6,473 research outputs found
Quantum causal histories
Quantum causal histories are defined to be causal sets with Hilbert spaces
attached to each event and local unitary evolution operators. The reflexivity,
antisymmetry, and transitivity properties of a causal set are preserved in the
quantum history as conditions on the evolution operators. A quantum causal
history in which transitivity holds can be treated as ``directed'' topological
quantum field theory. Two examples of such histories are described.Comment: 16 pages, epsfig latex. Some clarifications, minor corrections and
references added. Version to appear in Classical and Quantum Gravit
Exclusive channels in semi-inclusive production of pions and kaons
We investigate the role of exclusive channels in semi-inclusive
electroproduction of pions and kaons. Using the QCD factorization theorem for
hard exclusive processes we evaluate the cross sections for exclusive
pseudoscalar and vector meson production in terms of generalized parton
distributions and meson distribution amplitudes. We investigate the
uncertainties arising from the modeling of the nonperturbative input
quantities. Combining these results with available experimental data, we
compare the cross sections for exclusive channels to that obtained from quark
fragmentation in semi-inclusive deep inelastic scattering. We find that rho^0
production is the only exclusive channel with significant contributions to
semi-inclusive pion production at large z and moderate Q^2. The corresponding
contribution to kaon production from the decay of exclusively produced phi and
K^* is rather small.Comment: 33 pages, 18 figure
Regularization of the Hamiltonian constraint and the closure of the constraint algebra
In the paper we discuss the process of regularization of the Hamiltonian
constraint in the Ashtekar approach to quantizing gravity. We show in detail
the calculation of the action of the regulated Hamiltonian constraint on Wilson
loops. An important issue considered in the paper is the closure of the
constraint algebra. The main result we obtain is that the Poisson bracket
between the regulated Hamiltonian constraint and the Diffeomorphism constraint
is equal to a sum of regulated Hamiltonian constraints with appropriately
redefined regulating functions.Comment: 23 pages, epsfig.st
Uncorrelated and correlated nanoscale lattice distortions in the paramagnetic phase of magnetoresistive manganites
Neutron scattering measurements on a magnetoresistive manganite
La(CaSr)MnO show that uncorrelated
dynamic polaronic lattice distortions are present in both the orthorhombic (O)
and rhombohedral (R) paramagnetic phases. The uncorrelated distortions do not
exhibit any significant anomaly at the O-to-R transition. Thus, both the
paramagnetic phases are inhomogeneous on the nanometer scale, as confirmed
further by strong damping of the acoustic phonons and by the anomalous
Debye-Waller factors in these phases. In contrast, recent x-ray measurements
and our neutron data show that polaronic correlations are present only in the O
phase. In optimally doped manganites, the R phase is metallic, while the O
paramagnetic state is insulating (or semiconducting). These measurements
therefore strongly suggest that the {\it correlated} lattice distortions are
primarily responsible for the insulating character of the paramagnetic state in
magnetoresistive manganites.Comment: 10 pages, 8 figures embedde
KMS states on Quantum Grammars
We consider quantum (unitary) continuous time evolution of spins on a lattice
together with quantum evolution of the lattice itself. In physics such
evolution was discussed in connection with quantum gravity. It is also related
to what is called quantum circuits, one of the incarnations of a quantum
computer. We consider simpler models for which one can obtain exact
mathematical results. We prove existence of the dynamics in both Schroedinger
and Heisenberg pictures, construct KMS states on appropriate C*-algebras. We
show (for high temperatures) that for each system where the lattice undergoes
quantum evolution, there is a natural scaling leading to a quantum spin system
on a fixed lattice, defined by a renormalized Hamiltonian.Comment: 22 page
Graphical Evolution of Spin Network States
The evolution of spin network states in loop quantum gravity can be described
by introducing a time variable, defined by the surfaces of constant value of an
auxiliary scalar field. We regulate the Hamiltonian, generating such an
evolution, and evaluate its action both on edges and on vertices of the spin
network states. The analytical computations are carried out completely to yield
a finite, diffeomorphism invariant result. We use techniques from the
recoupling theory of colored graphs with trivalent vertices to evaluate the
graphical part of the Hamiltonian action. We show that the action on edges is
equivalent to a diffeomorphism transformation, while the action on vertices
adds new edges and re-routes the loops through the vertices.Comment: 24 pages, 21 PostScript figures, uses epsfig.sty, Minor corrections
in the final formula in the main body of the paper and in the formula for the
Tetrahedral net in the Appendi
Production of Polarized Vector Mesons off Nuclei
Using the light-cone QCD dipole formalism we investigate manifestations of
color transparency (CT) and coherence length (CL) effects in electroproduction
of longitudinally (L) and transversally (T) polarized vector mesons. Motivated
by forthcoming data from the HERMES experiment we predict both the A and Q^2
dependence of the L/T- ratios, for rho^0 mesons produced coherently and
incoherently off nuclei. For an incoherent reaction the CT and CL effects add
up and result in a monotonic A dependence of the L/T-ratio at different values
of Q^2. On the contrary, for a coherent process the contraction of the CL with
Q^2 causes an effect opposite to that of CT and we expect quite a nontrivial A
dependence, especially at Q^2 >> m_V^2.Comment: Revtex 24 pages and 14 figure
Causality in Spin Foam Models
We compute Teitelboim's causal propagator in the context of canonical loop
quantum gravity. For the Lorentzian signature, we find that the resultant power
series can be expressed as a sum over branched, colored two-surfaces with an
intrinsic causal structure. This leads us to define a general structure which
we call a ``causal spin foam''. We also demonstrate that the causal evolution
models for spin networks fall in the general class of causal spin foams.Comment: 19 pages, LaTeX2e, many eps figure
Closed-Flux Solutions to the Constraints for Plane Gravity Waves
The metric for plane gravitational waves is quantized within the Hamiltonian
framework, using a Dirac constraint quantization and the self-dual field
variables proposed by Ashtekar. The z axis (direction of travel of the waves)
is taken to be the entire real line rather than the torus (manifold
coordinatized by (z,t) is RxR rather than x R). Solutions to the
constraints proposed in a previous paper involve open-ended flux lines running
along the entire z axis, rather than closed loops of flux; consequently, these
solutions are annihilated by the Gauss constraint at interior points of the z
axis, but not at the two boundary points. The solutions studied in the present
paper are based on closed flux loops and satisfy the Gauss constraint for all
z.Comment: 18 pages; LaTe
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