Main topics of my research
Few Body Systems in Nuclear Physics back to main|Nuclear Physics and Few Body Systems|Nuclear Physics for everyone (pt)
Meson production reactions at threshold
Information on the short-range nuclear force is gained with experiments measuring meson production (pi and eta) near the threshold energies, where the colliding nucleons in the final state become essentially at rest, giving their initial kinetic energy as exchange of the produced meson mass. The rates of these reactions are crucially sensitive to the short-range parts of the nuclear forces.

New experiments (Uppsala, Julich) aim also to obtain additional information by using spin-polarized beams and to search for heavier mesons production thresholds. They
provide crucial tests for meson-nucleon interactions. Signals of possible etaNN quasi-bound states seem to appear in the eta production data when compared to the pion production data, and opened recently the speculation on the possible existence of stable eta-mesic exotic matter in stars.
Three-Nucleon Forces
A three-nucleon force is an interaction between three particles which is not the sum of pairwise interactions. An analogy that allows to understand the existence of a
three-nucleon force as a consequence of the structure and extension of the nucleons, is the earth-moon-satellite system: the tides induced by the moon in the oceans on the earth (like a polarization phenomena of a nucleon by another one) alter the interaction between the earth and the satellite, which depends not only on their relative position but also on the position of the moon. 

Three nucleon forces are weak but strong efforts exist (TUNL, Nikhef, Indiana) to measure their effects in scattering experiments of light nuclei, surveying a variety of
kinematical conditions. Microscopic models for the three-nucleon force are now under a finer experimental study. Theoretically, the several intermediate and short-range pieces of the three-nucleon force are not yet satisfactorily consistent.
Open problems
  • The pi N amplitude, constrained by chiral symmetry (breaking), determines through two-pion exchange diagrams, the intermediate nucleon-nucleon interaction and also the longest range of the small (but important!) three-nucleon force. But the short-range part of the nucleon-nucleon and three-nucleon interactions remains poorly known.
  • The role the of the s-wave term in the pi N interaction, a signature of chiral symmetry breaking, and of the sigma meson in such force, remains a controversial subject. 
  • Relativistic effects, associated to off-mass-shell propagation, retardation and negative-energy-states are also sources of short-range "exotics" currently under investigation. 
  • Enhancements at threshold of the cross-sections for certain meson production reactions are not yet fully understood and open speculations on the possibility of the existence of bound mesic nuclear matter.
back to main|Nuclear Physics and Few Body Systems|Nuclear Physics for everyone (pt)