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, 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

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

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