy contributions to the S-matrix
A simple modification of the definition of the S-matrix is proposed. It is
expected that the divergences related to nonzero self-energies are considerably
milder with the modified definition than with the usual one. This conjecture is
verified in a few examples using perturbation theory. The proposed formula is
written in terms of the total Hamiltonian operator and a free Hamiltonian
operator and is therefore applicable in any case when these Hamiltonian
operators are known.Comment: 24 pages, 1 figure; v2: revised version; v3: section 3 improved.
Accepted for publication in Central European Journal of Physics; v4: minor
text misprints correcte
Production of the Smallest QED Atom: True Muonium (mu^+ mu^-)
The "true muonium" (mu^+ mu-) and "true tauonium" (tau^+ tau^-) bound states
are not only the heaviest, but also the most compact pure QED systems. The
rapid weak decay of the tau makes the observation of true tauonium difficult.
However, as we show, the production and study of true muonium is possible at
modern electron-positron colliders.Comment: 4 pages, ReVTeX, 4 eps figures; minor wording changes and reordering
of a reference. Version accepted by Phys. Rev. Let
Facing the Spectator
We investigated the familiar phenomenon of the uncanny feeling that represented people in frontal pose invariably appear to ‘‘face you’’ from wherever you stand. We deploy two different methods. The stimuli include the conventional one—a flat portrait rocking back and forth about a vertical axis—augmented with two novel variations. In one alternative, the portrait frame rotates whereas the actual portrait stays motionless and fronto-parallel; in the other, we replace the (flat!) portrait with a volumetric object. These variations yield exactly the same optical stimulation in frontal view, but become grossly different in very oblique views. We also let participants sample their momentary awareness through ‘‘gauge object’’ settings in static displays. From our results, we conclude that the psychogenesis of visual awareness maintains a number—at least two, but most likely more—of distinct spatial frameworks simultaneously involving ‘‘cue–scission.’’ Cues may be effective in one of these spatial frameworks but ineffective or functionally different in other ones
Two fermion relativistic bound states: hyperfine shifts
We discuss the hyperfine shifts of the Positronium levels in a relativistic
framework, starting from a two fermion wave equation where, in addition to the
Coulomb potential, the magnetic interaction between spins is described by a
Breit term. We write the system of four first order differential equations
describing this model. We discuss its mathematical features, mainly in relation
to possible singularities that may appear at finite values of the radial
coordinate. We solve the boundary value problems both in the singular and non
singular cases and we develop a perturbation scheme, well suited for numerical
computations, that allows to calculate the hyperfine shifts for any level,
according to well established physical arguments that the Breit term must be
treated at the first perturbative order. We discuss our results, comparing them
with the corresponding values obtained from semi-classical expansions.Comment: 16 page
The bound mu+ mu- system
We consider the hyperfine structure, the atomic spectrum and the decay
channels of the bound mu+ mu- system (dimuonium). The annihilation lifetimes of
low-lying atomic states of the system lie in the nanosecond range range. The
decay rates could be measured by detection of the decay products (high energy
photons or electron-positron pairs). The hyperfine structure splitting of the
dimuonic system and its decay rate are influenced by electronic vacuum
polarization effects in the far time-like asymptotic region. This constitutes a
previously unexplored kinematic regime. We evaluate next--to-leading order
radiative corrections to the decay rate of low-lying atomic states. We also
obtain order alpha^5 corrections to the hyperfine splitting of the 1S and 2S
levels.Comment: 10 figures (eps format) attached, Scheduled tentatively by PRA for
Nov/Dec 199
Nonequilibrium thermodynamics of interacting tunneling transport: variational grand potential, density-functional formulation, and nature of steady-state forces
The standard formulation of tunneling transport rests on an open-boundary
modeling. There, conserving approximations to nonequilibrium Green function or
quantum-statistical mechanics provide consistent but computational costly
