Resonances and higher twist in polarized lepton-nucleon scattering

We present a detailed analysis of resonance contributions in the context of higher twist effects in the moments of the proton spin structure function g_1. For each of these moments, it is found that there exists a characteristic Q^2 region in which (perturbative) higher twist corrections coexist with (non-perturbative) resonance contribution of comparable magnitude.Comment: 17 pages LaTe

Nucleon Spin Structures from Lattice QCD: Flavor Singlet Axial and Tensor Charges

The flavor singlet axial and tensor charges of the nucleon are calculated in lattice QCD. We find $\Delta\Sigma=\Delta u+\Delta d+\Delta s=+0.638(54)-0.347(46)-0.109(30) = +0.18(10)$ for the axial charge and $\delta\Sigma=\delta u+\delta d+\delta s=+0.839(60)-0.231(55)-0.046(34) = +0.562(88)$ for the tensor charge. The result for the axial charge shows reasonable agreement with the experiment and that for the tensor charge is the first prediction from lattice QCD before experimental measurements.Comment: 10 pages, latex source-file, 5 figures as epsf-file, uses espcrc1.sty. Invited talk at the International Conference on Quark Lepton Nuclear Physics, 20-23 May 1997, Osaka, Japa

Proton Spin Content From Lattice QCD

We calculate the form factor of the quark energy momentum tensor and thereby extract the quark orbital angular momentum of the nucleon. The calculation is done on a quenched $16^3 \times 24$ lattice at $\beta = 6.0$ and with Wilson fermions at $\kappa$ = 0.148, 0.152, 0.154 and 0.155. We calculate the disconnected insertion stochastically which employs the $Z_2$ noise with an unbiased subtraction. This proves to be an efficient method of reduce the error from the noise. We find that the total quark contribution to the proton spin is $0.29 \pm 0.07$. From this we deduce that the quark orbital angular momentum is $0.17 \pm 0.08$ and predict the gluon spin to be $0.21 \pm 0.07$, i.e. about 40% of the proton spin is due to the glue.Comment: LATTICE99(Matrix Elements), 3 pages, 3 figure

Accessing directly the strange quark content of the proton at HERA

We investigate a double-spin asymmetry for the semi-inclusive $\Lambda$ hyperon production in the longitudinally deep inelastic lepton-proton scattering, the sign of which can provide us with important information about the strange quark helicity distribution in the proton.On the basis of the interpretation of the longitudinal deep inelastic lepton-nucleon scattering data as a negative strange quark polarization in the proton and the preliminary results on the measurement of the longitudinal $\Lambda$ polarization at the $Z$ resonance in electron-positron annihilation,we predict a minus sign for the suggested observable. The experimental condition required for our suggestion is met by the HERA facilities, so the asymmetry considered can be measured by the HERMES experiments at HERA in the near future.Comment: 10 pages, Revtex, completely rephrased, references adde

Recent Results from Jefferson Lab

Recent results on studies of the structure of nucleons and nuclei in the regime of strong interaction QCD are discussed. Use of high current polarized electron beams, polarized targets, and recoil polarimeters, in conjunction with modern spectrometers and detector instrumentation allow much more detailed studies of nucleon and nuclear structure than has been possible in the past. The CEBAF accelerator at Jefferson Lab was build to study the internal structure of hadrons in a regime where confinement is important and strong interaction QCD is the relevant theory. I discuss how the first experiments already make significant contributions towards an improved understanding of hadronic structure.Comment: Lecture presented at the International School of Nuclear Physics, Erice, Sicily, Italy, September 17 - 25, 199

An Analysis of the Next-to-Leading Order Corrections to the g_T(=g_1+g_2) Scaling Function

We present a general method for obtaining the quantum chromodynamical radiative corrections to the higher-twist (power-suppressed) contributions to inclusive deep-inelastic scattering in terms of light-cone correlation functions of the fundamental fields of quantum chromodynamics. Using this procedure, we calculate the previously unknown ${\cal O}(\alpha_s)$ corrections to the twist-three part of the spin scaling function $g_T(x_B,Q^2) (=g_1(x_B,Q^2)+g_2(x_B,Q^2))$ and the corresponding forward Compton amplitude $S_T(\nu,Q^2)$. Expanding our result about the unphysical point $x_B=\infty$, we arrive at an operator product expansion of the nonlocal product of two electromagnetic current operators involving twist-two and -three operators valid to ${\cal O}(\alpha_s)$ for forward matrix elements. We find that the Wandzura-Wilczek relation between $g_1(x_B,Q^2)$ and the twist-two part of $g_T(x_B,Q^2)$ is respected in both the singlet and non-singlet sectors at this order, and argue its validity to all orders. The large-$N_c$ limit does not appreciably simplify the twist-three Wilson coefficients.Comment: 41 pages, 9 figures, corrected minor erro

Helicity-dependent generalized parton distributions in constituent quark models

Helicity-dependent generalized parton distributions of the nucleon are derived from the overlap representation of generalized parton distributions using light-cone wave functions obtained in constituent quark models. Results from two different quark models are used also to study the angular momentum sum rule and the spin asymmetry in polarized electron scattering.Comment: 20 pages, 5 figures, 2 table

Target mass corrections and twist-3 in the nucleon spin structure functions

The Nachtmann moment is employed to study the contribution of twist-3 operator to the nucleon spin structure functions. Target mass corrections to the Cornwall-Norton moments of the spin structure functions $g_{1,2}$ are discussed. It is found that the corrections play a sizeable role to the contribution of the twist-3 $\tilde d_2$ extracted from the Cornwall-Norton moments.Comment: 11 pages, 1 figure

Quark Orbital-Angular-Momentum Distribution in the Nucleon

We introduce gauge-invariant quark and gluon angular momentum distributions after making a generalization of the angular momentum density operators. From the quark angular momentum distribution, we define the gauge-invariant and leading-twist quark {\it orbital} angular momentum distribution $L_q(x)$. The latter can be extracted from data on the polarized and unpolarized quark distributions and the off-forward distribution $E(x)$ in the forward limit. We comment upon the evolution equations obeyed by this as well as other orbital distributions considered in the literature.Comment: 8 pages, latex, no figures, minor corrections mad

Spin-Dependent Twist-Four Matrix Elements from g_1 Data in the Resonance Region

Matrix elements of spin-dependent twist-four operators are extracted from recent data on the spin-dependent g_1 structure function of the proton and deuteron in the resonance region. We emphasize the need to include the elastic contributions to the first moments of the structure functions at Q^2 < 2 GeV^2. The coefficients of the 1/Q^2 corrections to the Ellis-Jaffe sum rules are found to be 0.04 \pm 0.02 and 0.03 \pm 0.04 GeV^2 for the proton and neutron, respectively.Comment: 10 pages REVTeX, 4 figure