Ratio of Λˉ/Λ\bar{\Lambda}/\Lambda in Semi-inclusive Electroproduction

It is shown that the $\bar{\Lambda}/\Lambda$ cross section ratio in semi-inclusive electroproduction of $\Lambda$ and $\bar{\Lambda}$ hyperons in deep inelastic scattering of charged lepton on a nucleon target, can provide useful information on the quark to $\Lambda$ fragmentation functions. This ratio is calculated explicitly in a quark-diquark model, a pQCD based analysis, and an SU(3) symmetry model, with three different options for the contribution from the unfavored fragmentation functions. The $x$-dependence of this ratio is sensitive to the ratio of unfavored fragmentation functions over favored fragmentation functions, $D_{\bar{u}}^{\Lambda}(z)/D_{u}^{\Lambda}(z)$, whereas the $z$-dependence is sensitive to the flavor structure of the fragmentation functions, i.e., the ratio $D_u^{\Lambda}(z)/D_s^{\Lambda}(z)$. Future measurements by the HERMES Collaboration at DESY can discriminate between various cases.Comment: 11 latex files, 6 figure

Particle-Antiparticle Asymmetries of Λ\Lambda Production in Hadron-Nucleon Collisions

The particle-antiparticle asymmetries of $\Lambda$ production in 250 GeV/c $\pi^{\pm}$, $K^{\pm}$, and $p$ --nucleon collisions are studied with two model parametrizations of quark to $\Lambda$ fragmentation functions. It is shown that the available data can be qualitatively explained by the calculated results in both the quark-diquark model and a pQCD based analysis of fragmentation functions. The differences in the two model predictions are significant for $K^{\pm}$ beams, and high precision measurements of the asymmetries with detailed $x_F$ and $P_T$ information can discriminate between different predictions.Comment: 14 LaTex pages, 4 figures, to appear in Phys. Lett.

Parity-violating asymmetry of WW bosons produced in pp-pp collisions

The parity-violating asymmetry is an ideal tool to study the quark helicity distribution in the proton. We study the parity-violating asymmetry of $W^{\pm}$ bosons produced by longitudinally polarized $p$-$p$ collision in RHIC, based on predictions of quark distributions of the proton in the SU(6) quark-spectator-diquark model and a perturbative QCD based counting rule analysis. We find that the two models give nearly equal asymmetry for $W^+$ but that for $W^-$ quite different. Therefore future experiments on such quantity can help to clarify different predictions of the value $\Delta d(x)/d(x)$ at $x \to 1$ in the proton.Comment: 11 Latex pages, 9 figures, final version to appear in NP

Sivers function in light-cone quark model and azimuthal spin asymmetries in pion electroproduction

We perform a calculation of Sivers function in a light-cone SU(6) quark-diquark model with both scalar diquark and vector diquark spectators. We derive the transverse momentum dependent light-cone wave function of the proton by taking into account the Melosh-Wigner rotation. By adopting one-gluon exchange, we obtain a non-vanishing Sivers function of $down$ quark from interference of proton spin amplitudes. We analyze the $\frac{|P_{h\perp}|}{M}$ weighted Sivers asymmetries in $\pi^+$, $\pi^-$ and $\pi^0$ electroproduction off transverse polarized proton target, averaged and not averaged by the kinematics of HERMES experiment.Comment: 17 LaTex pages, 2 figures. Final version for journal publicatio

Q2Q^2--Dependence of the Gerasimov-Drell-Hearn Sum Rule

We test the Gerasimov-Drell-Hearn (GDH) sum rule numerically by calculating the total photon absorption cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$ on the nucleon via photon excitation of baryon resonances in the constituent quark model. A total of seventeen, low-lying, non-strange baryon resonances are included in this calculation. The transverse and longitudinal interference cross section, $\sigma_{1/2}^{TL}$, is found to play an important role in the study of the $Q^2$ variation of the sum rule. The results show that the GDH sum rule is saturated by these resonances at a confidence level of 94%. In particular, the $P_{33}(1232)$ excitation largely saturates the sum rule at $Q^2 = 0$, and dominates at small $Q^2$. The GDH integral has a strong $Q^2$-dependence below $Q^2= 1.0 {GeV}^2$ and changes its sign around $Q^2= 0.3 {GeV}^2$. It becomes weakly $Q^2$-dependent for $Q^2 > 1.0 {GeV}^2$ because of the quick decline of the resonance contributions. We point out that the $Q^2$ variation of the GDH sum rule is very important for understanding the nucleon spin structure in the non-perturbative QCD region.Comment: revtex, 17 pages, 3 ps figs include

