901 research outputs found
Einstein boundary conditions for the Einstein equations in the conformal-traceless decomposition
In relation to the BSSN formulation of the Einstein equations, we write down
the boundary conditions that result from the vanishing of the projection of the
Einstein tensor normally to a timelike hypersurface. Furthermore, by setting up
a well-posed system of propagation equations for the constraints, we show
explicitly that there are three constraints that are incoming at the boundary
surface and that the boundary equations are linearly related to them. This
indicates that such boundary conditions play a role in enforcing the
propagation of the constraints in the region interior to the boundary.
Additionally, we examine the related problem for a strongly hyperbolic
first-order reduction of the BSSN equations and determine the characteristic
fields that are prescribed by the three boundary conditions, as well as those
that are left arbitrary.Comment: 11 page
Generalized parton distributions: Status and perspectives
We summarize recent developments in understanding the concept of generalized
parton distributions (GPDs), its relation to nucleon structure, and its
application to high-Q2 electroproduction processes. Following a brief review of
QCD factorization and transverse nucleon structure, we discuss (a) new
theoretical methods for the analysis of deeply-virtual Compton scattering
(t-channel-based GPD parametrizations, dispersion relations); (b) the
phenomenology of hard exclusive meson production (experimental tests of
dominance of small-size configurations, model-independent comparative studies);
(c) the role of GPDs in small-x physics and pp scattering (QCD dipole model,
central exclusive diffraction). We emphasize the usefulness of the transverse
spatial (or impact parameter) representation for both understanding the
reaction mechanism in hard exclusive processes and visualizing the physical
content of the GPDs.Comment: 10 pages, 6 figures. Proceedings of SPIN2008, University of Virginia,
October 6-11, 200
The strong coupling constant at large distances
In this paper we discuss effective strong coupling constants. Those are well
behaved in the low-Q^2 domain, contrarily to alpha_s from pQCD. We present an
extraction of an effective strong coupling constant from Jefferson Lab
polarized data at intermediate and low Q^2. We also show how these data,
together with spin sum rules, allow us to obtain the effective coupling
constant over the entire Q^2 range. We then discuss the relation between the
experimentally extracted coupling constant and theoretical calculations at low
Q^2. We conclude on the importance of such study for the application of the
AdS/CFT correspondence to QCD.Comment: Proceedings for Spin2008 symposium. v2: added 2 referecences re.
infrared fixed poin
Generalized parton distributions, the hunt for quark orbital momenta
The Generalized Parton Distributions (GPDs) are the appropriate framework for
a universal description of the partonic structure of the nucleon. They
characterize the dynamics of quarks and gluons inside the nucleon and
consequently contain information about the spin of the nucleon. The current
experimental knowledge about GPDs is reviewed with the emphasis on the
determination of E^q(Q^2,x,xi,t), the least known and constrained GPD, of
particular importance in the nucleon spin puzzle. The perspectives of this
experimental program are also addressed.Comment: 6 pages, 2 figures Proceedings of the XVIIIth Symposium on Spin
Physics, Charlottesville (Virginia, USA), October 6-11,200
Parton Distributions in the Impact Parameter Space
Parton distributions in impact parameter space, which are obtained by Fourier
transforming GPDs, exhibit a significant deviation from axial symmetry when the
target and/or quark is transversely polarized. In combination with the final
state interactions, this transverse deformation provides a natural mechanism
for naive-T odd transverse single-spin asymmetries in semi-inclusive DIS. The
deformation can also be related to the transverse force acting on the active
quark in polarized DIS at higher twist.Comment: 9 pages, Proceedings Spin 200
Future Investigations of the Flavor Dependence of Sea Quark Helicities at STAR
The flavor dependence of polarized and unpolarized quark distributions in the
nucleon can lead to insights into the formation of the sea. Drell-Yan
measurements have pointed to flavor asymmetries in the unpolarized
distributions. Collisions at sqrt{s}=500 GeV with polarized protons at RHIC
will soon allow investigations of the flavor separated polarized quark
distributions via W production to complement measurements from semi-inclusive
DIS. We report on STAR's current plans, tracking upgrade, and expected
sensitivities.Comment: 4 pages, to appear in the proceedings of the 18th International Spin
Physics Symposium (Spin08), Charlottesville, Virginia, USA, October 6 - 11,
200
Electron Spin Precession at CEBAF
The nuclear physics experiments at the Thomas Jefferson National Accelerator
Facility often require longitudinally polarized electrons to be simultaneously
delivered to three experimental halls. The degree of longitudinal polarization
to each hall varies as function of the accelerator settings, making it
challenging in certain situations to deliver a high degree of longitudinal
polarization to all the halls simultaneously. Normally, the degree of
longitudinal polarization the halls receive is optimized by changing the
initial spin direction at the beginning of the machine with a Wien filter.
Herein, it is shown that it is possible to further improve the degree of
longitudinal polarization for multiple experimental halls by redistributing the
energy gain of the CEBAF linacs while keeping the total energy gain fixed.Comment: 4 pages, 3 figures, to appear in the proceedings of the 18th
International Symposium on Spin Physics (SPIN2008
Status of Longitudinal Polarized Parton Densities and Higher Twist
The present status of the longitudinal polarized parton densities (PDFs) and
the contribution of their first moments to the nucleon spin is discussed.
Special attention is paid to the role of higher twist effects in determining
the PDFs and to the polarized strange quark and gluon densities, which are
still not well determined from the present data.Comment: 6 pages, 5 figures; to appear in the proceedings of the 18th
International Symposium on Spin Physics (SPIN20008), October 6 - 11, 2008,
Charlottesville, Virginia, US
Ill-posedness in the Einstein equations
It is shown that the formulation of the Einstein equations widely in use in
numerical relativity, namely, the standard ADM form, as well as some of its
variations (including the most recent conformally-decomposed version), suffers
from a certain but standard type of ill-posedness. Specifically, the norm of
the solution is not bounded by the norm of the initial data irrespective of the
data. A long-running numerical experiment is performed as well, showing that
the type of ill-posedness observed may not be serious in specific practical
applications, as is known from many numerical simulations.Comment: 13 pages, 3 figures, accepted for publication in Journal of
Mathematical Physics (to appear August 2000
Soft-Fermion-Pole Mechanism to Single Spin Asymmetry in Hadronic Pion Production
Single spin asymmetry (SSA) is a twist-3 observable in the collinear
factorization approach. We present a twist-3 single-spin-dependent cross
section formula for the pion production in pp-collision, p^\uparrow p\to\pi X,
relevant to RHIC experiment. In particular, we calculate the soft-fermion-pole
(SFP) contribution to the cross section from the quark-gluon correlation
functions. We show that its effect can be as large as the soft-gluon-pole (SGP)
contribution owing to the large SFP partonic hard cross section, even though
the derivative of the SFP function does not participate in the cross section.Comment: 4 pages, 1 figure; to appear in the proceedings of the 18th
International Symposium on Spin Physics (SPIN2008), October 6 - 11, 2008,
Charlottesville, Virginia, US
- …