Partonic structure of the Nucleon in QCD and Nuclear Physics: new developments from old ideas

The nucleon is an ideal laboratory to solve QCD in the nonperturbative regime. There are several experimental observations that still lack a rigorous interpretation; they involve the nucleon as a (polarized) target as well as a beam (in collisions and Drell-Yan processes). These data look like big azimuthal and spin asymmetries, related to the transverse polarization and momentum of the nucleon and/or the final detected particles. They suggest internal reaction mechanisms that are suppressed in collinear perturbative QCD but that are "natural" in Nuclear Physics: quark helicity flips, residual final state interactions, etc.. In my talk, I will give a brief survey of the main results and I will flash the most recent developments and measurements.Comment: to appear in the proceedings of the X Convegno su problemi di Fisica Nucleare e Teorica, Cortona (Italy), 6-9 Oct. 2004; ws-proc9x6.cls include

Transversity and inclusive two-pion production

A model for dihadron fragmentation functions is briefly outlined, that describes the fragmentation of a quark in two unpolarized hadrons. The parameters are tuned to the output of the PYTHIA event generator for two-hadron semi-inclusive production in deep inelastic scattering at HERMES. Then, predictions are made for the unknown polarized fragmentation function and the related single-spin asymmetry in the azimuthal distribution of $\pi^+ \pi^-$ pairs in semi-inclusive deep inelastic scattering on transversely polarized targets at HERMES and COMPASS. This asymmetry can be used to extract the quark transversity distribution.Comment: 4 pages, 4 composite figures, talk presented at the 17th International Spin Physics Symposium (SPIN2006), Kyoto (Japan), October 2-7, 200

Two-hadron Inclusive DIS and Interference Fragmentation Functions

We investigate the properties of interference fragmentation functions arising from the emission of two leading hadrons inside the same jet for semi-inclusive lepton-nucleon deep-inelastic scattering. Using an extended spectator model we give numerical estimates for the example of the fragmentation into a proton-pion pair with its invariant mass on the Roper resonance.Comment: 10 pages, 12 figures in .eps format, epsfig and sprocl style files included, to appear in the Proceedings of the EPIC99 Workshop, April 8-11 1999, Bloomington (Indiana-USA

Alternative approaches to transversity: how convenient and feasible are they?

The complete knowledge of the nucleon spin structure at leading twist requires also addressing the transverse spin distribution of quarks, or transversity, which is yet unexplored because of its chiral-odd nature. Elaborating strategies to extract it from (spin) asymmetry data represents a unique opportunity to explore more generally the transverse spin structure of the nucleon and the transverse momentum dynamics of partons inside it. Here, we critically review some of the most promising approaches.Comment: 8 pages; 4 figures in .eps format; new version with modified fig.4; to appear in the Proceedings of the International Workshop on Transverse Polarisation Phenomena in Hard Processes (Transversity 2005), Villa Olmo (Como), 7-10th September 2005; World Scientific style file include

Modeling dihadron fragmentation functions

We present a model for dihadron fragmentation functions, describing the fragmentation of a quark into two unpolarized hadrons. We tune the parameters of our model to the output of the PYTHIA event generator for two-hadron semi-inclusive production in deep inelastic scattering at HERMES. Once the parameters of the model are fixed, we make predictions for other unknown fragmentation functions and for a single-spin asymmetry in the azimuthal distribution of pi+ pi- pairs in semi-inclusive deep inelastic scattering on a transversely polarized target at HERMES and COMPASS. Such asymmetry could be used to measure the quark transversity distribution function.Comment: 16 pages, 10 figure

First Extraction of Transversity from a Global Analysis of Electron-Proton and Proton-Proton Data

We present the first extraction of the transversity distribution in the framework of collinear factorization based on the global analysis of pion-pair production in deep-inelastic scattering off transversely polarized targets and in proton-proton collisions with one transversely polarized proton. The extraction relies on the knowledge of di-hadron fragmentation functions, which are taken from the analysis of electron-positron annihilation data. For the first time, the chiral-odd transversity is extracted from a global analysis similar to what is usually done for the chiral-even spin-averaged and helicity distributions. The knowledge of transversity is important for, among other things, detecting possible signals of new physics in high-precision low-energy experiments.Comment: version identical to published one; supplemental material available at http://link.aps.org/supplemental/10.1103/PhysRevLett.120.19200