On Transverse-Momentum Dependent Light-Cone Wave Functions of Light Mesons

Transverse-momentum dependent (TMD) light-cone wave functions of a light meson are important ingredients in the TMD QCD factorization of exclusive processes. This factorization allows one conveniently resum Sudakov logarithms appearing in collinear factorization. The TMD light-cone wave functions are not simply related to the standard light-cone wave functions in collinear factorization by integrating them over the transverse momentum. We explore relations between TMD light-cone wave functions and those in the collinear factorization. Two factorized relations can be found. One is helpful for constructing models for TMD light-cone wave functions, and the other can be used for resummation. These relations will be useful to establish a link between two types of factorization.Comment: add more discussions and reference

Perturbative Prediction for Parton Fragmentation into Heavy Hadron

By expanding functions of parton fragmentation into a heavy hadron in the inverse of the heavy quark mass $m_Q$ we attempt to factorize them into perturbative- and nonperturbative parts. In our approach the nonperturbative parts can be defined as matrix elements in heavy quark effective theory, the shape of the functions is predicted by perturbative QCD. In this work we neglect effect at order of $m_Q^{-2}$ and calculate the perturbative parts at one-loop level for heavy quark- and gluon fragmentation. We compare our results from leading log approximation with experimental results from $e^+e^-$ colliders and find a deviation below or at 10% level. Adding effect of higher order in $\alpha_s$ it can be expected to reduce the deviation. The size of matrix elements appearing at the order we consider for several types of heavy hadrons is determined.Comment: 21 pages + 3 pages figures, plain te

Melosh rotation: source of the proton's missing spin

It is shown that the observed small value of the integrated spin structure function for protons could be naturally understood within the naive quark model by considering the effect from Melosh rotation. The key to this problem lies in the fact that the deep inelastic process probes the light-cone quarks rather than the instant-form quarks, and that the spin of the proton is the sum of the Melosh rotated light-cone spin of the individual quarks rather than simply the sum of the light-cone spin of the quarks directly.Comment: 5 latex page

Light-Cone Representation of the Spin and Orbital Angular Momentum of Relativistic Composite Systems

The matrix elements of local operators such as the electromagnetic current, the energy momentum tensor, angular momentum, and the moments of structure functions have exact representations in terms of light-cone Fock state wavefunctions of bound states such as hadrons. We illustrate all of these properties by giving explicit light-cone wavefunctions for the two-particle Fock state of the electron in QED, thus connecting the Schwinger anomalous magnetic moment to the spin and orbital momentum carried by its Fock state constituents. We also compute the QED one-loop radiative corrections for the form factors for the graviton coupling to the electron and photon. Although the underlying model is derived from elementary QED perturbative couplings, it in fact can be used to simulate much more general bound state systems by applying spectral integration over the constituent masses while preserving all of the Lorentz properties, giving explicit realization of the spin sum rules and other local matrix elements. The role of orbital angular momentum in understanding the "spin crisis" problem for relativistic systems is clarified. We also prove that the anomalous gravitomagnetic moment B(0) vanishes for any composite system. This property is shown to follow directly from the Lorentz boost properties of the light-cone Fock representation and holds separately for each Fock state component. We show how the QED perturbative structure can be used to model bound state systems while preserving all Lorentz properties. We thus obtain a theoretical laboratory to test the consistency of formulae which have been proposed to probe the spin structure of hadrons.Comment: Version to be published in Nuclear Physics B. Includes illustrations of graviton-lepton form factors at one loop in QE