# P. Navratil

## Nuclear Structure from First Principles: Ab Initio No-Core Shell Model
for Light Nuclei

In recent years, construction of accurate nucleon-nucleon potentials and
increases
in computing power have led to new methods capable of
solving the nuclear structure problem for systems of more than four
nucleons.
In these lectures I will describe one of these methods, the ab initio
no-core shell
model (NCSM). The principal foundation of this approach is the use of
effective
interactions appropriate for the large, but finite, harmonic-oscillator
basis
employed in the calculations. These effective interactions are derived from
the underlying realistic inter-nucleon potentials in a way that guarantees
convergence to the exact solution as the basis size increases.

I will discuss convergence tests of the method and
nuclear structure results for light nuclei obtained
by using several modern nucleon-nucleon potentials, including those derived
from the effective field theory.

At present, the ab initio NCSM is capable of including the
much-less-explored
realistic three-nucleon forces. An important result of these
nuclear-structure
studies is the significance of three-nucleon interaction in determining
not only the binding
energy, but also the excitation spectra and other observables. Consequently,
nuclear-structure calculations are becoming a tool in discriminating
different
three-body interaction models and at the same time can put constraints
on the three-body force
parameters.

It is a challenging task to extend ab initio nuclear structure
approaches to the description of nuclear reactions. For the NCSM, this
is in particular
true concerning the low-energy reactions relevant for astrophysics.
The first step toward this goal is the cluster decomposition of the NCSM
eigenstates.
I will present results of cluster form factor calculations for light
p-shell nuclei,
e.g. 7Li->3H+4He, 6Li->4He+d etc. Concerning the direct reactions, a
possible answer is
the application of semi-microscopic folding approaches to construct
optical potentials
from nuclear densities obtained in the NCSM. I will describe
calculations performed
to interpret the new CNS 6He+p experiment with a polarized proton target.