6,027 research outputs found
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
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
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
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
Three-geometry and reformulation of the Wheeler-DeWitt equation
A reformulation of the Wheeler-DeWitt equation which highlights the role of
gauge-invariant three-geometry elements is presented. It is noted that the
classical super-Hamiltonian of four-dimensional gravity as simplified by
Ashtekar through the use of gauge potential and densitized triad variables can
furthermore be succinctly expressed as a vanishing Poisson bracket involving
three-geometry elements. This is discussed in the general setting of the
Barbero extension of the theory with arbitrary non-vanishing value of the
Immirzi parameter, and when a cosmological constant is also present. A proposed
quantum constraint of density weight two which is polynomial in the basic
conjugate variables is also demonstrated to correspond to a precise simple
ordering of the operators, and may thus help to resolve the factor ordering
ambiguity in the extrapolation from classical to quantum gravity. Alternative
expression of a density weight one quantum constraint which may be more useful
in the spin network context is also discussed, but this constraint is
non-polynomial and is not motivated by factor ordering. The article also
highlights the fact that while the volume operator has become a preeminient
object in the current manifestation of loop quantum gravity, the volume element
and the Chern-Simons functional can be of equal significance, and need not be
mutually exclusive. Both these fundamental objects appear explicitly in the
reformulation of the Wheeler-DeWitt constraint.Comment: 10 pages, LaTeX fil
Search for associated Higgs boson production using like charge dilepton events in p(p)over-bar collisions at root s=1.96 TeV
We present a search for associated Higgs boson production in the process p (p) over bar -> W/ZH -> l(+/-)l'(+/-) + X in ee, e mu, and mu mu final states. The search is based on data collected by the D0 experiment at the Fermilab Tevatron Collider at root s = 1.96 TeV corresponding to 5.3 fb(-1) of integrated luminosity. We require two isolated leptons (electrons or muons) with the same electric charge and additional kinematic requirements. No significant excess above background is observed, and we set 95% C. L. observed (expected) upper limits on ratio of the production cross section to the standard model prediction of 6.4 (7.3) for a Higgs boson mass of 165 GeV and 13.5 (19.8) for a mass of 115 GeV
The HERMES Back Drift Chambers
The tracking system of the HERMES spectrometer behind the bending magnet
consists of two pairs of large planar 6-plane drift chambers. The design and
performance of these chambers is described. This description comprises details
on the mechanical and electronical design, information about the gas mixture
used and its properties, results on alignment, calibration, resolution, and
efficiencies, and a discussion of the experience gained through the first three
years of operation.Comment: 21 pages, LaTex, 16 figures include
A generalized Hamiltonian Constraint Operator in Loop Quantum Gravity and its simplest Euclidean Matrix Elements
We study a generalized version of the Hamiltonian constraint operator in
nonperturbative loop quantum gravity. The generalization is based on admitting
arbitrary irreducible SU(2) representations in the regularization of the
operator, in contrast to the original definition where only the fundamental
representation is taken. This leads to a quantization ambiguity and to a family
of operators with the same classical limit. We calculate the action of the
Euclidean part of the generalized Hamiltonian constraint on trivalent states,
using the graphical notation of Temperley-Lieb recoupling theory. We discuss
the relation between this generalization of the Hamiltonian constraint and
crossing symmetry.Comment: 35 pp, 20 eps figures; minor corrections, references added; version
to appear in Class. Quant. Gra
Plane waves in quantum gravity: breakdown of the classical spacetime
Starting with the Hamiltonian formulation for spacetimes with two commuting
spacelike Killing vectors, we construct a midisuperspace model for linearly
polarized plane waves in vacuum gravity. This model has no constraints and its
degrees of freedom can be interpreted as an infinite and continuous set of
annihilation and creation like variables. We also consider a simplified version
of the model, in which the number of modes is restricted to a discrete set. In
both cases, the quantization is achieved by introducing a Fock representation.
We find regularized operators to represent the metric and discuss whether the
coherent states of the quantum theory are peaked around classical spacetimes.
It is shown that, although the expectation value of the metric on Killing
orbits coincides with a classical solution, its relative fluctuations become
significant when one approaches a region where null geodesics are focused. In
that region, the spacetimes described by coherent states fail to admit an
approximate classical description. This result applies as well to the vacuum of
the theory.Comment: 11 pages, no figures, version accepted for publication in Phys. Rev.
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