628 research outputs found
Current in the light-front Bethe-Salpeter formalism II: Applications
We pursue applications of the light-front reduction of current matrix
elements in the Bethe-Salpeter formalism. The normalization of the reduced wave
function is derived from the covariant framework and related to non-valence
probabilities using familiar Fock space projection operators. Using a simple
model, we obtain expressions for generalized parton distributions that are
continuous. The non-vanishing of these distributions at the crossover between
kinematic regimes (where the plus component of the struck quark's momentum is
equal to the plus component of the momentum transfer) is tied to higher Fock
components. Moreover continuity holds due to relations between Fock components
at vanishing plus momentum. Lastly we apply the light-front reduction to
time-like form factors and derive expressions for the generalized distribution
amplitudes in this model.Comment: 12 pages, 6 figures, RevTex
Exploring skewed parton distributions with two body models on the light front II: covariant Bethe-Salpeter approach
We explore skewed parton distributions for two-body, light-front wave
functions. In order to access all kinematical regimes, we adopt a covariant
Bethe-Salpeter approach, which makes use of the underlying equation of motion
(here the Weinberg equation) and its Green's function. Such an approach allows
for the consistent treatment of the non-wave function vertex (but rules out the
case of phenomenological wave functions derived from ad hoc potentials). Our
investigation centers around checking internal consistency by demonstrating
time-reversal invariance and continuity between valence and non-valence
regimes. We derive our expressions by assuming the effective qq potential is
independent of the mass squared, and verify the sum rule in a non-relativistic
approximation in which the potential is energy independent. We consider
bare-coupling as well as interacting skewed parton distributions and develop
approximations for the Green's function which preserve the general properties
of these distributions. Lastly we apply our approach to time-like form factors
and find similar expressions for the related generalized distribution
amplitudes.Comment: 25 pages, 12 figures, revised (minor changes but essential to
consistency
Neural network parametrization of the lepton energy spectrum in semileptonic B meson decays
We construct a parametrization of the lepton energy spectrum in inclusive
semileptonic decays of B mesons, based on the available experimental
information: moments of the spectrum with cuts, their errors and their
correlations, together with kinematical constraints. The result is obtained in
the form of a Monte Carlo sample of neural networks trained on replicas of the
experimental data, which represents the probability density in the space of
lepton energy spectra. This parametrization is then used to extract the b quark
mass m_b^{1S} in a way that theoretical uncertainties are minimized, for which
the value m_b^{1S}=4.84 \pm 0.14^{exp}\pm 0.05^{th} GeV is obtained.Comment: 32 pages, 22 figures, JHEP3 class. v4 version accepted for
publication in JHE
Finite volume corrections to the electromagnetic current of the nucleon
We compute corrections to both the isovector anomalous magnetic moment and
the isovector electromagnetic current of the nucleon to in the
framework of covariant two-flavor Baryon Chiral Perturbation Theory. We then
apply these corrections to lattice data for the anomalous magnetic moment from
the LHPC, RBC & UKQCD and QCDSF collaborations
Experimental Implementation of the Quantum Random-Walk Algorithm
The quantum random walk is a possible approach to construct new quantum
algorithms. Several groups have investigated the quantum random walk and
experimental schemes were proposed. In this paper we present the experimental
implementation of the quantum random walk algorithm on a nuclear magnetic
resonance quantum computer. We observe that the quantum walk is in sharp
contrast to its classical counterpart. In particular, the properties of the
quantum walk strongly depends on the quantum entanglement.Comment: 5 pages, 4 figures, published versio
Pedagogic model for Deeply Virtual Compton Scattering with quark-hadron duality
We show how quark-hadron duality can emerge for valence spin averaged
structure functions, and for the non-forward distributions of Deeply Virtual
Compton Scattering. Novel factorisations of the non-forward amplitudes are
proposed. Some implications for large angle scattering and deviations from the
quark counting rules are illustrated.Comment: Version accepted by Phys. Rev.
The Similarity Hypothesis in General Relativity
Self-similar models are important in general relativity and other fundamental
theories. In this paper we shall discuss the ``similarity hypothesis'', which
asserts that under a variety of physical circumstances solutions of these
theories will naturally evolve to a self-similar form. We will find there is
good evidence for this in the context of both spatially homogenous and
inhomogeneous cosmological models, although in some cases the self-similar
model is only an intermediate attractor. There are also a wide variety of
situations, including critical pheneomena, in which spherically symmetric
models tend towards self-similarity. However, this does not happen in all cases
and it is it is important to understand the prerequisites for the conjecture.Comment: to be submitted to Gen. Rel. Gra
Neutrino Emission from Goldstone Modes in Dense Quark Matter
We calculate neutrino emissivities from the decay and scattering of Goldstone
bosons in the color-flavor-locked (CFL) phase of quarks at high baryon density.
Interactions in the CFL phase are described by an effective low-energy theory.
For temperatures in the tens of keV range, relevant to the long-term cooling of
neutron stars, the emissivities involving Goldstone bosons dominate over those
involving quarks, because gaps in the CFL phase are MeV while the
masses of Goldstone modes are on the order of 10 MeV. For the same reason, the
specific heat of the CFL phase is also dominated by the Goldstone modes.
Notwithstanding this, both the emissivity and the specific heat from the
massive modes remain rather small, because of their extremely small number
densities. The values of the emissivity and the specific heat imply that the
timescale for the cooling of the CFL core in isolation is y,
which makes the CFL phase invisible as the exterior layers of normal matter
surrounding the core will continue to cool through significantly more rapid
processes. If the CFL phase appears during the evolution of a proto-neutron
star, neutrino interactions with Goldstone bosons are expected to be
significantly more important since temperatures are high enough (
MeV) to admit large number densities of Goldstone modes.Comment: 29 pages, no figures. slightly modified text, one new eqn. and new
refs. adde
PYTHIA 6.4 Physics and Manual
The PYTHIA program can be used to generate high-energy-physics `events', i.e.
sets of outgoing particles produced in the interactions between two incoming
particles. The objective is to provide as accurate as possible a representation
of event properties in a wide range of reactions, within and beyond the
Standard Model, with emphasis on those where strong interactions play a role,
directly or indirectly, and therefore multihadronic final states are produced.
The physics is then not understood well enough to give an exact description;
instead the program has to be based on a combination of analytical results and
various QCD-based models. This physics input is summarized here, for areas such
as hard subprocesses, initial- and final-state parton showers, underlying
events and beam remnants, fragmentation and decays, and much more. Furthermore,
extensive information is provided on all program elements: subroutines and
functions, switches and parameters, and particle and process data. This should
allow the user to tailor the generation task to the topics of interest.Comment: 576 pages, no figures, uses JHEP3.cls. The code and further
information may be found on the PYTHIA web page:
http://www.thep.lu.se/~torbjorn/Pythia.html Changes in version 2: Mistakenly
deleted section heading for "Physics Processes" reinserted, affecting section
numbering. Minor updates to take into account referee comments and new colour
reconnection option
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