QCD vacuum: nuclear forces, nucleons, pions ...

摘要

This contribution contains six sections, namely:1. from QCD to chiral perturbation theory - QCD is widely accepted as the theory of strong interactions, but direct applications to low-energy hadronic processes are difficult. In this regime, the light quarks u and d prevail, and one can employ a rigorously equivalent effective theory, known a chiral perturbation theory, based on hadronic degrees of freedom. 2. strong vacuum and the pion - Chiral symmetry is not exact in the real world. Nevertheless, the absence of of parity multiplets and the smallness of the pion mass suggest that it is a good approximate symmetry, realized in the Nambu-Goldstone mode. Its ground state, the vacuum, is filled with a condensate, made of quark-antiquark pairs. In sections 1-3, instances are presented of observables strongly influenced by the QCD vacuum. 3. nuclear forces - In the last few years, chiral perturbation theory has produced a very reliable picture of both two- and three-nucleon forces. In particular, the important isospin independent central potential V~+_C is well understood and known to be dominated by the scalar form factor of the nucleon, a function that describes the disturbance it produces over the vacuum. 4. nucleon scalar form factor - The spatial integration of the nucleon scalar form factor gives rise to aN, the nucleon a-term. The value of this quantity can be extracted from experiment and the empirical value accepted presently is 45±8 MeV. A simple model, based on the idea that the pion cloud of the nucleon is constructed at the expenses of the surrounding condensate, produces a aN in the range 43-49 MeV, with no free parameters. 5. scalar radius of the pion - The value of this radius can be extracted from pion-pion scattering data and the most reliable estimate is ( r~2 )s~T = 0.61 ± 0.04 fm~2. The extension of the model described in section 4 to the pion gives rise to a picture in which it is embedded into the condensate. As one moves towards its center, the condensate is gradually replaced by a mesonic cloud. When this process is completed, a phase transition occurs, at a distance R 0.6 fm. A version of the model, including scalar resonances, yields (r~2)s~x = 0.59 fm~2. 6. conclusion - The QCD vacuum is an active component of low-energy hadronic processes, essential to a consistent description of Nuclear Physics.