approaches; alternatively, use of density-dependent ballistic-transport
calculations [e.g., Phys. Rev. B 52, 5335 (1995)], here denoted `DBT', provide
computationally efficient (approximate) atomistic characterizations of the
electron behavior but has until now lacked a formal justification. This paper
presents an exact, variational nonequilibrium thermodynamic theory for fully
interacting tunneling and provides a rigorous foundation for frozen-nuclei DBT
calculations as a lowest order approximation to an exact nonequilibrium
thermodynamics density functional evaluation. The theory starts from the
complete electron nonequilibrium quantum statistical mechanics and I identify
the operator for the nonequilibrium Gibbs free energy. I demonstrate a minimal
property of a functional for the nonequilibrium thermodynamic grand potential
which thus uniquely identifies the solution as the exact nonequilibrium density
matrix. I also show that a uniqueness-of-density proof from a closely related
study [Phys. Rev. B 78, 165109 (2008)] makes it possible to provide a
single-particle formulation based on universal electron-density functionals. I
illustrate a formal evaluation of the thermodynamics grand potential value
which is closely related to the variation in scattering phase shifts and hence
to Friedel density oscillations. This paper also discusses the difference
between the here-presented exact thermodynamics forces and the often-used
electrostatic forces. Finally the paper documents an inherent adiabatic nature
of the thermodynamics forces and observes that these are suited for a
nonequilibrium implementation of the Born-Oppenheimer approximation.Comment: 37 pages, 3 Figure
Density-functional theory of nonequilibrium tunneling
Nanoscale optoelectronics and molecular-electronics systems operate with
current injection and nonequilibrium tunneling, phenomena that challenge
consistent descriptions of the steady-state transport. The current affects the
electron-density variation and hence the inter- and intra-molecular bonding
which in turn determines the transport magnitude. The standard approach for
efficient characterization of steady-state tunneling combines ground-state
density functional theory (DFT) calculations (of an effective scattering
potential) with a Landauer-type formalism and ignores all actual many-body
scattering. The standard method also lacks a formal variational basis. This
paper formulates a Lippmann-Schwinger collision density functional theory
(LSC-DFT) for tunneling transport with full electron-electron interactions.
Quantum-kinetic (Dyson) equations are used for an exact reformulation that
expresses the variational noninteracting and interacting many-body scattering
T-matrices in terms of universal density functionals. The many-body
Lippmann-Schwinger (LS) variational principle defines an implicit equation for
the exact nonequilibrium density.Comment: Title, abstract, and text are adjusted to precise formulations (the
original version contained a logical error
Compared efficacy of preservation solutions in liver transplantation: A long-term graft outcome study from the european liver transplant registry
International audienceBetween 2003 and 2012, 42 869 first liver transplantations performed in Europe with the use of either University of Wisconsin solution (UW; N = 24 562), histidine-tryptophan-ketoglutarate(HTK; N = 8696), Celsior solution (CE; N = 7756) or Institute Georges Lopez preservation solution (IGL-1; N = 1855) preserved grafts. Alternative solutions to the UW were increasingly used during the last decade. Overall, 3-year graft survival was higher with UW, IGL-1 and CE (75%, 75% and 73%, respectively), compared to the HTK (69%) (p 12 h or grafts used for patients with cancer (p < 0.0001). For partial grafts, 3-year graft survival was 89% for IGL-1, 67% for UW, 68% for CE and 64% for HTK (p = 0.009). Multivariate analysis identified HTK as an independent factor of graft loss, with recipient HIV (+), donor age ≥65 years, recipient HCV (+), main disease acute hepatic failure, use of a partial liver graft, recipient age ≥60 years, no identical ABO compatibility, recipient hepatitis B surface antigen (-), TIT ≥ 12 h, male recipient and main disease other than cirrhosis. HTK appears to be an independent risk factor of graft loss. Both UW and IGL-1, and CE to a lesser extent, provides similar results for full size grafts. For partial deceased donor liver grafts, IGL-1 tends to offer the best graft outcome
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