Search

Transverse single-spin asymmetries (SSA) in inclusive reactions are now considered to be directly related to the transverse momentum ${\bf k}_{T}$ of the fundamental partons involved in the process. We find that the ideal probe to extract information on the gluon Sivers function is the transverse SSA of prompt photon production $p p^{\uparrow} \to \gamma X$, at large $p_T$. The following related processes, $p p^{\uparrow} \to \gamma + jet + X$, $p p^{\uparrow} \to \gamma^* + X \to \mu^+ \mu^- + X$ and $\bar{p} p^{\uparrow} \to \gamma + X$ are also briefly discussed., Comment: 9 pages, 1 figure, to appear Physics Letters B

The shadowing and antishadowing of nuclear structure functions in the Gribov-Glauber picture is due respectively to the destructive and constructive interference of amplitudes arising from the multiple-scattering of quarks in the nucleus. The effective quark-nucleon scattering amplitude includes Pomeron and Odderon contributions from multi-gluon exchange as well as Reggeon quark-exchange contributions. We show that the coherence of these multiscattering nuclear processes leads to shadowing and antishadowing of the electromagnetic nuclear structure functions in agreement with measurements. This picture leads to substantially different antishadowing for charged and neutral current reactions, thus affecting the extraction of the weak-mixing angle $\theta_W$. We find that part of the anomalous NuTeV result for $\theta_W$ could be due to the nonuniversality of nuclear antishadowing for charged and neutral currents. Detailed measurements of the nuclear dependence of individual quark structure functions are thus needed to establish the distinctive phenomenology of shadowing and antishadowing and to make the NuTeV results definitive., Comment: 38 pages, 15 figures

We investigate the Collins effect in single spin asymmetries (SSAs) of the $p^{\uparrow}p \to \pi X$ process, by taking into account the transverse momentum dependence of the microscopic sub-processes cross sections, with the transverse momentum in the Collins function integrated over. We find that the asymmetries due to the Collins effect can only explain the available data at best qualitatively, by using our choices of quark distributions in the quark-diquark model and a pQCD-based analysis, together with several options of the Collins function. Our results indicate the necessity to take into account contributions from other effects such as the Sivers effect or twist-3 contributions., Comment: 10 LaTeX pages, 2 figures. Version in journal publication

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. B

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 figures

We perform a calculation of the Sivers function in a spectator model of the nucleon, with scalar and axial-vector diquarks. We make use of gluon rescattering to produce the nontrivial phases necessary to generate the Sivers function. The inclusion of axial-vector diquarks enables us to obtain a nonzero Sivers function for down quarks. Using the results of our model, we discuss the phenomenology of transvere single spin asymmetries in pi+, pi-, and pi0 production, which are currently analysed by the HERMES and COMPASS collaborations. We find that the inclusion of axial-vector diquarks substantially reduces the asymmetries., Comment: 8 pages, 7 figures, uses RevTex, added calculation of h1^perp, final version accepted for publication in PLB

We estimate the Collins fragmentation function by introducing the effect of gluon rescattering in a model calculation of the fragmentation process. We include all necessary diagrams to the one-loop level and compute the nontrivial phases giving rise to the Collins function. We compare our results to the ones obtained from pion rescattering. We conclude that three out of four one-loop diagrams give sizeable contributions to the Collins function, and that the effect of gluon rescattering has a magnitude comparable to that of pion rescattering, but has opposite sign., Comment: 6 pages, 5 figures, uses RevTex. Typos corrected and a sentence added in the conclusions. To be published in PLB

There is a striking suppression of the cross section for production of octet baryons in $e^+ e ^-$ annihilation, as the mass of the produced hadron increases. We present a simple parametrization for the fragmentation functions into octet baryons guided by two input models: the SU(3) flavor symmetry part is given by a quark-diquark model, and the baryon mass suppression part is inspired by the string model. We need only eight free parameters to describe the fragmentation functions for all octet baryons. These free parameters are determined by a fit to the experimental data of octet baryon production in $e^+ e ^-$ annihilation. Then we apply the obtained fragmentation functions to predict the cross section of the octet baryon production in charged lepton DIS and find consistency with the available experimental data. Furthermore, baryon production in $pp$ collisions is suggested to be an ideal domain to check the predicted mass suppression., Comment: 20 pages, 5 figures

We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional $3q$ structure, different $(3q)(Q\bar{Q})$ admixtures in the nucleon wavefunction are also taken into account. The contributions to the nucleon spin from various components of the nucleon wavefunction are discussed. The effect due to the Melosh-Wigner rotation is also studied. It is shown that the Jaffe-Lipkin term is only important when antiquarks are negatively polarized. We arrive at a new "minimal" quark model, which is close to the naive quark model, in order to understand the proton spin "puzzle"., Comment: 19 latex pages, Published in Eur.Phys.J.A