Narrow Results

We compare the Parton distributions deduced in the framework of a quantum statistical approach for both the longitudinal and transverse degrees of freedom with the unpolarized distributions measured at HERA and with the polarized ones proposed in a previous paper, which have been shown to be in very good agreement also with the results of experiments performed after that proposal. The agreement with HERA data in correspondence to very similar values for the “temperature” and the “potentials” found in the previous work gives a robust confirm of the statistical model. The unpolarized distributions are compared also with the result of NNPDF. The free parameters are fixed mainly by data in the range (0.1, 0.5) for the x variable, where the valence Partons dominate, and in the small x region for the diffractive contribution. This feature makes the parametrization proposed here very attractive.

The key ingredients of the Gribov program of attacking quark confinement are reviewed.

The report concerns only one of the issues discussed in my talk at ICFP 2007: the significance of recent lattice results for our understanding of the nonperturbative phenomena in the low energy domain of QCD. All of these results are consistent with the phenomenological estimates for the coupling constants of the chiral SU(2)×SU(2) Lagrangian and thereby corroborate the predictions based on these. In the case of SU(3)×SU(3), the results also confirm the old estimates obtained from chiral perturbation theory. In particular, the first two terms in the expansion of the masses and decay constants in powers of m_s appear to represent a decent approximation for the full series. Some of the lattice results indicate that the quark condensate violates the OZI-rule rather strongly, but as there are some discrepancies concerning this issue, it is too early to draw conclusions.

The anomalous chromomagnetic dipole moment, with the gluon off-shell, of the Standard Model (SM) light quarks ($u$, $d$, $s$, $c$ and $b$), at the $Z$ gauge boson mass scale, is computed by using the $\overline{\mathrm{\text{MS}}}$ scheme. The numerical results disagree with all the previous predictions reported in the literature and show a discrepancy of up to two orders of magnitude in certain situations.

Remarks on strange quarks in the nucleon.

A peculiar prediction of perturbative QCD, with the assumption of Local Parton Hadron Duality, is that the difference in mean hadron multiplicity between heavy and light quark initiated events in e⁺e^- annihilation, δ_Qℓ, is energy independent. In the Modified Leading Logarithmic Approximation (MLLA), the numerical value of the constant is derived in terms of a few experimentally measurable quantities. While the energy independence of δ_Qℓ has been succesfully verified experimentally for b-quarks up to the highest LEP2 energy, the numerical prediction originally published in 1992, , clearly overestimates the experimental results. In this work, done in collaboration with Yuri L. Dokshitzer, Valery A. Khoze and Wolfgang Ochs, we show that the original MLLA prediction needs a revision, in the light of new experimental results and the improvement in the understanding of the experimental data. We now find , in better agreement with experiment, and we show that the remaining difference can be attributed largely to next-to-MLLA contributions, an important subset of which are identified and evaluated. The situation for charmed quarks is also reviewed.