Parton distributions for the pion in a chiral quark model

Parton distributions for the pion are studied in a chiral quark model characterized by a quark propagator for which a spectral representation is assumed. Electromagnetic and chiral symmetry constraints are imposed through the relevant Ward-Takahashi identities for flavoured vertex functions. Finiteness of the theory, requires the spectral function to be non-positive definite. Straightforward calculation yields the result that the pion structure function becomes one in the chiral limit, regardless of the details of the spectral function. LO and NLO evolution provide a satisfactory description of phenomenological parameterizations of the valence distribution functions but fails to describe gluon and sea distributions.Comment: Latex, World Scientific, 8 pages, 1 figures. Talk given at the Workshop on " Lepton Scattering, Hadrons and QCD " March 26 -- April 6, 2001. Adelaide (Australia

Meson Resonances at large Nc: Complex Poles vs Breit-Wigner Masses

The rigorous quantum mechanical definition of a resonance requires determining the pole position in the second Riemann sheet of the analytically continued partial wave scattering amplitude in the complex Mandelstam s-variable plane. For meson resonances we investigate the alternative Breit-Wigner (BW) definition within the large Nc expansion. By assuming that the pole position is ${\cal O} (N_C^{0})$ and exploiting unitarity, we show that the BW determination of the resonance mass differs from the pole position by ${\cal O} (N_C^{-2})$ terms, which can be extracted from pi-pi scattering data. For the case of the f0(600) pole, the BW scalar mass is predicted to occur at about 700 MeV while the true value is located at about 800 MeV.Comment: 7 pages. No figures. (elsevier preprint

Partonic quasi-distributions of the pion in chiral quark models

The evaluation of partonic distributions presents a challenge for QCD, and in particular for its Euclidean lattice realization. Recently, objects called quasi-distributions (which become standard distributions in a limit of the longitudinal momentum of the target hadron going to infinity) have been proposed. We present a non-perturbative, dynamical evaluation of the quark quasi-distribution amplitude (QDA) of the pion in the framework of chiral quark models (the Nambu--Jona-Lasinio model and the spectral quark model). We arrive at simple but nontrivial analytic expressions, where the dependence on the longitudinal momentum, the momentum fraction, or the transverse-momentum (for the unintegrated objects) can be explicitly assessed. For the parton distribution amplitude (PDA), we carry out the necessary QCD evolution from the constituent quark model scale to higher scales accessible on the lattice, and compare favorably to the LaMET data.Comment: 6 pages, talk presented by WB at XVII International Conference on Hadron Spectroscopy and Structure - Hadron2017, Salamanca, 25-29 September 201