Justifying the exotic Theta+ pentaquark

The existence of a light S=+1 baryon resonance follows from Quantum Field Theory applied to baryons. This is illustrated in the Skyrme model (where Theta+ exists but is too strong) and in a new mean field approach where Theta+ arises as a consequence of three known resonances: Lambda(1405), N(1440) and N(1535).Comment: 3 p., contribution to the PANIC-08 proceeding

Future Polarised DIS Fixed Target Experiments

New experiments in polarised deep inelastic scattering will mainly concentrate on the measurement of semi-inclusive asymmetries. Especially, the upgraded HERMES experiment at DESY and the newly build COMPASS experiment at CERN will investigate the gluon polarisation via open charm and high p_T hadron pair production, study in detail the flavour decomposition of the quark helicity distributions and measure the tranversity distributions with tranversely polarised targets.Comment: 5 pages, 2 figures, LaTeX, Contribution to the proceedings of the DIS99, DESY-Zeuthen, Germany, April 199

Q^2 Evolution of the Neutron Spin Structure Moments using a ^3He Target

We have measured the spin structure functions g_1 and g_2 of ^3He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ^3He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q^2 evolution of Γ_1(Q^2)=∫_0^1g_1(x,Q^2)dx, Γ_2(Q^2)=∫_0^1g_2(x,Q^2)dx, and d_2(Q^2)=∫_0^1x^2[2g_1(x,Q^2)+3g_2(x,Q^2)]dx for the neutron in the range 0.1 ≤ Q^2 ≤0.9  GeV^2 with good precision. Γ_1(Q^2) displays a smooth variation from high to low Q^2. The Burkhardt-Cottingham sum rule holds within uncertainties and d_2 is nonzero over the measured range

Drell-Hearn-Gerasimov Sum-Rule for the Deuteron in Nuclear Effective Field Theory

The Drell-Hearn-Gerasimov sum rule for the deuteron is studied in nuclear effective field theory. The low-energy theorem for the spin-dependent Compton amplitude $f_1(\omega)$ is derived to the next-to-leading order in low-energy expansion. The spin-dependent photodisintegration cross section $\sigma^P-\sigma^A$ is calculated to the same order, and its contribution to the dispersive integral is evaluated.Comment: 8 pages, 2 figure

Q^2 Evolution of the Generalized Gerasimov-Drell-Hearn Integral for the Neutron using a ^3He Target

We present data on the inclusive scattering of polarized electrons from a polarized ^3He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5°. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference σ_(1/2)-σ_(3/2). We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q^2 of 0.1–0.9   GeV^2

Experimental determination of the effective strong coupling constant

We present a first attempt to experimentally extract an effective strong coupling constant that we define to be a low Q2 extension of a previous definition by S. Brodsky et al. following an initial work of G. Grunberg. Using Jefferson Lab data and sum rules, we establish its Q2-behavior over the complete Q2-range. The result is compared to effective coupling constants inferred from different processes and to calculations based on Schwinger-Dyson equations, hadron spectroscopy or lattice QCD. Although the connection between the experimentally extracted effective coupling constants and the calculations is not established it is interesting to note that their behaviors are similar.Comment: Published in Physics Letters B 650 4 24

NLO corrections to the twist-3 amplitude in DVCS on a nucleon in the Wandzura-Wilczek approximation: quark case

We computed the NLO corrections to twist-3, $L \to T$, flavor non-singlet amplitude in DVCS on a nucleon in the Wandzura-Wilczek approximation. Explicit calculation shows that factorization holds for NLO contribution to this amplitude, although the structure of the factorized amplitude at the NLO is more complicated than in the leading-order formula. Next-to-leading order coefficient functions for matrix elements of twist-3 vector and axial-vector quark string operators and their LO evolution equations are presented.Comment: 15 pages, 4 figure

Higher moments of nucleon spin structure functions in heavy baryon chiral perturbation theory and in a resonance model

The third moment $d_2$ of the twist-3 part of the nucleon spin structure function $g_2$ is generalized to arbitrary momentum transfer $Q^2$ and is evaluated in heavy baryon chiral perturbation theory (HBChPT) up to order ${\mathcal{O}}(p^4)$ and in a unitary isobar model (MAID). We show how to link $d_2$ as well as higher moments of the nucleon spin structure functions $g_1$ and $g_2$ to nucleon spin polarizabilities. We compare our results with the most recent experimental data, and find a good description of these available data within the unitary isobar model. We proceed to extract the twist-4 matrix element $f_2$ which appears in the $1/Q^2$ suppressed term in the twist expansion of the spin structure function $g_1$ for proton and neutron.Comment: 30 pages, 7 figure

Virtual Compton Scattering at Low Energy and the Generalized Polarizabilities of the Nucleon o

Virtual Compton Scattering on the nucleon: $\gamma^* N \to \gamma N$ is a new and rapidly developing field at low and high energies. This lecture is about the low energy part, i.e. for energies in the $(\gamma p)$ center-of-mass mainly up to the $\Delta(1232)$ resonance region. I review the concept of Generalized Polarizabilities of the Nucleon, and the experiments dedicated to their measurement.Comment: 16 pages, 8 figures. lecture given at the Erice School "Lepton Scattering and the Structure of Hadrons and Nuclei," Erice, 16 - 24 Sept. 2004; to appear in "Progress in Particle and Nuclear Physics

Accessing the Longitudinally Polarized Photon Content of the Proton

We investigate the QED Compton process (QEDCS) in longitudinally polarized lepton-proton scattering both in the elastic and inelastic channels and show that the cross section can be expressed in terms of the polarized equivalent photon distribution of the proton. We provide the necessary kinematical constraints to extract the polarized photon content of the proton using this process at HERMES, COMPASS and eRHIC. We also discuss the suppression of the major background process coming from virtual Compton scattering. We point out that such an experiment can give valuable information on $g_1(x_B, Q^2)$ in the small $x_B$, broad $Q^2$ region at the future polarized collider eRHIC and especially in the lower $Q^2$, medium $x_B$ region in fixed target experiments.Comment: Version to appear in PR