268 research outputs found

    Addressing transversity with interference fragmentation functions

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    The class of interference fragmentation functions, arising from interference among different hadron production channels, is reconsidered. Their symmetry properties with respect to chiral transformations allow building spin asymmetries where the quark transversity distribution can be factorized out at leading twist. For the case of two leading spinless hadrons inside the same current jet, the pair system is expanded in relative partial waves. The cross section is represented on the helicity basis of the target and the fragmenting quark, as well as on the relative orbital angular momentum of the pair. From the decay matrix being positive semi-definite, new bounds on the interference fragmentation functions can be derived. The expansion in partial waves allows to naturally incorporate in a unified formalism specific cases already studied in the literature, such as the fragmentation functions arising from the interference of two mesons in relative s and p waves, as well as the fragmentation of a spin-1 hadron.Comment: Proceedings of GDH2002, July 3-6, Genova, Italy. 5 pages, 2 figures in .eps format, style file include

    Interference fragmentation functions and spin asymmetries

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    A new class of fragmentation functions, arising from the interference of different hadron production channels, is analyzed. Their symmetry properties with respect to naive time-reversal transformations allow for the exploration of final-state interactions occurring during and after the hadronization. Their symmetry properties with respect to chiral transformations allow building spin asymmetries where the quark transversity distribution can be factorized out at leading twist. Explicit calculations will be shown for the interference fragmentation functions arising from final-state interactions of two pions detected in the same current jet for the case of semi-inclusive Deep-Inelastic Scattering (DIS).Comment: 4 pages, 2 eps figures included, World Scientific style file included; to appear in proceedings of DIS2001, Bologna, Apr.27th - May 1st, 200

    T-odd fragmentation functions

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    We discuss the properties of fragmentation functions arising from the interference of two leading hadrons produced inside the same jet in the current fragmentation region of a hard process. For the case of semi-inclusive Deep Inelastic Scattering (DIS), a proper folding of the cross section, integrated over the azimuthal position of the detected hadrons, produces a factorized form that allows for the extraction of the quark transversity distribution at leading twist. Using an extended spectator model, explicit calculations are shown for the hadron pair being two pions with invariant mass inside the ρ\rho resonance width.Comment: 4 pages, .eps figures included; Elsevier espcrc1.sty used; proceedings of the 3rd International Conference on Perspectives in Hadronic Physics, ICTP-Trieste, May 7-11 2001, to appear in Nucl. Phys. A (Proc. Suppl.

    Elastic Form Factors in Point-form Approach

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    Using the point-form approach to relativistic quantum mechanics, a covariant framework is presented for the calculation of proton and neutron electromagnetic form factors. Results for charge radii, magnetic moments, and electric as well as magnetic form factors are produced using the wave functions obtained in the so-called Goldstone-boson-exchange constituent quark model. All the predictions are found in remarkable agreement with existing experimental data.Comment: 8 pages, 5 .eps figures, formatted with World Scientific LaTeX2e document class style file (included), to appear in "Proceedings of the VIII Convegno su Problemi di Fisica Nucleare e Teorica", Oct. 18-21/2000, Cortona (Italy), World Scientific Pub

    Baryon Structure

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    A review of the theoretical activity in Italy in the research field of Hadronic Physics is given. Specific focus is put on phenomenological models based on the effective degrees of freedom of constituent quarks, on parton distributions in hard processes in the Bjorken limit and on the possibility of linking the two concepts via evolution equations. A brief introduction is given also about the socalled generalized parton distributions.Comment: 20 pages, no figures, invited general report at the "IX Convegno su Problemi di Fisica Nucleare e Teorica", Cortona, Oct. 9-12, 2002 (Italy), to appear in World Scientific Proceeding

    Angular Distributions for Knockout and Scattering of Protons in the Eikonal Approximation

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    The advent of new electron accelerators with few-GeV beam energies makes the (e,e'p) reaction a promising tool for investigating new aspects of the electromagnetic interaction. To this purpose it is crucial to set the scale of Final-State Interactions (FSI) at high ejectile energies. We have analyzed the constraints which allow for a meaningful generalization to (e,e'p) of the Glauber method used in elastic (p,p) scattering. The FSI dominance at large deflection angles produces in the distributions a universal behaviour resembling the coherent diffractive scattering between the ejected proton and the (residual) nucleus. It is argued that the comparison between (e,e'p) and (p,p), when a meaningful one, may represent a convenient tool to disentangle (exotic) effects due to FSI from those related to the (hard) electromagnetic vertex.Comment: LaTeX, 21 pages, 4 figures uuencoded in a separate file. Submitted to Phys. Rev.

    Monte Carlo simulation of single spin asymmetries in pion-proton collisions

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    We present Monte Carlo simulations of both the Sivers and the Boer-Mulders effects in the polarized Drell-Yan π±p↑→Ό+Ό−X\pi^\pm p^\uparrow \to \mu^+ \mu^- X process at the center-of-mass energy s∌14\sqrt{s} \sim 14 GeV reachable at COMPASS with pion beams of energy 100 GeV. For the Sivers effect, we adopt two different parametrizations for the Sivers function to explore the statistical accuracy required to extract unambiguous information on this parton density. In particular, we verify the possibility of checking its predicted sign change between Semi-Inclusive Deep-Inelastic Scattering (SIDIS) and Drell-Yan processes, a crucial test of nonperturbative QCD. For the Boer-Mulders effect, because of the lack of parametrizations we can make only guesses. The goal is to explore the possibility of extracting information on the transversity distribution, the missing piece necessary to complete the knowledge of the nucleon spin structure at leading twist, and the Boer-Mulders function, which is related to the long-standing problem of the violation of the Lam-Tung sum rule in the unpolarized Drell-Yan cross section.Comment: RevTeX, 13 pages, 7 figures included in .eps forma

    Helicity Asymmetry for Proton Emission from Polarized Electrons in the Eikonal Regime

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    The nuclear response to longitudinally polarized electrons, detected in coincidence with out-of-plane high-energy protons, is discussed in a simple model where the ejectile wave function is approximated as a plane wave with a complex wave vector. This choice is equivalent to solve the problem of Final-State Interactions (FSI) in homogeneous nuclear matter, as the residual nucleus can be described to a first approximation when dealing with very fast emitted protons. The main advantage of the present method is that in the framework of the Distorted-Wave Impulse Approximation (DWIA) it allows for an analytical derivation of all the components of the nuclear response. It emerges that cancellations among the leading contributions determine the very small absolute size of the socalled fifth structure function and produce a nontrivial asymptotic scaling of the related helicity asymmetry for large values of the momentum transfer.Comment: 22 pages, RevTeX, 5 postscript figures encoded in separate file, submitted to Phys. Rev.

    Partonic angular momentum and the Sivers asymmetry

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    We assume a connection between the forward limit of the GPD E and the first moment of the Sivers transverse-momentum distribution, inspired by Burkardt’s idea of chromodynamic lensing. Then, we show that it is possible to fit at the same time the values of the nucleon anomalous magnetic moments and the data for semi-inclusive single-spin asymmetries originating from the Sivers effect. This opens a plausible way to quantifying the contribution of the partonic angular momentum to the spin of the nucleon, according to Ji’s definition