We have seen there is good understanding of some of the tree-level couplings of 4D string models. Also non-renormalization theorems guarantee that the superpotential computed at tree-level is exact at all orders in string perturbation theory. This powerful result depends crucially on the fact that the superpotential is a holomorphic function of the fields, so if by the Peccei-Quinn symmetry cannot depend on the imaginary part of the dilaton field , then it cannot depend neither on the real part of . This fact cannot be used for the Kähler potential, which in general will be corrected order by order in string perturbation theory. This is then the least known part of any string theory effective action. On the other hand the gauge kinetic function is also holomorhic and we know it exactly at the tree-level (). Since this function determines the gauge coupling itself, it is very interesting to consider the loop corrections to .

During the past several years, explicit one-loop corrections to have been computed, especially for some orbifold models. First it was shown that string loop diagrams reproduce the standard running of the gauge couplings in field theory, as expected. More interesting though, was to find the finite corrections given by threshold effects which include heavy string modes running in the loop. These corrections will be functions of the moduli fields, such as the geometric moduli but also other moduli such as continuous Wilson lines of orbifold models.

For factorized orbifold models,
the explicit dependence of the one-loop corrections
on the
moduli fields takes the form [83]:

(29) |

(30) |

(31) |

The calculation of the string one-loop corrections to the gauge couplings is
better compared to the Lagrangian for the dilaton in a linear multiplet .
After performing the duality transformation it was found that those are not
only corrections to the function but also to the Kähler potential of
the form (for factorized orbifolds)[51]:

(32) |

The knowledge of loop corrections to is not only important for studying
questions of gauge coupling unification and supersymmetry breaking
by condensation of hidden sector gauge fermions.
It is also important because there is also a non-renormalization theorem
for staying that *there are no further
corrections to f beyond one-loop*
[50]. This is again as in standard
supersymmetric theories [53]. The only thing to keep in mind is that
for it is important to state clearly that
we are working with the `Wilson' effective action rather than the 1PI
effective action.
In this case the gauge kinetic function is holomorphic
and does not get renormalized beyond one loop [53].
On the other hand, the 1PI gauge coupling is *not* holomorphic
and does get corrections from higher loops but, since it gives
the physical coupling, it is invariant under duality
symmetries.