268 research outputs found

### Addressing transversity with interference fragmentation functions

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

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

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 $\rho$
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

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

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

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

We present Monte Carlo simulations of both the Sivers and the Boer-Mulders
effects in the polarized Drell-Yan $\pi^\pm p^\uparrow \to \mu^+ \mu^- X$
process at the center-of-mass energy $\sqrt{s} \sim 14$ 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

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

We assume a connection between the forward limit of the GPD E and the first moment of the Sivers transverse-momentum distribution, inspired by Burkardtâs 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 Jiâs definition

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